Network Working Group M. Nystroem Internet-Draft RSA, The Security Division of EMC Intended status: Informational S. Machani Expires: December 13, 2007 Diversinet Corp. M. Pei VeriSign, Inc. A. Doherty RSA, The Security Division of EMC June 11, 2007 Dynamic Symmetric Key Provisioning Protocol (DSKPP) draft-doherty-keyprov-dskpp-00 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on December 13, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Nystroem, et al. Expires December 13, 2007 [Page 1] Internet-Draft DSKPP June 2007 Abstract DSKPP is a client-server protocol for initialization (and configuration) of cryptographic tokens. The protocol requires neither private-key capabilities in the cryptographic tokens, nor an established public-key infrastructure. The four-pass variant of the protocol ensures that a provisioned (or generated) symmetric key will only be available to the server and the cryptographic token itself. Two-pass (i.e., one round-trip) and one-pass (i.e., one message) variants enable secure and efficient download and installation of a symmetric key to a cryptographic token. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 6 1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2. Background . . . . . . . . . . . . . . . . . . . . . . . 6 1.3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 7 1.3.1. A mobile device user obtains a symmetric key . . . . 7 1.3.2. A user acquires multiple symmetric keys of different types . . . . . . . . . . . . . . . . . . 7 1.3.3. A key provisioning service imposes a validity period policy for provisioning sessions . . . . . . 7 1.3.4. A symmetric key issuer uses a third pary provisioning service provider . . . . . . . . . . . 8 1.3.5. A client application uses a pre-shared transport key to communicate with the provisioning service provider . . . . . . . . . . . . . . . . . . . . . . 8 1.3.6. A user renews its symmetric key with the same key ID . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.3.7. An administrator initiates a symmetric key replacement before it can be used . . . . . . . . . 9 1.3.8. A user acquires a symmetric key through SMS . . . . 9 1.3.9. A client acquires a symmetric key over a transport protocol that does not ensure data confidentiality . . . . . . . . . . . . . . . . . . 9 1.3.10. A client acquires a symmetric key over a transport protocol that does not provide authentication . . . . . . . . . . . . . . . . . . . 10 1.4. Requirements . . . . . . . . . . . . . . . . . . . . . . 10 1.4.1. Mandatory Requirements . . . . . . . . . . . . . . . 10 1.4.2. Desirable Requirements . . . . . . . . . . . . . . . 11 1.5. Non-Goals . . . . . . . . . . . . . . . . . . . . . . . . 11 1.6. Document organization . . . . . . . . . . . . . . . . . . 12 2. Acronyms and Notation . . . . . . . . . . . . . . . . . . . . 13 2.1. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . 13 2.2. Notation and Terminology . . . . . . . . . . . . . . . . 13 Nystroem, et al. Expires December 13, 2007 [Page 2] Internet-Draft DSKPP June 2007 2.3. XML Namespaces . . . . . . . . . . . . . . . . . . . . . 14 3. DSKPP . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 15 3.2. Entities . . . . . . . . . . . . . . . . . . . . . . . . 15 3.3. Principles of Operation . . . . . . . . . . . . . . . . . 15 3.3.1. Four-pass DSKPP . . . . . . . . . . . . . . . . . . 15 3.3.2. Two-pass DSKPP . . . . . . . . . . . . . . . . . . . 19 3.3.3. One-pass DSKPP . . . . . . . . . . . . . . . . . . . 19 3.4. Authentication . . . . . . . . . . . . . . . . . . . . . 20 3.4.1. Client Authentication . . . . . . . . . . . . . . . 20 3.4.2. Server Authentication . . . . . . . . . . . . . . . 22 3.5. Symmetric Key Container Format . . . . . . . . . . . . . 22 3.6. The DSKPP One-Way Pseudorandom Function, DSKPP-PRF . . . 22 3.6.1. Introduction . . . . . . . . . . . . . . . . . . . . 22 3.6.2. Declaration . . . . . . . . . . . . . . . . . . . . 23 3.7. Generation of Symmetric Keys for Cryptographic Tokens . . 23 3.8. Encryption of Pseudorandom Nonces Sent from the DSKPP Client . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.9. MAC calculations . . . . . . . . . . . . . . . . . . . . 24 3.9.1. Four-pass DSKPP . . . . . . . . . . . . . . . . . . 24 3.9.2. Two-pass DSKPP . . . . . . . . . . . . . . . . . . . 25 3.9.3. One-pass DSKPP . . . . . . . . . . . . . . . . . . . 26 3.10. DSKPP Schema Basics . . . . . . . . . . . . . . . . . . . 27 3.10.1. Introduction . . . . . . . . . . . . . . . . . . . . 27 3.10.2. General XML Schema Requirements . . . . . . . . . . 28 3.10.3. The AbstractRequestType Type . . . . . . . . . . . . 28 3.10.4. The AbstractResponseType Type . . . . . . . . . . . 28 3.10.5. The VersionType Type . . . . . . . . . . . . . . . . 29 3.10.6. The IdentifierType Type . . . . . . . . . . . . . . 29 3.10.7. The StatusCode Type . . . . . . . . . . . . . . . . 29 3.10.8. The DeviceIdentifierDataType Type . . . . . . . . . 32 3.10.9. The TokenPlatformInfoType and PlatformType Types . . 32 3.10.10. The NonceType Type . . . . . . . . . . . . . . . . . 33 3.10.11. The AlgorithmsType Type . . . . . . . . . . . . . . 33 3.10.12. The ProtocolVariantsType and the TwoPassSupportType Types . . . . . . . . . . . . . . 33 3.10.13. The SecretContainersFormatTypeType . . . . . . . . . 34 3.10.14. The AuthenticationDataType Type . . . . . . . . . . 35 3.10.15. The PayloadType Type . . . . . . . . . . . . . . . . 37 3.10.16. The MacType Type . . . . . . . . . . . . . . . . . . 37 3.10.17. The SecretContainerType Type . . . . . . . . . . . . 38 3.10.18. The ExtensionsType and the AbstractExtensionType Types . . . . . . . . . . . . . . . . . . . . . . . 38 3.11. DSKPP Messages . . . . . . . . . . . . . . . . . . . . . 39 3.11.1. Introduction . . . . . . . . . . . . . . . . . . . . 39 3.11.2. DSKPP Initialization (OPTIONAL) . . . . . . . . . . 39 3.11.3. The DSKPP Client's Initial PDU (2- and 4-Pass) . . . 41 3.11.4. The DSKPP Server's Initial PDU (4-Pass Only) . . . . 44 Nystroem, et al. Expires December 13, 2007 [Page 3] Internet-Draft DSKPP June 2007 3.11.5. The DSKPP Client's Second PDU (4-Pass Only) . . . . 46 3.11.6. The DSKPP Server's Final PDU (1-, 2-, and 4-Pass) . 48 3.12. Protocol Extensions . . . . . . . . . . . . . . . . . . . 49 3.12.1. The ClientInfoType Type . . . . . . . . . . . . . . 49 3.12.2. The ServerInfoType Type . . . . . . . . . . . . . . 50 3.12.3. The KeyInitializationDataType Type . . . . . . . . . 50 4. Protocol Bindings . . . . . . . . . . . . . . . . . . . . . . 52 4.1. General Requirements . . . . . . . . . . . . . . . . . . 52 4.2. HTTP/1.1 Binding for DSKPP . . . . . . . . . . . . . . . 52 4.2.1. Introduction . . . . . . . . . . . . . . . . . . . . 52 4.2.2. Identification of DSKPP Messages . . . . . . . . . . 52 4.2.3. HTTP Headers . . . . . . . . . . . . . . . . . . . . 52 4.2.4. HTTP Operations . . . . . . . . . . . . . . . . . . 53 4.2.5. HTTP Status Codes . . . . . . . . . . . . . . . . . 53 4.2.6. HTTP Authentication . . . . . . . . . . . . . . . . 53 4.2.7. Initialization of DSKPP . . . . . . . . . . . . . . 53 4.2.8. Example Messages . . . . . . . . . . . . . . . . . . 53 5. Security considerations . . . . . . . . . . . . . . . . . . . 55 5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 55 5.2. Active Attacks . . . . . . . . . . . . . . . . . . . . . 55 5.2.1. Introduction . . . . . . . . . . . . . . . . . . . . 55 5.2.2. Message Modifications . . . . . . . . . . . . . . . 55 5.2.3. Message Deletion . . . . . . . . . . . . . . . . . . 57 5.2.4. Message Insertion . . . . . . . . . . . . . . . . . 57 5.2.5. Message Replay . . . . . . . . . . . . . . . . . . . 57 5.2.6. Message Reordering . . . . . . . . . . . . . . . . . 58 5.2.7. Man-in-the-Middle . . . . . . . . . . . . . . . . . 58 5.3. Passive Attacks . . . . . . . . . . . . . . . . . . . . . 58 5.4. Cryptographic Attacks . . . . . . . . . . . . . . . . . . 58 5.5. Attacks on the Interaction between DSKPP and User Authentication . . . . . . . . . . . . . . . . . . . . . 59 5.6. Additional Considerations Specific to 2- and 1-pass DSKPP . . . . . . . . . . . . . . . . . . . . . . . . . . 59 5.6.1. Client Contributions to K_TOKEN Entropy . . . . . . 59 5.6.2. Key Confirmation . . . . . . . . . . . . . . . . . . 60 5.6.3. Server Authentication . . . . . . . . . . . . . . . 60 5.6.4. Client Authentication . . . . . . . . . . . . . . . 60 5.6.5. Key Protection in the Passphrase Profile . . . . . . 61 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 62 7. Intellectual Property Considerations . . . . . . . . . . . . 63 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 64 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 65 9.1. Normative references . . . . . . . . . . . . . . . . . . 65 9.2. Informative references . . . . . . . . . . . . . . . . . 65 Appendix A. DSKPP Schema . . . . . . . . . . . . . . . . . . . . 67 Appendix B. Key Initialization Profiles of DSKPP . . . . . . . . 76 B.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 76 B.2. Key Transport Profile . . . . . . . . . . . . . . . . . . 76 Nystroem, et al. Expires December 13, 2007 [Page 4] Internet-Draft DSKPP June 2007 B.2.1. Introduction . . . . . . . . . . . . . . . . . . . . 76 B.2.2. Identification . . . . . . . . . . . . . . . . . . . 76 B.2.3. Payloads . . . . . . . . . . . . . . . . . . . . . . 76 B.3. Key wrap profile . . . . . . . . . . . . . . . . . . . . 77 B.3.1. Introduction . . . . . . . . . . . . . . . . . . . . 77 B.3.2. Identification . . . . . . . . . . . . . . . . . . . 77 B.3.3. Payloads . . . . . . . . . . . . . . . . . . . . . . 77 B.4. Passphrase-based key wrap profile . . . . . . . . . . . . 79 B.4.1. Introduction . . . . . . . . . . . . . . . . . . . . 79 B.4.2. Identification . . . . . . . . . . . . . . . . . . . 79 B.4.3. Payloads . . . . . . . . . . . . . . . . . . . . . . 79 Appendix C. Example Messages . . . . . . . . . . . . . . . . . . 81 C.1. Example Messages in a Four-pass Exchange . . . . . . . . 81 C.1.1. Example of a DSKPP Initialization (Trigger) Message . . . . . . . . . . . . . . . . . . . . . . 81 C.1.2. Example of a Message . . . . . . . . . 82 C.1.3. Example of a Message . . . . . . . . . 83 C.1.4. Example of a Message . . . . . . . . . 83 C.1.5. Example of a Message . . . . . . . 83 C.2. Example Messages in a Two- or One-pass Exchange . . . . . 84 C.2.1. Example of a Message Indicating Support for Two-pass DSKPP . . . . . . . . . . . . . 84 C.2.2. Example of a Message Using the Key Transport Profile . . . . . . . . . . . . . . . 86 C.2.3. Example of a Message Using the Key Wrap Profile . . . . . . . . . . . . . . . . . . 88 C.2.4. Example of a Message using the Passphrase-based Key Wrap Profile . . . . . . . . . 89 Appendix D. Integration with PKCS #11 . . . . . . . . . . . . . 92 D.1. The 4-pass Variant . . . . . . . . . . . . . . . . . . . 92 D.2. The 2-pass Variant . . . . . . . . . . . . . . . . . . . 92 D.3. The 1-pass Variant . . . . . . . . . . . . . . . . . . . 95 Appendix E. Example of DSKPP-PRF Realizations . . . . . . . . . 98 E.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 98 E.2. DSKPP-PRF-AES . . . . . . . . . . . . . . . . . . . . . . 98 E.2.1. Identification . . . . . . . . . . . . . . . . . . . 98 E.2.2. Definition . . . . . . . . . . . . . . . . . . . . . 98 E.2.3. Example . . . . . . . . . . . . . . . . . . . . . . 99 E.3. DSKPP-PRF-SHA256 . . . . . . . . . . . . . . . . . . . . 99 E.3.1. Identification . . . . . . . . . . . . . . . . . . . 99 E.3.2. Definition . . . . . . . . . . . . . . . . . . . . . 100 E.3.3. Example . . . . . . . . . . . . . . . . . . . . . . 101 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 102 Intellectual Property and Copyright Statements . . . . . . . . . 103 Nystroem, et al. Expires December 13, 2007 [Page 5] Internet-Draft DSKPP June 2007 1. Introduction 1.1. Scope This document describes a client-server protocol for initialization (and configuration) of cryptographic tokens. The protocol requires neither private-key capabilities in the cryptographic tokens, nor an established public-key infrastructure. The objectives of this protocol are to: o Provide a secure method of initializing cryptographic tokens with symmetric keys without exposing generated, secret material to any other entities than the server and the cryptographic token itself. o Avoid, as much as possible, any impact on existing cryptographic token manufacturing processes. o Provide a solution that is easy to administer and scales well. The mechanism is intended for general use within computer and communications systems employing connected cryptographic tokens (or software emulations thereof). 1.2. Background A cryptographic token MAY be a hand-held hardware device, a hardware device connected to a personal computer through an electronic interface, such as USB, or a software module resident on a personal computer, which offers cryptographic functionality that MAY be used to authenticate a user towards some service. Increasingly, these tokens work in a connected fashion, enabling their programmatic initialization as well as programmatic retrieval of their output values. This document intends to meet the need for an open and inter-operable mechanism to programmatically initialize and configure symmetric keys to connected cryptographic tokens. The target mechanism addressed herein is a symmetric key provisioning server. In an ideal deployment scenario, direct and near real-time communication is possible between the provisioning server and the cryptographic token. In such an environment, it is possible for the cryptographic token and provisioning server to mutually generate a symmetric key as defined in [4]). This is the strongest approach to symmetric key provisioning, as the key is not transported between systems, and is therefore not vulnerable to man-in-the-middle (MITM) attacks. There are, however, several deployment scenarios where direct Nystroem, et al. Expires December 13, 2007 [Page 6] Internet-Draft DSKPP June 2007 communication between the symmetric key provisioning server and the cryptographic token is not possible, where work-flow constraints otherwise would limit real-time communications (e.g. need for administrators to authorize processes), or where network latency or other design constraints (such as when initialization of tokens using existing keys from legacy systems is required) makes mutual key generation less suitable. In these situations, the token is required to download and install a symmetric key from the provisioning server in a secure and efficient manner. This document tries to meet the needs of these scenarios by describing variations to DSKPP for the provisioning of symmetric keys in two round trips or less. 1.3. Use Cases The applicability of DSKPP is described by the use cases in this section. The next section describes the primary requirements that were derived from these use cases. 1.3.1. A mobile device user obtains a symmetric key A user with a mobile device wants to acquire a symmetric key to use with a software-based cryptographic token in the device. The symmetric key MAY be pre-generated by a back-end issuance server, or generated by the provisioning server during the provisioning process. A unique Secret ID is assigned to the symmetric key by the provisioning server. This protocol enables the client device to request the symmetric key, authenticate to the provisioning server, download the symmetric key over-the-air (OTA), and install it on the mobile device. 1.3.2. A user acquires multiple symmetric keys of different types A user wants to provision multiple symmetric keys on a device. The symmetric keys MAY or MAY NOT be of the same type. The keys MAY be used with different algorithms, such as the HMAC-Based One-Time Password (HOTP), RSA SecurID, symmetric challenge-response, or others. The protocol MUST provide for a mechanism to uniquely identify a specific symmetric key in the device using token identification to allow device authentication before provisioning. 1.3.3. A key provisioning service imposes a validity period policy for provisioning sessions Once a user initiates a symmetric key request, the key provisioning service may require that any subsequent actions to complete the provisioning cycle occur within a certain time window. For example, Nystroem, et al. Expires December 13, 2007 [Page 7] Internet-Draft DSKPP June 2007 a provisioning issuer MAY provide an authentication code to a user upon the user's initial request for a secret key. Such an authentication code is associated with a validity period; a user MUST consume the pick-up code to initialize or download a symmetric key within the validity window. 1.3.4. A symmetric key issuer uses a third pary provisioning service provider A symmetric key issuer outsources its key provisioning to a third party key provisioning server provider. The issuer is responsible for authenticating and granting rights to users to acquire keys while it may delegate the actual key generation and provisioning to a third party provisioning service. The issuer may acquire symmetric keys on behalf of its users from the provisioning service provider or redirect the user to acquire the secrets directly from provisioning service provider. In the latter case, it is often necessary for a user to authenticate to the provisioning service provider. 