Network Working Group N. Madden Internet-Draft ForgeRock Intended status: Standards Track February 11, 2020 Expires: August 14, 2020 Public Key Authenticated Encryption for JOSE: ECDH-1PU draft-madden-jose-ecdh-1pu-03 Abstract This document describes the ECDH-1PU public key authenticated encryption algorithm for JWE. The algorithm is similar to the existing ECDH-ES encryption algorithm, but adds an additional ECDH key agreement between static keys of the sender and recipient. This additional step allows the recipient to be assured of sender authenticity without requiring a nested signed-then-encrypted message structure. 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 https://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 August 14, 2020. Copyright Notice Copyright (c) 2020 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 (https://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 to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of Madden Expires August 14, 2020 [Page 1] Internet-Draft JOSE ECDH-1PU February 2020 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Terminology . . . . . . . . . . . . . . . . 3 2. Key Agreement with Elliptic Curve Diffie-Hellman One-Pass Unified Model (ECDH-1PU) . . . . . . . . . . . . . . . . . . 3 2.1. Header Parameters used for ECDH Key Agreement . . . . . . 4 2.1.1. "skid" Header Parameter . . . . . . . . . . . . . . . 5 2.2. Key Derivation for ECDH-1PU Key Agreement . . . . . . . . 5 3. IANA considerations . . . . . . . . . . . . . . . . . . . . . 6 3.1. JSON Web Signature and Encryption Algorithms Registration 6 3.1.1. ECDH-1PU . . . . . . . . . . . . . . . . . . . . . . 7 3.2. JSON Web Signature and Encryption Header Parameters Registration . . . . . . . . . . . . . . . . . . . . . . 7 3.2.1. skid . . . . . . . . . . . . . . . . . . . . . . . . 7 4. Security Considerations . . . . . . . . . . . . . . . . . . . 7 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 5.1. Normative References . . . . . . . . . . . . . . . . . . 8 5.2. Informative References . . . . . . . . . . . . . . . . . 9 Appendix A. Example ECDH-1PU Key Agreement Computation with A256GCM . . . . . . . . . . . . . . . . . . . . . . 9 Appendix B. Document History . . . . . . . . . . . . . . . . . . 11 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12 1. Introduction JSON Object Signing and Encryption (JOSE) defines a number of encryption (JWE) [RFC7516] and digital signature (JWS) [RFC7515] algorithms. When symmetric cryptography is used, JWE provides authenticated encryption that ensures both confidentiality and sender authentication. However, for public key cryptography the existing JWE encryption algorithms provide only confidentiality and some level of ciphertext integrity. When sender authentication is required, users must resort to nested signed-then-encrypted structures, which increases the overhead and size of resulting messages. This document describes an alternative encryption algorithm called ECDH-1PU that provides public key authenticated encryption, allowing the benefits of authenticated encryption to be enjoyed for public key JWE as it currently is for symmetric cryptography. ECDH-1PU is based on the One-Pass Unified Model for Elliptic Curve Diffie-Hellman key agreement described in [NIST.800-56A]. Madden Expires August 14, 2020 [Page 2] Internet-Draft JOSE ECDH-1PU February 2020 The advantages of public key authenticated encryption with ECDH-1PU compared to using nested signed-then-encrypted documents include the following: o The resulting message size is more compact as an additional layer of headers and base64url-encoding is avoided. A 500-byte payload when encrypted and authenticated with ECDH-1PU (with P-256 keys and "A256GCM" Content Encryption Method) results in a 1087-byte JWE in Compact Encoding. An equivalent nested signed-then- encrypted JOSE message using the same keys and encryption method is 1489 bytes (37% larger). o The same primitives are used for both confidentiality and authenticity, providing savings in code size for constrained environments. o The generic composition of signatures and public key encryption involves a number of subtle details that are essential to security [PKAE]. Providing a dedicated algorithm for public key authenticated encryption reduces complexity for users of JOSE libraries. o ECDH-1PU provides only authenticity and not the stronger security properties of non-repudiation or third-party verifiability. This can be an advantage in applications where privacy, anonymity, or plausible deniability are goals. 