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draft-ounsworth-pq-composite-sigs.txt
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LAMPS M. Ounsworth
Internet-Draft J. Gray
Intended status: Standards Track Entrust
Expires: 17 August 2024 M. Pala
CableLabs
J. Klaussner
D-Trust GmbH
14 February 2024
Composite Signatures For Use In Internet PKI
draft-ounsworth-pq-composite-sigs-12
Abstract
The migration to post-quantum cryptography is unique in the history
of modern digital cryptography in that neither the old outgoing nor
the new incoming algorithms are fully trusted to protect data for the
required data lifetimes. The outgoing algorithms, such as RSA and
elliptic curve, may fall to quantum cryptanalysis, while the incoming
post-quantum algorithms face uncertainty about both the underlying
mathematics as well as hardware and software implementations that
have not had sufficient maturing time to rule out classical
cryptanalytic attacks and implementation bugs.
Cautious implementers may wish to layer cryptographic algorithms such
that an attacker would need to break all of them in order to
compromise the data being protected using either a Post-Quantum /
Traditional Hybrid, Post-Quantum / Post-Quantum Hybrid, or
combinations thereof. This document, and its companions, defines a
specific instantiation of hybrid paradigm called "composite" where
multiple cryptographic algorithms are combined to form a single key
or signature such that they can be treated as a single atomic object
at the protocol level.
This document defines the structures CompositeSignaturePublicKey,
CompositeSignaturePrivateKey and CompositeSignatureValue, which are
sequences of the respective structure for each component algorithm.
Composite signature algorithm identifiers are specified in this
document which represent the explicit combinations of the underlying
component algorithms.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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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 17 August 2024.
Copyright Notice
Copyright (c) 2024 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 Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Changes in version -12 . . . . . . . . . . . . . . . . . . . 3
2. Changes in version -11 . . . . . . . . . . . . . . . . . . . 3
3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Composite Design Philosophy . . . . . . . . . . . . . . . 6
3.3. Composite Signatures . . . . . . . . . . . . . . . . . . 6
3.3.1. Composite KeyGen . . . . . . . . . . . . . . . . . . 7
3.3.2. Composite Sign . . . . . . . . . . . . . . . . . . . 7
3.3.3. Composite Verify . . . . . . . . . . . . . . . . . . 9
3.4. OID Concatenation . . . . . . . . . . . . . . . . . . . . 11
3.5. PreHashing the Message . . . . . . . . . . . . . . . . . 13
3.6. Algorithm Selection Criteria . . . . . . . . . . . . . . 13
4. Composite Signature Structures . . . . . . . . . . . . . . . 14
4.1. pk-CompositeSignature . . . . . . . . . . . . . . . . . . 14
4.2. CompositeSignaturePublicKey . . . . . . . . . . . . . . . 15
4.3. CompositeSignaturePrivateKey . . . . . . . . . . . . . . 15
4.4. Encoding Rules . . . . . . . . . . . . . . . . . . . . . 16
4.5. Key Usage Bits . . . . . . . . . . . . . . . . . . . . . 17
5. Composite Signature Structures . . . . . . . . . . . . . . . 17
5.1. sa-CompositeSignature . . . . . . . . . . . . . . . . . . 17
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5.2. CompositeSignatureValue . . . . . . . . . . . . . . . . . 18
6. Algorithm Identifiers . . . . . . . . . . . . . . . . . . . . 18
6.1. Notes on id-MLDSA44-RSA2048-PSS-SHA256 . . . . . . . . . 21
6.2. Notes on id-MLDSA65-RSA3072-PSS-SHA512 . . . . . . . . . 22
7. ASN.1 Module . . . . . . . . . . . . . . . . . . . . . . . . 22
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
8.1. Object Identifier Allocations . . . . . . . . . . . . . . 30
8.1.1. Module Registration - SMI Security for PKIX Module
Identifier . . . . . . . . . . . . . . . . . . . . . 30
8.1.2. Object Identifier Registrations - SMI Security for PKIX
Algorithms . . . . . . . . . . . . . . . . . . . . . 30
9. Security Considerations . . . . . . . . . . . . . . . . . . . 33
9.1. Policy for Deprecated and Acceptable Algorithms . . . . . 33
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 33