1.3.5. A client application uses a pre-shared transport key to communicate with the provisioning service provider A software-based cryptographic token application is loaded onto a smart card after the card is issued to a user. The symmetric key for the cryptographic token application will then be provisioned using a secure channel mechanism present in many smart card platforms. This allows a direct secure channel to be established between the smart card chip and the provisioning server. For example, the card commands (i.e., Application Protocol Data Units, or APDUs) are encrypted with a pre-shared transport key and sent directly to the smart card chip, allowing secure post-issuance in-the-field provisioning. This secure flow can pass Transport Layer Security (TLS) and other transport security boundaries. Note that this use case requires DSKPP to be tunneled and the provisioning server to know the correct pre-established transport key. 1.3.6. A user renews its symmetric key with the same key ID A user wants to renew its symmetric key with the same key ID. Such a need may occur in the case when a user wants to upgrade its cryptographic token device or when a key has expired. When a user uses the same cryptographic token to, for example, perform strong authentication at multiple Web login sites, keeping the same key ID removes the need for the user to register a new key ID at each site. Nystroem, et al. Expires December 13, 2007 [Page 8] Internet-Draft DSKPP June 2007 1.3.7. An administrator initiates a symmetric key replacement before it can be used This use case represents a special case of symmetric key renewal in which a local administrator can authenticate the user procedurally before initiating DSKPP. It also allows for keys on physical cryptographic tokens to be issued with a restriction that the key MUST be replaced with a new key prior to token use. Bulk initialization under controlled conditions during manufacture is likely to meet the security needs of most applications. However, reliance on a pre-disclosed secret is unacceptable in some circumstances. One circumstance is when cryptographic tokens are issued for classified government use or high security applications. In such cases, the token issuer requires the ability to remove all secret information installed on the token during manufacture and replace it with secret keys established under conditions controlled by the issuer. It is, however, in most cases impractical for the administrator to apply a physical marking to the token itself, such as a serial number. It is, therefore, necessary for the enrollment process to communicate the token serial number to the provisioning service. Another variation of this use case is that some enterprises may prefer to re-provision a new secret to an existing cryptographic token if they decide to reuse the token that was with one user and for a new user. Note that this use case is essentially the same as the last use case wherein the same key ID is used for renewal. 1.3.8. A user acquires a symmetric key through SMS A mobile device may support Short Message Service (SMS) but is not able to support a data service allowing for HTTP or HTTPS transports. In such a case, the user may initiate a symmetric key request from a desktop computer and ask the server to send the key to a mobile phone through SMS. The online communication between the desktop computer and the server can carry out user authentication. 1.3.9. A client acquires a symmetric key over a transport protocol that does not ensure data confidentiality Some devices are not able to support a secure transport channel such as TLS to provide data confidentiality. A user wants to provision a symmetric key to such a device. It is up to DSKPP to ensure data confidentiality over non-secure networks. Nystroem, et al. Expires December 13, 2007 [Page 9] Internet-Draft DSKPP June 2007 1.3.10. A client acquires a symmetric key over a transport protocol that does not provide authentication Some devices are not able to use a transport protocol that provides server authentication such as TLS. A user wants to be sure that it acquires a symmetric key from an authentic provisioning service provider. It is up to DSKPP to provide proper client and server authentication. 1.4. Requirements This section specifies mandatory and desirable protocol requirements. 1.4.1. Mandatory Requirements R1: The protocol SHOULD support multiple types of keys for symmetric key-based authentication methods. R2: The protocol SHOULD support re-generated symmetric keys (by separate key issuance service) or locally generated keys in real-time (by provisioning server). R3: The protocol SHOULD support mutually generated symmetric keys by both client and server. R4: The protocol SHOULD allow devices to host multiple symmetric keys; each key MAY be acquired in a separate provisioning session. R5: The protocol SHOULD support renewal of a symmetric key with the same key ID. R6: The protocol SHOULD allow clients to specify their cryptographic and security capabilities to the server and the server to indicate the cryptography and algorithm types that it will be using. R7: The protocol SHOULD support mutual authentication and confidentiality of sensitive provisioning data R8: The protocol SHOULD NOT require a public-key infrastructure and the use of client certificates for device authentication or symmetric key data protection. It MUST allow for other mechanisms, such as symmetric key-based techniques, to be used. Nystroem, et al. Expires December 13, 2007 [Page 10] Internet-Draft DSKPP June 2007 R9: The protocol SHOULD NOT rely on transport layer security (e.g., SSL/TLS) for core security requirements. It SHOULD be compatible with transport layer security when available. R10: The protocol SHOULD allow for the transport of the symmetric key expiration date set by the key issuer. R11: The protocol SHOULD allow the server to use pre-loaded symmetric transport keys on a device-by-device basis (i.e., smart card update keys, such as used by Global Platform for establishing a secure channel). R12: The protocol SHOULD enable simple user experience for the provisioning process. R13: The protocol SHOULD protect against replay attacks. R14: The protocol SHOULD protect against MITM attacks. 1.4.2. Desirable Requirements D1: The protocol MAY support a device request to acquire multiple symmetric keys in the same session. D2: The protocol MAY allow the provisioning server to verify that the key has been correctly provisioned to the client. D3: The protocol MAY support a client to notify the server upon symmetric key deletion. 1.5. Non-Goals The following is a list of features that are not required of the protocol: NR1: Support for client generated symmetric key upload to a provisioning server. NR2: Support for other key lifecycle management functions, such as key suspension, lock, and activation. These functions are supported in symmetric key-based application, such as the authentication system. NR3: Support for asymmetric key pair provisioning. Nystroem, et al. Expires December 13, 2007 [Page 11] Internet-Draft DSKPP June 2007 1.6. Document organization The organization of this document is as follows: o Section 1 is an introduction. o Section 2 defines acronyms and notation used in this document. o Section 3 defines the protocol mechanism in detail. o Section 4 defines a binding of the protocol to the transport layer. o Section 5 discusses security considerations. o Appendix A defines the XML schema for the protocol mechanism. o Appendix B contains key initialization profiles for the 1- and 2-pass versions of DSKPP defined herein. o Appendix C provides example messages. o Appendix D discusses integration with PKCS #11 [5]. o Appendix E provides example realizations of an abstract pseudorandom function defined in Section 3.6. Nystroem, et al. Expires December 13, 2007 [Page 12] Internet-Draft DSKPP June 2007 2. Acronyms and Notation 2.1. Acronyms DSKPP Dynamic Symmetric Key Provisioning Protocol HMAC Hashing for Message Authentication HOTP HMAC-Based One-Time Password MAC Message Authentication Code MITM Man-in-the-Middle OTP One-Time Password PDU Protocol Data Unit PRF Pseudo-Random Function PSKC Portable Symmetric Key Container SOAP Simple Object Access Protocol TLS Transport Layer Security XML Extensible Markup Language 2.2. Notation and Terminology The following notations are used in this document: || String concatenation [x] Optional element x A ^ B Exclusive-OR operation on strings A and B (where A and B are of equal length) ID_C Identifier for DSKPP client ID_S Identifier for DSKPP server K Key used to encrypt R_C (either K_SERVER or K_SHARED) Nystroem, et al. Expires December 13, 2007 [Page 13] Internet-Draft DSKPP June 2007 K_AUTH Secret key used for authentication purposes in 4-pass DSKPP K_CLIENT Public key of the cryptographic token K_DERIVED Secret key derived from a passphrase that is known to both the cryptographic token or user and the DSKPP server K_MAC Secret key used for key confirmation and authentication purposes, generated in DSKPP K_SERVER Public key of the DSKPP server K_SHARED Secret key shared between the cryptographic token and the DSKPP server K_TOKEN Secret key used for token computations, generated in DSKPP R Pseudorandom value chosen by the cryptographic token and used for MAC computations R_C Pseudorandom value chosen by the cryptographic token R_S Pseudorandom value chosen by the DSKPP server The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [6]. The following typographical convention is used in the body of the text: . 2.3. XML Namespaces The target XML namespace for data types defined in this document is as follows: xmlns:dskpp="urn:ietf:params:xml:ns:keyprov:protocol" The protocol also relies on the following namespace for XML types defined in [7]: xmlns:pskc="urn:ietf:params:xml:ns:keyprov:container" It also uses the following namespace for data types defined in [1]: xmlns:ds="http://www.w3.org/2000/09/xmldsig#" Nystroem, et al. Expires December 13, 2007 [Page 14] Internet-Draft DSKPP June 2007 3. DSKPP 3.1. Overview The DSKPP is a client-server protocol for the secure initialization and configuration of cryptographic tokens. The protocol is meant to provide high assurance for both the provisioning server and the client (i.e., the cryptographic token) that generated keys have been correctly and randomly generated and not exposed to other entities. The protocol does not require the existence of a public-key infrastructure. 3.2. Entities In principle, the protocol involves a DSKPP client and a DSKPP server. It is assumed that a desktop/laptop or a wireless device (e.g., a mobile phone or a PDA) will host an application communicating with the DSKPP server as well as the cryptographic token, and collectively, the cryptographic token and the communicating application form the DSKPP client. When there is a need to point out if an action is to be performed by the communicating application or by the token, the text will make this explicit. The manner in which the communicating application will transfer DSKPP protocol elements to and from the cryptographic token is transparent to the DSKPP server. One method for this transfer is described in [8]. 3.3. Principles of Operation To initiate a DSKPP session, a user MAY use a browser to connect to a web server (or service) running on some host. The user MAY then identify and OPTIONALLY authenticate herself and possibly indicate how the DSKPP client SHALL contact the DSKPP server. There are also other alternatives for DSKPP session initiation, such as the DSKPP client being pre-configured to contact a certain DSKPP server, or the user being informed out-of-band about the location of the DSKPP server. In any event, once the location of the DSKPP server is known, the DSKPP client and the DSKPP server engage in a 4-pass, 2-pass, or 1-pass protocol, depending upon the deployment scenario. 3.3.1. Four-pass DSKPP The 4-pass protocol flow is suitable for environments wherein there is a direct and near real-time communication possible between the DSKPP client and DSKPP server. It is not suitable for environments Nystroem, et al. Expires December 13, 2007 [Page 15] Internet-Draft DSKPP June 2007 wherein administrative approval is a required step in the flow, nor for provisioning of legacy keys. The 4-pass protocol flow, shown in Figure 1 and expanded in Figure 2, consists of two round-trips between the DSKPP client and server. +---------------+ +---------------+ | | | | | DSKPP client | | DSKPP server | | | | | +---------------+ +---------------+ | | | [ <---- DSKPP trigger ----- ] | | | | ------- Client Hello -------> | | | | <------ Server Hello -------- | | | | ------- Client Nonce -------> | | | | <----- Server Finished ------ | | | Figure 1: The 4-pass DSKPP protocol (with OPTIONAL preceding trigger) a. The DSKPP client sends a message to the DSKPP server. The contents of the message provide information to the DSKPP server about the cryptographic token's identity, supported DSKPP versions, cryptographic algorithms supported by the token and key types that MAY be generated using this protocol, and encryption and MAC algorithms supported by the cryptographic token for the purposes of this protocol. b. The DSKPP server responds to the DSKPP client with a message, whose content includes a random nonce, R_S, along with information about the type of key to generate, and the encryption algorithm chosen to protect sensitive data sent in the protocol. In addition, the message provides either information about a shared secret key to use for encrypting the cryptographic token's random nonce (see below), or its own public key. The length of the nonce R_S MAY depend on the selected key type. c. Based on information contained in the message, the cryptographic token generates a random nonce, R_C, and encrypts it using the selected encryption algorithm and with a key, K, that is either the DSKPP server's public key, K_SERVER, or a shared secret key, K_SHARED, as indicated by the DSKPP server. The length of the nonce R_C MAY depend on the selected key type. The DSKPP client then sends the encrypted random Nystroem, et al. Expires December 13, 2007 [Page 16] Internet-Draft DSKPP June 2007 nonce to the DSKPP server in a message. The token also calculates a cryptographic key, K_TOKEN, of the selected type from the combination of the two random nonces R_S and R_C, the encryption key K, and possibly some other data, using the DSKPP-PRF function defined herein. d. The DSKPP server decrypts R_C, calculates K_TOKEN from the combination of the two random nonces R_S and R_C, the encryption key K, and possibly some other data, using the DSKPP-PRF function defined herein. The server then associates K_TOKEN with the cryptographic token in a server-side data store. The intent is that the data store later on will be used by some service that needs to verify or decrypt data produced by the cryptographic token and the key. e. Once the association has been made, the DSKPP server sends a confirmation message to the DSKPP client called . The confirmation message includes an identifier for the generated key and MAY also contain additional configuration information, e.g., the identity of the DSKPP server. f. Upon receipt of the DSKPP server's confirmation message, the cryptographic token associates the provided key identifier with the generated key K_TOKEN, and stores the provided configuration data, if any. Note: Conceptually, although R_C is one pseudorandom string, it MAY be viewed as consisting of two components, R_C1 and R_C2, where R_C1 is generated during the protocol run, and R_C2 can be generated at the cryptographic token manufacturing time and stored in the cryptographic token. In that case, the latter string, R_C2, SHOULD be unique for each cryptographic token for a given manufacturer. The inclusion of the two random nonces R_S and R_C in the key generation provides assurance to both sides (the token and the DSKPP server) that they have contributed to the key's randomness and that the key is unique. The inclusion of the encryption key K ensures that no man-in-the-middle MAY be present, or else the cryptographic token will end up with a key different from the one stored by the legitimate DSKPP server. Note: A man-in-the-middle (in the form of corrupt client software or a mistakenly contacted server) MAY present his own public key to the token. This will enable the attacker to learn the client's version of K_TOKEN. However, the attacker is not able to persuade the legitimate server to derive the same value for K_TOKEN, since K_TOKEN is a function of the public key involved, and the attacker's public Nystroem, et al. Expires December 13, 2007 [Page 17] Internet-Draft DSKPP June 2007 key must be different than the correct server's (or else the attacker would not be able to decrypt the information