Network Working Group A. Jivsov Internet Draft PGP Corporation Intended status: Internet Draft April 28, 2008 Expires: October 25, 2008 ECC in OpenPGP draft-jivsov-openpgp-ecc-00.txt 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 October 25, 2008. Copyright Notice Copyright (C) The IETF Trust (2008). A. Jivsov Expires October 25, 2008 [Page 1] Internet-Draft ECC in OpenPGP April 2008 Abstract This document proposes an Elliptic Curve Cryptography extension to the OpenPGP public key format and specifies three Elliptic Curves that enjoy broad support by other standards, including NIST standards. The document aims to standardize an optimal but narrow set of parameters for best interoperability and it does so within the framework it defines that can be expanded in the future to allow more choices. Conventions used in this document 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]. An application MAY implement this draft; note that any [RFC2119] keyword within this draft applies to an OpenPGP application only if it chooses to implement this draft. Table of Contents 1. Introduction.................................................2 2. Elliptic Curve Cryptography..................................3 3. Supported ECC curves.........................................3 4. Supported public key algorithms..............................3 5. Conversion primitives........................................4 6. Key Derivation Function......................................4 7. EC DH Algorithm (ECDH).......................................5 8. Encoding of public and private keys..........................7 9. Data encoding with public keys...............................8 10. ECC curve ID................................................8 11. Compatibility profiles......................................9 11.1. OpenPGP ECC profile....................................9 11.2. Suite-B profile........................................9 11.2.1. Secret information................................9 11.2.2. Top Secret information............................9 11.3. Interoperability with Suite-B profile..................9 12. Security Considerations....................................10 13. IANA Considerations........................................12 14. Normative references.......................................13 1. Introduction The OpenPGP protocol [RFC4880] supports RSA and DSA public key formats. This document defines the extension to incorporate A. Jivsov Expires October 25, 2008 [Page 2] Internet-Draft ECC in OpenPGP April 2008 support for public keys that are based on Elliptic Curve Cryptography (ECC). 2. Elliptic Curve Cryptography This specification establishes the minimum set of Elliptic Curve Cryptography public key parameters and cryptographic methods that will likely satisfy the widest range of platforms and applications and facilitate interoperability. The set meets the requirements of Suite-B and includes an additional Elliptic Curve (EC) beyond Suite-B requirements, allowing users to match the level of security of every type of AES algorithm specified in [RFC4880]. This document defines a path to expand ECC support in the future. 3. Supported ECC curves This standard defines three named prime field curves, that are defined in [FIPS 186-2] as "Curve P-256", "Curve P-384", "Curve P-521". To identify the named curves new ECC public key algorithm-specific parameter is introduced: the ECC curve ID, defined in section 10. 4. Supported public key algorithms Supported public key algorithms are Elliptic Curve Digital Signature Algorithm (ECDSA), defined in [FIPS 186-2], and Elliptic Curve Diffie-Hellman (ECDH), defined in section 7. Other compatible definition of ECDSA can be found in [SEC1]. The section 9.1. Public-Key Algorithms of [RFC4880] is expanded to define the following public key algorithm IDs: ID Description of algorithm 19 ECDSA public key algorithm [to be ECDH public key algorithm ASSIGNED] presumably 22 A. Jivsov Expires October 25, 2008 [Page 3] Internet-Draft ECC in OpenPGP April 2008 Applications MUST support ECDSA and ECDH. 5. Conversion primitives The method to convert an EC point to the octet string is defined in [SEC1]. This specification only defines uncompressed point format. For convenience, the synopsis of the encoding method is given below, however, the [SEC1] is the normative source of the definition. The point is encoded in MPI format. The content of the MPI is the following: B = B0 || x || y where x and y are coordinates of the point P = (x, y), each encoded in big endian format and zero-padded to the underlying field size. B0 is a byte with following values: value description 0 Point O. In this case there is no x or y octets present. 