10.1. Normative References . . . . . . . . . . . . . . . . . . 33
10.2. Informative References . . . . . . . . . . . . . . . . . 35
Appendix A. Samples . . . . . . . . . . . . . . . . . . . . . . 37
A.1. Explicit Composite Signature Examples . . . . . . . . . . 37
A.1.1. MLDSA44-ECDSA-P256-SHA256 Public Key . . . . . . . . 38
A.1.2. MLDSA44-ECDSA-P256 Private Key . . . . . . . . . . . 38
A.1.3. MLDSA44-ECDSA-P256 Self-Signed X509 Certificate . . . 40
Appendix B. Implementation Considerations . . . . . . . . . . . 42
B.1. FIPS certification . . . . . . . . . . . . . . . . . . . 42
B.2. Backwards Compatibility . . . . . . . . . . . . . . . . . 42
B.2.1. Parallel PKIs . . . . . . . . . . . . . . . . . . . . 43
B.2.2. Hybrid Extensions (Keys and Signatures) . . . . . . . 44
Appendix C. Intellectual Property Considerations . . . . . . . . 44
Appendix D. Contributors and Acknowledgements . . . . . . . . . 44
D.1. Making contributions . . . . . . . . . . . . . . . . . . 45
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 45
1. Changes in version -12
* Fixed the ASN.1 module pk-CompositeSignature Information Object
Class so it now compiles
2. Changes in version -11
* Remove ambiguity and made it clear that all component signature
MUST be verified
* Added language to ensure that component keys MUST not be used in
any other context
* Changed the content of the OID artifact to the DER encoded OID
* Reduced number of pre-hashing algorithm by removing SHA384 and
SHAKE and replacing those with SHA512
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* Updated the prototype OIDs since the changes in this draft are not
compatible with version -10
* Fixed other nits
3. Introduction
During the transition to post-quantum cryptography, there will be
uncertainty as to the strength of cryptographic algorithms; we will
no longer fully trust traditional cryptography such as RSA, Diffie-
Hellman, DSA and their elliptic curve variants, but we will also not
fully trust their post-quantum replacements until they have had
sufficient scrutiny and time to discover and fix implementation bugs.
Unlike previous cryptographic algorithm migrations, the choice of
when to migrate and which algorithms to migrate to, is not so clear.
Even after the migration period, it may be advantageous for an
entity's cryptographic identity to be composed of multiple public-key
algorithms.
Cautious implementers may wish to combine cryptographic algorithms
such that an attacker would need to break all of them in order to
compromise the data being protected. Such mechanisms are referred to
as Post-Quantum / Traditional Hybrids
[I-D.driscoll-pqt-hybrid-terminology].
PQ/T Hybrid cryptography can, in general, provide solutions to two
migration problems:
* Algorithm strength uncertainty: During the transition period, some
post-quantum signature and encryption algorithms will not be fully
trusted, while also the trust in legacy public key algorithms will
start to erode. A relying party may learn some time after
deployment that a public key algorithm has become untrustworthy,
but in the interim, they may not know which algorithm an adversary
has compromised.
* Ease-of-migration: During the transition period, systems will
require mechanisms that allow for staged migrations from fully
classical to fully post-quantum-aware cryptography.
* Safeguard against faulty algorithm implementations and compromised
keys: Even for long known algorithms there is a non-negligible
risk of severe implementation faults. Latest examples are the
ROCA attack and ECDSA psychic signatures. Using more than one
algorithms will mitigate these risks.
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This document defines a specific instantiation of the PQ/T Hybrid
paradigm called "composite" where multiple cryptographic algorithms
are combined to form a single signature such that it can be treated
as a single atomic algorithm at the protocol level. Composite
algorithms address algorithm strength uncertainty because the
composite algorithm remains strong so long as one of its components
remains strong. Concrete instantiations of composite signature
algorithms are provided based on ML-DSA, Falcon, RSA and ECDSA.
Backwards compatibility is not directly covered in this document, but
is the subject of Appendix B.2.
