| Internet-Draft | Blind BBS Signatures | October 2024 |
| Kalos & Bernstein | Expires 23 April 2025 | [Page] |
This document defines an extension to the BBS Signature scheme that supports blind digital signatures, i.e., signatures over messages not known to the Signer.¶
This note is to be removed before publishing as an RFC.¶
Source for this draft and an issue tracker can be found at https://github.com/BasileiosKal/blind-bbs-signatures.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
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This Internet-Draft will expire on 23 April 2025.¶
Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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The BBS digital signature scheme, as defined in [I-D.irtf-cfrg-bbs-signatures], can be extended to support blind signatures functionality. In a blind signatures setting, the user (called the Prover in the context of the BBS scheme) will request a signature on a list of messages, without revealing those messages to the Signer (who can optionally also include messages of their choosing to the signature).¶
By allowing the Prover to acquire a valid signature over messages not known to the Signer, blind signatures address some limitations of their plain digital signature counterparts. In the BBS scheme, knowledge of a valid signature allows generation of BBS proofs. As a result, a signature compromise (by an eavesdropper, a phishing attack, a leakage of the Signer's logs etc.,) can lead to impersonation of the Prover by malicious actors (especially in cases involving "long-lived" signatures, as in digital credentials applications etc.,). Using Blind BBS Signatures on the other hand, the Prover can commit to a secret message (for example, a private key) before issuance, guaranteeing that no one will be able to generate a valid proof without knowledge of their secret.¶
Furthermore, applications like Privacy Pass ([I-D.ietf-privacypass-protocol]) may require a signature to be "scoped" to a specific audience or session (as to require "fresh" signatures for different sessions etc.,). However, simply sending an audience or session identifier to the Signer (to be included in the signature), will compromise the privacy guarantees that these applications try to enforce. Using blind signing, the Prover will be able to require signatures bound to those values, without having to reveal them to the Signer.¶
The presented protocol, compared to the scheme defined in [I-D.irtf-cfrg-bbs-signatures], introduces an additional communication step between the Prover and the Signer. The Prover will start by constructing a "hiding" commitment to the messages they want to get a signature on (i.e., a commitment which reveals no information about the committed values), together with a proof of correctness of that commitment. They will send the (commitment, proof) pair to the Signer, who, upon receiving the pair, will attempt to verify the commitment's proof of correctness. If successful, they will use it in generating a BBS signature over the messages committed by the Prover, including their own messages if any.¶
This document, in addition to defining the operation for creating and verifying a commitment, also details a core signature generation operation, different from the one presented in [I-D.irtf-cfrg-bbs-signatures], meant to handle the computation of the blind signature. The document will also define a new BBS Interface, which is needed to handle the different inputs, i.e., messages committed by the Prover or chosen by the Signer etc... The signature verification and proof generation core cryptographic operations however, will work as described in [I-D.irtf-cfrg-bbs-signatures]. To further facilitate deployment, both the exposed interface as well as the core cryptographic operation of proof verification will be the same as the one detailed in [I-D.irtf-cfrg-bbs-signatures].¶
Below is a basic diagram describing the main entities involved in the scheme.¶