4 Uncompressed point. x and y of EC point values follow. Note that point O shall not appear in a public or a private key. Therefore, the size of the MPI payload is always curve_size*2 + 3 bits. For example, for "Curve P-256" the point is represented as a bit string of length 515 bits. If other conversion methods are defined in the future, the application MAY use them only when it is certain that every recipient of the data supports the other format. 6. Key Derivation Function A key derivation function (KDF) is necessary to implement EC encryption. The Concatenation Key Derivation Function (Approved Alternative 1) defined in [NIST SP800-56A] is REQUIRED with the following restriction: the KDF hash function MAY be any of the following hash functions specified by [FIPS 180-2]: SHA2-256, SHA2-384, SHA2-512. See section 12 for the details regarding the choice of the hash function. A. Jivsov Expires October 25, 2008 [Page 4] Internet-Draft ECC in OpenPGP April 2008 For convenience, the synopsis of the encoding method is given below, however, [NIST SP800-56A] is the normative source of the definition. // Implements KDF( X, oBits, P ); // Input: point X = (x,y) // oBits - the desired size of output // hBits - the size of output of hash function Hash // P - octets representing the parameters counter=1; threshold = (oBits + hBits - 1) / hBits; // Convert the point P to octet string as defined in section 6: // ZB' = 04 || x || y // and extract the x portion from ZB': ZB = x; do { C32 = (uint32)big_endian(counter); HB = Hash ( ZB || C32 || P ); MB = MB || HB; } while( counter <= threshold ); return oBits leftmost bits of MB 7. EC DH Algorithm (ECDH) The method is a combination of ECC Diffie-Hellman method to establish a shared secret and a key wrapping method that uses the shared secret to protect symmetric encryption key. One-Pass Diffie-Hellman method C(1, 1, ECC CDH), defined in [NIST SP800-56A], SHOULD be implemented with the following restrictions: ECC CDH primitive employed by this method is modified to always assume the cofactor as 1, KDF specified in section 6 is used, and KDF parameters specified below are used. Key derivation parameters MUST be encoded as 40 octets. These 40 octets are the result of concatenation of the following 7 fields, each of them is considered a fixed-length field of corresponding size: o a one-octet curve ID defined in section 10 o a one-octet public key algorithm ID defined in section 4 o a one-octet value 01, reserved for future extensions A. Jivsov Expires October 25, 2008 [Page 5] Internet-Draft ECC in OpenPGP April 2008 o a one-octet hash function ID used in KDF; according to section 6, this octet is 08 for SHA2-256, 09 for SHA2-384, or 10 for SHA2-512 o a one-octet algorithm ID for the symmetric algorithm used to wrap the symmetric key for message encryption; the method is defined later in this section o 15 octets representing the UTF-8 encoding of the string "AnonymousSender" o 20 octets representing recipient encryption subkey or master key fingerprint, identifying the key material that is needed for decryption The key wrapping method is based on [RFC3394]. KDF produces the AES key that is used as KEK according to [RFC3394]. Refer to section 12 for the details regarding the choice of the KEK algorithm, which MUST be one of three AES algorithms. The input to key wrapping method is the value "m" derived from the session key as described in section 5.1. Public-Key Encrypted Session Key Packets (Tag 1) of [RFC4880], except, the PKCS#1.5 padding step is omitted. The output of the method consists of two fields. The first field is the MPI with the ephemeral key used to establish shared secret. The second field is composed of the following two fields: o a one octet, encoding the size in octets of the result of the key wrapping method; the value 255 is reserved for future extensions o up to 254 octets representing the result of the key wrapping method applied to session