This document is intended for general applicability anywhere that
digital signatures are used within PKIX and CMS structures. For a
more detailed use-case discussion for composite signatures, the
reader is encouraged to look at [I-D.vaira-pquip-pqc-use-cases]
3.1. 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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The following terms are used in this document:
ALGORITHM: A standardized cryptographic primitive, as well as any
ASN.1 structures needed for encoding data and metadata needed to use
the algorithm. This document is primarily concerned with algorithms
for producing digital signatures.
BER: Basic Encoding Rules (BER) as defined in [X.690].
CLIENT: Any software that is making use of a cryptographic key. This
includes a signer, verifier, encrypter, decrypter.
COMPONENT ALGORITHM: A single basic algorithm which is contained
within a composite algorithm.
COMPOSITE ALGORITHM: An algorithm which is a sequence of two or more
component algorithms, as defined in Section 4.
DER: Distinguished Encoding Rules as defined in [X.690].
LEGACY: For the purposes of this document, a legacy algorithm is any
cryptographic algorithm currently is use which is not believe to be
resistant to quantum cryptanalysis.
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PKI: Public Key Infrastructure, as defined in [RFC5280].
POST-QUANTUM ALGORITHM: Any cryptographic algorithm which is believed
to be resistant to classical and quantum cryptanalysis, such as the
algorithms being considered for standardization by NIST.
PUBLIC / PRIVATE KEY: The public and private portion of an asymmetric
cryptographic key, making no assumptions about which algorithm.
SIGNATURE: A digital cryptographic signature, making no assumptions
about which algorithm.
STRIPPING ATTACK: An attack in which the attacker is able to
downgrade the cryptographic object to an attacker-chosen subset of
original set of component algorithms in such a way that it is not
detectable by the receiver. For example, substituting a composite
public key or signature for a version with fewer components.
3.2. Composite Design Philosophy
[I-D.driscoll-pqt-hybrid-terminology] defines composites as:
_Composite Cryptographic Element_: A cryptographic element that
incorporates multiple component cryptographic elements of the same
type in a multi-algorithm scheme.
Composite keys as defined here follow this definition and should be
regarded as a single key that performs a single cryptographic
operation such key generation, signing, verifying, encapsulating, or
decapsulating -- using its internal sequence of component keys as if
they form a single key. This generally means that the complexity of
combining algorithms can and should be handled by the cryptographic
library or cryptographic module, and the single composite public key,
private key, and ciphertext can be carried in existing fields in
protocols such as PKCS#10 [RFC2986], CMP [RFC4210], X.509 [RFC5280],
CMS [RFC5652], and the Trust Anchor Format [RFC5914]. In this way,
composites achieve "protocol backwards-compatibility" in that they
will drop cleanly into any protocol that accepts signature algorithms
without requiring any modification of the protocol to handle multiple
keys.
3.3. Composite Signatures
Here we define the signature mechanism in which a signature is a
cryptographic primitive that consists of three algorithms:
* KeyGen() -> (pk, sk): A probabilistic key generation algorithm,
which generates a public key pk and a secret key sk.
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* Sign(sk, Message) -> (signature): A signing algorithm which takes
as input a secret key sk and a Message, and outputs a signature
* Verify(pk, Message, signature) -> true or false: A verification
algorithm which takes as input a public key, a Message and
signature and outputs true if the signature and public key can be
used to verify the message. Thus it proves the Message was signed
with the secret key associated with the public key and verifies
the integrity of the Message. If the signature and public key
cannot verify the Message, it returns false.
A composite signature allows two or more underlying signature
algorithms to be combined into a single cryptographic signature
operation and can be used for applications that require signatures.
3.3.1. Composite KeyGen
The KeyGen() -> (pk, sk) of a composite signature algorithm will
perform the KeyGen() of the respective component signature algorithms
and it produces a composite public key pk as per Section 4.2 and a
composite secret key sk is per Section 4.3. The component keys MUST
be uniquely generated for each component key of a Composite and MUST
NOT be used in any other keys or as a standalone key.
3.3.2. Composite Sign
Generation of a composite signature involves applying each component
algorithm's signature process to the input message according to its
specification, and then placing each component signature value into
the CompositeSignatureValue structure defined in Section 5.1.
The following process is used to generate composite signature values.