1.1. Requirements Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC8174] when, and only when, they appear in all capitals, as shown here. 2. Key Agreement with Elliptic Curve Diffie-Hellman One-Pass Unified Model (ECDH-1PU) This section defines the specifics of key agreement with Elliptic Curve Diffie-Hellman One-Pass Unified Model, in combination with the one-step KDF, as defined in Section 5.8.2.1 of [NIST.800-56A] using the Concatenation Format of Section 5.8.2.1.1. This is identical to the ConcatKDF function used by the existing JWE ECDH-ES algorithm defined in Section 4.6 of [RFC7518]. As for ECDH-ES, the key agreement result can be used in one of two ways: 1. directly as the Content Encryption Key (CEK) for the "enc" algorithm, in the Direct Key Agreement mode, or Madden Expires August 14, 2020 [Page 3] Internet-Draft JOSE ECDH-1PU February 2020 2. as a symmetric key used to wrap the CEK with the "A128KW", "A192KW", or "A256KW" algorithms, in the Key Agreement with Key Wrapping mode. A new ephemeral public key value MUST be generated for each key agreement operation. In Direct Key Agreement mode, the output of the KDF MUST be a key of the same length as that used by the "enc" algorithm. In this case, the empty octet sequence is used as the JWE Encrypted Key value. The "alg" (algorithm) Header Parameter value "ECDH-1PU" is used in Direct Key Agreement mode. In Key Agreement with Key Wrapping mode, the output of the KDF MUST be a key of the length needed for the specified key wrapping algorithm. In this case, the JWE Encrypted Key is the CEK wrapped with the agreed-upon key. The following "alg" (algorithm) Header Parameter values are used to indicate the JWE Encrypted Key is the result of encrypting the CEK using the result of the key agreement algorithm as the key encryption key for the corresponding key wrapping algorithm: +-----------------+-------------------------------------------------+ | "alg" Param | Key Management Algorithm | | Value | | +-----------------+-------------------------------------------------+ | ECDH-1PU+A128KW | ECDH-1PU using one-pass KDF and CEK wrapped | | | with "A128KW" | | ECDH-1PU+A192KW | ECDH-1PU using one-pass KDF and CEK wrapped | | | with "A192KW" | | ECDH-1PU+A256KW | ECDH-1PU using one-pass KDF and CEK wrapped | | | with "A256KW" | +-----------------+-------------------------------------------------+ 2.1. Header Parameters used for ECDH Key Agreement The "epk" (ephemeral public key), "apu" (Agreement PartyUInfo), and "apv" (Agreement PartyVInfo) header parameters are used in ECDH-1PU exactly as defined in Section 4.6.1 of [RFC7518]. When no other values are supplied, it is RECOMMENDED that the producer software initializes the "apu" header to the base64url- encoding of the SHA-256 hash of the concatenation of the sender's static public key and the ephemeral public key, and the "apv" header to the base64url-encoding of the SHA-256 hash of the recipient's static public key. This ensures that all keys involved in the key agreement are cryptographically bound to the derived keys. Madden Expires August 14, 2020 [Page 4] Internet-Draft JOSE ECDH-1PU February 2020 2.1.1. "skid" Header Parameter A new Header Parameter "skid" (Sender Key ID) is registered as a hint as to which of the sender's keys was used to authenticate the JWE. The structure of the "skid" value is unspecified. Its value MUST be a case-sensitive string. Use of this Header Parameter is OPTIONAL. When used with a JWK, the "skid" value is used to match a JWK "kid" parameter value [RFC7517]. 