received from the client). Therefore, once the attacker is no longer "in the middle," the client and server will detect that they are "out of synch" when they try to use their keys. In the case of encrypting R_C with K_SERVER, it is therefore important to verify that K_SERVER really is the legitimate server's key. One way to do this is to independently validate a newly generated K_TOKEN against some validation service at the server (e.g. by using a connection independent from the one used for the key generation). +----------------------+ +-------+ +----------------------+ | +------------+ | | | | | | | Server key | | | | | | | +<-| Public |------>------------->-------------+---------+ | | | | Private | | | | | | | | | | +------------+ | | | | | | | | | | | | | | | | | | V V | | | | V V | | | +---------+ | | | | +---------+ | | | | | Decrypt |<-------<-------------<-----------| Encrypt | | | | | +---------+ | | | | +---------+ | | | | | +--------+ | | | | ^ | | | | | | Server | | | | | | | | | | | | Random |--->------------->------+ +----------+ | | | | | +--------+ | | | | | | Client | | | | | | | | | | | | | Random | | | | | | | | | | | | +----------+ | | | | | | | | | | | | | | | | V V | | | | V V | | | | +------------+ | | | | +------------+ | | | +-->| DSKPP PRF | | | | | | DSKPP PRF |<----+ | | +------------+ | | | | +------------+ | | | | | | | | | | V | | | | V | | +-------+ | | | | +-------+ | | | Key | | | | | | Key | | | +-------+ | | | | +-------+ | | +-------+ | | | | +-------+ | | |Key Id |-------->------------->------|Key Id | | | +-------+ | | | | +-------+ | +----------------------+ +-------+ +----------------------+ DSKPP Server DSKPP Client DSKPP Client (Token) (PC Host) Figure 2: Principal data flow for DSKPP key generation - using public server key Nystroem, et al. Expires December 13, 2007 [Page 18] Internet-Draft DSKPP June 2007 3.3.2. Two-pass DSKPP In two-pass DSKPP, the client's initial message is directly followed by a message. There is no exchange of the message or the message. Essentially, two-pass DSKPP is a transport of key material from the DSKPP server to the DSKPP client. However, as the two-pass variant of DSKPP consists of one round trip to the server, the client is still able to specify algorithm preferences and supported key types in the message. Note that the DSKPP "trigger" message MAY be used to trigger the client's sending of the message. +---------------+ +---------------+ | | | | | DSKPP client | | DSKPP server | | | | | +---------------+ +---------------+ | | | [ <---- DSKPP trigger ----- ] | | | | ------- Client Hello -------> | | | | <----- Server Finished ------ | | | Figure 1: The 2-pass DSKPP protocol (with OPTIONAL preceding trigger) 3.3.3. One-pass DSKPP In one-pass DSKPP, the server simply sends a message to the DSKPP client. In this case, there is no exchange of the , , and DSKPP messages, and hence there is no way for the client to express supported algorithms or key types. Before attempting one-pass DSKPP, the server MUST therefore have prior knowledge not only that the client is able and willing to accept this variant of DSKPP, but also of algorithms and key types supported by the client. Nystroem, et al. Expires December 13, 2007 [Page 19] Internet-Draft DSKPP June 2007 +---------------+ +---------------+ | | | | | DSKPP client | | DSKPP server | | | | | +---------------+ +---------------+ | | | <----- Server Finished ------ | | | Figure 1: The 1-pass DSKPP protocol Outside the specific cases where one-pass DSKPP is desired, clients SHOULD be constructed and configured to only accept DSKPP server messages in response to client-initiated transactions. 3.4. Authentication 3.4.1. Client Authentication To ensure that a generated K_TOKEN ends up associated with the correct token and user, the DSKPP server MAY couple an initial user authentication to the DSKPP execution in several ways, as discussed in the following sub-sections. Whatever the method, the DSKPP server MUST ensure that a generated key is associated with the correct token, and if applicable, the correct user. For a further discussion of this, and threats related to man-in-the-middle attacks in this context, see Section 5. 3.4.1.1. Device Certificate Instead of requiring an Authentication Code for in-band authentication, a device certificate could be used, which was supplied with the cryptographic token by its issuer. 3.4.1.2. Device Identifier The provisioning server could be pre-configured with a device identifier. The DSKPP server MAY then include this token identifier in the DSKPP initialization trigger, and the DSKPP client would include it in its message(s) to the DSKPP server for authentication. Note that it is also legitimate for a DSKPP client to initiate the DSKPP protocol run without having received an initialization message from a server, but in this case any provided device identifier SHALL NOT be accepted by the DSKPP server unless the server has access to a unique key for the identified device and that key will be used in the protocol. Nystroem, et al. Expires December 13, 2007 [Page 20] Internet-Draft DSKPP June 2007 3.4.1.3. One-time Use Authentication Code A token issuer MAY provide a one-time value, called an Authentication Code, to the user or device out-of-band and require this value to be used by the DSKPP client when contacting the DSKPP server. The DSKPP client includes the authentication data in its request message, and the DSKPP server MUST verify the data before continuing with the protocol run. Note: An alternate method for getting the Authentication Code to the client, is for the DSKPP server to place the value in the element of the DSKPP initialization trigger (if triggers are used; see Section 4.2.7) . +------------+ Get Authentication Code +------------+ | User |<------------------------->| Issuer | +------------+ +------------+ | | | | | | V V +--------------+ +--------------+ | Provisioning | Authentication Data | Provisioning | | Client |----------------------->| Server | +--------------+ +--------------+ Figure 3: User Authentication with One-Time Code Considering an Authentication Code as a special form of shared secret between a user and a provisioning server, Authentication Data can have one of the following forms: o AuthenticationData = Hash (Authentication Code) When an Authentication Code is used to initiate the protocol run, the Authentication Code MUST be sent to the DSKPP server in a secure manner. If the underlying transport channel is secure, the authentication data MAY contain the plaintext format or the hashed format of the Authentication Code using a hash function. o AuthenticationData = HMAC (Authentication Code, R_S) If the underlying transport is not secure, the client MUST use both the server nonce R_S and the Authentication Code to derive authentication data. o AuthenticationData = Nystroem, et al. Expires December 13, 2007 [Page 21] Internet-Draft DSKPP June 2007 When a certificate is used for authentication, the authentication data MAY be client-signed. Authentication data MAY be omitted if client certificate authentication has been provided by the transport channel such as TLS. When a token issuer delegates symmetric key provisioning to a third party provisioning service provider, both client authentication and issuer authentication are required by the provisioning server. Client authentication to the Issuer MAY be in-band or out-of-band as described above. The issuer acts as a proxy for the provisioning server. The issuer authenticates to the provisioning service provider either using a certificate or a pre-established secret key. 3.4.2. Server Authentication A DSKPP server MUST authenticate itself to avoid a false "Commit" of a symmetric key that which could cause the cryptographic token to end up in an initialized state for which the server does not know the stored key. To do this, the DSKPP server authenticates itself by including a MAC in each of its responses to the client. In 2-pass and 1-pass DSKPP, servers authenticate themselves by including a second MAC value in the response message. In addition, a DSKPP server can leverage transport layer authentication if it is available. 3.5. Symmetric Key Container Format The default symmetric key container format that is used in the message is based on the Portable Symmetric Key Container (PSKC) defined in [7]. Alternative formats MAY include PKCS#12 [9] or PKCS#5 XML [10] format. 3.6. The DSKPP One-Way Pseudorandom Function, DSKPP-PRF 3.6.1. Introduction The general requirements on DSKPP-PRF are the same as on keyed hash functions: It SHALL take an arbitrary length input, and be one-way and collision-free (for a definition of these terms, see, e.g., [11]). Further, the DSKPP-PRF function SHALL be capable of generating a variable-length output, and its output SHALL be unpredictable even if other outputs for the same key are known. It is assumed that any realization of DSKPP-PRF takes three input parameters: A secret key k, some combination of variable data, and the desired length of the output. The combination of variable data can, without loss of generalization, be considered as a salt value (see PKCS#5 Version 2.0 [12], Section 4), and this characterization Nystroem, et al. Expires December 13, 2007 [Page 22] Internet-Draft DSKPP June 2007 of DSKPP-PRF SHOULD fit all actual PRF algorithms implemented by tokens. From the point of view of this specification, DSKPP-PRF is a "black-box" function that, given the inputs, generates a pseudorandom value. Separate specifications MAY define the implementation of DSKPP-PRF for various types of cryptographic tokens. Appendix E contains two example realizations of DSKPP-PRF. 3.6.2. Declaration DSKPP-PRF (k, s, dsLen) Input: k secret key in octet string format s octet string of varying length consisting of variable data distinguishing the particular string being derived dsLen desired length of the output Output: DS pseudorandom string, dsLen-octets long For the purposes of this document, the secret key k SHALL be 16 octets long. 3.7. Generation of Symmetric Keys for Cryptographic Tokens In DSKPP, keys are generated using the DSKPP-PRF function, a secret random value R_C chosen by the DSKPP client, a random value R_S chosen by the DSKPP server, and the key k used to encrypt R_C. The input parameter s of DSKPP-PRF is set to the concatenation of the (ASCII) string "Key generation", k, and R_S, and the input parameter dsLen is set to the desired length of the key, K_TOKEN (the length of K_TOKEN is given by the key's type): dsLen = (desired length of K_TOKEN) K_TOKEN = DSKPP-PRF (R_C, "Key generation" || k || R_S, dsLen) When computing K_TOKEN above, the output of DSKPP-PRF MAY be subject to an algorithm-dependent transform before being adopted as a key of the selected type. One example of this is the need for parity in DES keys. Nystroem, et al. Expires December 13, 2007 [Page 23] Internet-Draft DSKPP June 2007 3.8. Encryption of Pseudorandom Nonces Sent from the DSKPP Client DSKPP client random nonce(s) are either encrypted with the public key provided by the DSKPP server or by a shared secret key. For example, in the case of a public RSA key, an RSA encryption scheme from PKCS #1 [13] MAY be used. In the case of a shared secret key, to avoid dependence on other algorithms, the DSKPP client MAY use the DSKPP-PRF function described herein with the shared secret key K_SHARED as input parameter k (in this case, K_SHARED SHOULD be used solely for this purpose), the concatenation of the (ASCII) string "Encryption" and the server's nonce R_S as input parameter s, and dsLen set to the length of R_C: dsLen = len(R_C) DS = DSKPP-PRF(K_SHARED, "Encryption" || R_S, dsLen) This will produce a pseudorandom string DS of length equal to R_C. Encryption of R_C MAY then be achieved by XOR-ing DS with R_C: Enc-R_C = DS ^ R_C The DSKPP server will then perform the reverse operation to extract R_C from Enc-R_C. Note: It may appear that an attacker, who learns a previous value of R_C, may be able to replay the corresponding R_S and, hence, learn a new R_C as well. However, this attack is mitigated by the requirement for a server to show knowledge of K_AUTH (see below) in order to successfully complete a key re-generation. 3.9. MAC calculations 3.9.1. Four-pass DSKPP 3.9.1.1. Server Authentication: The MAC value SHALL be computed on the (ASCII) string "MAC 1 computation", the client's nonce R (if sent), and the server's nonce R_S using an authentication key K_AUTH that SHOULD be a special authentication key used only for this purpose but MAY be the current K_TOKEN. The MAC value MAY be computed by using the DSKPP-PRF function of Section 3.6, in which case the input parameter s SHALL be set to the concatenation of the (ASCII) string "MAC 1 computation", R (if sent by the client), and R_S, and k SHALL be set to K_AUTH. The input Nystroem, et al. Expires December 13, 2007 [Page 24] Internet-Draft DSKPP June 2007 parameter dsLen SHALL be set to the length of R_S: dsLen = len(R_S) MAC = DSKPP-PRF (K_AUTH, "MAC 1 computation" || [R ||] R_S, dsLen) 3.9.1.2. Server Authentication: The MAC value SHALL be computed on the (ASCII) string "MAC 2 computation" and R_C using an authentication key K_AUTH. Again, this SHOULD be a special authentication key used only for this purpose, but MAY also be an existing K_TOKEN. (In this case, implementations MUST protect against attacks where K_TOKEN is used to pre-compute MAC values.) If no authentication key is present in the token, and no K_TOKEN existed before the DSKPP run, K_AUTH SHALL be the newly generated K_TOKEN. If DSKPP-PRF is used as the MAC algorithm, then the input parameter s SHALL be consist of the concatenation of the (ASCII) string "MAC 2 computation", R_C, the parameter dsLen SHALL be set to the length of R_C: dsLen = len(R_C) MAC = DSKPP-PRF (K_AUTH, "MAC 2 computation" || R_C, dsLen) 3.9.2. Two-pass DSKPP 3.9.2.1. Key Confirmation In two-pass DSKPP, the client is REQUIRED to include a nonce R in the message. Further, the server is REQUIRED to include an identifier, ID_S, for itself (in the element) in the message. The MAC value in the message SHALL be computed on the (ASCII) string "MAC 1 computation", the server identifier ID_S, and R using a MAC key K_MAC. Again, in contrast with the MAC calculation in the four-pass DSKPP, this key SHALL be provided together with K_TOKEN to the token, and hence there is no need for a K_AUTH for key confirmation purposes. If DSKPP-PRF is used as the MAC algorithm, then the input parameter s SHALL consist of the concatenation of the (ASCII) string "MAC 1 computation" and R, and the parameter dsLen SHALL be set to the length of R: dsLen = len(R) MAC = DSKPP-PRF (K_MAC, "MAC 1 computation" || ID_S || R, dsLen) Nystroem, et al. Expires December 13, 2007 [Page 25] Internet-Draft DSKPP June 2007 3.9.2.2. Server Authentication As discussed in Section 3.4.2, servers need to authenticate themselves when attempting to replace an existing K_TOKEN. In 2-pass DSKPP, servers authenticate themselves by including a second MAC value in the AuthenticationDataType extension. The MAC value in the AuthenticationDataType extension SHALL be computed on the (ASCII) string "MAC 1 computation", the server identifier ID_S, and R, using the existing MAC key K_MAC' (the MAC key that existed before this protocol run). The MAC algorithm SHALL be the same as the algorithm used for key confirmation purposes. If DSKPP-PRF is used as the MAC algorithm, then the input parameter s SHALL consist of the concatenation of the (ASCII) string "MAC 1 computation" ID_S, and R. The parameter dsLen SHALL be set to at least 16 (i.e. the length of the MAC SHALL be at least 16 octets): dsLen >= 16 MAC = DSKPP-PRF (K_MAC', "MAC 1 computation" || ID_S || R, dsLen) 3.9.3. One-pass DSKPP 3.9.3.1. Key Confirmation In one-pass DSKPP, the server is REQUIRED to include an identifier, ID_S, for itself (in the element) in the message. The MAC value in the message SHALL be computed on the (ASCII) string "MAC 1 computation", the server identifier ID_S, and an unsigned integer value I, using a MAC key K_MAC. The value I MUST be monotonically increasing and guaranteed not to be used again by this server towards this token. It could for example be the number of seconds since some point in time with sufficient granularity, a counter value, or a combination of the two where the counter value is reset for each new time value. In contrast to the MAC calculation in four-pass DSKPP, the MAC key K_MAC SHALL be provided together with K_TOKEN to the token, and hence there is no need for a K_AUTH for key confirmation purposes. Note: The integer I does not necessarily need to be maintained per token by the DSKPP server (it is enough if the server can guarantee that the same value is never being sent twice to the same token). If DSKPP-PRF is used as the MAC algorithm, then the input parameter s SHALL consist of the concatenation of the (ASCII) string "MAC 1 computation", ID_S, and I. The parameter dsLen SHALL be set to at least 16 (i.e. the length of the MAC SHALL be at least 16 octets): Nystroem, et al. Expires December 13, 2007 [Page 26] Internet-Draft DSKPP June 2007 dsLen >= 16 MAC = DSKPP-PRF (K_MAC, "MAC 1 computation" || ID_S || I, dsLen) The server SHALL provide I to the client in the Nonce attribute of the element of the message using the AuthenticationCodeMacType defined in Section 3.10.14. 