Internet Engineering Task Force M. Pritikin, Ed. Internet-Draft Cisco Systems, Inc. Intended status: Standards Track July 11, 2011 Expires: January 12, 2012 Enrollment over Secure Transport draft-pritikin-est-02 Abstract This document specifies a protocol for certificate Enrollment over Secure Transport (EST). EST is a certificate enrollment protocol that operates over HTTPS, and thus should be trivially accessible by modern clients. The Certificate Management over CMS (CMC) "Simple PKI Request" and "Simple PKI Response" messages are leveraged. EST is designed to be easily implemented by clients and servers running other common enrollment mechanisms such as Simple Certificate Enrollment Protocol (SCEP). Renewal and rekey mechanisms are described consistent with Certificate Management Protocol (CMP). Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on January 12, 2012. Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect Pritikin Expires January 12, 2012 [Page 1] Internet-Draft EST July 2011 to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 7 2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 7 3. Secure Transport . . . . . . . . . . . . . . . . . . . . . . . 9 3.1. HTTPS-Based Server Authentication . . . . . . . . . . . . 9 3.2. Server Authentication and Authorization . . . . . . . . . 10 3.3. HTTPS-Based Client Authentication . . . . . . . . . . . . 11 3.4. HTTP-Based Client Authentication . . . . . . . . . . . . . 11 3.5. Client Authorization . . . . . . . . . . . . . . . . . . . 12 3.6. Proof-of-Possession . . . . . . . . . . . . . . . . . . . 12 3.7. Peer Authentication . . . . . . . . . . . . . . . . . . . 13 4. HTTP URLs . . . . . . . . . . . . . . . . . . . . . . . . . . 13 5. Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 5.1. Distribution of CA certificates . . . . . . . . . . . . . 15 5.1.1. Distribution of CA certificates response . . . . . . . 15 5.2. Simple Enrollment of Clients . . . . . . . . . . . . . . . 16 5.2.1. Simple Re-Enrollment of Clients . . . . . . . . . . . 16 5.2.2. Simple Enroll and Re-Enroll Response . . . . . . . . . 17 5.3. Full CMC . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.3.1. Full CMC Request . . . . . . . . . . . . . . . . . . . 18 5.3.2. Full CMC Response . . . . . . . . . . . . . . . . . . 18 6. Cryptographic Algorithms . . . . . . . . . . . . . . . . . . . 18 7. Contributors/Acknowledgements . . . . . . . . . . . . . . . . 19 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 9. Security Considerations . . . . . . . . . . . . . . . . . . . 19 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21 10.1. Normative References . . . . . . . . . . . . . . . . . . . 21 10.2. Informative References . . . . . . . . . . . . . . . . . . 22 Appendix A. Server Discovery . . . . . . . . . . . . . . . . . . 22 Appendix B. External TLS concentrator . . . . . . . . . . . . . . 22 Appendix C. CGI Server implementation . . . . . . . . . . . . . . 23 Appendix D. Updating SCEP implementations . . . . . . . . . . . . 23 Appendix E. Key Update mechanisms . . . . . . . . . . . . . . . . 25 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25 Pritikin Expires January 12, 2012 [Page 2] Internet-Draft EST July 2011 1. Introduction This document profiles certificate enrollment for clients using Certificate Management over CMS [RFC5272] messages over a secure transport. This profile describes a simple yet functional certificate management protocol targeting simple PKI clients that need to acquire client certificate(s) and associated infrastructure certificate(s). "CMC: Transport Protocols" [RFC5273] provides some guidance for running CMC over HTTP [RFC2617] but notes only that "clients MAY attempt to send HTTP requests using TLS 1.0 [TLS] or later, although servers are not required to support TLS". No attempt is made in that document to specify how the client and server might take advantage of a secured transport to better leverage the Simple PKI messages. This profile specifies secure transport mechanisms and how values from the TLS exchange, the HTTP exchange, and the CMC Simple PKI messages layers are used for authentication (and authorization) purposes by the server. The aspects profiled from TLS/HTTPS, CMS and CMP are summarized in Figure 1: Pritikin Expires January 12, 2012 [Page 3] Internet-Draft EST July 2011 Profiled Layers: Protocol: +---------------------------------------------------+ | | | 3) Message types | | CMC "Simple PKI" messages. | | PEM encoded certifiate chains. | | Optionally "Full" CMC messages. | | | +---------------------------------------------------+ | | | 2) HTTP headers and URLs for control | | URLs used to specify the PKI operation | | (including renew/rekey). | | Content-Type headers specify the message type. | | Headers profiled for control/error messages. | | Username/password methods supported for | | client proof-of-identity. | | | | | +- ----(combination is known as HTTPS)--+ | | | | | 1) TLS for transport security | | Provides proof-of-identity for | | EST Server authentication and | | EST Client authentication. | | "Channel binding" type techniques used to | | during Proof-of-Possesion. | | | +---------------------------------------------------+ | | | TCP/IP layer etc included in diagram for context | | | +---------------------------------------------------+ Application Logic: +------------------------------------+ | | | 4) Certificate Chain Validation | | Certificate chains that include | | rekey/renewed certificates as | | specified in CMP. | | | +------------------------------------+ Pritikin Expires January 12, 2012 [Page 4] Internet-Draft EST July 2011 Figure 1 The following provides a high level overview describing how these layers are used. Each aspect is profiled in detail in the sections below. 