key encoded as described above Note that for session key sizes 128, 192, and 256 bits the size of the result of the key wrapping method is, respectfully, 32, 40, and 48 octets. For convenience, the synopsis of the encoding method is given below, however, this section, [NIST SP800-56A], and [RFC3394] are the normative sources of the definition. Obtain authenticated recipient public key R Generate ephemeral key pair {v, V=vG} Compute shared point S = vR; m = symm_alg_ID || session key. A. Jivsov Expires October 25, 2008 [Page 6] Internet-Draft ECC in OpenPGP April 2008 Param = curve_ID || public_key_alg_ID || 01 || KDF_hash_ID || AES_alg_ID for AESKeyWrap || "AnonymousSender" || recipient_fingerprint; Z_len = key size for AES_alg_ID to be used with AESKeyWrap Compute Z = KDF( S, Z_len, Param ); Compute C = AESKeyWrap( Z, m ) as per [RFC3394] VB = convert point V to octet string Output (MPI(VB) || len(C) || C). The decryption is the inverse of the method given. Note that the recipient obtains the shared secret by calculating S = rV = rvG, where (r,R) is the recipient's key pair. Consistent with section 5.13 Sym. Encrypted Integrity Protected Data Packet (Tag 18) of [RFC4880], the MDC SHOULD be used anytime symmetric key is protected by ECDH. 8. Encoding of public and private keys The following algorithm-specific packets are added to Section 5.5.2 Public-Key Packet Formats of [RFC4880] to support ECDH and ECDSA. This algorithm-specific portion is: Algorithm-Specific Fields for ECDH keys: o a one-octet curve ID number, defined in section 10 o a one-octet value 01, reserved for future extension o a one-octet hash function ID used with KDF o a one-octet algorithm ID for the symmetric algorithm used to wrap the symmetric key for message encryption, see section 7 for details o MPI of EC point representing public key Algorithm-Specific Fields for ECDSA keys: o a one-octet curve ID number, defined in section 10 o MPI of EC point representing public key The following algorithm-specific packets are added to section 5.5.3. Secret-Key Packet Formats of [RFC4880] to support ECDH and ECDSA. A. Jivsov Expires October 25, 2008 [Page 7] Internet-Draft ECC in OpenPGP April 2008 Algorithm-Specific Fields for ECDH or ECDSA secret keys: o MPI of an integer representing the secret key, which is a scalar of the EC point 9. Data encoding with public keys Section 5.2.2. Version 3 Signature Packet Format defines signature formats. No changes in format are needed for ECDSA. Section 5.1. Public-Key Encrypted Session Key Packets (Tag 1) is extended to support ECDH. The following two fields are result of applying KDF, as described in section 7. Algorithm Specific Fields for ECDH: o an MPI of EC point representing ephemeral public key o a one octet size, followed by symmetric key encoded using the method described in section [RFC3394]. 10. ECC curve ID The parameter ECC curve ID is an integer that defines the named curve. ID Curve description Curve name 0 Reserved 1 NIST Curve P-256 [FIPS 186-2] "NIST P256" 2 NIST Curve P-384 [FIPS 186-2] "NIST P384" 3 NIST Curve P-521 [FIPS 186-2] "NIST P521" 100-110 Private/Experimental curves 255 Reserved for future expansion A. Jivsov Expires October 25, 2008 [Page 8] Internet-Draft ECC in OpenPGP April 2008 11. Compatibility profiles 11.1. OpenPGP ECC profile Application MUST implement curve with ID 1, MAY implement curve with ID 2, and SHOULD implement curve with ID 3, defined in section 10. Application MUST implement SHA2-256 and SHOULD implement SHA2-512. Application MUST implement AES-128 and SHOULD implement AES-256. Application SHOULD follow section 12 regarding the choice of the following algorithms for each curve o the KDF hash algorithm o KEK algorithm o message digest algorithm and hash algorithm used in key certifications o message encryption symmetric algorithm. It is recommended that the chosen symmetric algorithm for message encryption be no less secure than the KEK algorithm. 11.2. Suite-B profile A subset of algorithms allowed by this specification can be used to achieve NSA Suite-B compatibility. 