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Sign (sk, Message) -> (signature)
Input:
K1, K2 Signing private keys for each component. See note below on
composite inputs.
A1, A2 Component signature algorithms. See note below on
composite inputs.
Message The Message to be signed, an octet string
HASH The Message Digest Algorithm used for pre-hashing. See section
on pre-hashing below.
OID The Composite Signature String Algorithm Name converted
from ASCII to bytes. See section on OID concatenation
below.
Output:
signature The composite signature, a CompositeSignatureValue
Signature Generation Process:
1. Compute a Hash of the Message
M' = HASH(Message)
2. Generate the n component signatures independently,
according to their algorithm specifications.
S1 := Sign( K1, A1, DER(OID) || M' )
S2 := Sign( K2, A2, DER(OID) || M' )
3. Encode each component signature S1 and S2 into a BIT STRING
according to its algorithm specification.
signature ::= Sequence { S1, S2 }
4. Output signature
Figure 1: Composite Sign(sk, Message)
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Note on composite inputs: the method of providing the list of
component keys and algorithms is flexible and beyond the scope of
this pseudo-code. When passed to the Composite Sign(sk, Message) API
the sk is a CompositePrivateKey. It is possible to construct a
CompositePrivateKey from component keys stored in separate software
or hardware keystores. Variations in the process to accommodate
particular private key storage mechanisms are considered to be
conformant to this document so long as it produces the same output as
the process sketched above.
Since recursive composite public keys are disallowed, no component
signature may itself be a composite; ie the signature generation
process MUST fail if one of the private keys K1 or K2 is a composite.
A composite signature MUST produce, and include in the output, a
signature value for every component key in the corresponding
CompositePublicKey, and they MUST be in the same order; ie in the
output, S1 MUST correspond to K1, S2 to K2.
3.3.3. Composite Verify
Verification of a composite signature involves applying each
component algorithm's verification process according to its
specification.
Compliant applications MUST output "Valid signature" (true) if and
only if all component signatures were successfully validated, and
"Invalid signature" (false) otherwise.
The following process is used to perform this verification.
Composite Verify(pk, Message, signature)
Input:
P1, P2 Public verification keys. See note below on
composite inputs.
Message Message whose signature is to be verified,
an octet string
signature CompositeSignatureValue containing the component
signature values (S1 and S2) to be verified.
A1, A2 Component signature algorithms. See note
below on composite inputs.
HASH The Message Digest Algorithm for pre-hashing. See
section on pre-hashing the message below.
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OID The Composite Signature String Algorithm Name converted
from ASCII to bytes. See section on OID concatenation
below
Output:
Validity (bool) "Valid signature" (true) if the composite
signature is valid, "Invalid signature"
(false) otherwise.
Signature Verification Procedure::
1. Check keys, signatures, and algorithms lists for consistency.
If Error during Desequencing, or the sequences have
different numbers of elements, or any of the public keys
P1 or P2 and the algorithm identifiers A1 or A2 are
composite then output "Invalid signature" and stop.
2. Compute a Hash of the Message
M' = HASH(Message)
3. Check each component signature individually, according to its
algorithm specification.
If any fail, then the entire signature validation fails.
if not verify( P1, DER(OID) || M', S1, A1 ) then
output "Invalid signature"
if not verify( P2, DER(OID) || M', S2, A2 ) then
output "Invalid signature"
if all succeeded, then
output "Valid signature"
Figure 2: Composite Verify(pk, Message, signature)
Note on composite inputs: the method of providing the list of
component keys and algorithms is flexible and beyond the scope of
this pseudo-code. When passed to the Composite Verify(pk, Message,
signature) API the pk is a CompositePublicKey. It is possible to
construct a CompositePublicKey from component keys stored in separate
software or hardware keystores. Variations in the process to
accommodate particular private key storage mechanisms are considered
to be conformant to this document so long as it produces the same
output as the process sketched above.
Since recursive composite public keys are disallowed, no component
signature may itself be a composite; ie the signature generation
process MUST fail if one of the private keys K1 or K2 is a composite.