1) TLS for transport security: CMC section 3.1 notes that "the Simple PKI Request MUST NOT be used if a proof-of-identity needs to be included". This precludes use of these messages if inline authentication and/or authorization is required, unless a secured transport that provides proof-of-identity is also specified. Many simple clients engaged in certificate enrollment operations will have a TLS client implementation available for secure transport, so use of TLS is specified herein. This document specifies a method for authorizing client enrollment requests using existing certificates. Such existing certificates may have been issued by the CA (from which the client is requesting a certificate) or they may have been issued under a distinct PKI (e.g. an IEEE 802.1AR IDevID [IDevID] credential). Additionally this document specifies username/password authentication methods beyond those included in CMC. Authentication and authorization mechanisms required for certificate issuance (and renew/rekey) are provided by HTTP and TLS (HTTPS) mechanisms as described in this document. Proof-of-possession is a distinct issue from proof-of-identity and is addressed in Section 3.6. This document also defines an appropriate transport for the full CMC specification compliant with CMC Transport Protocols. 2) HTTP Headers and URLs for control: This profile supports two operations indicated by specific URLs: * Distribution of CA certificates * Authorized enrollment and re-enrollment of clients This document profiles HTTP headers to indicate the message type and to provide the protocol control messages. Support for the HTTP username/password methods is profiled. CMC also states that: "No special services are provided for doing either renewal (new certificates with the same key) or rekeying Pritikin Expires January 12, 2012 [Page 5] Internet-Draft EST July 2011 (new certificates on new keys) of clients. Instead a renewal/ rekey message looks the same as any enrollment message, with the identity proof being supplied by existing certificates from the CA." This profile clarifies the renewal and rekey behavior of both the client and server. It does so by specifying the HTTP control mechanisms required of the client and server without require a distinct message type. 3) Message Types: Some messages types used here are defined in CMC and include subsets of the PKCS#10 Certification Request [RFC2986] and the PKCS#7 [RFC2315] message specifications. This document profiles the use of two Certificate Management over CMS messages: "Simple PKI Request" and "Simple PKI Response" and does not require full implementation of all CMC features. This is consistent with the CMC protocol specification of "simple" messages for clients to use "in the event no other services are needed". Additional simple message formats are defined in this document. To support distribution of the CA certificate chain a simple PEM format is specified. Full CMC messages MAY be used as specified below. HTTP Content-Type headers are as specified in CMC: Transport Protocols, Table 1. This document introduces new content types for the simple format messages not specified by CMC. 4) Certificate Chain Validation: A small clarification of the application layer certificate chain validation logic is provided by a normative reference to CMP. The certificate renewal and rekey certificate chaining mechanisms documented in CMP [RFC4210] are referenced. An EST server providing certificate management functions is operated by (or on behalf of) a CA or RA. An EST server MAY provide additional, non-EST services on other URLs. The server also MAY support full CMC messages over HTTP. [[EDNOTE: Comments such as this one, included within double brackets and initiated with an 'EDNOTE', are for editorial use and shall be removed as the document is polished.]] Pritikin Expires January 12, 2012 [Page 6] Internet-Draft EST July 2011 1.1. Requirements Language 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 [RFC2119]. 2. Requirements [[EDNOTE: The following section is still included here for succinctness. It will eventually be published independently as draft-pritikin-estr-00.]] The following describes goals and technical requirements for initial PKI certificate enrollment along with a rationale for each requirement. G1 "Completeness". Server and client implementations compliant with this document MUST be able to interoperate without reference to subsequent profiles or additional future specifications. The goal of this enrollment protocol is to provide a simple and easy- to-implement method for end-entities to request, obtain and update a certificate issued from a specified Certification Authority. The following certificate management operations are required. Additional operations NEED NOT be supported (via this protocol) although the protocol design SHOULD be extensible: M1 "Distribution of current CA certificates". Clients MUST be able to obtain the current certificate for the CA under which the client's certificate was issued. Certificates have a finite lifetime and will need to be updated on a periodic basis. It must be possible for the client to obtain the updated CA certificates. M2 "Enrollment". A client MUST be able to use the protocol to submit a certificate request and obtain a certificate. M3 "Renew/Rekey". Certificates have a finite lifetime and will need to be updated on a periodic basis. A client MUST be able to use the protocol for certificate renewal or rekey operations. End-Entity Proof of Identity authentication mechanisms: A1 "Username/Password". It MUST be possible to identify a username specified client as being in possession of an associated symmetric key. This allows users currently in possession of a username and password to obtain a certificate. Pritikin Expires January 12, 2012 [Page 7] Internet-Draft EST July 2011 A2 "Password". It MUST be possible to identity a client wihtout reference to a "username". A common operational model is to distribute a "one time password" that is presented to a CA or RA to authorize enrollment. A3 "Existing Certificate". It MUST be possible to authenticate a client by making use of an existing certificate associated with