3.9.3.2. Server Authentication As discussed in Section 3.4.2, servers need to authenticate themselves when attempting to replace an existing K_TOKEN. In 1-pass DSKPP, servers authenticate themselves by including a second MAC value in the AuthenticationDataType extension. The MAC value in the AuthenticationDataType extension SHALL be computed on the (ASCII) string "MAC 1 computation", the server identifier ID_S, and a new value I', I' > I, using the existing MAC key K_MAC' (the MAC key that existed before this protocol run). The MAC algorithm SHALL be the same as the algorithm used for key confirmation purposes. If DSKPP-PRF is used as the MAC algorithm, then the input parameter s SHALL consist of the concatenation of the (ASCII) string "MAC 1 computation" ID_S, and I'. The parameter dsLen SHALL be set to at least 16 (i.e. the length of the MAC SHALL be at least 16 octets): dsLen >= 16 MAC = DSKPP-PRF (K_MAC', "MAC 1 computation" || ID_S || I', dsLen) The server SHALL provide I' to the client in the Nonce attribute of the element of the AuthenticationDataType extension. If the protocol run is successful, the client stores I' as the new value of I for this server. 3.10. DSKPP Schema Basics 3.10.1. Introduction Core parts of the XML schema for DSKPP, found in Appendix A, are explained in this section. Specific protocol message elements are defined in Section 3.11. Examples can be found in Appendix C. The XML format for DSKPP messages have been designed to be extensible. However, it is possible that the use of extensions will harm interoperability; therefore, any use of extensions SHOULD be carefully considered. For example, if a particular implementation relies on the presence of a proprietary extension, then it may not be able to interoperate with independent implementations that have no Nystroem, et al. Expires December 13, 2007 [Page 27] Internet-Draft DSKPP June 2007 knowledge of this extension. XML types defined in this sub-section are not DSKPP messages; rather they provide building blocks that are used by DSKPP messages. 3.10.2. General XML Schema Requirements Some DSKPP elements rely on the parties being able to compare received values with stored values. Unless otherwise noted, all elements in this document that have the XML Schema "xs:string" type, or a type derived from it, MUST be compared using an exact binary comparison. In particular, DSKPP implementations MUST NOT depend on case-insensitive string comparisons, normalization or trimming of white space, or conversion of locale-specific formats such as numbers. Implementations that compare values that are represented using different character encodings MUST use a comparison method that returns the same result as converting both values to the Unicode character encoding, Normalization Form C [2], and then performing an exact binary comparison. No collation or sorting order for attributes or element values is defined. Therefore, DSKPP implementations MUST NOT depend on specific sorting orders for values. 3.10.3. The AbstractRequestType Type All DSKPP requests are defined as extensions to the abstract AbstractRequestType type. The elements of the AbstractRequestType, therefore, apply to all DSKPP requests. All DSKPP requests MUST contain a Version attribute. For this version of this specification, Version SHALL be set to "1.0". 3.10.4. The AbstractResponseType Type All DSKPP responses are defined as extensions to the abstract AbstractResponseType type. The elements of the AbstractResponseType, therefore, apply to all DSKPP responses. All DSKPP responses contain a Version attribute indicating the version that was used. A Status attribute, which indicates whether the preceding request was successful or not MUST also be present. Finally, all responses MAY contain a SessionID attribute identifying the particular DSKPP session. The SessionID attribute needs only be present if more than Nystroem, et al. Expires December 13, 2007 [Page 28] Internet-Draft DSKPP June 2007 one roundtrip is REQUIRED for a successful protocol run (this is the case with the protocol version described herein). 3.10.5. The VersionType Type The VersionType type is used within DSKPP messages to identify the highest version of this protocol supported by the DSKPP client and server. 3.10.6. The IdentifierType Type The IdentifierType type is used to identify various DSKPP elements, such as sessions, users, and services. Identifiers MUST NOT be longer than 128 octets. 3.10.7. The StatusCode Type The StatusCode type enumerates all possible return codes: Nystroem, et al. Expires December 13, 2007 [Page 29] Internet-Draft DSKPP June 2007 Upon transmission or receipt of a message for which the Status attribute's value is not "Success" or "Continue", the default behavior, unless explicitly stated otherwise below, is that both the DSKPP server and the DSKPP client SHALL immediately terminate the DSKPP session. DSKPP servers and DSKPP clients MUST delete any secret values generated as a result of failed runs of the DSKPP protocol. Session identifiers MAY be retained from successful or failed protocol runs for replay detection purposes, but such retained identifiers SHALL not be reused for subsequent runs of the protocol. When possible, the DSKPP client SHOULD present an appropriate error message to the user. These status codes are valid in all DSKPP Response messages unless explicitly stated otherwise: o "Continue" indicates that the DSKPP server is ready for a subsequent request from the DSKPP client. It cannot be sent in the server's final message. o "Success" indicates successful completion of the DSKPP session. It can only be sent in the server's final message. o "Abort" indicates that the DSKPP server rejected the DSKPP client's request for unspecified reasons. o "AccessDenied" indicates that the DSKPP client is not authorized to contact this DSKPP server. Nystroem, et al. Expires December 13, 2007 [Page 30] Internet-Draft DSKPP June 2007 o "MalformedRequest" indicates that the DSKPP server failed to parse the DSKPP client's request. o "UnknownRequest" indicates that the DSKPP client made a request that is unknown to the DSKPP server. o "UnknownCriticalExtension" indicates that a critical DSKPP extension (see below) used by the DSKPP client was not supported or recognized by the DSKPP server. o "UnsupportedVersion" indicates that the DSKPP client used a DSKPP protocol version not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. o "NoSupportedKeyTypes" indicates that the DSKPP client only suggested key types that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. Note that the error will only occur if the DSKPP server does not support any of the DSKPP client's suggested key types. o "NoSupportedEncryptionAlgorithms" indicates that the DSKPP client only suggested encryption algorithms that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. Note that the error will only occur if the DSKPP server does not support any of the DSKPP client's suggested encryption algorithms. o "NoSupportedMACAlgorithms" indicates that the DSKPP client only suggested MAC algorithms that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. Note that the error will only occur if the DSKPP server does not support any of the DSKPP client's suggested MAC algorithms. o "NoProtocolVariants" indicates that the DSKPP client only suggested a protocol variant (either 2-pass or 4-pass) that is not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. Note that the error will only occur if the DSKPP server does not support any of the DSKPP client's suggested protocol variants. o "NoSupportedSecretContainers" indicates that the DSKPP client only suggested secret container formats that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. Note that the error will only occur if the DSKPP server does not support any of the DSKPP client's suggested secret container formats. Nystroem, et al. Expires December 13, 2007 [Page 31] Internet-Draft DSKPP June 2007 o "AuthenticationDataInvalid" indicates that the DSKPP client supplied user or device authentication data that the DSKPP server failed to validate. o "InitializationFailed" indicates that the DSKPP server could not generate a valid key given the provided data. When this status code is received, the DSKPP client SHOULD try to restart DSKPP, as it is possible that a new run will succeed. 3.10.8. The DeviceIdentifierDataType Type The DeviceIdentifierDataType type is used to uniquely identify the device that houses the cryptographic token, e.g., a mobile phone. The device identifier allows the DSKPP server to find, e.g., a pre- shared transport key for 2-pass DSKPP and/or the correct shared secret for MAC'ing purposes. The default DeviceIdentifierDataType is defined in [7]. 3.10.9. The TokenPlatformInfoType and PlatformType Types The TokenPlatformInfoType type is used to carry characteristics of the intended token platform, and applies in the public-key variant of DSKPP in situations when the client potentially needs to select a cryptographic token to initialize. Nystroem, et al. Expires December 13, 2007 [Page 32] Internet-Draft DSKPP June 2007 3.10.10. The NonceType Type The NonceType type is used to carry pseudorandom values in DSKPP messages. A nonce, as the name implies, MUST be used only once. For each DSKPP message that requires a nonce element to be sent, a fresh nonce SHALL be generated each time. Nonce values MUST be at least 16 octets long. 3.10.11. The AlgorithmsType Type The AlgorithmsType type is a list of type-value pairs that define algorithms supported by a DSKPP client or server. Algorithms are identified through URIs. 3.10.12. The ProtocolVariantsType and the TwoPassSupportType Types The ProtocolVariantsType type is OPTIONALLY used by the DSKPP client to indicate the number of passes of the DSKPP protocol that it supports (see Section 3.3). The ProtocolVariantsType MAY be used to indicate support for 4-pass or 2-pass DSKPP. Because 1-pass DSKPP does include a client request to the server, the ProtocolVariantsType type MAY NOT be used to indicate support for 1-pass DSKPP. If the ProtocolVariantsType is not used, then the DSKPP server will proceed with ordinary 4-pass DSKPP. However, it does not support 4-pass DSKPP, then the server MUST find a suitable two-pass variant or else the protocol run will fail. Nystroem, et al. Expires December 13, 2007 [Page 33] Internet-Draft DSKPP June 2007 The TwoPassSupportType type signals client support for the 2-pass version of DSKPP, informs the server of supported two-pass variants, and provides OPTIONAL payload data to the DSKPP server. The payload is sent in an opportunistic fashion, and MAY be discarded by the DSKPP server if the server does not support the two-pass variant the payload is associated with. The elements of this type have the following meaning: o : A two-pass key initialization method supported by the DSKPP client. Multiple supported methods MAY be present, in which case they SHALL be listed in order of precedence. o : An OPTIONAL payload associated with each supported key initialization method. A DSKPP client that indicates support for two-pass DSKPP MUST also include the nonce R in its message (this will enable the client to verify that the DSKPP server it is communicating with is alive). 3.10.13. The SecretContainersFormatTypeType The SecretContainersFormatType type is a list of type-value pairs that are OPTIONALLY used to define secret container formats supported by a DSKPP client or server. Secret container formats are identified through URIs, e.g., the PSKC URI "http://www.openauthentication.org/OATH/2006/10/PSKC#SecretContainer" (see [7]. Nystroem, et al. Expires December 13, 2007 [Page 34] Internet-Draft DSKPP June 2007 3.10.14. The AuthenticationDataType Type The AuthenticationDataType type is OPTIONALLY used to carry client or server authentication values in DSKPP messages (see Section 3.4). The element MAY be used as follows: a A DSKPP client MAY include a one-time use AuthenticationCode that was given by the token issuer to the user for acquiring a symmetric key. An AuthenticationCode MAY or MAY NOT contain alphanumeric characters in addition to numeric digits depending on the device type and policy of the token issuer. For example, if the device is a mobile phone, a code that the user enters on the keypad would typically be restricted to numeric digits for ease of use. An activation code can be sent to the DSKPP server in plaintext form, hashed data form, or keyed hash data form depending on the underlying transport protocol. b A DSKPP client MAY include an AuthenticationCertificate that contains a certificate issued with the device by the token issuer. c A DSKPP server MAY use the AuthenticationDataType element AuthenticationCodeMac to carry a MAC for authenticating itself to the client. For example, when a successful 1- or 2-pass DSKPP protocol run will result in an existing key being replaced, then the DSKPP server MUST include a MAC proving to the DSKPP client that the server knows the value of the key it is about to replace. Nystroem, et al. Expires December 13, 2007 [Page 35] Internet-Draft DSKPP June 2007 The element of the AuthenticationDataType type have the following meaning: o : A requestor's identifier. The value MAY be a user ID, a device ID, or a keyID associated with the requestor's authentication value. When the authentication data is based on a Nystroem, et al. Expires December 13, 2007 [Page 36] Internet-Draft DSKPP June 2007 certificate, can be omitted, as the certificate itself is typically sufficient to identify the requestor. Also, if a message was provided by the server to initiate the DSKPP protocol run, can be omitted, as the DeviceID, KeyID, and/or nonce provided in the element ought to be sufficient to identify the requestor. o : A one-time use value sent in the clear to the DSKPP server. o : A one-time use value sent in digest form to the DSKPP server. o : An authentication MAC and OPTIONAL additional information (e.g., MAC algorithm). The value could be a one-time use value sent as a MAC value to the DSKPP server; or, it could be a MAC value sent to the DSKPP client, where the MAC is calculated as described in Section 3.9. o : A device certificate sent to the DSKPP server. 3.10.15. The PayloadType Type The PayloadType type is used to carry data in a DSKPP client or server message. For this version of the protocol, only one payload is defined, the pseudorandom string R_S, for one message, the DSKPP . 3.10.16. The MacType Type The MacType type is used by the DSKPP server to carry a MAC value that the DSKPP server uses to authenticate itself to the client. Nystroem, et al. Expires December 13, 2007 [Page 37] Internet-Draft DSKPP June 2007 3.10.17. The SecretContainerType Type The SecretContainerType type is used by the DSKPP server in its final message to carry symmetric key(s) (in the 2- and 1-pass exchanges) and configuration data. The default element defined for the SecretContainerType is contained in the namespace defined in the PSKC namespace as SecretContainerType (see [7]. 3.10.18. The ExtensionsType and the AbstractExtensionType Types The ExtensionsType type is a list of type-value pairs that define OPTIONAL DSKPP features supported by a DSKPP client or server. Extensions MAY be sent with any DSKPP message. Please see the description of individual DSKPP messages in Section 3.12 of this document for applicable extensions. All DSKPP extensions are defined as extensions to the AbstractExtensionType type. The elements of the AbstractExtensionType, therefore, apply to all DSKPP extensions. Unless an extension is marked as Critical, a receiving party need not be able to interpret it. A receiving party is always free to disregard any (non-critical) extensions. Nystroem, et al. Expires December 13, 2007 [Page 38] Internet-Draft DSKPP June 2007 3.11. DSKPP Messages 3.11.1. Introduction In this section, DSKPP messages, including their parameters, encoding and semantics are defined. 3.11.2. DSKPP Initialization (OPTIONAL) The DSKPP server MAY initialize the DSKPP protocol by sending a message. This message MAY, e.g., be sent in response to a user requesting token initialization in a browsing session. Nystroem, et al. Expires December 13, 2007 [Page 39] Internet-Draft DSKPP June 2007 Message used to trigger the device to initiate a DSKPP protocol run. The element is intended for the DSKPP client and MAY inform the DSKPP client about the identifier for the device that houses the cryptographic token to be initialized, and, OPTIONALLY, of the identifier for the key on that token. The latter would apply to key renewal. The trigger always contains a nonce to allow the DSKPP server to couple the trigger with a later DSKPP request. Finally, the trigger MAY contain a URL to use when contacting the DSKPP server. The elements are for future extensibility. Any provided or values SHALL be used by the DSKPP client in the subsequent request. The OPTIONAL element informs the DSKPP client about the characteristics of the intended token platform, and applies in the public-key variant of DSKPP in situations when the client potentially needs to decide which one of several tokens to initialize. Nystroem, et al. Expires December 13, 2007 [Page 40] Internet-Draft DSKPP June 2007 The Version attribute SHALL be set to "1.0" for this version of DSKPP. 