11.2.1. Secret information Applications MUST use curve ID 1. KEK SHOULD be used with AES-128, but MAY be used with AES-256. SHA2-256 SHOULD be used for KDF, but SHA2-384 MAY be used for KDF. 11.2.2. Top Secret information Application MUST use curve ID 2. KEK MUST be used with AES-256. SHA2-384 MUST be used for KDF. 11.3. Interoperability with Suite-B profile For brevity, in this section applications complying with [RFC4880] and OpenPGP profile defined in section 11.1 are called compliant with OpenPGP and ECC specifications. A. Jivsov Expires October 25, 2008 [Page 9] Internet-Draft ECC in OpenPGP April 2008 The set of symmetric key encryption, hash, and public key algorithms allowed by Suite-B is a subset of algorithms allowed by OpenPGP and ECC specifications. Care must be taken to ensure interoperability between applications implementing OpenPGP and ECC and applications following Suite-B. Encryption to multiple recipients is one example in which incompatibilities are possible. According to [RFC4880], even though there is no shared symmetric encryption algorithm in the OpenPGP recipients' preferences, the specification requires TripleDES to be effectively in the intersection of the encryption preferences. TripleDES as implicit default is inherited from [RFC4880] by this specification to improve interoperability. While TripleDES ensures interoperability between applications complaint with OpenPGP and ECC specifications, it doesn't help interoperability with Suite-B profile. Suppose TripleDES is the only shared algorithm within a set of recipients. If Suite-B compliant recipient is added to the mentioned recipient set, the sender SHALL NOT send out a message. This is because TripleDES is excluded from Suite-B and sending out two copies of the same message, one encrypted with TripleDES and another with AES-128 or AES-256, would mean that the same information that must have been protected with Suite-B compliant algorithm was protected instead with non-compliant TripleDES. This restriction covers other cases in which none of recipients' shared algorithms are allowed by Suite-B. One of available methods to a recipient to help ensure interoperability with Suite-B is to include one of two Suite-B symmetric algorithms, AES-128 or AES-256, or both, in the set of preferred algorithms. Only hash algorithms defined in section 11.2 must be used in key certifications, including key self-signatures, and in message digests for Suite-B interoperability. 12. Security Considerations The curves proposed in this document correspond to the symmetric key sizes 128 bits, 192 bits, and 256 bits as described in the table below. This allows OpenPGP application to offer security comparable with the strength of each AES algorithms allowed by [RFC4880]. The following table defines the hash and symmetric encryption algorithm that SHOULD be used with specific curve for ECDSA or ECDH. Stronger hash algorithm or symmetric key algorithm MAY be used for a given ECC curve. However, note that the increase in the A. Jivsov Expires October 25, 2008 [Page 10] Internet-Draft ECC in OpenPGP April 2008 strength of the hash algorithm or symmetric key algorithm may not increase the overall security offered by the given ECC key. Curve Curve name ECC RSA Hash size Symmetric ID strength strength, key size informative 1 "NIST P256" 256 3072 256 128 2 "NIST P384" 384 7680 384 192 3 "NIST P521" 521 15360 512 256 Requirement levels indicated elsewhere in this document result in the effective support for the following combinations of algorithms in OpenPGP profile: MUST implement curve ID 1 / SHA2-256 / AES-128, SHOULD implement curve ID 3 / SHA2-512 / AES-256, MAY implement curve ID 2 / SHA2-384 / AES-256, among other allowed combinations. Consistent with the table above, the following table defines the KDF hash algorithm and AES KEK encryption algorithm that SHOULD be used with specific curve for ECDH. Stronger KDF hash algorithm or KEK algorithm MAY be used for a given ECC curve. Curve Curve name Recommended KDF hash Recommended KEK ID algorithm encryption algorithm 1 "NIST P256" SHA2-256 AES-128 2 "NIST P384" SHA2-384 AES-192 3 "NIST P521" SHA2-512 AES-256 Applications SHOULD implement, advertise through key preferences, and use in compliance with [RFC4880] strongest algorithms specified in this document. Note that [RFC4880] symmetric algorithm preference list may restrict the use of balanced strength of symmetric key algorithms for corresponding public key. For example, the presence of A. Jivsov Expires October 25, 2008 [Page 11] Internet-Draft ECC in