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3.4. OID Concatenation
As mentioned above, the OID input value for the Composite Signature
Generation and verification process is the DER encoding of the OID
represented in Hexidecimal bytes. The following table shows the HEX
encoding for each Signature AlgorithmID
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+=================================+============================+
| Composite Signature AlgorithmID | DER Encoding to be |
| | prepended to each Message |
+=================================+============================+
| id-MLDSA44-RSA2048-PSS-SHA256 | 060B6086480186FA6B50080101 |
+---------------------------------+----------------------------+
| id- | 060B6086480186FA6B50080102 |
| MLDSA44-RSA2048-PKCS15-SHA256 | |
+---------------------------------+----------------------------+
| id-MLDSA44-Ed25519-SHA512 | 060B6086480186FA6B50080103 |
+---------------------------------+----------------------------+
| id-MLDSA44-ECDSA-P256-SHA256 | 060B6086480186FA6B50080104 |
+---------------------------------+----------------------------+
| id-MLDSA44-ECDSA- | 060B6086480186FA6B50080105 |
| brainpoolP256r1-SHA256 | |
+---------------------------------+----------------------------+
| id-MLDSA65-RSA3072-PSS-SHA512 | 060B6086480186FA6B50080106 |
+---------------------------------+----------------------------+
| id- | 060B6086480186FA6B50080107 |
| MLDSA65-RSA3072-PKCS15-SHA512 | |
+---------------------------------+----------------------------+
| id-MLDSA65-ECDSA-P256-SHA512 | 060B6086480186FA6B50080108 |
+---------------------------------+----------------------------+
| id-MLDSA65-ECDSA- | 060B6086480186FA6B50080109 |
| brainpoolP256r1-SHA512 | |
+---------------------------------+----------------------------+
| id-MLDSA65-Ed25519-SHA512 | 060B6086480186FA6B5008010A |
+---------------------------------+----------------------------+
| id-MLDSA87-ECDSA-P384-SHA512 | 060B6086480186FA6B5008010B |
+---------------------------------+----------------------------+
| id-MLDSA87-ECDSA- | 060B6086480186FA6B5008010C |
| brainpoolP384r1-SHA512 | |
+---------------------------------+----------------------------+
| id-MLDSA87-Ed448-SHA512 | 060B6086480186FA6B5008010D |
+---------------------------------+----------------------------+
| id-Falon512-ECDSA-P256-SHA256 | 060B6086480186FA6B5008010E |
+---------------------------------+----------------------------+
| id-Falcon512-ECDSA- | 060B6086480186FA6B5008010F |
| brainpoolP256r1-SHA256 | |
+---------------------------------+----------------------------+
| id-Falcon512-Ed25519-SHA512 | 060B6086480186FA6B50080110 |
+---------------------------------+----------------------------+
Table 1: Composite Signature OID Concatenations
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3.5. PreHashing the Message
As noted in the composite signature generation process and composite
signature verification process, the Message should be pre-hashed into
M' with the digest algorithm specified in the composite signature
algorithm identifier. The choice of the digest algorithm was chosen
with the following criteria:
1. For composites paired with RSA or ECDSA, the hashing algorithm
SHA256 or SHA512 is used as part of the RSA or ECDSA signature
algorithm and is therefore also used as the composite prehashing
algorithm.
2. For ML-DSA signing a digest of the message is allowed as long as
the hash function provides at least y bits of classical security
strength against both collision and second preimage attacks. For
MLDSA44 y is 128 bits, MLDSA65 y is 192 bits and for MLDSA87 y is
256 bits. Therefore SHA256 is paired with RSA and ECDSA with
MLDSA44 and SHA512 is paired with RSA and ECDSA with MLDSA65 and
MLDSA87 to match the appropriate security strength.
3. Ed25519 [RFC8032] uses SHA512 internally, therefore SHA512 is
used to pre-hash the message when Ed25519 is a component
algorithm.
4. Ed448 [RFC8032] uses SHAKE256 internally, but to reduce the set
of prehashing algorihtms, SHA512 was selected to pre-hash the
message when Ed448 is a component algorithm.
5. TODO: For Falcon signing it is expected prehashing digest
accomodations will be allowed.
3.6. Algorithm Selection Criteria
The composite algorithm combinations defined in this document were
chosen according to the following guidelines:
1. A single RSA combination is provided at a key size of 3072 bits,
matched with NIST PQC Level 3 algorithms.