the client. A certificate used for client identification need not be issued under the same PKI as the certificate that is being requested. This allows clients that are already in a PKI to use a certificate from that PKI to obtain additional certificates. Additionally this capability allows a client who has a certificate issued by a 3rd party, such as a certificate issued by a device manufacturer, to leverage that credential during initial enrollment. A4 "Username/password and Certificate". It MUST be possible to authenticate a client by using a combination of a username and password and an existing certificate. This is a combination of A1 and A3. This supports "two factor authentication" where the client proves possession of the private keys for an existing certificate stored within a hardware device and knowledge of a username/password. A5 "Password and certificate". It MUST be possible to authenticate a client by using a combination of a secrete value that is not associated with a "username" and an existing certificate. This is a combination of A2 and A3. This requirement is similar to A4 except that the client is in possession of a "one time password". End-Entity Proof of Possession: P1 Proof-of-Possession of subject keys MUST be supported. As discussed in Appendix C of [RFC4211] Proof-of-Possession "means that the CA is adequately convinced that the entity requesting a certificate for the public key Y, has access to the corresponding private key X". Key algorithms: K1 "Algorithm agility". The protocol MUST support algorithm agility. It must be possible to employ different cryptographic algorithms for securing the transport or for signing the certificates. The protocol SHOULD demonstrate this agility by being shown to work with existing RSA based solutions as well as providing for other algorithms such as Elliptic Curve cryptography. Server Identity mechanism: Pritikin Expires January 12, 2012 [Page 8] Internet-Draft EST July 2011 I1 "RA certificate". It MUST be possible for a client to verify authorization of the EST server as a representative of the CA. The protocol operations allow the client to send a username/ password or (one time) password to the EST server. These values cannot be safely transmitted to an unauthorized server. 3. Secure Transport HTTPS MUST be used. TLS 'session resumption' SHOULD be supported. HTTPS is defined in HTTP Over TLS [RFC2818] and is a definition of how HTTP messages are carried over TLS. HTTPS (HTTP over TLS) is a commonly used transport and can be easily layered on top of extremely simple client or server code. In some environments HTTPS can be utilized through an external process. Specifying HTTPS as the secured transport for PKI enrollment messages introduces two potential 'layers' for communication of authorization data or for status/informative responses during the protocol exchange: TLS and HTTPS. This profile specifies when information is used from each layer. 3.1. HTTPS-Based Server Authentication The client MUST validate the HTTPS server certificate presented during the TLS [RFC5246] defined Server Certificate message or the client MUST independently validate the response contents. The cipher suites are detailed in Section 6. There are multiple methods of validation depending on the current state of the client: 1. If the client has a store of trust anchors, which may be in the form of certificates, for validating HTTPS connections the client MAY validate the HTTPS server certificate using the standard HTTP logic of checking the server's identity as presented in the server's Certificate message against the URL provisioned for the EST server (see HTTPS Over TLS, Section 3.1 Server Identity. This method makes it possible for clients with a large store of HTTPS certificates to securely obtain the CA server certificate by leveraging the HTTPS security model. Note that the EST server URL MUST be made available to the client in a secure fashion and many systems are configured with many trust anchors from a wide range of CAs and this would make such systems vulnerable to spoofing of the ETS server certificate by an attacker that is ablet to obtain an erroneous certificate from a lax CA. As detailed in Section 9 clients are RECOMMENDED to ship with a carefully chosen list of initial trust anchors. Proper selection Pritikin Expires January 12, 2012 [Page 9] Internet-Draft EST July 2011 of initial trust anchors is out of scope of this document. [[EDNOTE: is there an RFC discussing this problem in the HTTPS space that we can reference?]] 2. If the client already has one or more trust anchors associated with this EST server, the client MAY validate the EST server certificate using these trust anchors. The EST server URL MAY be made available to the client in an insecure fashion. 3. If the client does not yet have a trust anchor associated with this EST server then the client MAY provisionally accept the TLS connection, but the HTTP content data must be accepted manually as described in Section 5.1. HTTP authentication requests MUST NOT be responded to. If one of these validation methods succeeds the CA certificate are stored and made available for future use. If none of these validation methods succeeds the client MUST reject the EST server response and SHOULD log or inform the end user. The EST server MUST present an end-entity certificate such as the CMC Local Registration Authority (LRA) certificate. The client MUST support validating the EST server certificate using the "Verifying Certificates" logic specified in CMP section 4.4. Appendix E provides an informative summary of key renewal and the associated validation logic. 