(3) Blind Sign (1) Commit
+----- +-----
| | | |
| | | |
| \ / | \ /
+----------+ +-----------+
| | | |
| | | |
| |<-(2)* Commitment + Proof of Correctness--| |
| Signer | | Prover |
| |-------(4)* Send signature + msgs-------->| |
| | | |
| | | |
+----------+ +-----------+
|
|
|
(5)* Send proof
+
disclosed msgs
|
|
\ /
+-----------+
| |
| |
| |
| Verifier |
| |
| |
| |
+-----------+
| / \
| |
| |
+-----
(6) ProofVerify
Note The protocols implied by the items annotated by an asterisk are out of scope for this specification¶
Terminology defined by [I-D.irtf-cfrg-bbs-signatures] applies to this draft.¶
Additionally, the following terminology is used throughout this document:¶
Notation defined by [I-D.irtf-cfrg-bbs-signatures] applies to this draft.¶
Additionally, the following notation and primitives are used:¶
list = [a, b, c] and elements = [d, a], the result of list.append(elements) will be [a, b, c, d, a].¶
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.¶
This document makes use of various operations defined by the BBS Signature Scheme document [I-D.irtf-cfrg-bbs-signatures]. For clarity, whenever an operation will be used defined in [I-D.irtf-cfrg-bbs-signatures], it will be prefixed by "BBS." (e.g., "BBS.CoreProofGen" etc.). More specifically, the operations used are the following:¶
BBS.CoreVerify: Refers to the CoreVerify operation defined in Section 3.6.2 of [I-D.irtf-cfrg-bbs-signatures].¶
BBS.CoreProofGen: Refers to the CoreProofGen operation defined in Section 3.6.3 of [I-D.irtf-cfrg-bbs-signatures].¶
BBS.create_generators: Refers to the create_generators operation defined in Section 4.1.1 of [I-D.irtf-cfrg-bbs-signatures].¶
BBS.messages_to_scalars: Refers to the messages_to_scalars operation defined in Section 4.1.2 of [I-D.irtf-cfrg-bbs-signatures].¶
BBS.get_random_scalars: Refers to the get_random_scalars operation defined in Section 4.2.1 of [I-D.irtf-cfrg-bbs-signatures].¶
BBS.hash_to_scalar: Refers to the hash_to_scalar operation defined in Section 4.2.2 of [I-D.irtf-cfrg-bbs-signatures].¶
This operation is used by the Prover to create a commitment to a set of messages (committed_messages), that they intend to include in the blind signature. Note that this operation returns both the serialized combination of the commitment and its proof of correctness (commitment_with_proof), as well as the random scalar used to blind the commitment (secret_prover_blind).¶
(commitment_with_proof, secret_prover_blind) = commit(
committed_messages,
api_id)
Inputs:
- committed_messages (OPTIONAL), a vector of octet strings. If not
supplied it defaults to the empty
array ("()").
- api_id (OPTIONAL), octet string. If not supplied it defaults to the
empty octet string ("").
Outputs:
- (commitment_with_proof, secret_prover_blind), a tuple comprising from
an octet string and a
random scalar in that
order.
Procedure:
1. committed_message_scalars = BBS.messages_to_scalars(
committed_messages, api_id)
2. blind_generators = BBS.create_generators(
length(committed_message_scalars) + 1,
"BLIND_" || api_id)
3. return CoreCommit(committed_message_scalars,
blind_generators, api_id)
¶
The following is a helper operation used by the BlindSign procedure (Section 4.2.1) to validate an optional commitment. If a commitment is not supplied, or if it is the Identity_G1, the following operation will return the Identity_G1 as the "default" commitment point, which will be ignored by all computations during BlindSign.¶
commit = deserialize_and_validate_commit(commitment_with_proof,
blind_generators, api_id)
Inputs:
- commitment_with_proof (OPTIONAL), octet string. If it is not supplied
it defaults to the empty octet
string ("").
- blind_generators (OPTIONAL), vector of points of G1. If it is not
supplied it defaults to the empty set
("()").
- api_id (OPTIONAL), octet string. If not supplied it defaults to the
empty octet string ("").
Outputs:
- commit, a point of G1; or INVALID.