3.11.3. The DSKPP Client's Initial PDU (2- and 4-Pass) This message is the initial message sent from the DSKPP client to the DSKPP server. Nystroem, et al. Expires December 13, 2007 [Page 41] Internet-Draft DSKPP June 2007 Message sent from DSKPP client to DSKPP server to initiate a DSKPP session. The components of this message have the following meaning: o Version: (attribute inherited from the AbstractRequestType type) The highest version of this protocol the client supports. Only version one ("1.0") is currently specified. o : An identifier for the cryptographic token as defined in Section 3.4.1 above. The identifier SHALL only be present if such shared secrets exist or if the identifier was Nystroem, et al. Expires December 13, 2007 [Page 42] Internet-Draft DSKPP June 2007 provided by the server in a element (see Section 4.2.7 below). In the latter case, it MUST have the same value as the identifier provided in that element. o : An identifier for the key that will be overwritten if the protocol run is successful. The identifier SHALL only be present if the key exists or was provided by the server in a element (see Section 4.2.7 below). In the latter case, it MUST have the same value as the identifier provided in that element. o : This is the nonce R, which, when present, SHALL be used by the server when calculating MAC values (see below). It is RECOMMENDED that clients include this element whenever the element is present. o : This OPTIONAL element SHALL be present if and only if the DSKPP run was initialized with a message (see Section 4.2.7 below), and SHALL, in that case, have the same value as the child of that message. A server using nonces in this way MUST verify that the nonce is valid and that any device or key identifier values provided in the message match the corresponding identifier values in the message. o : A sequence of URIs indicating the key types for which the token is willing to generate keys through DSKPP. o : A sequence of URIs indicating the encryption algorithms supported by the cryptographic token for the purposes of DSKPP. The DSKPP client MAY indicate the same algorithm both as a supported key type and as an encryption algorithm. o : A sequence of URIs indicating the MAC algorithms supported by the cryptographic token for the purposes of DSKPP. The DSKPP client MAY indicate the same algorithm both as an encryption algorithm and as a MAC algorithm (e.g., urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes defined in Appendix E). o : This OPTIONAL element is used by the DSKPP client to indicate support for four-pass or two-pass DSKPP. If two-pass support is specified, then MUST be set to nonce R in the message unless is already present. Nystroem, et al. Expires December 13, 2007 [Page 43] Internet-Draft DSKPP June 2007 o : This OPTIONAL element is a sequence of URIs indicating the secret container formats supported by the DSKPP client. If this element is not provided, then the DSKPP server SHALL proceed with "http://www.openauthentication.org/OATH/ 2006/10/PSKC#SecretContainer" (see [7]. o : This OPTIONAL element contains data that the DSKPP client uses to authenticate the user or device to the DSKPP server. The element is set as specified in Section 3.4.1. o : A sequence of extensions. One extension is defined for this message in this version of DSKPP: the ClientInfoType (see Section 3.12). 3.11.4. The DSKPP Server's Initial PDU (4-Pass Only) This message is the first message sent from the DSKPP server to the DSKPP client (assuming a trigger message has not been sent to initiate the protocol, in which case, this message is the second message sent from the DSKPP server to the DSKPP client). It is sent upon reception of a message. Nystroem, et al. Expires December 13, 2007 [Page 44] Internet-Draft DSKPP June 2007 Message sent from DSKPP server to DSKPP client in response to a received ClientHello PDU. The components of this message have the following meaning: o Version: (attribute inherited from the AbstractResponseType type) The version selected by the DSKPP server. MAY be lower than the version indicated by the DSKPP client, in which case, local policy at the client SHALL determine whether or not to continue the session. o SessionID: (attribute inherited from the AbstractResponseType type) An identifier for this session. o Status: (attribute inherited from the AbstractResponseType type) Return code for the . If Status is not "Continue", only the Status and Version attributes will be present; otherwise, all the other element MUST be present as well. Nystroem, et al. Expires December 13, 2007 [Page 45] Internet-Draft DSKPP June 2007 o : The type of the key to be generated. o : The encryption algorithm to use when protecting R_C. o : The MAC algorithm to be used by the DSKPP server. o : Information about the key to use when encrypting R_C. It will either be the server's public key (the alternative of ds:KeyInfoType) or an identifier for a shared secret key (the alternative of ds:KeyInfoType). o : The secret container format type to be used by the DSKPP server. The default setting relies on the SecretContainerType element defined in "urn:ietf:params:xml:schema:keyprov:container" [7]. o : The actual payload. For this version of the protocol, only one payload is defined: the pseudorandom string R_S. o : A list of server extensions. Two extensions are defined for this message in this version of DSKPP: the ClientInfoType and the ServerInfoType (see Section 3.12). o : The MAC MUST be present if the DSKPP run will result in the replacement of an existing symmetric key with a new one (i.e., if the element was present in the Second message sent from DSKPP client to DSKPP server in a DSKPP session. The components of this message have the following meaning: o Version: (inherited from the AbstractRequestType type) SHALL be the same version as in the message. o : SHALL have the same value as the SessionID attribute in the received message. o : The nonce generated and encrypted by the token. The encryption SHALL be made using the selected encryption algorithm and identified key, and as specified in Section 3.6. o : The authentication data value, which MAY OPTIONALLY be the same as provided in the , SHALL be set as specified in Section 3.4.1. o : A list of extensions. Two extensions are defined for this message in this version of DSKPP: the ClientInfoType and the ServerInfoType (see Section 3.12). Nystroem, et al. Expires December 13, 2007 [Page 47] Internet-Draft DSKPP June 2007 3.11.6. The DSKPP Server's Final PDU (1-, 2-, and 4-Pass) This message is the last message of the DSKPP protocol run. In a 4-pass exchange, the DSKPP server sends this message in response to a message, whereas in a 2-pass exchange, the DSKPP server sends this message in response to a message. In a 1-pass exchange, the DSKPP server sends only this message to the client. Final message sent from DSKPP server to DSKPP client in a DSKPP session. The components of this message have the following meaning: o Version: (inherited from the AbstractResponseType type) The DSKPP version used in this session. o SessionID: (inherited from the AbstractResponseType type) The previously established identifier for this session. o Status: (inherited from the AbstractResponseType type) Return code for the message. If Status is not "Success", only the Status, SessionID, and Version attributes will be present (the presence of the SessionID attribute is dependent on the type of reported error); otherwise, all the other elements MUST be present as well. In this latter case, the message can be seen as a "Commit" message, instructing the cryptographic token to store the generated key and associate the Nystroem, et al. Expires December 13, 2007 [Page 48] Internet-Draft DSKPP June 2007 given key identifier with this key. o : The secret container containing symmetric key values (in the case of a 2- or 1-pass exchange) and configuration data. The default container format is based on the SecretContainerType type from PSKC, as defined in [7]. o : A list of extensions chosen by the DSKPP server. For this message, this version of DSKPP defines one extension, the ClientInfoType (see Section 3.12). o : To avoid a false "Commit" message causing the token to end up in an initialized state for which the server does not know the stored key, messages MUST always be authenticated with a MAC. The MAC SHALL be made using the already established MAC algorithm. The MAC value SHALL be computed as specified in Section 3.9.1.2. When receiving a message with Status="Success" for which the MAC verifies, the DSKPP client SHALL associate the generated key K_TOKEN with the provided key identifier and store this data permanently. After this operation, it SHALL not be possible to overwrite the key unless knowledge of an authorizing key is proven through a MAC on a later (and ) message. The DSKPP client MUST verify the MAC. The DSKPP client MUST terminate the DSKPP session if the MAC does not verify, and MUST, in this case, also delete any nonces, keys, and/or secrets associated with the failed run of the DSKPP protocol. The MacType's MacAlgorithm attribute SHALL, when present, identify the negotiated MAC algorithm. 3.12. Protocol Extensions 3.12.1. The ClientInfoType Type When present in a or a message, the OPTIONAL ClientInfoType extension contains DSKPP client-specific information. DSKPP servers MUST support this extension. DSKPP servers MUST NOT attempt to interpret the data it carries and, if received, MUST include it unmodified in the current protocol run's next server response. Servers need not retain the ClientInfoType's data after that response has been generated. Nystroem, et al. Expires December 13, 2007 [Page 49] Internet-Draft DSKPP June 2007 3.12.2. The ServerInfoType Type When present, the OPTIONAL ServerInfoType extension contains DSKPP server-specific information. This extension is only valid in messages for which Status = "Continue". DSKPP clients MUST support this extension. DSKPP clients MUST NOT attempt to interpret the data it carries and, if received, MUST include it unmodified in the current protocol run's next client request (i.e., the message). DSKPP clients need not retain the ServerInfoType's data after that request has been generated. This extension MAY be used, e.g., for state management in the DSKPP server. 3.12.3. The KeyInitializationDataType Type This extension is used for 2- and 1-pass DSKPP exchange; it carries an identifier for the selected key initialization method as well as key initialization method-dependent payload data. Servers MAY include this extension in a message that is being sent in response to a received message if and only if that message selected TwoPassSupport as the ProtocolVariantType and the client indicated support for the selected key initialization method. Servers MUST include this extension in a message that is sent as part of a 1-pass DSKPP. Nystroem, et al. Expires December 13, 2007 [Page 50] Internet-Draft DSKPP June 2007 This extension is only valid in ServerFinished PDUs. It contains key initialization data and its presence results in a two-pass (or one-pass, if no ClientHello was sent) DSKPP exchange. The elements of this type have the following meaning: o : A two-pass key initialization method supported by the DSKPP client. o : A payload associated with the key initialization method. Since the syntax is a shorthand for , any well-formed payloads can be carried in this element. Nystroem, et al. Expires December 13, 2007 [Page 51] Internet-Draft DSKPP June 2007 4. Protocol Bindings 4.1. General Requirements DSKPP assumes a reliable transport. 4.2. HTTP/1.1 Binding for DSKPP 4.2.1. Introduction This section presents a binding of the previous messages to HTTP/1.1 [14]. Note that the HTTP client normally will be different from the DSKPP client, i.e., the HTTP client will only exist to "proxy" DSKPP messages from the DSKPP client to the DSKPP server. Likewise, on the HTTP server side, the DSKPP server MAY receive DSKPP PDUs from a "front-end" HTTP server. 4.2.2. Identification of DSKPP Messages The MIME-type for all DSKPP messages SHALL be application/vnd.ietf.keyprov.dskpp+xml 4.2.3. HTTP Headers HTTP proxies MUST NOT cache responses carrying DSKPP messages. For this reason, the following holds: o When using HTTP/1.1, requesters SHOULD: * Include a Cache-Control header field set to "no-cache, no- store". * Include a Pragma header field set to "no-cache". o When using HTTP/1.1, responders SHOULD: * Include a Cache-Control header field set to "no-cache, no-must- revalidate, private". * Include a Pragma header field set to "no-cache". * NOT include a Validator, such as a Last-Modified or ETag header. There are no other restrictions on HTTP headers, besides the requirement to set the Content-Type header value to application/ vnd.ietf.keyprov.dskpp+xml. Nystroem, et al. Expires December 13, 2007 [Page 52] Internet-Draft DSKPP June 2007 4.2.4. HTTP Operations Persistent connections as defined in HTTP/1.1 are assumed but not required. DSKPP requests are mapped to HTTP POST operations. DSKPP responses are mapped to HTTP responses. 4.2.5. HTTP Status Codes A DSKPP HTTP responder that refuses to perform a message exchange with a DSKPP HTTP requester SHOULD return a 403 (Forbidden) response. In this case, the content of the HTTP body is not significant. In the case of an HTTP error while processing a DSKPP request, the HTTP server MUST return a 500 (Internal Server Error) response. This type of error SHOULD be returned for HTTP-related errors detected before control is passed to the DSKPP processor, or when the DSKPP processor reports an internal error (for example, the DSKPP XML namespace is incorrect, or the DSKPP schema cannot be located). If the type of a DSKPP request cannot be determined, the DSKPP responder MUST return a 400 (Bad request) response. In these cases (i.e., when the HTTP response code is 4xx or 5xx), the content of the HTTP body is not significant. Redirection status codes (3xx) apply as usual. Whenever the HTTP POST is successfully invoked, the DSKPP HTTP responder MUST use the 200 status code and provide a suitable DSKPP message (possibly with DSKPP error information included) in the HTTP body. 4.2.6. HTTP Authentication No support for HTTP/1.1 authentication is assumed. 4.2.7. Initialization of DSKPP The DSKPP server MAY initialize the DSKPP protocol by sending an HTTP response with Content-Type set to application/ vnd.ietf.keyprov.dskpp+xml and response code set to 200 (OK). This message MAY, e.g., be sent in response to a user requesting token initialization in a browsing session. The initialization message MAY carry data in its body. If this is the case, the data SHALL be a valid instance of a element. 4.2.8. Example Messages Nystroem, et al. Expires December 13, 2007 [Page 53] Internet-Draft DSKPP June 2007 a. Initialization from DSKPP server: HTTP/1.1 200 OK Cache-Control: no-store Content-Type: application/vnd.ietf.keyprov.dskpp+xml Content-Length: DSKPP initialization data in XML form... b. Initial request from DSKPP client: POST http://example.com/cgi-bin/DSKPP-server HTTP/1.1 Cache-Control: no-store Pragma: no-cache Host: example.com Content-Type: application/vnd.ietf.keyprov.dskpp+xml Content-Length: DSKPP data in XML form (supported version, supported algorithms...) c. Initial response from DSKPP server: HTTP/1.1 200 OK Cache-Control: no-store Content-Type: application/vnd.ietf.keyprov.dskpp+xml Content-Length: DSKPP data in XML form (server random nonce, server public key, ...) Nystroem, et al. Expires December 13, 2007 [Page 54] Internet-Draft DSKPP June 2007 5. Security considerations 5.1. General DSKPP is designed to protect generated key material from exposure. No other entities than the DSKPP server and the cryptographic token will have access to a generated K_TOKEN if the cryptographic algorithms used are of sufficient strength and, on the DSKPP client side, generation and encryption of R_C and generation of K_TOKEN take place as specified and in the token. This applies even if malicious software is present in the DSKPP client. However, as discussed in the following, DSKPP does not protect against certain other threats resulting from man-in-the-middle attacks and other forms of attacks. DSKPP SHOULD, therefore, be run over a transport providing privacy and integrity, such as HTTP over Transport Layer Security (TLS) with a suitable ciphersuite, when such threats are a concern. Note that TLS ciphersuites with anonymous key exchanges are not suitable in those situations. 