3.2. Server Authentication and Authorization The client MUST check the EST server authorization. If the client has a securely configured and authorized URI for the EST server it SHOULD check the URI "against the server's identity as presented in the server's Certificate message" (Section 3.1 Server Identity [RFC2818]). The securely configured URI provides the authorization statement and the server's authenticated identity confirms it is the authorized server. If this check fails, or if the URI was configured using an insecure method, then the client MUST verify the server's authorization by checking that the [RFC5280] defined certificate policy extension sequence contains the 'LRA Authorization' policy OID. The LRA Authorization policy OID is defined as: id-cmc [[EDNOTE: TBD, perhaps 35]]. The LRA Authorization policy information MUST NOT contain any optional qualifiers. Pritikin Expires January 12, 2012 [Page 10] Internet-Draft EST July 2011 3.3. HTTPS-Based Client Authentication The server MUST send a TLS section 7.4.4 "Certificate Request" and the client MUST respond. The client MUST respond with a certificate that allows it to subsequently send the a TLS Section 7.4.8 "Certificate Verify" (i.e. the client MUST use an end entity "client certificate that has signing capability"). The server MUST verify the Certificate Verify message. The certificate presented by the client MAY be from the same PKI as the Server Certificate, from a completely different PKI, or as a last resort the client MAY respond with a self-signed certificate. The certificate supplied during authentication is used during client authorization (Section 3.5). The server MUST support the validation of the EST client certificate using normal certificate validation logic including rekey/renew support as specified in CMP section 4.4. Appendix E provides an informative summary of key renewal and the associated validation logic. 3.4. HTTP-Based Client Authentication As specified in CMC: Transport Protocols the server "MUST NOT assume client support for any type of HTTP authentication such as cookies, Basic authentication, or Digest authentication". Clients intended for deployments where password authentication is advantageous SHOULD support the Basic and Digest authentication mechanism. Servers MAY provide configuration mechanisms for administrators to enable Basic and Digest authentication methods. Servers that support Basic and Digest authentication methods reject requests using the HTTP defined WWW-Authenticate response-header (Section 14.47). At which point the client SHOULD repeat the request, including the appropriate HTTP [RFC2617] Authorization Request Header (Section 3.2.2). Support for Basic authentication as specified in HTTP allows the server access to the client's cleartext password. This provides integration with legacy username password databases but requires exposing the plaintext password to the EST server. The client MUST NOT respond to this request unless the EST server has been authenticated (as per Section 3.2). Clients MAY set the username to the empty string ("") if they wish to present a "one time password" or "PIN" that is not associated with a username. Pritikin Expires January 12, 2012 [Page 11] Internet-Draft EST July 2011 3.5. Client Authorization When the CA server receives a CMC Simple PKI Enrollment or re- enrollment message, the decision to issue a certificates is always a matter of local policy. Thus the CA can use any data it wishes in making that determination. The EST protocol exchange provides the EST server access to the TLS client Certificate in addition to the HTTP user authentication credentials to help in that determination. The communication channel between the TLS implementation and the EST software implementation is out-of-scope of this document. 3.6. Proof-of-Possession As discussed in Appendix C of CRMF [RFC4211] Proof-of-Possession "means that the CA is adequately convinced that the entity requesting a certificate for the public key Y, has access to the corresponding private key X". This specification provides proof-of-possession by including information specific to the current TLS session within the signed certification request. This proves to the server that the TLS client has possession of the private key associated with the certification request and that the client was able to sign the certification request after the TLS session was established. The value included within the certification request is very similar to "tls-unique" as defined in Channel Bindings for TLS [RFC5929]. The value is defined as: tls-unique-securerenegotiation: The first TLS Finished message sent in the _first_ TLS handshake of the TLS connection that is being bound to is the TLS "channel binding" value. Any TLS renegotiation MUST use "secure_renegotiation" [RFC5746] (thus maintaining the binding). Mandating secure renegotiation allows implementations to avoid the synchronization issues encountered with tls-unique. The client generating the request SHOULD obtain the tls-unique- securerenegotation value, encode it using base64 encoding, and place the resulting string in the certification request challenge password field. The server SHOULD verify the tls-unique-securerenegotation information. This ensures that the authenticated TLS client is in possession of the private key used to sign the certification request. The tls-unique-securerenegotiation value is encoded into the certification request by the client but back-end infrastructure elements that process the request might not have access to the Pritikin Expires January 12, 2012 [Page 12] Internet-Draft EST July 2011 initial TLS session. Such infrastructure elements validate the source of the certification request to determine if proof-of- possession checks have already been performed. For example if the client authentication results in an authenticated client identity of an RA that is known to independently verify the proof-of-possession, then the back-end infrastructure does not need to perform proof-of- possession checks a second time. Implementation Note: The tls-unique value is consistent with tls- unique-securerenegotiation until after a renegotiation (at which point the tls-unique value is the TLS Finished message of the "most recent TLS handshake" instead of the "first handshake"). A valid tls-unique-securerenegotiation value can be obtained by careful use of the implementation's tls-unique channel binding TLS APIs so long as renegotiation has not yet taken place. The use of tls-unique-securerenegotiation makes it possible for servers to wait to request TLS client authentication until after the URI has been parsed, as is commonly implemented. 