Procedure:
1. if commitment_with_proof is the empty string (""), return Identity_G1
2. com_res = octets_to_commitment_with_proof(commitment_with_proof)
3. if com_res is INVALID, return INVALID
4. (commit, commit_proof) = com_res
5. if length(commit_proof[1]) + 1 != length(blind_generators),
return INVALID
6. validation_res = CoreCommitVerify(commit, commit_proof,
blind_generators, api_id)
7. if validation_res is INVALID, return INVALID
8. commitment
¶
The following section defines a BBS Interface for blind BBS signatures. The identifier of the Interface is defined as ciphersuite_id || BLIND_H2G_HM2S_, where ciphersuite_id the unique identifier of the BBS ciphersuite used, as is defined in Section 6 of [I-D.irtf-cfrg-bbs-signatures]). Each BBS Interface MUST define operations to map the input messages to scalar values and to create the generator set, required by the core operations. The input messages to the defined Interface will be mapped to scalars using the messages_to_scalars operation defined in Section 4.1.2 of [I-D.irtf-cfrg-bbs-signatures]. The generators will be created using the create_generators operation defined in Section 4.1.1 of [I-D.irtf-cfrg-bbs-signatures].¶
Other than the BlindSign operation defined in Section 4.2.1, which uses the FinalizeBlindSign procedure, defined in Section 4.3.3, all other interface operations defined in this section use the core operations defined in Section 3.6 of [I-D.irtf-cfrg-bbs-signatures].¶
This operation returns a BBS blind signature from a secret key (SK), over a header, a set of messages and optionally a commitment value (see Section 1.1). If supplied, the commitment value must be accompanied by its proof of correctness (commitment_with_proof, as outputted by the Commit operation defined in Section 4.1.1).¶
The BlindSign operation makes use of the FinalizeBlindSign procedure defined in Section 4.3.3 and the B_calculate procedure defined in Section 5.1. The B_calculate is defined to return an array of elements, to establish extendability of the scheme by allowing the B_calculate operation to return more elements than just the point to be signed.¶
blind_signature = BlindSign(SK, PK, commitment_with_proof, header,
messages)
Inputs:
- SK (REQUIRED), a secret key in the form outputted by the KeyGen
operation.
- PK (REQUIRED), an octet string of the form outputted by SkToPk
provided the above SK as input.
- commitment_with_proof (OPTIONAL), an octet string, representing a
serialized commitment and
commitment_proof, as the first
element outputted by the Commit
operation. If not supplied, it
defaults to the empty string ("").
- header (OPTIONAL), an octet string containing context and application
specific information. If not supplied, it defaults
to an empty string ("").
- messages (OPTIONAL), a vector of octet strings. If not supplied, it
defaults to the empty array ("()").
Parameters:
- api_id, the octet string ciphersuite_id || "BLIND_H2G_HM2S_", where
ciphersuite_id is defined by the ciphersuite and
"BLIND_H2G_HM2S_"is an ASCII string composed of 15 bytes.
- (octet_point_length, octet_scalar_length), defined by the ciphersuite.
Outputs:
- blind_signature, a blind signature encoded as an octet string; or
INVALID.
Deserialization:
1. L = length(messages)
// calculate the number of blind generators used by the commitment,
// if any.
2. M = length(commitment_with_proof)
3. if M != 0, M = M - octet_point_length - octet_scalar_length
4. M = M / octet_scalar_length
5. if M < 0, return INVALID
Procedure:
1. generators = BBS.create_generators(L + 1, api_id)
2. blind_generators = BBS.create_generators(M, "BLIND_" || api_id)
3. commit = deserialize_and_validate_commit(commitment_with_proof,
blind_generators, api_id)
4. if commit is INVALID, return INVALID
5. (msg_1, ..., msg_L) = BBS.messages_to_scalars(messages, api_id)
6. res = B_calculate(generators, commit, messages)
7. if res is INVALID, return INVALID
8. (B) = res
9. blind_sig = FinalizeBlindSign(SK,
PK,
B,
generators,
blind_generators,
header,
api_id)
10. if blind_sig is INVALID, return INVALID
11. return blind_sig
¶
This operation validates a blind BBS signature (signature), given the Signer's public key (PK), a header (header), a set of, known to the Signer, messages (messages) and if used, a set of committed messages (committed_messages) and the secret_prover_blind as returned by the Commit operation (Section 4.1.1).¶
This operation makes use of the CoreVerify operation as defined in Section 3.6.2 of [I-D.irtf-cfrg-bbs-signatures].¶
result = Verify(PK, signature, header, messages, committed_messages,
secret_prover_blind)
Inputs:
- PK (REQUIRED), an octet string of the form outputted by the SkToPk
operation.
- signature (REQUIRED), an octet string of the form outputted by the
Sign operation.
- header (OPTIONAL), an octet string containing context and application
specific information. If not supplied, it defaults
to an empty string.
- messages (OPTIONAL), a vector of octet strings. If not supplied, it
defaults to the empty array "()".
- committed_messages (OPTIONAL), a vector of octet strings. If not
supplied, it defaults to the empty
array "()".
