Public Key Authenticated Encryption for JOSE: ECDH-1PU
draft-madden-jose-ecdh-1pu-00
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| Last updated | 2019-05-09 | ||
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draft-madden-jose-ecdh-1pu-00
Network Working Group N. Madden
Internet-Draft ForgeRock
Intended status: Standards Track May 8, 2019
Expires: November 9, 2019
Public Key Authenticated Encryption for JOSE: ECDH-1PU
draft-madden-jose-ecdh-1pu-00
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. The mode is also a useful building block for constructing
interactive handshake protocols on top of JOSE.
Status of This Memo
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This Internet-Draft will expire on November 9, 2019.
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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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Terminology . . . . . . . . . . . . . . . . 3
2. Key Agreement with Elliptic Curve Diffie-Hellman Ephemeral-
Static Static-Static (ECDH-1PU) . . . . . . . . . . . . . . . 3
2.1. Header Parameters used for ECDH Key Agreement . . . . . . 4
2.2. Key Derivation for ECDH-1PU Key Agreement . . . . . . . . 4
3. Two-way interactive handshake . . . . . . . . . . . . . . . . 6
4. IANA considerations . . . . . . . . . . . . . . . . . . . . . 7
4.1. ECDH-1PU . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . 8
6.2. Informative References . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
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:
o The resulting message size is more compact as an additional layer
of headers and base64url-encoding is avoided.
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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 Ephemeral-Static
Static-Static (ECDH-1PU)
This section defines the specifics of key agreement with Elliptic
Curve Diffie-Hellman Ephemeral-Static Static-Static, 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
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"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 Concat KDF and CEK wrapped with |
| | "A128KW" |
| ECDH-1PU+A192KW | ECDH-1PU using Concat KDF and CEK wrapped with |
| | "A192KW" |
| ECDH-1PU+A256KW | ECDH-1PU using Concat 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.
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
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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.
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.
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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 specifiy 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 doucument, e.g., by using values that identify the producer and
consumer.
3. Two-way interactive handshake
A party that has received a JWE encrypted with ECDH-1PU MAY reply to
that message by creating a new JWE using ECDH-1PU, but using the
ephemeral public key ("epk") from the first message as if it was the
originating party's static public key. In this case, key agreement
proceeds exactly as for Section 2, but with the originator's
ephemeral public key used as the recipient (Party V) static public
key. The "alg" (algorithm) Header Parameter in the response MUST be
identical to the "alg" Header Parameter of the original message.
The value of the "apu" (Agreement PartyUInfo) Header Parameter value
from the original message SHOULD be reflected as the "apv" (Agreement
PartyVInfo) Header Parameter value in the new message. Applications
need to specify how the new "apu" Header Parameter should be
constructed.
If a "kid" claim was included in the ephemeral public key of the
original message, then a "kid" Header Parameter with the same value
MUST be included in the reply JWE.
After the initial message and a reply have been exchanged, the two
parties may communicate using the derived key from the second message
as the encryption key for any number of additional messages. When
ECDH-1PU is used in Direct Key Agreement mode, then subsequent
messages using the derived key MUST be encrypted using the "dir"
(Direct) JWE algorithm. When used in Key Agreement with Key Wrapping
mode, subsequent messages using the derived key MUST be encrypted
using the associated Key Wrapping algorithm, as shown in the
following table:
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+----------------------------+------------------------------+
| ECDH-1PU "alg" Param Value | Subsequent "alg" Param Value |
+----------------------------+------------------------------+
| ECDH-1PU+A128KW | A128KW |
| ECDH-1PU+A192KW | A192KW |
| ECDH-1PU+A256KW | A256KW |
+----------------------------+------------------------------+
4. IANA considerations
This section registers JWE algorithms as per the registry established
in [RFC7518].
4.1. ECDH-1PU
Algorithm Name: "ECDH-1PU"
Algorithm Description: ECDH One-Pass Unified Model using Concat
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]
5. Security Considerations
The security considerations of [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. The second and any
subsequent messages in the two-way interactive handshake described in
Section 3 are not vulnerable to KCI. If resistance to KCI is desired
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in a single message, then it is RECOMMENDED to 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 and will be trusted by any of the other recipients.
It is RECOMMENDED that a unique CEK is used for each recipient.
6. References
6.1. 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, <https://www.rfc-editor.org/info/rfc7515>.
[RFC7516] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
RFC 7516, DOI 10.17487/RFC7516, May 2015,
<https://www.rfc-editor.org/info/rfc7516>.
[RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
DOI 10.17487/RFC7518, May 2015,
<https://www.rfc-editor.org/info/rfc7518>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/rfc7748>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
6.2. Informative References
[PKAE] An, J., "Authenticated Encryption in the Public-Key
Setting: Security Notions and Analyses", IACR ePrint
2001/079, 2001.
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Author's Address
Neil Madden
ForgeRock
Broad Quay House
Prince Street
Bristol BS1 4DJ
United Kingdom
Email: neil.madden@forgerock.com
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