3.7. Peer Authentication The EST server can itself be an EST client when an RA uses EST to communicate with back-end infrastructure elements. Authentication of credentials identifying an EST peer is in scope in that appropriate generic credential authentication in an environment supporting Root CA Key Update is mandated. EST clients validating peer (other EST client) certificates MUST support the Root CA Key Update verification mechanisms specified in CMP section 4.4 when validating the peer certificates. Appendix E provides an informative summary on key renewal. 4. HTTP URLs EST uses the HTTP "GET" and "POST" messages to communicate with the EST server. The following describes the syntax of these messages: "GET" BASEPATH OPERATIONPATH "POST" BASEPATH OPERATIONPATH where: o BASEPATH is a common path for all EST operations o OPERATIONPATH specifies the specific operation. When an URL is formed the BASEPATH and OPERATIONPATH are combined to form the abs_path [RFC2616]. The server and port and MUST be pre- Pritikin Expires January 12, 2012 [Page 13] Internet-Draft EST July 2011 configured or otherwise discovered by the client as described in Appendix A. The means by which clients acquire the base URL are outside the scope of this document. The following are two example base URLs: o https://example.org/BASEPATH o https://example.org:8080/arbitrary/base/path These can be conveniently distributed as they are in a form with which many end users are already familiar. The following operation URLs for client to access are defined relative to the EST base URL: o /CACerts - The server responds to an HTTPS GET with the CA certificates as defined in Distribution of CA certificates (Section 5.1). o /simpleEnroll - The client sends a CMC Simple PKI Enrollment message as specified in Enrollment of Clients (Section 5.2), the response is a CMC Simple PKI Response. message as specified in Enroll Response (Section 5.2.2). o /simpleReEnroll - Exactly the same as 'simpleEnroll' except that the request is for re-enrollment or re-issuance purposes. o /fullCMC - Provides for a CMC transport (optional). The following examples are valid complete URLs under this specification: o https://example.org/BASEPATH/CACerts o https://example2.org/arbitrary/base/path/simpleEnroll o https://example2.org/arbitrary/base/path/simpleReEnroll o https://example3.org/example/ca/fullCMC The mechanisms by which the EST server interacts with an HTTPS server to handle GET and POST operations at these URLs is outside the scope of this document. The use of distinct URLs simplifies implementation for servers that do not perform client authentication when distributing "CACerts" responses. Implementation note: A simple Common Gateway Interface (CGI) application at each fully specified path, with the HTTPS server configured to provide Section 3.3, is sufficient for a working example (the web service can forward the Section 3.6 proof-of- Pritikin Expires January 12, 2012 [Page 14] Internet-Draft EST July 2011 possession information to the application using the CGI interface). [[EDNOTE: This does not use the mechanism specified in "Defining Well-Known Uniform Resource Identifiers (URIs)" [RFC5785]. That would be a possibility here for a base URL of "https://example.org/.well-known/EST" but such would preclude the flexibility associated with multiple base urls being handled by the same server unless some form of "?designator=value" is included.]] 5. Messages 5.1. Distribution of CA certificates Before engaging in enrollment operations, clients MUST request trust anchor information by sending an HTTPS GET message to the EST base URL with the relative path extension 'CACerts'. Clients SHOULD request an up to date response before stored information has expired. The EST server SHOULD NOT require client authentication or authorization to reply to this request. The client MUST authenticate the EST server as specified in Authentication and Authorization (Section 3). If the authentication and authorization is successful, the client accepts the response and stores it. If the authentication and authorization is not successful, then when the response is received the client MUST extract the CA certificate and engage the end-user or otherwise authorize the credential using out-of-band pre-configuration data such as a CA certificate "fingerprint" (e.g., a SHA-1, SHA-256, SHA- 512, or MD5 hash on the whole CA certificate). The client MUST NOT accept the CA certificate without validating it via one of the mechanisms described above. Subsequent connections to the EST server validate the TLS server certificate using the stored CA certificates as described in Authentication and Authorization (Section 3). 5.1.1. Distribution of CA certificates response The EST server MUST respond to the client HTTPS GET message with trust anchor information in the form of a certificate. Additionally the server MUST include any "Root CA Key Update" CMP certificates (Appendix E provides an informative summary of "Root CA Key Update"). The response format is a text file containing a list of certificates each formatted as specified in Section 6.1 of [RFC4945]. Each Pritikin Expires January 12, 2012 [Page 15] Internet-Draft EST July 2011 certificate is delimited by a newline. The content-type of "application/x-est-cacerts" MUST be specified. 