- secret_prover_blind (OPTIONAL), a scalar value. If not supplied it
defaults to zero "0".
Parameters:
- api_id, the octet string ciphersuite_id || "BLIND_H2G_HM2S_", where
ciphersuite_id is defined by the ciphersuite and
"BLIND_H2G_HM2S_"is an ASCII string composed of 15 bytes.
Outputs:
- result: either VALID or INVALID
Procedure:
1. message_scalars = BBS.messages_to_scalars(messages, api_id)
2. committed_message_scalars = ()
4. committed_message_scalars.append(secret_prover_blind)
5. committed_message_scalars.append(BBS.messages_to_scalars(
committed_messages, api_id))
6. generators = BBS.create_generators(length(message_scalars) + 1, api_id)
7. blind_generators = BBS.create_generators(length(committed_message_scalars), "BLIND_" || api_id)
8. res = BBS.CoreVerify(
PK,
signature,
generators.append(blind_generators),
header,
message_scalars.append(committed_message_scalars),
api_id)
9. return res
¶
This operation creates a BBS proof, which is a zero-knowledge, proof-of-knowledge, of a BBS signature, while optionally disclosing any subset of the signed messages. Note that in contrast to the ProofGen operation of [I-D.irtf-cfrg-bbs-signatures] (see Section 3.5.3), the ProofGen operation defined in this section accepts 2 different lists of messages and disclosed indexes, one for the messages known to the Signer (messages) and the corresponding disclosed indexes (disclosed_indexes) and one for the messages committed by the Prover (committed_messages) and the corresponding disclosed indexes (disclosed_commitment_indexes).¶
Furthermore, the operation also expects the secret_prover_blind (as returned from the Commit operation defined in Section 4.1.1) value. If the BBS signature is generated using a commitment value, then the secret_prover_blind returned by the Commit operation used to generate the commitment should be provided to the ProofGen operation (otherwise the resulting proof will be invalid).¶
This operation makes use of the CoreProofGen operation as defined in Section 3.6.3 of [I-D.irtf-cfrg-bbs-signatures].¶
proof = BlindProofGen(PK,
signature,
header,
ph,
messages,
committed_messages,
disclosed_indexes,
disclosed_commitment_indexes,
secret_prover_blind)
Inputs:
- PK (REQUIRED), an octet string of the form outputted by the SkToPk
operation.
- signature (REQUIRED), an octet string of the form outputted by the
Sign operation.
- header (OPTIONAL), an octet string containing context and application
specific information. If not supplied, it defaults
to an empty string.
- ph (OPTIONAL), an octet string containing the presentation header. If
not supplied, it defaults to an empty string.
- messages (OPTIONAL), a vector of octet strings. If not supplied, it
defaults to the empty array "()".
- committed_messages (OPTIONAL), a vector of octet strings. If not
supplied, it defaults to the empty
array "()".
- disclosed_indexes (OPTIONAL), vector of unsigned integers in ascending
order. Indexes of disclosed messages. If
not supplied, it defaults to the empty
array "()".
- disclosed_commitment_indexes (OPTIONAL), vector of unsigned integers
in ascending order. Indexes
of disclosed committed
messages. If not supplied, it
defaults to the empty array
"()".
- secret_prover_blind (OPTIONAL), a scalar value. If not supplied it
defaults to zero "0".
Parameters:
- api_id, the octet string ciphersuite_id || "BLIND_H2G_HM2S_", where
ciphersuite_id is defined by the ciphersuite and
"BLIND_H2G_HM2S_"is an ASCII string composed of 15 bytes.
Outputs:
- proof, an octet string; or INVALID.