2.2. Key Derivation for ECDH-1PU Key Agreement The key derivation process derives the agreed-upon key from the shared secret Z established through the ECDH algorithm, per Section 6.2.1.2 of [NIST.800-56A]. For the NIST prime order curves "P-256", "P-384", and "P-521", the ECC CDH primitive for cofactor Diffie-Hellman defined in Section 5.7.1.2 of [NIST.800-56A] is used (taking note that the cofactor for all these curves is 1). For curves "X25519" and "X448" the appropriate ECDH primitive from Section 5 of [RFC7748] is used. Key derivation is performed using the one-step KDF, as defined in Section 5.8.1 and Section 5.8.2.1 of [NIST.800-56A] using the Concatenation Format of Section 5.8.2.1.1, where the Auxilary Function H is SHA-256. The KDF parameters are set as follows: Z This is set to the representation of the shared secret Z as an octet sequence. As per Section 6.2.1.2 of [NIST.800-56A] Z is the concatenation of Ze and Zs, where Ze is the shared secret derived from applying the ECDH primitive to the sender's ephemeral private key and the recipient's static public key. Zs is the shared secret derived from applying the ECDH primitive to the sender's static private key and the recipient's static public key. keydatalen This is set to the number of bits in the desired output key. For "ECDH-1PU", this is the length of the key used by the "enc" algorithm. For "ECDH-1PU+A128KW", "ECDH-1PU+A192KW", and "ECDH-1PU+A256KW", this is 128, 192, and 256, respectively. AlgorithmID The AlgorithmID values is of the form Datalen || Data, where Data is a variable-length string of zero or more octets, and Datalen is a fixed-length, big-endian 32-bit counter that indicates the length (in octets) of Data. In the Direct Key Agreement case, Data is set to the octets of the ASCII representation of the "enc" Header Parameter value. In the Key Agreement with Key Wrapping case, Data is set to the octets of the ASCII representation of the "alg" (algorithm) Header Parameter value. Madden Expires August 14, 2020 [Page 5] Internet-Draft JOSE ECDH-1PU February 2020 PartyUInfo The PartyUInfo value is of the form Datalen || Data, where Data is a variable-length string of zero or more octets, and Datalen is a fixed-length, big-endian 32-bit counter that indicates the length (in octets) of Data. If an "apu" (agreement PartyUInfo) Header Parameter is present, Data is set to the result of base64url decoding the "apu" value and Datalen is set to the number of octets in Data. Otherwise, Datalen is set to 0 and Data is set to the empty octet sequence. PartyVInfo The PartyVInfo value is of the form Datalen || Data, where Data is a variable-length string of zero or more octets, and Datalen is a fixed-length, big-endian 32-bit counter that indicates the length (in octets) of Data. If an "apv" (agreement PartyVInfo) Header Parameter is present, Data is set to the result of base64url decoding the "apv" value and Datalen is set to the number of octets in Data. Otherwise, Datalen is set to 0 and Data is set to the empty octet sequence. SuppPubInfo This is set to the keydatalen represented as a 32-bit big-endian integer. SuppPrivInfo This is set to the empty octet sequence. Applications need to specify how the "apu" and "apv" Header Parameters are used for that application. The "apu" and "apv" values MUST be distinct, when used. Applications wishing to conform to [NIST.800-56A] need to provide values that meet the requirements of that document, e.g., by using values that identify the producer and consumer. See Appendix A for an example key agreement computation using this method. 3. IANA considerations This section registers identifiers under the IANA JSON Web Signature and Encryption Algorithms Registry established by [RFC7518] and the IANA JSON Web Signature and Encryption Header Parameters registry established by [RFC7515]. 3.1. JSON Web Signature and Encryption Algorithms Registration This section registers JWE algorithms as per the registry established in [RFC7518]. Madden Expires August 14, 2020 [Page 6] Internet-Draft JOSE ECDH-1PU February 2020 3.1.1. ECDH-1PU Algorithm Name: "ECDH-1PU" Algorithm Description: ECDH One-Pass Unified Model using one-pass KDF Algorithm Usage Location(s): "alg" JOSE Implementation Requirements: Optional Change Controller: IESG Specification Document(s): Section 2 Algorithm Analysis Document(s): [NIST.800-56A] (Section 7.3), [PKAE] 3.2. JSON Web Signature and Encryption Header Parameters Registration This section registers new Header Parameters as per the registry established in [RFC7515]. 