5.2. Active Attacks 5.2.1. Introduction An active attacker MAY attempt to modify, delete, insert, replay, or reorder messages for a variety of purposes including service denial and compromise of generated key material. Section 5.2.2 through Section 5.2.7. 5.2.2. Message Modifications Modifications to a message will either cause denial- of-service (modifications of any of the identifiers or the nonce) or the DSKPP client to contact the wrong DSKPP server. The latter is in effect a man-in-the-middle attack and is discussed further in Section 5.2.7. An attacker may modify a message. This means that the attacker could indicate a different key or device than the one intended by the DSKPP client, and could also suggest other cryptographic algorithms than the ones preferred by the DSKPP client, e.g., cryptographically weaker ones. The attacker could also suggest earlier versions of the DSKPP protocol, in case these versions have been shown to have vulnerabilities. These modifications could lead to an attacker succeeding in initializing or modifying another token than the one intended (i.e., the server assigning the generated key to the wrong token), or gaining access to a generated key through the use of weak cryptographic algorithms or protocol versions. DSKPP implementations MAY protect against the latter by having strict Nystroem, et al. Expires December 13, 2007 [Page 55] Internet-Draft DSKPP June 2007 policies about what versions and algorithms they support and accept. The former threat (assignment of a generated key to the wrong token) is not possible when the shared-key variant of DSKPP is employed (assuming existing shared keys are unique per token), but is possible in the public-key variant. Therefore, DSKPP servers MUST NOT accept unilaterally provided device identifiers in the public-key variant. This is also indicated in the protocol description. In the shared- key variant, however, an attacker may be able to provide the wrong identifier (possibly also leading to the incorrect user being associated with the generated key) if the attacker has real-time access to the token with the identified key. In other words, the generated key is associated with the correct token but the token is associated with the incorrect user. See further Section 5.5 for a discussion of this threat and possible countermeasures. An attacker may also modify a message. This means that the attacker could indicate different key types, algorithms, or protocol versions than the legitimate server would, e.g., cryptographically weaker ones. The attacker could also provide a different nonce than the one sent by the legitimate server. Clients will protect against the former through strict adherence to policies regarding permissible algorithms and protocol versions. The latter (wrong nonce) will not constitute a security problem, as a generated key will not match the key generated on the legitimate server. Also, whenever the DSKPP run would result in the replacement of an existing key, the element protects against modifications of R_S. Modifications of messages are also possible. If an attacker modifies the SessionID attribute, then, in effect, a switch to another session will occur at the server, assuming the new SessionID is valid at that time on the server. It still will not allow the attacker to learn a generated K_TOKEN since R_C has been wrapped for the legitimate server. Modifications of the element, e.g., replacing it with a value for which the attacker knows an underlying R'C, will not result in the client changing its pre-DSKPP state, since the server will be unable to provide a valid MAC in its final message to the client. The server MAY, however, end up storing K'TOKEN rather than K_TOKEN. If the token has been associated with a particular user, then this could constitute a security problem. For a further discussion about this threat, and a possible countermeasure, see Section 5.5 below. Note that use of Secure Socket Layer (SSL) or TLS does not protect against this attack if the attacker has access to the DSKPP client (e.g., through malicious software, "trojans"). Finally, attackers may also modify the message. Replacing the element will only result in denial-of-service. Replacement of any other element may cause the DSKPP client to Nystroem, et al. Expires December 13, 2007 [Page 56] Internet-Draft DSKPP June 2007 associate, e.g., the wrong service with the generated key. DSKPP SHOULD be run over a transport providing privacy and integrity when this is a concern. 5.2.3. Message Deletion Message deletion will not cause any other harm than denial-of- service, since a token SHALL NOT change its state (i.e., "commit" to a generated key) until it receives the final message from the DSKPP server and successfully has processed that message, including validation of its MAC. A deleted message will not cause the server to end up in an inconsistent state vis-a-vis the token if the server implements the suggestions in Section 5.5. 5.2.4. Message Insertion An active attacker may initiate a DSKPP run at any time, and suggest any device identifier. DSKPP server implementations MAY receive some protection against inadvertently initializing a token or inadvertently replacing an existing key or assigning a key to a token by initializing the DSKPP run by use of the . The element allows the server to associate a DSKPP protocol run with, e.g., an earlier user-authenticated session. The security of this method, therefore, depends on the ability to protect the element in the DSKPP initialization message. If an eavesdropper is able to capture this message, he may race the legitimate user for a key initialization. DSKPP over a transport providing privacy and integrity, coupled with the recommendations in Section 5.5, is RECOMMENDED when this is a concern. Insertion of other messages into an existing protocol run is seen as equivalent to modification of legitimately sent messages. 5.2.5. Message Replay During 4-pass DSKPP, attempts to replay a previously recorded DSKPP message will be detected, as the use of nonces ensures that both parties are live. For example, a DSKPP client knows that a server it is communicating with is "live" since the server MUST create a MAC on information sent by the client. The same is true for 2-pass DSKPP thanks to the requirement that the client sends R in the message and that the server includes R in the MAC computation. In 1-pass DSKPP clients (tokens) that record the latest I used by a particular server (as identified by ID_S) will be able to detect replays. Nystroem, et al. Expires December 13, 2007 [Page 57] Internet-Draft DSKPP June 2007 5.2.6. Message Reordering An attacker may attempt to re-order 4-pass DSKPP messages but this will be detected, as each message is of a unique type. Note: Message re-ordering attacks cannot occur in 2- and 1-pass DSKPP since each party sends at most one message each. 5.2.7. Man-in-the-Middle In addition to other active attacks, an attacker posing as a man in the middle may be able to provide his own public key to the DSKPP client. This threat and countermeasures to it are discussed in Section 3.3. An attacker posing as a man-in-the-middle may also be acting as a proxy and, hence, may not interfere with DSKPP runs but still learn valuable information; see Section 5.3. 5.3. Passive Attacks Passive attackers may eavesdrop on DSKPP runs to learn information that later on may be used to impersonate users, mount active attacks, etc. If DSKPP is not run over a transport providing privacy, a passive attacker may learn: o What tokens a particular user is in possession of; o The identifiers of keys on those tokens and other attributes pertaining to those keys, e.g., the lifetime of the keys; and o DSKPP versions and cryptographic algorithms supported by a particular DSKPP client or server. Whenever the above is a concer, DSKPP SHOULD be run over a transport providing privacy. If man-in-the-middle attacks for the purposes described above are a concern, the transport SHOULD also offer server-side authentication. 5.4. Cryptographic Attacks An attacker with unlimited access to an initialized token may use the token as an "oracle" to pre-compute values that later on may be used to impersonate the DSKPP server. Section 3.8 and Section 3.11 contain discussions of this threat and steps RECOMMENDED to protect against it. Nystroem, et al. Expires December 13, 2007 [Page 58] Internet-Draft DSKPP June 2007 5.5. Attacks on the Interaction between DSKPP and User Authentication If keys generated in DSKPP will be associated with a particular user at the DSKPP server (or a server trusted by, and communicating with the DSKPP server), then in order to protect against threats where an attacker replaces a client-provided encrypted R_C with his own R'C (regardless of whether the public-key variant or the shared-secret variant of DSKPP is employed to encrypt the client nonce), the server SHOULD not commit to associate a generated K_TOKEN with the given token (user) until the user simultaneously has proven both possession of a token containing K_TOKEN and some out-of-band provided authenticating information (e.g., a temporary password). For example, if the token is a one-time password token, the user could be required to authenticate with both a one-time password generated by the token and an out-of-band provided temporary PIN in order to have the server "commit" to the generated token value for the given user. Preferably, the user SHOULD perform this operation from another host than the one used to initialize the token, in order to minimize the risk of malicious software on the client interfering with the process. Note: This scenario, wherein the attacker replaces a client-provided R_C with his own R'C, does not apply to 2- and 1-pass DSKPP as the client does not provide any entropy to K_TOKEN. The attack as such (and its countermeasures) still applies to 2- and 1-pass DSKPP, however, as it essentially is a man-in-the-middle attack. Another threat arises when an attacker is able to trick a user to authenticate to the attacker rather than to the legitimate service before the DSKPP protocol run. If successful, the attacker will then be able to impersonate the user towards the legitimate service, and subsequently receive a valid DSKPP trigger. If the public-key variant of DSKPP is used, this may result in the attacker being able to (after a successful DSKPP protocol run) impersonate the user. Ordinary precautions MUST, therefore, be in place to ensure that users authenticate only to legitimate services. 5.6. Additional Considerations Specific to 2- and 1-pass DSKPP 5.6.1. Client Contributions to K_TOKEN Entropy In 4-pass DSKPP, both the client and the server provide randomizing material to K_TOKEN , in a manner that allows both parties to verify that they did contribute to the resulting key. In the 1- and 2-pass DSKPP versions defined herein, only the server contributes to the entropy of K_TOKEN. This means that a broken or compromised (pseudo-)random number generator in the server may cause more damage than it would in the 4-pass variant. Server implementations SHOULD Nystroem, et al. Expires December 13, 2007 [Page 59] Internet-Draft DSKPP June 2007 therefore take extreme care to ensure that this situation does not occur. 5.6.2. Key Confirmation 4-pass DSKPP servers provide key confirmation through the MAC on R_C in the message. In the 1- and 2-pass DSKPP variants described herein, key confirmation is provided by the MAC including I (in the 1-pass case) or R (2-pass case), using K_MAC. 5.6.3. Server Authentication DSKPP servers MUST authenticate themselves whenever a successful DSKPP 1- or 2-pass protocol run would result in an existing K_TOKEN being replaced by a K_TOKEN', or else a denial-of-service attack where an unauthorized DSKPP server replaces a K_TOKEN with another key would be possible. In 1- and 2-pass DSKPP, servers authenticate by including the AuthenticationDataType extension containing a MAC as described in Section 3.9 above. 5.6.4. Client Authentication A DSKPP server MUST authenticate a client to ensure that K_TOKEN is delivered to the intended device. The following measures SHOULD be considered: o When a device certificate is used for client authentication, the DSKPP server SHOULD follow standard certificate verification processes to ensure that it is a trusted device. o When an Authentication Code is used for client authentication, a password dictionary attack on the authentication data is possible. When a secure channel, e.g., SSL or TLS, is established between a DSKPP client and server, an attacker could successfully brute- force guess an Authentication Code, allowing him to illegitimately receive K_TOKEN. o The length the of the Authentication Code when used over a non- secure channel SHOULD be longer than what is used over a secure channel. When a device, e.g., some mobile phones with small screens, cannot handle a long Authentication Code in a user- friendly manner, DSKPP SHOULD rely on a secure channel for communication. o In the case that a non-secure channel has to be used, the Authentication Code SHOULD be sent to the server MAC's with a DSKPP server's nonce value. The Authentication Code and nonce value MUST be strong enough to prevent offline brute-force Nystroem, et al. Expires December 13, 2007 [Page 60] Internet-Draft DSKPP June 2007 recovery of the Authentication Code from the HMAC data. Because the nonce value is almost public across a non-secure channel, the key strength is dependent on the Authentication Code. 5.6.5. Key Protection in the Passphrase Profile The passphrase-based key wrap profile uses the PBKDF2 function from [12] to generate an encryption key from a passphrase and salt string. The derived key, K_DERIVED is used by the server to encrypt K_TOKEN and by the token to decrypt the newly delivered K_TOKEN. It is important to note that passphrase-based encryption is generally limited in the security that it provides despite the use of salt and iteration count in PBKDF2 to increase the complexity of attack. Implementations SHOULD therefore take additional measures to strengthen the security of the passphrase-based key wrap profile. The following measures SHOULD be considered where applicable: o The passphrase SHOULD be selected well, and usage guidelines such as the ones in [15] SHOULD be taken into account. o A different passphrase SHOULD be used for every key initialization wherever possible (the use of a global passphrase for a batch of tokens SHOULD be avoided, for example). One way to achieve this is to use randomly-generated passphrases. o The passphrase SHOULD be protected well if stored on the server and/or on the token and SHOULD be delivered to the token's user using secure methods. o User pre-authentication SHOULD be implemented to ensure that K_TOKEN is not delivered to a rogue recipient. o The iteration count in PBKDF2 SHOULD be high to impose more work for an attacker using brute-force methods (see [12] for recommendations). However, it MUST be noted that the higher the count, the more work is required on the legitimate token to decrypt the newly delivered K_TOKEN. Servers MAY use relatively low iteration counts to accommodate tokens with limited processing power such as some PDA and cell phones when other security measures are implemented and the security of the passphrase-based key wrap method is not weakened. o Transport level security (e.g. TLS) SHOULD be used where possible to protect a 2-pass or 1-pass protocol run. Transport level security provides a second layer of protection for the newly generated K_TOKEN. Nystroem, et al. Expires December 13, 2007 [Page 61] Internet-Draft DSKPP June 2007 6. IANA Considerations This document calls for registration of new URNs within the IETF sub- namespace per RFC3553 [16]. The following URNs are RECOMMENDED: o DSKPP XML schema: "urn:ietf:params:xml:schema:keyprov:protocol" o DSKPP XML namespace: "urn:ietf:params:xml:ns:keyprov:protocol" Nystroem, et al. Expires December 13, 2007 [Page 62] Internet-Draft DSKPP June 2007 7. Intellectual Property Considerations RSA and RSA Security are registered trademarks or trademarks of RSA Security Inc. in the United States and/or other countries. The names of other products and services mentioned may be the trademarks of their respective owners. Nystroem, et al. Expires December 13, 2007 [Page 63] Internet-Draft DSKPP June 2007 8. Acknowledgements Thanks to all the members of OATH [17] and participants of OTPS workshops for their review and comments related to this document. Nystroem, et al. Expires December 13, 2007 [Page 64] Internet-Draft DSKPP June 2007 9. References 9.1. Normative references [1] W3C, "XML Signature Syntax and Processing", W3C Recommendation, February 2002, . [2] Davis, M. and M. Duerst, "Unicode Normalization Forms", March 2001, . [3] W3C, "XML Encryption Syntax and Processing", W3C Recommendation, December 2002, . 