Deserialization:
1. L = length(messages)
2. M = length(committed_messages)
3. if length(disclosed_indexes) > L, return INVALID
4. for i in disclosed_indexes, if i < 0 or i >= L, return INVALID
5. if length(disclosed_commitment_indexes) > M, return INVALID
6. for j in disclosed_commitment_indexes,
if i < 0 or i >= M, return INVALID
Procedure:
1. message_scalars = BBS.messages_to_scalars(messages, api_id)
2. committed_message_scalars = ()
3. committed_message_scalars.append(secret_prover_blind)
4. committed_message_scalars.append(BBS.messages_to_scalars(
committed_messages, api_id))
5. generators = BBS.create_generators(length(message_scalars) + 1, api_id)
6. blind_generators = BBS.create_generators(length(committed_message_scalars) + 1, "BLIND_" || api_id)
7. indexes = ()
8. indexes.append(disclosed_indexes)
9. for j in disclosed_commitment_indexes: indexes.append(j + L + 1)
10. proof = BBS.CoreProofGen(
PK,
signature,
generators.append(blind_generators),
header,
ph,
message_scalars.append(committed_message_scalars),
indexes,
api_id)
11. return proof
¶
The ProofVerify operation validates a BBS proof, given the Signer's public key (PK), a header and presentation header values, two arrays of disclosed messages (the ones known to the Signer and the ones committed by the prover) and two corresponding arrays of indexes those messages had in the original vectors of signed messages. In addition, the BlindProofVerify operation defined in this section accepts the integer L, representing the total number of signed messages known by the Signer.¶
This operation makes use of the CoreProofVerify operation as defined in Section 3.6.4 of [I-D.irtf-cfrg-bbs-signatures].¶
result = BlindProofVerify(PK,
proof,
header,
ph,
L,
disclosed_messages,
disclosed_committed_messages,
disclosed_indexes,
disclosed_committed_indexes)
Inputs:
- PK (REQUIRED), an octet string of the form outputted by the SkToPk
operation.
- proof (REQUIRED), an octet string of the form outputted by the
ProofGen operation.
- header (OPTIONAL), an optional octet string containing context and
application specific information. If not supplied,
it defaults to the empty octet string ("").
- ph (OPTIONAL), an octet string containing the presentation header. If
not supplied, it defaults to the empty octet
string ("").
- L (OPTIONAL), an integer, representing the total number of Signer
known messages if not supplied it defaults to 0.
- disclosed_messages (OPTIONAL), a vector of octet strings. If not
supplied, it defaults to the empty
array ("()").
- disclosed_indexes (OPTIONAL), vector of unsigned integers in ascending
order. Indexes of disclosed messages. If
not supplied, it defaults to the empty
array ("()").
Parameters:
- api_id, the octet string ciphersuite_id || "H2G_HM2S_", where
ciphersuite_id is defined by the ciphersuite and "H2G_HM2S_"is
an ASCII string comprised of 9 bytes.
- (octet_point_length, octet_scalar_length), defined by the ciphersuite.
Outputs:
- result, either VALID or INVALID.
Deserialization:
1. proof_len_floor = 2 * octet_point_length + 3 * octet_scalar_length
2. if length(proof) < proof_len_floor, return INVALID
3. U = floor((length(proof) - proof_len_floor) / octet_scalar_length)
4. total_no_messages = length(disclosed_indexes) +
length(disclosed_committed_indexes) + U
5. M = total_no_messages - L
Procedure:
1. generators = BBS.create_generators(L + 1, api_id)
2. blind_generators = BBS.create_generators(M + 1, "BLIND_" || api_id)
3. disclosed_message_scalars = messages_to_scalars(
disclosed_messages, api_id)
4. disclosed_committed_message_scalars = messages_to_scalars(
disclosed_committed_messages, api_id)
5. message_scalars = disclosed_message_scalars.append(
disclosed_committed_message_scalars)
6. indexes = ()
7. indexes.append(disclosed_indexes)
8. for j in disclosed_commitment_indexes: indexes.append(j + L + 1)
9. result = BBS.CoreProofVerify(PK,
proof,
generators.append(blind_generators),
header,
ph,
message_scalars,
indexes,
api_id)
10. return result
¶
commit_with_proof = CoreCommit(blind_generators, committed_messages, api_id)
Inputs:
- blind_generators (REQUIRED), vector of pseudo-random points in G1.
- committed_messages (OPTIONAL), a vector of scalars. If not supplied,
it defaults to the empty array ("()").
- api_id (OPTIONAL), an octet string. If not supplied it defaults to the
empty octet string ("").