5.2. Simple Enrollment of Clients At any time the client MAY request a certificate from the EST base URL with the relative path extension "simpleEnroll'. When HTTPS POSTing to the 'Enroll' location the client MUST include a CMC Simple PKI Enrollment request as specified in CMC Section 3.1 (a PKCS#10 Certification Request). The content-type of "application/x-est-pkcs10" MUST be specified. The format of the request is as specified in Section 6.4 of [RFC4945]. The server MUST authenticate the client as specified in Authentication and Authorization (Section 3). The server applies whatever authorization or policy logic it chooses determining if the certificate should be issued. The server MAY choose to issue a certificate different from the certificate request as specified in CMC Section 3.1. The client MAY request an additional certificate even when using an existing certificate in the TLS client authentication. The client MUST authenticate the EST server as specified in Section 3.1. If the EST server forwards the request to a back-end process it SHOULD communicate the authentication results. For example using the CMC "RA POP Witness Control" in a CMC Full PKI Request message. 5.2.1. Simple Re-Enrollment of Clients At any time a client MAY request renew/rekey of its certificate from the EST base URL with the relative path extension "simpleReEnroll'. The certificate request is the same format as for the "simpleEnroll" path extension with the same content-type. The EST server MUST handle enrollment requests submitted to the "simpleReEnroll" URL as renewal or rekey requests rather than depending only on the method of identifying a renewal or rekey request specified in Section 2 of RFC5272 [RFC5272], that "renewal and rekey requests look the same as any certification request, except that the identity proof is supplied by existing certificates from a trusted CA". The proof of client identity is supplied by client authentication during the HTTPS handshake. When attempting to renew or rekey the client MUST use its existing certificate for TLS client Pritikin Expires January 12, 2012 [Page 16] Internet-Draft EST July 2011 authentication. [[EDNOTE: draft-turner-suiteb-cmc defines a method of recognizing an re-enroll based on PKCS10 contents, see section 4.1. The method described herein is explicit.]] If the server forwards the request to a back-end process it SHOULD communicate that this is a renew/rekey attempt. Implementation note: if using this protocol to communicate with a CA the /simpleReEnroll URL is used. 5.2.2. Simple Enroll and Re-Enroll Response The server responds to a 'simpleEnroll' or 'simpleReEnroll' request with the client's newly issued certificate or it provides an error response. If the enrollment is successful the server response MUST have a response code of 200 with a content-type of "application/x-est-x509". The response data is the certificate formatted as specified in Section 6.1 of [RFC4945]. The issued certificate MAY be signed by a new CA key established as described in CMP. When rejecting a request the server MUST specify either an HTTP 4xx/ 401 error, or an HTTP 5xx error. A simple CMC response with content- type of "application/pkcs7-mime" MAY be included in the response data for any error response. If the content-type is not set the response data MUST be a plain text human readable error message. A client MAY elect to not parse a CMC error response in favor of a generic error message. If the server responds with an HTTP 202 this indicates that the request has been accepted for processing but that a response is not yet available. The server MUST include a Retry-After header as defined for 503 responses and MAY include informative human readable content. The client MUST wait at least the specified 'retry-after' time before repeating the same request. The client repeats the initial enrollment request after the appropriate 'retry-after' interval as expired. The client SHOULD log or inform the end user of this event. The server is responsible for maintaining all state necessary to recognize and handle retry operations as the client is stateless in this regard (it simply sends the same request repeatedly until it receives a different response code). All other return codes are handled as specified in HTTP. Pritikin Expires January 12, 2012 [Page 17] Internet-Draft EST July 2011 5.3. Full CMC At any time the client MAY request a certificate from the EST base URL with the relative path extension "fullCMC". The client MUST authenticate the server as specified in Server Authentication (Section 3.1) if an HTTPS url is used. The server SHOULD authenticate the client as specified in Authentication and Authorization (Section 3). The server MAY depend on CMC client authentication methods instead. In addition to the normal CMC proof-of-identity mechanisms the client SHOULD include the Section 3.6 value. 5.3.1. Full CMC Request When HTTP(S) POSTing to the 'fullCMC' location the client MUST include a valid CMC message. The content-type MUST be set to "application/pkcs7-mime" as specified in CMC: Transport Protocols. 5.3.2. Full CMC Response The server responds with the client's newly issued certificate or provides an error response. If the enrollment is successful the server response MUST have a response code of 200 with a content-type of "application/pkcs7-mime" as specified in CMC: Transport Protocols. The response data includes either the CMC Simple PKI Response or the CMC Full PKI Response. When rejecting a request the server MAY specify either an HTTP 4xx/ 401 error, an HTTP 5xx error or a response code 200. A CMC response with content-type of "application/pkcs7-mime" MUST be included in the response data for any error response. The client MUST parse the CMC response to determine the current status. All other return codes are handled as specified in Section 5.2.2 or HTTP [RFC2616]. 