2. Elliptic curve algorithms are provided with combinations on each
of the NIST [RFC6090], Brainpool [RFC5639], and Edwards [RFC7748]
curves. NIST PQC Levels 1 - 3 algorithms are matched with
256-bit curves, while NIST levels 4 - 5 are matched with 384-bit
elliptic curves. This provides a balance between matching
classical security levels of post-quantum and traditional
algorithms, and also selecting elliptic curves which already have
wide adoption.
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3. NIST level 1 candidates are provided, matched with 256-bit
elliptic curves, intended for constrained use cases.
If other combinations are needed, a separate specification should be
submitted to the IETF LAMPS working group. To ease implementation,
these specifications are encouraged to follow the construction
pattern of the algorithms specified in this document.
The composite structures defined in this specification allow only for
pairs of algorithms. This also does not preclude future
specification from extending these structures to define combinations
with three or more components.
4. Composite Signature Structures
In order for signatures to be composed of multiple algorithms, we
define encodings consisting of a sequence of signature primitives
(aka "component algorithms") such that these structures can be used
as a drop-in replacement for existing signature fields such as those
found in PKCS#10 [RFC2986], CMP [RFC4210], X.509 [RFC5280], CMS
[RFC5652].
4.1. pk-CompositeSignature
The following ASN.1 Information Object Class is a template to be used
in defining all composite Signature public key types.
pk-CompositeSignature {OBJECT IDENTIFIER:id,
FirstPublicKeyType,SecondPublicKeyType}
PUBLIC-KEY ::= {
IDENTIFIER id
KEY SEQUENCE {
firstPublicKey BIT STRING (CONTAINING FirstPublicKeyType),
secondPublicKey BIT STRING (CONTAINING SecondPublicKeyType)
}
PARAMS ARE absent
CERT-KEY-USAGE { digitalSignature, nonRepudiation, keyCertSign, cRLSign}
}
As an example, the public key type pk-MLDSA65-ECDSA-P256-SHA256 is
defined as:
pk-MLDSA65-ECDSA-P256-SHA256 PUBLIC-KEY ::=
pk-CompositeSignature{ id-MLDSA65-ECDSA-P256-SHA256,
OCTET STRING, ECPoint}
The full set of key types defined by this specification can be found
in the ASN.1 Module in Section 7.
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4.2. CompositeSignaturePublicKey
Composite public key data is represented by the following structure:
CompositeSignaturePublicKey ::= SEQUENCE SIZE (2) OF BIT STRING
A composite key MUST contain two component public keys. The order of
the component keys is determined by the definition of the
corresponding algorithm identifier as defined in section Section 6.
Some applications may need to reconstruct the SubjectPublicKeyInfo
objects corresponding to each component public key. Table 3 in
Section 6 provides the necessary mapping between composite and their
component algorithms for doing this reconstruction. This also
motivates the design choice of SEQUENCE OF BIT STRING instead of
SEQUENCE OF OCTET STRING; using BIT STRING allows for easier
transcription between CompositeSignaturePublicKey and
SubjectPublicKeyInfo.
When the CompositeSignaturePublicKey must be provided in octet string
or bit string format, the data structure is encoded as specified in
Section 4.4.
Component keys of a CompositeSignaturePublicKey MUST NOT be used in
any other type of key or as a standalone key.
4.3. CompositeSignaturePrivateKey
Usecases that require an interoperable encoding for composite private
keys, such as when private keys are carried in PKCS #12 [RFC7292],
CMP [RFC4210] or CRMF [RFC4211] MUST use the following structure.
CompositeSignaturePrivateKey ::= SEQUENCE SIZE (2) OF OneAsymmetricKey
Each element is a OneAsymmetricKey` [RFC5958] object for a component
private key.
The parameters field MUST be absent.
The order of the component keys is the same as the order defined in
Section 4.2 for the components of CompositeSignaturePublicKey.