Deserialization:
1. M = length(committed_messages)
2. if length(blind_generators) != M + 1, return INVALID
3. (Q_2, J_1, ..., J_M) = blind_generators
Procedure:
1. (secret_prover_blind, s~, m~_1, ..., m~_M)
= BBS.get_random_scalars(M + 2)
2. C = Q_2 * secret_prover_blind + J_1 * msg_1 + ... + J_M * msg_M
3. Cbar = Q_2 * s~ + J_1 * m~_1 + ... + J_M * m~_M
4. challenge = calculate_blind_challenge(C, Cbar, blind_generators,
api_id)
5. s^ = s~ + secret_prover_blind * challenge
6. for m in (1, 2, ..., M): m^_i = m~_1 + msg_i * challenge
7. proof = (s^, (m^_1, ..., m^_M), challenge)
8. commit_with_proof = commitment_with_proof_to_octets(C, proof)
9. return (commit_with_proof, secret_prover_blind)
¶
This operation is used by the Signer to verify the correctness of a commitment_proof for a supplied commitment, over a list of points of G1 called the blind_generators, used to compute that commitment.¶
result = CoreCommitVerify(commitment, commitment_proof,
blind_generators, api_id)
Inputs:
- commitment (REQUIRED), a commitment (see (#terminology)).
- commitment_proof (REQUIRED), a commitment_proof (see (#terminology)).
- blind_generators (REQUIRED), vector of pseudo-random points in G1.
- api_id (OPTIONAL), octet string. If not supplied it defaults to the
empty octet string ("").
Outputs:
- result: either VALID or INVALID
Deserialization:
1. (s^, commitments, cp) = commitment_proof
2. M = length(commitments)
3. (m^_1, ..., m^_M) = commitments
4. if length(blind_generators) != M + 1, return INVALID
5. (Q_2, J_1, ..., J_M) = blind_generators
Procedure:
1. Cbar = Q_2 * s^ + J_1 * m^_1 + ... + J_M * m^_M + commitment * (-cp)
2. cv = calculate_blind_challenge(commitment, Cbar, blind_generators,
api_id)
3. if cv != cp, return INVALID
4. return VALID
¶
This operation computes a blind BBS signature, from a secret key (SK), a set of generators (points of G1), a supplied commitment with its proof of correctness (commitment_with_proof), a header (header) and a set of messages (messages). The operation also accepts the identifier of the BBS Interface, calling this core operation.¶
blind_signature = FinalizeBlindSign(SK,
PK,
B,
generators,
blind_generators,
header,
api_id)
Inputs:
- SK (REQUIRED), a secret key in the form outputted by the KeyGen
operation.
- PK (REQUIRED), an octet string of the form outputted by SkToPk
provided the above SK as input.
- B (REQUIRED), a point of G1, different than Identity_G1.
- generators (REQUIRED), vector of pseudo-random points in G1.
- blind_generators (OPTIONAL), vector of pseudo-random points in G1. If
not supplied it defaults to the empty
array.
- header (OPTIONAL), an octet string containing context and application
specific information. If not supplied, it defaults
to an empty string.
- api_id (OPTIONAL), an octet string. If not supplied it defaults to the
empty octet string ("").
Outputs:
- blind_signature, a blind signature encoded as an octet string; or
INVALID.
Definitions:
1. signature_dst, an octet string representing the domain separation
tag: api_id || "H2S_" where "H2S_" is an ASCII string
composed of 4 bytes.
Deserialization:
1. L = length(generators) - 1
2. M = length(blind_generators)
3. if L <= 0 or M <=0, return INVALID
4. (Q_1, H_1, ..., H_L) = generators
5. (J_1, ..., J_M) = blind_generators
Procedure:
1. domain = calculate_domain(PK, Q_1, (H_1, ..., H_L, J_1, ..., J_M),
header, api_id)
2. e_octs = serialize((SK, B, domain))
3. e = BBS.hash_to_scalar(e_octs, signature_dst)
4. A = B * (1 / (SK + e))
5. return signature_to_octets((A, e))
¶
res = B_calculate(generators, commitment, message_scalars)
Inputs:
- generators (REQUIRED), an array of at least one point from the
G1 group.
- commitment (OPTIONAL), a point from the G1 group. If not supplied it
defaults to the Identity_G1 point.