3.2.1. skid Header Parameter Name: "skid" Header Parameter Description: Sender Key ID Header Parameter Usage Location(s): JWE Change Controller: IESG Specification Document(s): Section 2.1.1 4. Security Considerations The security considerations of [RFC7516] and [RFC7518] relevant to ECDH-ES also apply to this specification. The security considerations of [NIST.800-56A] apply here. When performing an ECDH key agreement between a static private key and any untrusted public key, care should be taken to ensure that the public key is a valid point on the same curve as the private key. Failure to do so may result in compromise of the static private key. For the NIST curves P-256, P-384, and P-521, appropriate validation routines are given in Section 5.6.2.3.3 of [NIST.800-56A]. For the curves used by X25519 and X448, consult the security considerations of [RFC7748]. The ECDH-1PU algorithm is vulnerable to Key Compromise Impersonation (KCI) attacks. If the long-term static private key of a party is compromised, then the attacker can not only impersonate that party to other parties, but also impersonate any other party when communicating with the compromised party. If resistance to KCI is desired in a single message, then the sender SHOULD use a nested JWS signature over the content. Madden Expires August 14, 2020 [Page 7] Internet-Draft JOSE ECDH-1PU February 2020 When Key Agreement with Key Wrapping is used, with the same Content Encryption Key (CEK) reused for multiple recipients, any of those recipients can produce a new message that appears to come from the original sender. The new message will be indistinguishable from a genuine message from the original sender to any of the other participants. To avoid this attack, the content SHOULD be encrypted separately to each recipient with a unique CEK or a nested signature over the content SHOULD be used. The security properties of the one-pass unified model are given in Section 7.3 of [NIST.800-56A]. 5. References 5.1. Normative References [NIST.800-56A] Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R. Davis, "Recommendation for Pair-Wise Key Establishment Using Discrete Logarithm Cryptography Revision 3.", NIST Special Publication 800-56A, April 2018. [RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May 2015, . [RFC7516] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)", RFC 7516, DOI 10.17487/RFC7516, May 2015, . [RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517, DOI 10.17487/RFC7517, May 2015, . [RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518, DOI 10.17487/RFC7518, May 2015, . [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves for Security", RFC 7748, DOI 10.17487/RFC7748, January 2016, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . Madden Expires August 14, 2020 [Page 8] Internet-Draft JOSE ECDH-1PU February 2020 5.2. Informative References [PKAE] An, J., "Authenticated Encryption in the Public-Key Setting: Security Notions and Analyses", IACR ePrint 2001/079, 2001. Appendix A. Example ECDH-1PU Key Agreement Computation with A256GCM This example uses ECDH-1PU in Direct Key Agreement mode ("alg" value "ECDH-1PU") to produce an agreed-upon key for AES GCM with a 256-bit key ("enc" value "A256GCM"). The example re-uses the keys and parameters of the example computation in Appendix C of [RFC7518], with the addition of an extra static key-pair for Alice. In this example, a producer Alice is encrypting content to a consumer Bob. Alice's static key-pair (in JWK format) used for the key agreement in this example (including the private part) is: {"kty":"EC", "crv":"P-256", "x":"WKn-ZIGevcwGIyyrzFoZNBdaq9_TsqzGl96oc0CWuis", "y":"y77t-RvAHRKTsSGdIYUfweuOvwrvDD-Q3Hv5J0fSKbE", "d":"Hndv7ZZjs_ke8o9zXYo3iq-Yr8SewI5vrqd0pAvEPqg"} Bob's static key-pair (in JWK format) is: {"kty":"EC", "crv":"P-256", "x":"weNJy2HscCSM6AEDTDg04biOvhFhyyWvOHQfeF_PxMQ", "y":"e8lnCO-AlStT-NJVX-crhB7QRYhiix03illJOVAOyck", "d":"VEmDZpDXXK8p8N0Cndsxs924q6nS1RXFASRl6BfUqdw"} The producer (Alice) generates an ephemeral key for the key agreement computation. Alice's ephemeral key (in JWK format) is: {"kty":"EC", "crv":"P-256", "x":"gI0GAILBdu7T53akrFmMyGcsF3n5dO7MmwNBHKW5SV0", "y":"SLW_xSffzlPWrHEVI30DHM_4egVwt3NQqeUD7nMFpps", "d":"0_NxaRPUMQoAJt50Gz8YiTr8gRTwyEaCumd-MToTmIo"} Header Parameter values used in this example are as follows. The "apu" (agreement PartyUInfo) Header Parameter value is the base64url encoding of the UTF-8 string "Alice" and the "apv" (agreement PartyVInfo) Header Parameter value is the base64url encoding of the UTF-8 string "Bob". The "epk" (ephemeral public key) Header Parameter is used to communicate the producer's (Alice's) ephemeral public key value to the consumer (Bob). Madden Expires August 14, 2020 [Page 9] Internet-Draft JOSE ECDH-1PU February 2020 {"alg":"ECDH-1PU", "enc":"A256GCM", "apu":"QWxpY2U", "apv":"Qm9i", "epk": {"kty":"EC", "crv":"P-256", "x":"gI0GAILBdu7T53akrFmMyGcsF3n5dO7MmwNBHKW5SV0", "y":"SLW_xSffzlPWrHEVI30DHM_4egVwt3NQqeUD7nMFpps" } } The resulting one-pass KDF [NIST.800-56A] parameter values are: Ze This is set to the output of the ECDH key agreement between Alice's ephemeral private key and Bob's static public key. In this example, Ze is the following octet sequence (in hexadecimal notation): 9e 56 d9 1d 81 71 35 d3 72 83 42 83 bf 84 26 9c fb 31 6e a3 da 80 6a 48 f6 da a7 79 8c fe 90 c4 Zs This is set to the output of the ECDH key agreement between Alice's static private key and Bob's static public key. In this example, Zs is the following octet sequence (in hexadecimal notation): e3 ca 34 74 38 4c 9f 62 b3 0b fd 4c 68 8b 3e 7d 41 10 a1 b4 ba dc 3c c5 4e f7 b8 12 41 ef d5 0d Z This is set to the concatenation of Ze followed by Zs. In this example, Z is the following octet sequence (in hexadecimal notation): 9e 56 d9 1d 81 71 35 d3 72 83 42 83 bf 84 26 9c fb 31 6e a3 da 80 6a 48 f6 da a7 79 8c fe 90 c4 e3 ca 34 74 38 4c 9f 62 b3 0b fd 4c 68 8b 3e 7d 41 10 a1 b4 ba dc 3c c5 4e f7 b8 12 41 ef d5 0d keydatalen This value is 256 - the number of bits in the desired output key (because "A256GCM" uses a 256-bit key). AlgorithmID This is set to the octets representing the 32-bit big- endian value 7 - 00 00 00 07 in hexadecimal notation - the number of octets in the AlgorithmID content "A256GCM", followed by the octets representing the ASCII string "A256GCM" - 41 32 35 36 47 43 4d (in hex). The complete value is therefore: 00 00 00 07 41 32 35 36 47 43 4d Madden Expires August 14, 2020 [Page 10] Internet-Draft JOSE ECDH-1PU February 2020 PartyUInfo This is set to the octets representing the 32-bit big- endian value 5, followed by the octets representing the UTF-8 string "Alice". In hexadecimal notation: 00 00 00 05 41 6c 69 63 65 PartyVInfo This is set to the octets representing the 32-bit big- endian value 3, followed by the octets representing the UTF-8 string "Bob". In hexadecimal notation: 00 00 00 03 42 6f 62 SuppPubInfo This is set to the octets representing the 32-bit big- endian value 256 - the keydatalen value. In hexadecimal notation: 00 00 01 00 SuppPrivInfo This is set to the empty octet sequence. Concatenating the parameters AlgorithmID through SuppPrivInfo results in a FixedInfo value in Concatenation Format (as per Section 5.8.2.1.1 of [NIST.800-56A]) of (in hexidecimal notation): 00 00 00 07 41 32 35 36 47 43 4d 00 00 00 05 41 6c 69 63 65 00 00 00 03 42 6f 62 00 00 01 00 Concatenating the round number 1 (00 00 00 01), Z, and the FixedInfo value results in a one-pass KDF round 1 hash input of (hexadecimal): 00 00 00 01 9e 56 d9 1d 81 71 35 d3 72 83 42 83 bf 84 26 9c fb 31 6e a3 da 80 6a 48 f6 da a7 79 8c fe 90 c4 e3 ca 34 74 38 4c 9f 62 b3 0b fd 4c 68 8b 3e 7d 41 10 a1 b4 ba dc 3c c5 4e f7 b8 12 41 ef d5 0d 00 00 00 07 41 32 35 36 47 43 4d 00 00 00 05 41 6c 69 63 65 00 00 00 03 42 6f 62 00 00 01 00 The resulting derived key, which is the full 256 bits of the round 1 hash output is: 6c af 13 72 3d 14 85 0a d4 b4 2c d6 dd e9 35 bf fd 2f ff 00 a9 ba 70 de 05 c2 03 a5 e1 72 2c a7 The base64url-encoded representation of this derived key is: bK8Tcj0UhQrUtCzW3ek1v_0v_wCpunDeBcIDpeFyLKc Appendix B. Document History -03 Corrected typos and clarified wording. Removed unnecessary references. -02 Removed two-way interactive handshake protocol section and example after discussion with Hannes Tschofenig. Madden Expires August 14, 2020 [Page 11] Internet-Draft JOSE ECDH-1PU February 2020 -01 Added examples in Appendix A and a two-way handshake example. Added "skid" Header Parameter and registration. Fleshed out Security Considerations. Author's Address Neil Madden ForgeRock Broad Quay House Prince Street Bristol BS1 4DJ United Kingdom Email: neil.madden@forgerock.com Madden Expires August 14, 2020 [Page 12]