9.2. Informative references [4] RSA, The Security Division of EMC, "Cryptographic Token Key Initialization Protocol (CT-KIP)", November 2006, . [5] RSA Laboratories, "Cryptographic Token Interface Standard", PKCS #11 Version 2.20, June 2004, . [6] "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997, . [7] "Portable Symmetric Key Container", 2005, . [8] RSA Laboratories, "PKCS #11 Mechanisms for the Cryptographic Token Key Initialization Protocol", PKCS #11 Version 2.20 Amd.2, December 2005, . [9] "Personal Information Exchange Syntax Standard", PKCS #12 Version 1.0, 2005, . [10] RSA Laboratories, "XML Schema for PKCS #5 Version 2.0", PKCS #5 Version 2.0 Amd.1 (FINAL DRAFT), October 2006, . [11] RSA Laboratories, "Frequently Asked Questions About Today's Nystroem, et al. Expires December 13, 2007 [Page 65] Internet-Draft DSKPP June 2007 Cryptography", Version 4.1, 2000. [12] RSA Laboratories, "Password-Based Cryptography Standard", PKCS #5 Version 2.0, March 1999, . [13] RSA Laboratories, "RSA Cryptography Standard", PKCS #1 Version 2.1, June 2002, . [14] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999, . [15] National Institute of Standards and Technology, "Password Usage", FIPS 112, May 1985, . [16] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An IETF URN Sub-namespace for Registered Protocol Parameters", RFC 3553, BCP 73, June 2003. [17] "Initiative for Open AuTHentication", 2005, . [18] National Institute of Standards and Technology, "Specification for the Advanced Encryption Standard (AES)", FIPS 197, November 2001, . [19] Krawzcyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997. [20] Iwata, T. and K. Kurosawa, "OMAC: One-Key CBC MAC. In Fast Software Encryption", FSE 2003, Springer-Verlag , 2003, . [21] National Institute of Standards and Technology, "Secure Hash Standard", FIPS 180-2, February 2004, . Nystroem, et al. Expires December 13, 2007 [Page 66] Internet-Draft DSKPP June 2007 Appendix A. DSKPP Schema Nystroem, et al. Expires December 13, 2007 [Page 67] Internet-Draft DSKPP June 2007 Nystroem, et al. Expires December 13, 2007 [Page 68] Internet-Draft DSKPP June 2007 Nystroem, et al. Expires December 13, 2007 [Page 69] Internet-Draft DSKPP June 2007 This extension is only valid in ServerFinished PDUs. It contains key initialization data and its presence results in a two-pass (or one-pass, if no ClientHello was sent) DSKPP exchange. Nystroem, et al. Expires December 13, 2007 [Page 71] Internet-Draft DSKPP June 2007 Message used to trigger the device to initiate a DSKPP protocol run. Message sent from DSKPP client to DSKPP server to initiate a DSKPP session. Message sent from DSKPP server to DSKPP client in response to a received ClientHello PDU. Second message sent from DSKPP client to DSKPP server in a DSKPP session. Final message sent from DSKPP server to DSKPP client in a DSKPP session. Nystroem, et al. Expires December 13, 2007 [Page 74] Internet-Draft DSKPP June 2007 Nystroem, et al. Expires December 13, 2007 [Page 75] Internet-Draft DSKPP June 2007 Appendix B. Key Initialization Profiles of DSKPP B.1. Introduction This appendix introduces three profiles of DSKPP for key initialization. They MAY all be used for two- as well as one-pass initialization of cryptographic tokens. Further profiles MAY be defined by external entities or through the IETF process. B.2. Key Transport Profile B.2.1. Introduction This profile initializes the cryptographic token with a symmetric key, K_TOKEN, through key transport and key derivation. The key transport is carried out using a public key, K_CLIENT, whose private key part resides in the token as the transport key. A key K from which two keys, K_TOKEN and K_MAC are derived SHALL be transported. B.2.2. Identification This profile SHALL be identified with the following URN: urn:ietf:params:xml:schema:keyprov:protocol#transport B.2.3. Payloads In the two-pass version of DSKPP, the client SHALL send a payload associated with this key initialization method. The payload SHALL be of type ds:KeyInfoType ([1]), and only those choices of the ds: KeyInfoType that identify a public key are allowed. The ds: X509Certificate option of the ds:X509Data alternative is RECOMMENDED when the public key corresponding to the private key on the cryptographic token has been certified. The server payload associated with this key initialization method SHALL be of type xenc:EncryptedKeyType ([3]), and only those encryption methods utilizing a public key that are supported by the DSKPP client (as indicated in the element of the message in the case of 2-pass DSKPP, or as otherwise known in the case of 1-pass DSKPP) are allowed as values for the element. Further, in the case of 2-pass DSKPP, the element SHALL contain the same value (i.e. identify the same public key) as the of the corresponding supported key initialization method in the message that triggered the response. The element MAY be present, but SHALL, when present, contain the same value as the element of the Nystroem, et al. Expires December 13, 2007 [Page 76] Internet-Draft DSKPP June 2007 message. The Type attribute of the xenc:EncryptedKeyType SHALL be present and SHALL identify the type of the wrapped token key. The type SHALL be one of the types supported by the DSKPP client (as reported in the of the preceding message in the case of 2-pass DSKPP, or as otherwise known in the case of 1-pass DSKPP). The transported key SHALL consist of two parts of equal length. The first half constitutes K_MAC and the second half constitutes K_TOKEN. The length of K_TOKEN (and hence also the length of K_MAC) is determined by the type of K_TOKEN. DSKPP servers and tokens supporting this profile MUST support the http://www.w3.org/2001/04/xmlenc#rsa-1_5 key-wrapping mechanism defined in [3]. When this profile is used, the MacAlgorithm attribute of the element of the message MUST be present and MUST identify the selected MAC algorithm. The selected MAC algorithm MUST be one of the MAC algorithms supported by the DSKPP client (as indicated in the element of the message in the case of 2-pass DSKPP, or as otherwise known in the case of 1-pass DSKPP). The MAC SHALL be calculated as described in Section 3.9 In addition, DSKPP servers MUST include the AuthenticationDataType element (see further Section 3.9) in their messages whenever a successful protocol run will result in an existing K_TOKEN being replaced. B.3. Key wrap profile B.3.1. Introduction This profile initializes the cryptographic token with a symmetric key, K_TOKEN, through key wrap and key derivation. The key wrap SHALL be carried out using a (symmetric) key-wrapping key, K_SHARED, known in advance by both the token and the DSKPP server. A key K from which two keys, K_TOKEN and K_MAC are derived SHALL be wrapped. B.3.2. Identification This profile SHALL be identified with the following URI: urn:ietf:params:xml:schema:keyprov:protocol#wrap B.3.3. Payloads In the 2-pass version of DSKPP, the client SHALL send a payload associated with this key initialization method. The payload SHALL be Nystroem, et al. Expires December 13, 2007 [Page 77] Internet-Draft DSKPP June 2007 of type ds:KeyInfoType ([1]), and only those choices of the ds: KeyInfoType that identify a symmetric key are allowed. The ds: KeyName alternative is RECOMMENDED. The server payload associated with this key initialization method SHALL be of type xenc:EncryptedKeyType ([3]), and only those encryption methods utilizing a symmetric key that are supported by the DSKPP client (as indicated in the element of the message in the case of 2-pass DSKPP, or as otherwise known in the case of 1-pass DSKPP) are allowed as values for the element. Further, in the case of 2-pass DSKPP, the element SHALL contain the same value (i.e. identify the same symmetric key) as the of the corresponding supported key initialization method in the message that triggered the response. The element MAY be present, and SHALL, when present, contain the same value as the element of the message. The Type attribute of the xenc:EncryptedKeyType SHALL be present and SHALL identify the type of the wrapped token key. The type SHALL be one of the types supported by the DSKPP client (as reported in the of the preceding message in the case of 2-pass DSKPP, or as otherwise known in the case of 1-pass DSKPP). The wrapped key SHALL consist of two parts of equal length. The first half constitutes K_MAC and the second half constitutes K_TOKEN. The length of K_TOKEN (and hence also the length of K_MAC) is determined by the type of K_TOKEN. DSKP servers and tokens supporting this profile MUST support the http://www.w3.org/2001/04/xmlenc#kw-aes128 key-wrapping mechanism defined in [3]. When this profile is used, the MacAlgorithm attribute of the element of the message MUST be present and MUST identify the selected MAC algorithm. The selected MAC algorithm MUST be one of the MAC algorithms supported by the DSKPP client (as indicated in the element of the message in the case of 2-pass DSKPP, or as otherwise known in the case of 1-pass DSKPP). The MAC SHALL be calculated as described in Section 3.9 In addition, DSKPP servers MUST include the AuthenticationDataType element (see further Section 3.9) in their messages whenever a successful protocol run will result in an existing K_TOKEN being replaced. Nystroem, et al. Expires December 13, 2007 [Page 78] Internet-Draft DSKPP June 2007 B.4. Passphrase-based key wrap profile B.4.1. Introduction This profile is a variation of the key wrap profile. It initializes the cryptographic token with a symmetric key, K_TOKEN, through key wrap and key derivation, using a passphrase-derived key-wrapping key, K_DERIVED. The passphrase is known in advance by both the token user and the DSKPP server. To preserve the property of not exposing K_TOKEN to any other entity than the DSKPP server and the token itself, the method SHOULD be employed only when the token contains facilities (e.g. a keypad) for direct entry of the passphrase. A key K from which two keys, K_TOKEN and K_MAC are derived SHALL be wrapped. B.4.2. Identification This profile SHALL be identified with the following URI: urn:ietf:params:xml:schema:keyprov:protocol#passphrase-wrap B.4.3. Payloads In the 2-pass version of DSKPP, the client SHALL send a payload associated with this key initialization method. The payload SHALL be of type ds:KeyInfoType ([1]). The ds:KeyName option SHALL be used and the key name SHALL identify the passphrase that will be used by the server to generate the key-wrapping key. As an example, the identifier could be a user identifier or a registration identifier issued by the server to the user during a session preceding the DSKPP protocol run. The server payload associated with this key initialization method SHALL be of type xenc:EncryptedKeyType ([3]), and only those encryption methods utilizing a passphrase to derive the key-wrapping key that are supported by the DSKPP client (as indicated in the element of the message in the case of 2-pass DSKPP, or as otherwise known in the case of 1-pass DSKPP) are allowed as values for the element. Further, in the case of 2-pass DSKPP, the element SHALL contain the same value (i.e. identify the same passphrase) as the of the corresponding supported key initialization method in the message that triggered the response. The element MAY be present, and SHALL, when present, contain the same value as the element of the message. The Type attribute of the xenc: EncryptedKeyType SHALL be present and SHALL identify the type of the wrapped token key. The type SHALL be one of the types supported by Nystroem, et al. Expires December 13, 2007 [Page 79] Internet-Draft DSKPP June 2007 the DSKPP client (as reported in the of the preceding message in the case of 2-pass DSKPP, or as otherwise known in the case of 1-pass DSKPP). The wrapped key SHALL consist of two parts of equal length. The first half constitutes K_MAC and the second half constitutes K_TOKEN. The length of K_TOKEN (and hence also the length of K_MAC) is determined by the type of K_TOKEN. DSKPP servers and tokens supporting this profile MUST support the PBES2 password based encryption scheme defined in [12] (and identified as http://www.rsasecurity.com/rsalabs/pkcs/schemas/pkcs-5#pbes2 in [10]), the PBKDF2 passphrase-based key derivation function also defined in [12] (and identified as http://www.rsasecurity.com/rsalabs/pkcs/schemas/pkcs-5#pbkdf2 in [10]), and the http://www.w3.org/2001/04/xmlenc#kw-aes128 key- wrapping mechanism defined in [3]. When this profile is used, the MacAlgorithm attribute of the element of the message MUST be present and MUST identify the selected MAC algorithm. The selected MAC algorithm MUST be one of the MAC algorithms supported by the DSKPP client (as indicated in the element of the message in the case of 2-pass DSKPP, or as otherwise known in the case of 1-pass DSKPP). The MAC SHALL be calculated as described in Section 3.9 In addition, DSKPP servers MUST include the AuthenticationDataType element (see further Section 3.9) in their messages whenever a successful protocol run will result in an existing K_TOKEN being replaced. Nystroem, et al. Expires December 13, 2007 [Page 80] Internet-Draft DSKPP June 2007 Appendix C. Example Messages All examples are syntactically correct. MAC and cipher values are fictitious however. C.1. Example Messages in a Four-pass Exchange The examples below illustrate a complete four-pass DSKPP exchange. C.1.1. Example of a DSKPP Initialization (Trigger) Message ManufacturerABC XL0000000001234 U2 112dsdfwf312asder394jw== Nystroem, et al. Expires December 13, 2007 [Page 81] Internet-Draft DSKPP June 2007 C.1.2. Example of a Message ManufacturerABC XL0000000001234 U2 112dsdfwf312asder394jw== http://www.rsa.com/rsalabs/otps/schemas/2005/09/ otps-wst#SecurID-AES http://www.openauthentication.org/OATH/2006/10/PSKC# HOTP http://www.w3.org/2001/05/xmlenc#rsa_1_5 urn:ietf:params:xml:schema:keyprov:protocol# dskpp-prf-aes urn:ietf:params:xml:schema:keyprov:protocol# dskpp-prf-aes FourPass urn:ietf:params:xml:schema:keyprov:container 1erd354657689102abcd Nystroem, et al. Expires December 13, 2007 [Page 82] Internet-Draft DSKPP June 2007 C.1.3. Example of a Message http://www.rsa.com/rsalabs/otps/schemas/2005/09/otps-wst# SecurID-AES urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes KEY-1 urn:ietf:params:xml:schema:keyprov:container qw2ewasde312asder394jw== C.1.4. Example of a Message VXENc+Um/9/NvmYKiHDLaErK0gk= 1erd354657689102abcd C.1.5. Example of a Message Nystroem, et al. Expires December 13, 2007 [Page 83] Internet-Draft DSKPP June 2007 CredentialIssuer MyFirstToken Time 10/30/2009 miidfasde312asder394jw== C.2. Example Messages in a Two- or One-pass Exchange The examples illustrate a complete two-pass DSKPP exchange. The server messages MAY also constitute the only messages in a one-pass DSKPP exchange. C.2.1. Example of a Message Indicating Support for Two- pass DSKPP The client indicates support both for the two-pass key transport variant as well as the two-pass key wrap variant. Nystroem, et al. Expires December 13, 2007 [Page 84] Internet-Draft DSKPP June 2007 ManufacturerABC XL0000000001234 U2 http://www.rsa.com/rsalabs/otps/schemas/2005/09/otps-wst# SecurID-AES http://www.openauthentication.org/OATH/2006/10/PSKC#HOTP http://www.w3.org/2001/05/xmlenc#rsa_1_5 http://www.w3.org/2001/04/xmlenc#kw-aes128 http://www.rsasecurity.com/rsalabs/pkcs/schemas/pkcs-5# pbes2 urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes urn:ietf:params:xml:schema:keyprov:protocol# dskpp-prf-aes urn:ietf:params:xml:schema:keyprov:protocol#wrap Key_001 urn:ietf:params:xml:schema:keyprov:protocol#transport miib Nystroem, et al. Expires December 13, 2007 [Page 85] Internet-Draft DSKPP June 2007 urn:ietf:params:xml:schema:keyprov:container 1erd354657689102abcd C.2.2. Example of a Message Using the Key Transport Profile In this example, the server responds to the previous request using the key transport profile. Nystroem, et al. Expires December 13, 2007 [Page 86] Internet-Draft DSKPP June 2007 43212093< miib CredentialIssuer MyFirstToken 7JHUyp3azOkqJENSsh6b2vxXzwGBYypzJxEr+ikQAa229KV/BgZhGA== 1 10/30/2009 miidfasde312asder394jw== Nystroem, et al. Expires December 13, 2007 [Page 87] Internet-Draft DSKPP June 2007 C.2.3. Example of a Message Using the Key Wrap Profile In this example, the server responds to the previous request using the key wrap profile. Nystroem, et al. Expires December 13, 2007 [Page 88] Internet-Draft DSKPP June 2007 43212093 Key-001 CredentialIssuer MyFirstToken 7JHUyp3azOkqJENSsh6b2vxXzwGBYypzJxEr+ikQAa229KV/BgZhGA== 1 10/30/2009 miidfasde312asder394jw== C.2.4. Example of a Message using the Passphrase-based Key Wrap Profile In this example, the server responds to the previous request using the passphrase-based key wrap profile. Nystroem, et al. Expires December 13, 2007 [Page 89] Internet-Draft DSKPP June 2007 32113435 1024 128 43212093 Passphrase1 CredentialIssuer MyFirstToken 7JHUyp3azOkqJENSsh6b2vxXzwGBYypzJxEr+ikQAa229KV/BgZhGA== Nystroem, et al. Expires December 13, 2007 [Page 90] Internet-Draft DSKPP June 2007 1 10/30/2009 miidfasde312asder394jw== Nystroem, et al. Expires December 13, 2007 [Page 91] Internet-Draft DSKPP June 2007 Appendix D. Integration with PKCS #11 A DSKPP client that needs to communicate with a conncected cryptographic token to perform a DSKPP exchange MAY use PKCS #11 [5]as a programming interface. D.1. The 4-pass Variant When performing 4-pass DSKPP with a cryptographic token using the PKCS #11 programming interface, the procedure described in [8], Appendix B, is RECOMMENDED. D.2. The 2-pass Variant A suggested procedure to perform 2-pass DSKPP with a cryptographic token through the PKCS #11 interface using the mechanisms defined in [8] is as follows: a. On the client side, 1. The client selects a suitable slot and token (e.g. through use of the or the element of the DSKPP trigger message). 