6. Cryptographic Algorithms This section details the specific cryptographic algorithms and cipher suite requirements. When the TLS connection is established the supported cipher suite codes are exchanged in the ClientHello and ServerHello messages. The Pritikin Expires January 12, 2012 [Page 18] Internet-Draft EST July 2011 negotiated cipher suite denotes the algorithms used during client and server authentication and these are negotiated to match the credentials available to the peers. The client SHOULD offer the Suite B compliant cipher suites as indicated in [RFC5430], Section 4 "Suite B Compliance and Interoperability Requirements". For greatest interoperability the client SHOULD also offer TLS_RSA_WITH_AES_128_CBC_SHA. When the client accesses the "simpleReEnroll" method the cipher suite MUST be appropriate for the existing certificate. The certificate type used determines the appropriate signatureAlgorithm for the PKCS#10 Certification Request. For example if a [RFC5430] cipher suite is used the signatureAlgorithm MAY be ecdsa-with-sha256 for P-256 certification requests, or MAY be ecdsa-with-sha384 for P-384 certification requests. [[EDNOTE: This is in alignment with draft-turner-suitb-cmc-03 section 4.1. To encourage algorithm agility, discussions of the MUST/SHOULD algorithms should be in a distinct document.]] 7. Contributors/Acknowledgements The editor would like to thank Stephen Kent, Vinod Arjun, Jan Vilhuber and others for their feedback and prototypes of early drafts. 8. IANA Considerations (This section is incomplete) The following aspects should be registered with IANA Considerations: The LRA Authorization certificate policy extension OID as discussed in Section 3.2 requires registration with IANA. [[EDNOTE: The URLs specified in Section 1 probably do not need to be registered with IANA.]] 9. Security Considerations (This section is incomplete) "Badges? We ain't got no badges. We don't need no badges! I don't have to show you any stinkin' badges!" -- The Treasure of the Sierra Pritikin Expires January 12, 2012 [Page 19] Internet-Draft EST July 2011 Madre. As described in CMC Section 6.7, "For keys that can be used as signature keys, signing the certification request with the private key serves as a POP on that key pair". The inclusion of tls-unique- securerenegotiation within the certification request provides timeliness to the proof-of-possession. For support of keys that can not be used for signing the certification request the full CMC specification MUST be used. As described in Section 3.3 clients use an existing certificate for TLS client authentication. If a certificate with appropriate key usage is not available the client MAY generate one. If a self-signed certificate with appropriate key usage is used the server SHOULD require HTTP based client authentication according to server policy as described in Section 3.3 and Section 3.5. The server MAY fall back on manual authorization by the server administrator. As described in Section 3.1 servers use an existing certificate for TLS server authentication. When the server certificate is issued by a mutually trusted PKI hierarchy validation proceeds as specified in Section 3.2. In this situation the client has validated the server as being a valid responder for the URI configured but can not directly verify that the responder is authorized as an RA within the to-be-enrolled PKI hierarchy. A client may thus be enticed to expose username/password or certificate enrollment requests to an unauthorized server (if the server presents a valid HTTPS certificate for an erroneous URL that the client has been tricked into using). Proof-of-Identity and Proof-of-Possession checks by the CA prevent an illegitimate RA from leveraging such misconfigured clients to act as a man-in-the-middle during client authenticated operations but it is possible for such illegitimate RAs to send the client doctored messages or erroneous CA certificate lists. If the illegitimate RA has successfully phished a username/password or PIN from the client it might try to use these values to enroll its own keypair with the real PKI hierarchy. EST servers identified with an externally issued server certificate SHOULD require HTTPS based client authentication (Section 3.3). Similarly EST clients SHOULD use an existing client certificate to identify themselves and otherwise prevent "private data" (obviously including passwords but also including private identity information) from being exposed during the enrollment exchange a weak server authorization method is used. 10. References Pritikin Expires January 12, 2012 [Page 20] Internet-Draft EST July 2011 10.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2315] Kaliski, B., "PKCS #7: Cryptographic Message Syntax Version 1.5", RFC 2315, March 1998. [RFC2616] 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. [RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, "HTTP Authentication: Basic and Digest Access Authentication", RFC 2617, June 1999. [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification Request Syntax Specification Version 1.7", RFC 2986, November 2000. [RFC4210] Adams, C., Farrell, S., Kause, T., and T. Mononen, "Internet X.509 Public Key Infrastructure Certificate Management Protocol (CMP)", RFC 4210, September 2005. [RFC4945] Korver, B., "The Internet IP Security PKI Profile of IKEv1/ISAKMP, IKEv2, and PKIX", RFC 4945, August 2007. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008. [RFC5272] Schaad, J. and M. Myers, "Certificate Management over CMS (CMC)", RFC 5272, June 2008. [RFC5273] Schaad, J. and M. Myers, "Certificate Management over CMS (CMC): Transport Protocols", RFC 5273, June 2008. [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, May 2008. [RFC5430] Salter, M., Rescorla, E., and R. Housley, "Suite B Profile for Transport Layer Security (TLS)", RFC 5430, March 2009. [RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov, Pritikin Expires January 12, 2012 [Page 21] Internet-Draft EST July 2011 "Transport Layer Security (TLS) Renegotiation Indication Extension", RFC 5746, February 2010. [RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings for TLS", RFC 5929, July 2010. 