OpenPGP April 2008 symmetric key algorithms and their order in key preference list affects the choices available to encoding side for compliance with the table above. Therefore, applications need to be concerned with this compliance throughout the life of the key, starting immediately after key generation when the key preferences are first added to a key. It is generally advisable to have at the head of the key preference list a symmetric algorithm of strength corresponding to the public key. Often encryption to multiple recipients results in an unordered intersection subset. For example, given two recipients, if first recipient's set is {A, B} and second's is {B, A}, the intersection is unordered set of two algorithms A and B. In this case application SHOULD choose stronger encryption algorithm. Resource constraint, such as limited computational power, is the likely reason why an application might prefer to use weakest algorithms. On the other side of the spectrum are applications that can implement every algorithm defined in this document. Most of applications are expected to fall into either of two categories. An application in the second or strongest category SHOULD prefer AES-256 to AES-192. While some statements in this specification refer to TripleDES algorithm, this is only done to help interoperability with existing application and already generated keys; AES-256 is the recommended alternative to TripleDES in all circumstances when AES-256 is available. SHA-1 MUST NOT be used for ECDSA or as part of ECDH method. MDC MUST be used when symmetric encryption key is protected by ECDH. None of the ECC methods described in this document are allowed with deprecated V3 keys. The application MUST only use Iterated and Salted S2K to protect private keys, as defined in section 3.7.1.3 Iterated and Salted S2K of [RFC4880]. 13. IANA Considerations This document asks IANA to assign an algorithm number from OpenPGP Public-Key Algorithms range, or "name space" in the terminology of [RFC2434], that was created by [RFC4880]. Two ID numbers are requested, as defined in section 4. The first one with value 19 is already designated for ECDSA and currently unused, while another one is new (and expected to be 22). Finally, this document creates the name space for curve IDs defined in section 10. Its initial content is defined in the section 10 A. Jivsov Expires October 25, 2008 [Page 12] Internet-Draft ECC in OpenPGP April 2008 and includes IDs for newly introduced curves, private space for experimental work, and the ID reserved for future name space expansion. Future allocations in the registry will be done by IETF Expert Review process after general consensus between implementors of the standard is reached. Most important motivation to add new curve to the registry is expected to be the need for stronger curves. 14. Normative references [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", March 1997 [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. Thayer, "OpenPGP Message Format", November 2007 [FIPS 186-2] US Dept. of Commerce / NIST, "DIGITAL SIGNATURE STANDARD (DSS)", 2001 October 5 [SEC1] Certicom Research, "SEC 1: Elliptic Curve Cryptography", September 20, 2000 [NIST SP800-56A] Elaine Barker, Don Johnson, and Miles Smid, "Recommendation for Pair-WiseKey Establishment Schemes Using Discrete Logarithm Cryptography (Revised)", March, 2007 [FIPS 180-2] NIST, SECURE HASH STANDARD, 2002 August 1 [RFC3394] J. Schaad, R. Housley, "Advanced Encryption Standard (AES) Key Wrap Algorithm", September 2002 [RFC2434] Narten, T., Alvestrand, H., "Guidelines for Writing an IANA Considerations Section in RFCs", Contributors Hal Finney provided important criticism on compliance with [NIST SP800-56A] and NSA Suite-B, and pointed out a few other mistakes. Acknowledgment The author would like to acknowledge the help of many individuals who kindly voiced their opinions on IETF OpenPGP Working Group mailing list and, in particular the help of Jon Callas, David Crick, Ian G. [to be continued] Author's Address A. Jivsov Expires October 25, 2008 [Page 13] Internet-Draft ECC in OpenPGP April 2008 Andrey Jivsov PGP Corporation Email: ajivsov@pgp.com Intellectual Property Statement 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. 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