2. A nonce R is generated, e.g. by calling C_SeedRandom and C_GenerateRandom. 3. The client sends its first message to the server, including the nonce R. b. On the server side, 1. A generic key K = K_TOKEN | K _MAC (where '|' denotes concatenation) is generated, e.g. by calling C_GenerateKey (using key type CKK_GENERIC_SECRET). The template for K SHALL allow it to be exported (but only in wrapped form, i.e. CKA_SENSITIVE SHALL be set to CK_TRUE and CKA_EXTRACTABLE SHALL also be set to CK_TRUE), and also to be used for further key derivation. From K, a token key K_TOKEN of suitable type is derived by calling C_DeriveKey using the PKCS #11 mechanism CKM_EXTRACT_KEY_FROM_KEY and setting the CK_EXTRACT_PARAMS to the first bit of the generic secret key (i.e. set to 0). Likewise, a MAC key K_MAC is derived from K by calling C_DeriveKey using the CKM_EXTRACT_KEY_FROM_KEY mechanism, this time setting CK_EXTRACT_PARAMS to the length of K (in bits) divided by two. Nystroem, et al. Expires December 13, 2007 [Page 92] Internet-Draft DSKPP June 2007 2. The server wraps K with either the token's public key K_CLIENT, the shared secret key K_SHARED, or the derived shared secret key K_DERIVED by using C_WrapKey. If use of the DSKPP key wrap algorithm has been negotiated then the CKM_KIP_WRAP mechanism SHALL be used to wrap K. When calling C_WrapKey, the hKey handle in the CK_KIP_PARAMS structure SHALL be set to NULL_PTR. The pSeed parameter in the CK_KIP_PARAMS structure SHALL point to the nonce R provided by the DSKPP client, and the ulSeedLen parameter SHALL indicate the length of R. The hWrappingKey parameter in the call to C_WrapKey SHALL be set to refer to the wrapping key. 3. Next, the server needs to calculate a MAC using K_MAC. If use of the DSKPP MAC algorithm has been negotiated, then the MAC is calculated by calling C_SignInit with the CKM_KIP_MAC mechanism followed by a call to C_Sign. In the call to C_SignInit, K_MAC SHALL be the signature key, the hKey parameter in the CK_KIP_PARAMS structure SHALL be set to NULL_PTR, the pSeed parameter of the CT_KIP_PARAMS structure SHALL be set to NULL_PTR, and the ulSeedLen parameter SHALL be set to zero. In the call to C_Sign, the pData parameter SHALL be set to the concatenation of the string ID_S and the nonce R, and the ulDataLen parameter SHALL be set to the length of the concatenated string. The desired length of the MAC SHALL be specified through the pulSignatureLen parameter and SHALL be set to the length of R. 4. If the server also needs to authenticate its message (due to an existing K_TOKEN being replaced), the server SHALL calculate a second MAC. Again, if use of the DSKPP MAC algorithm has been negotiated, then the MAC is calculated by calling C_SignInit with the CKM_KIP_MAC mechanism followed by a call to C_Sign. In this call to C_SignInit, the K_MAC existing before this DSKPP protocol run SHALL be the signature key, the hKey parameter in the CK_KIP_PARAMS structure SHALL be set to NULL, the pSeed parameter of the CT_KIP_PARAMS structure SHALL be set to NULL_PTR, and the ulSeeidLen parameter SHALL be set to zero. In the call to C_Sign, the pData parameter SHALL be set to the concatenation of the string ID_S and the nonce R, and the ulDataLen parameter SHALL be set to the length of concatenated string. The desired length of the MAC SHALL be specified through the pulSignatureLen parameter and SHALL be set to the length of R. Nystroem, et al. Expires December 13, 2007 [Page 93] Internet-Draft DSKPP June 2007 5. The server sends its message to the client, including the wrapped key K, the MAC and possibly also the authenticating MAC. c. On the client side, 1. The client calls C_UnwrapKey to receive a handle to K. After this, the client calls C_DeriveKey twice: Once to derive K_TOKEN and once to derive K_MAC. The client SHALL use the same mechanism (CKM_EXTRACT_KEY_FROM_KEY) and the same mechanism parameters as used by the server above. When calling C_UnwrapKey and C_DeriveKey, the pTemplate parameter SHALL be used to set additional key attributes in accordance with local policy and as negotiated and expressed in the protocol. In particular, the value of the element in the server's response message MAY be used as CKA_ID for K_TOKEN. The key K SHALL be destroyed after deriving K_TOKEN and K_MAC. 2. The MAC is verified in a reciprocal fashion as it was generated by the server. If use of the CKM_KIP_MAC mechanism has been negotiated, then in the call to C_VerifyInit, the hKey parameter in the CK_KIP_PARAMS structure SHALL be set to NULL_PTR, the pSeed parameter SHALL be set to NULL_PTR, and ulSeedLen SHALL be set to 0. The hKey parameter of C_VerifyInit SHALL refer to K_MAC. In the call to C_Verify, pData SHALL be set to the concatenation of the string ID_S and the nonce R, and the ulDataLen parameter SHALL be set to the length of the concatenated string, pSignature to the MAC value received from the server, and ulSignatureLen to the length of the MAC. If the MAC does not verify the protocol session ends with a failure. The token SHALL be constructed to not "commit" to the new K_TOKEN or the new K_MAC unless the MAC verifies. 3. If an authenticating MAC was received (REQUIRED if the new K_TOKEN will replace an existing key on the token), then it is verified in a similar vein but using the K_MAC associated with this server and existing before the protocol run. Again, if the MAC does not verify the protocol session ends with a failure, and the token MUST be constructed no to "commit" to the new K_TOKEN or the new K_MAC unless the MAC verifies. Nystroem, et al. Expires December 13, 2007 [Page 94] Internet-Draft DSKPP June 2007 D.3. The 1-pass Variant A suggested procedure to perform 1-pass DSKPP with a cryptographic token through the PKCS #11 interface using the mechanisms defined in [8] is as follows: a. On the server side, 1. A generic key K = K_TOKEN | K _MAC (where '|' denotes concatenation) is generated, e.g. by calling C_GenerateKey (using key type CKK_GENERIC_SECRET). The template for K SHALL allow it to be exported (but only in wrapped form, i.e. CKA_SENSITIVE SHALL be set to CK_TRUE and CKA_EXTRACTABLE SHALL also be set to CK_TRUE), and also to be used for further key derivation. From K, a token key K_TOKEN of suitable type is derived by calling C_DeriveKey using the PKCS #11 mechanism CKM_EXTRACT_KEY_FROM_KEY and setting the CK_EXTRACT_PARAMS to the first bit of the generic secret key (i.e. set to 0). Likewise, a MAC key K_MAC is derived from K by calling C_DeriveKey using the CKM_EXTRACT_KEY_FROM_KEY mechanism, this time setting CK_EXTRACT_PARAMS to the length of K (in bits) divided by two. 2. The server wraps K with either the token's public key, K_CLIENT, the shared secret key, K_SHARED, or the derived shared secret key, K_DERIVED by using C_WrapKey. If use of the DSKPP key wrap algorithm has been negotiated, then the CKM_KIP_WRAP mechanism SHALL be used to wrap K. When calling C_WrapKey, the hKey handle in the CK_KIP_PARAMS structure SHALL be set to NULL_PTR. The pSeed parameter in the CK_KIP_PARAMS structure SHALL point to the octet-string representation of an integer I whose value SHALL be incremented before each protocol run, and the ulSeedLen parameter SHALL indicate the length of the octet-string representation of I. The hWrappingKey parameter in the call to C_WrapKey SHALL be set to refer to the wrapping key. Note: The integer-to-octet string conversion SHALL be made using the I2OSP primitive from [13]. There SHALL be no leading zeros. 3. For the server's message to the client, if use of the DSKPP MAC algorithm has been negotiated, then the MAC is calculated by calling C_SignInit with the CKM_KIP_MAC mechanism followed by a call to C_Sign. In the call to C_SignInit, K_MAC SHALL be the signature key, the hKey parameter in the CK_KIP_PARAMS structure SHALL be set to NULL_PTR, the pSeed parameter of the CT_KIP_PARAMS structure SHALL be set to NULL_PTR, and the Nystroem, et al. Expires December 13, 2007 [Page 95] Internet-Draft DSKPP June 2007 ulSeedLen parameter SHALL be set to zero. In the call to C_Sign, the pData parameter SHALL be set to the concatenation of the string ID_S and the octet-string representation of the integer I, and the ulDataLen parameter SHALL be set to the length of concatenated string. The desired length of the MAC SHALL be specified through the pulSignatureLen parameter as usual, and SHALL be equal to, or greater than, sixteen (16). 4. If the server also needs to authenticate its message (due to an existing K_TOKEN being replaced), the server calculates a second MAC. If the DSKPP MAC mechanism is used, the server does this by calling C_SignInit with the CKM_KIP_MAC mechanism followed by a call to C_Sign. In the call to C_SignInit, the K_MAC existing on the token before this protocol run SHALL be the signature key, the hKey parameter in the CK_KIP_PARAMS structure SHALL be set to NULL_PTR, the pSeed parameter of the CT_KIP_PARAMS structure SHALL be set to NULL_PTR, and the ulSeedLen parameter SHALL be set to zero. In the call to C_Sign, the pData parameter SHALL be set to the concatenation of the string ID_S and the octet- string representation of the integer I+1 (i.e. I SHALL be incremented before each use), and the ulDataLen parameter SHALL be set to the length of the concatenated string. The desired length of the MAC SHALL be specified through the pulSignatureLen parameter as usual, and SHALL be equal to, or greater than, sixteen (16). 5. The server sends its message to the client, including the MAC and possibly also the authenticating MAC. b. On the client side, 1. The client calls C_UnwrapKey to receive a handle to K. After this, the client calls C_DeriveKey twice: Once to derive K_TOKEN and once to derive K_MAC. The client SHALL use the same mechanism (CKM_EXTRACT_KEY_FROM_KEY) and the same mechanism parameters as used by the server above. When calling C_UnwrapKey and C_DeriveKey, the pTemplate parameter SHALL be used to set additional key attributes in accordance with local policy and as negotiated and expressed in the protocol. In particular, the value of the element in the server's response message MAY be used as CKA_ID for K_TOKEN. The key K SHALL be destroyed after deriving K_TOKEN and K_MAC. Nystroem, et al. Expires December 13, 2007 [Page 96] Internet-Draft DSKPP June 2007 2. The MAC is verified in a reciprocal fashion as it was generated by the server. If use of the CKM_KIP_MAC mechanism has been negotiated, then in the call to C_VerifyInit, the hKey parameter in the CK_KIP_PARAMS structure SHALL be set to NULL_PTR, the pSeed parameter SHALL be set to NULL_PTR, and ulSeedLen SHALL be set to 0. The hKey parameter of C_VerifyInit SHALL refer to K_MAC. In the call to C_Verify, pData SHALL be set to the concatenation of the string ID_S and the octet-string representation of the provided value for I, and the ulDataLen parameter SHALL be set to the length of the concatenated string, pSignature to the MAC value received from the server, and ulSignatureLen to the length of the MAC. If the MAC does not verify or if the provided value of I is not larger than any stored value I' for the identified server ID_S the protocol session ends with a failure. The token SHALL be constructed to not "commit" to the new K_TOKEN or the new K_MAC unless the MAC verifies. If the verification succeeds, the token SHALL store the provided value of I as a new I' for ID_S. 3. If an authenticating MAC was received (REQUIRED if K_TOKEN will replace an existing key on the token), it is verified in a similar vein but using the K_MAC existing before the protocol run. Again, if the MAC does not verify the protocol session ends with a failure, and the token MUST be constructed no to "commit" to the new K_TOKEN or the new K_MAC unless the MAC verifies. Nystroem, et al. Expires December 13, 2007 [Page 97] Internet-Draft DSKPP June 2007 Appendix E. Example of DSKPP-PRF Realizations E.1. Introduction This example appendix defines DSKPP-PRF in terms of AES [18] and HMAC [19]. E.2. DSKPP-PRF-AES E.2.1. Identification For tokens supporting this realization of DSKPP-PRF, the following URI MAY be used to identify this algorithm in DSKPP: urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes When this URI is used to identify the encryption algorithm to use, the method for encryption of R_C values described in Section 3.8 SHALL be used. E.2.2. Definition DSKPP-PRF-AES (k, s, dsLen) Input: k Encryption keyto use s Octet string consisting of randomizing material. The length of the string s is sLen. dsLen Desired length of the output Output: DS A pseudorandom string, dsLen-octets long Steps: 1. Let bLen be the output block size of AES in octets: bLen = (AES output block length in octets) (normally, bLen = 16) 2. If dsLen > (2**32 - 1) * bLen, output "derived data too long" and stop Nystroem, et al. Expires December 13, 2007 [Page 98] Internet-Draft DSKPP June 2007 3. Let n be the number of bLen-octet blocks in the output data, rounding up, and let j be the number of octets in the last block: n = ROUND( dsLen / bLen) j = dsLen - (n - 1) * bLen 4. For each block of the pseudorandom string DS, apply the function F defined below to the key k, the string s and the block index to compute the block: B1 = F (k, s, 1) , B2 = F (k, s, 2) , ... Bn = F (k, s, n) The function F is defined in terms of the OMAC1 construction from [20], using AES as the block cipher: F (k, s, i) = OMAC1-AES (k, INT (i) || s) where INT (i) is a four-octet encoding of the integer i, most significant octet first, and the output length of OMAC1 is set to bLen. Concatenate the blocks and extract the first dsLen octets to product the desired data string DS: DS = B1 || B2 || ... || Bn<0..j-1> Output the derived data DS. E.2.3. Example If we assume that dsLen = 16, then: n = 16 / 16 = 1 j = 16 - (1 - 1) * 16 = 16 DS = B1 = F (k, s, 1) = OMAC1-AES (k, INT (1) || s) E.3. DSKPP-PRF-SHA256 E.3.1. Identification For tokens supporting this realization of DSKPP-PRF, the following URI MAY be used to identify this algorithm in DSKPP: Nystroem, et al. Expires December 13, 2007 [Page 99] Internet-Draft DSKPP June 2007 urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-sha256 When this URI is used to identify the encryption algorithm to use, the method for encryption of R_C values described in Section 3.8 SHALL be used. E.3.2. Definition DSKPP-PRF-SHA256 (k, s, dsLen) Input: k Encryption key to use s Octet string consisting of randomizing material. The length of the string s is sLen. dsLen Desired length of the output Output: DS A pseudorandom string, dsLen-octets long Steps: 1. Let bLen be the output size of SHA-256 in octets of [21] (no truncation is done on the HMAC output): bLen = 32 (normally, bLen = 16) 2. If dsLen > (2**32 - 1) * bLen, output "derived data too long" and stop 3. Let n be the number of bLen-octet blocks in the output data, rounding up, and let j be the number of octets in the last block: n = ROUND( dsLen / bLen) j = dsLen - (n - 1) * bLen 4. For each block of the pseudorandom string DS, apply the function F defined below to the key k, the string s and the block index to compute the block: B1 = F (k, s, 1) , B2 = F (k, s, 2) , ... Bn = F (k, s, n) Nystroem, et al. Expires December 13, 2007 [Page 100] Internet-Draft DSKPP June 2007 The function F is defined in terms of the HMAC construction from [19], using SHA-256 as the digest algorithm: F (k, s, i) = HMAC-SHA256 (k, INT (i) || s) where INT (i) is a four-octet encoding of the integer i, most significant octet first, and the output length of HMAC is set to bLen. Concatenate the blocks and extract the first dsLen octets to product the desired data string DS: DS = B1 || B2 || ... || Bn<0..j-1> Output the derived data DS. E.3.3. Example If we assume that sLen = 256 (two 128-octet long values) and dsLen = 16, then: n = ROUND ( 16 / 32 ) = 1 j = 16 - (1 - 1) * 32 = 16 B1 = F (k, s, 1) = HMAC-SHA256 (k, INT (1) || s) DS = B1<0 ... 15> That is, the result will be the first 16 octets of the HMAC output. Nystroem, et al. Expires December 13, 2007 [Page 101] Internet-Draft DSKPP June 2007 Authors' Addresses Magnus Nystroem RSA, The Security Division of EMC Email: magnus@rsa.com Salah Machani Diversinet Corp. Email: smachani@diversinet.com Mingliang Pei VeriSign, Inc. Email: mpei@verisign.com Andrea Doherty RSA, The Security Division of EMC Email: adoherty@rsa.com Nystroem, et al. Expires December 13, 2007 [Page 102] Internet-Draft DSKPP June 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Nystroem, et al. Expires December 13, 2007 [Page 103]