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.

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Table of Contents

• 1. Introduction
• 1.1. Requirements Terminology
• 2. Key Agreement with Elliptic Curve Diffie-Hellman One-Pass Unified Model (ECDH-1PU)
• 2.1. Header Parameters used for ECDH Key Agreement
• 2.1.1. "skid" Header Parameter
• 2.2. Key Derivation for ECDH-1PU Key Agreement
• 3. IANA considerations
• 3.1. JSON Web Signature and Encryption Algorithms Registration
• 3.1.1. ECDH-1PU
• 3.2. JSON Web Signature and Encryption Header Parameters Registration
• 3.2.1. skid
• 4. Security Considerations
• 5. References
• 5.1. Normative References
• 5.2. Informative References
• Appendix A. Example ECDH-1PU Key Agreement Computation with A256GCM
• Appendix B. Document History
• Author's Address

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].

The advantages of public key authenticated encryption with ECDH-1PU compared to using nested signed-then-encrypted documents include the following:

• 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).
• The same primitives are used for both confidentiality and authenticity, providing savings in code size for constrained environments.
• 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.
• 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
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:

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.

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: [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.

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.

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

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].

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.

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

5.2. Informative References

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).

     {"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

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.

-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