10.2. Informative References [IDevID] IEEE Std, "IEEE 802.1AR Secure Device Identifier", December 2009, . [RFC4211] Schaad, J., "Internet X.509 Public Key Infrastructure Certificate Request Message Format (CRMF)", RFC 4211, September 2005. Appendix A. Server Discovery (informative) (This section is incomplete) Clients MAY use DNS-SD or similar discovery algorithms to determine the EST base URL. In such cases it is expected that method 2 (Section 3.1) be used during server authentication. Appendix B. External TLS concentrator (informative) In some deployments it may be beneficial to use a TLS concentrator to offload the TLS processing from the server. In such a deployment the TLS client authentication result must, in some way, be forwarded to the server. The TLS server SHOULD NOT reject the connection based on PKIX validation of the client certificate. The client certificate SHOULD be passed to the EST layer for verification and authorization. This allows support of external TLS concentrators, or an external web server, that might provide an independent TLS implementation. The TLS concentrator MUST validate the TLS Section 7.4.8 'Certificate Verify'. A TLS concentrator MUST insert the client certificate into the HTTP header. The TLS concentrator MUST first remove any existing client Pritikin Expires January 12, 2012 [Page 22] Internet-Draft EST July 2011 certificates, possibly inserted by a nefarious client, from the HTTP headers before forwarding the HTTP connection to the server. [TBD - need to better understand what would happen in the case of proxy's or multiple concentrators. Or specifically state that as out of scope.] [TBD - the HTTP header field names etc shall be specified here] The EST server MUST be specifically configured by the administrator to accept this mechanism. Appendix C. CGI Server implementation (informative) In some deployments it may be beneficial to use a HTTPS server that runs the EST server as a CGI application. In such a deployment the HTTPS server client authentication result must, in some way, be forwarded to the server. An HTTPS server MUST insert the client certificate into environment variables before calling a server CGI application. [TBD - describe the CGI environment variables here. Can likely follow the apache example]. An HTTP server MUST insert the client certificate into environment variables before calling a server CGI application. [TBD - describe the CGI environment variables here. Can likely follow the apache example]. Appendix D. Updating SCEP implementations (informative) SCEP has been used instead of a full implementation of CMC for the same simplicity reasons discussed in Section 1. Such implementations would benefit from being updated to this specification in the following ways: o Implementing a subset of CMC provides an enhancement path if the full CMC functionality is required. Pritikin Expires January 12, 2012 [Page 23] Internet-Draft EST July 2011 o The use of HTTPS as a transport is often perceived as more secure. Although the SCEP protocol specification includes mechanisms (and complexity) to address security issues avoiding a vendor requirement to educate systems administrators is beneficial. Implementors can benefit from the wide availability of existing HTTPS/TLS libraries. o SCEP servers can use their CA certificate to protect SCEP traffic in ways that are not appropriate. (See SCEP draft Section 8.2). This specification precludes those misuses. o The SCEP draft Appendix D renew and rekey functionalities imply a 'flag moment' where the PKI infrastructure transitions from an (expired) CA certificate to a new CA certificate. This specification specifies the better mechanism defined in CMP. Updating an SCEP client implementation to support this protocol involves the following changes to the SCEP implementation. There is no server side indication that SCEP clients should be so modified so this depends on a client side configuration: o The SCEP client supports HTTPS server authentication and authorization as detailed Section 3.1. o The SCEP client supports HTTPS client authentication as detailed in Section 3.3. o When performing the "Get CA Cert" SCEP transaction the client supports the Section 5.1 described CMC Simple PKI Response (ref CMC 4.1, which is extremely similar to the SCEP "CA/RA Certificate Response Message Format" if not exactly the same). o When performing the certificate enrollment via SCEP PKCSReq the outgoing message is simplified to be only the inner PKCS10 (ref CMC section 3.2.1.2.1). o When handling the certificate enrollment response the response format is simplified to be only the SCEP inner 'messageData' containing the actual certificates in the degenerate PKCS7 form. (ref CMC 4.1) The only 'authenticatedAttributes' value of remaining importance is the 'pkiStatus' and this value is now found in the HTTP header as defined in Section 5.2.2. o Polling is simplified with clients repeatedly establishing the full HTTPS connection; no polling specific state information is encoded into the EST messages. Pritikin Expires January 12, 2012 [Page 24] Internet-Draft EST July 2011 o GetCert is deprecated. o GetCRL is deprecated. These simplifications to an existing SCEP implementation result in an SCEP client that is compliant with CMC when using the EST transport. Implementation note: The use of tls-unique-securerenegotiation precludes the use of SCEP 'challenge-password' within the pkcs10 for password/PIN assertion. Instead these values must be asserted with the Section 3.4 described mechanism. A side effect of this is that a client communicating with an EST server can not embed an SCEP 'challenge-password' within the PKCS#10. An EST service running as an RA thus can not forward the PKCS#10 using SCEP to an SCEP server that expects the 'challenge-password' to be populated. Appendix E. Key Update mechanisms (informative) (This section is incomplete) The CMP section 4.4 defined Root CA Key Update mechanisms are repeated here for easier reference. Author's Address Max Pritikin (editor) Cisco Systems, Inc. 510 McCarthy Drive Milpitas, CA USA Email: pritikin@cisco.com Pritikin Expires January 12, 2012 [Page 25]