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When a CompositeSignaturePrivateKey is conveyed inside a
OneAsymmetricKey structure (version 1 of which is also known as
PrivateKeyInfo) [RFC5958], the privateKeyAlgorithm field SHALL be set
to the corresponding composite algorithm identifier defined according
to Section 6, the privateKey field SHALL contain the
CompositeSignaturePrivateKey, and the publicKey field MUST NOT be
present. Associated public key material MAY be present in the
CompositeSignaturePrivateKey.
In some usecases the private keys that comprise a composite key may
not be represented in a single structure or even be contained in a
single cryptographic module; for example if one component is within
the FIPS boundary of a cryptographic module and the other is not; see
{sec-fips} for more discussion. The establishment of correspondence
between public keys in a CompositeSignaturePublicKey and private keys
not represented in a single composite structure is beyond the scope
of this document.
Component keys of a CompositeSignaturePrivateKey MUST NOT be used in
any other type of key or as a standalone key.
4.4. Encoding Rules
Many protocol specifications will require that the composite public
key and composite private key data structures be represented by an
octet string or bit string.
When an octet string is required, the DER encoding of the composite
data structure SHALL be used directly.
CompositeSignaturePublicKeyOs ::= OCTET STRING (CONTAINING CompositeSignaturePublicKey ENCODED BY der)
When a bit string is required, the octets of the DER encoded
composite data structure SHALL be used as the bits of the bit string,
with the most significant bit of the first octet becoming the first
bit, and so on, ending with the least significant bit of the last
octet becoming the last bit of the bit string.
CompositeSignaturePublicKeyBs ::= BIT STRING (CONTAINING CompositeSignaturePublicKey ENCODED BY der)
In the interests of simplicity and avoiding compatibility issues,
implementations that parse these structures MAY accept both BER and
DER.
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4.5. Key Usage Bits
For protocols such as X.509 [RFC5280] that specify key usage along
with the public key, then the composite public key associated with a
composite signature MUST have a signing-type key usage.
If the keyUsage extension is present in a Certification Authority
(CA) certificate that indicates a composite key, then any combination
of the following values MAY be present:
digitalSignature;
nonRepudiation;
keyCertSign; and
cRLSign.
If the keyUsage extension is present in an End Entity (EE)
certificate that indicates a composite key, then any combination of
the following values MAY be present:
digitalSignature; and
nonRepudiation;
5. Composite Signature Structures
5.1. sa-CompositeSignature
The ASN.1 algorithm object for a composite signature is:
sa-CompositeSignature {
OBJECT IDENTIFIER:id,
PUBLIC-KEY:publicKeyType }
SIGNATURE-ALGORITHM ::= {
IDENTIFIER id
VALUE CompositeSignatureValue
PARAMS ARE absent
PUBLIC-KEYS { publicKeyType }
}
The following is an explanation how SIGNATURE-ALGORITHM elements are
used to create Composite Signatures:
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+=============================+===================================+
| SIGNATURE-ALGORITHM element | Definition |
+=============================+===================================+
| IDENTIFIER | The Object ID used to identify |
| | the composite Signature Algorithm |
+-----------------------------+-----------------------------------+
| VALUE | The Sequence of BIT STRINGS for |
| | each component signature value |
+-----------------------------+-----------------------------------+
| PARAMS | Parameters are absent |
+-----------------------------+-----------------------------------+
| PUBLIC-KEYS | The composite key required to |
| | produce the composite signature |
+-----------------------------+-----------------------------------+
Table 2
5.2. CompositeSignatureValue
The output of the composite signature algorithm is the DER encoding
of the following structure:
CompositeSignatureValue ::= SEQUENCE SIZE (2) OF BIT STRING
Where each BIT STRING within the SEQUENCE is a signature value
produced by one of the component keys. It MUST contain one signature
value produced by each component algorithm, and in the same order as
specified in the object identifier.
The choice of SEQUENCE SIZE (2) OF BIT STRING, rather than for
example a single BIT STRING containing the concatenated signature
values, is to gracefully handle variable-length signature values by
taking advantage of ASN.1's built-in length fields.
6. Algorithm Identifiers
This section defines the algorithm identifiers for explicit
combinations. For simplicity and prototyping purposes, the signature
algorithm object identifiers specified in this document are the same
as the composite key object Identifiers. A proper implementation
should not presume that the object ID of a composite key will be the
same as its composite signature algorithm.