- message_scalars (OPTIONAL), an array of scalar values. If not
supplied, it defaults to the empty
array ("()").
Outputs:
- res, an array of a single element from the G1 subgroup, or INVALID.
Deserialization:
1. L = length(messages)
2. if length(generators) != L + 1, return INVALID
3. (Q_1, H_1, ..., H_L) = generators
4. (msg_1, ..., msg_L) = messages
Procedure:
1. B = Q_1 + H_1 * msg_1 + ... + H_L * msg_L + commitment
4. if B is Identity_G1, return INVALID
5. return B
¶
challenge = calculate_blind_challenge(C, Cbar, generators, api_id)
Inputs:
- C (REQUIRED), a point of G1.
- Cbar (REQUIRED), a point of G1.
- generators (REQUIRED), an array of points from G1, of length at
least 1.
- api_id (OPTIONAL), octet string. If not supplied it defaults to the
empty octet string ("").
Definition:
- blind_challenge_dst, an octet string representing the domain
separation tag: api_id || "H2S_" where
ciphersuite_id is defined by the ciphersuite and
"H2S_" is an ASCII string composed of 4 bytes.
Deserialization:
1. if length(generators) == 0, return INVALID
2. M = length(generators) - 1
Procedure:
1. c_arr = (M)
2. c_arr.append(generators)
3. c_octs = serialize(c_arr.append(C, Cbar))
4. return BBS.hash_to_scalar(c_octs, blind_challenge_dst)
¶
commitment_octets = commitment_with_proof_to_octets(commitment, proof)
Inputs:
- commitment (REQUIRED), a point of G1.
- proof (REQUIRED), a vector comprising of a scalar, a possibly empty
vector of scalars and another scalar in that order.
Outputs:
- commitment_octets, an octet string or INVALID.
Procedure:
1. commitment_octs = serialize(commitment)
2. if commitment_octs is INVALID, return INVALID
3. proof_octs = serialize(proof)
4. if proof_octs is INVALID, return INVALID
5. return commitment_octs || proof_octs
¶
commitment = octets_to_commitment_with_proof(commitment_octs)
Inputs:
- commitment_octs (REQUIRED), an octet string in the form outputted from
the commitment_to_octets operation.
Parameters:
- (octet_point_length, octet_scalar_length), defined by the ciphersuite.
Outputs:
- commitment, a commitment in the form (C, proof), where C a point of G1
and a proof vector comprising of a scalar, a possibly
empty vector of scalars and another scalar in that order.
Procedure:
1. commit_len_floor = octet_point_length + 2 * octet_scalar_length
2. if length(commitment_octs) < commit_len_floor, return INVALID
3. C_octets = commitment_octs[0..(octet_point_length - 1)]
4. C = octets_to_point_g1(C_octets)
5. if C is INVALID, return INVALID
6. if C == Identity_G1, return INVALID
7. j = 0
8. index = octet_point_length
9. while index < length(commitment_octs):
10. end_index = index + octet_scalar_length - 1
11. s_j = OS2IP(commitment_octets[index..end_index])
12. if s_j = 0 or if s_j >= r, return INVALID
13. index += octet_scalar_length
14. j += 1
15. if index != length(commitment_octs), return INVALID
16. if j < 2, return INVALID
17. msg_commitment = ()
18. if j >= 3, set msg_commitment = (s_2, ..., s_(j-1))
19. return (C, (s_0, msg_commitments, s_j))
¶
Security considerations detailed in Section 6 of [I-D.irtf-cfrg-bbs-signatures] apply to this draft as well.¶
The random scalar value secret_prover_blind calculated and returned by the Commit operation is responsible for "hiding" the committed messages (otherwise, in many practical applications, the Signer may be able to retrieve them). Furthermore, it guarantees that the entity generating the BBS proof (see BlindProofGen defined in Section 4.2.3) has knowledge of that factor. As a result, the secret_prover_blind MUST remain private by the Prover and it MUST be generated using a cryptographically secure pseudo-random number generator. See Section 6.7 of [I-D.irtf-cfrg-bbs-signatures] on recommendations and requirements for implementing the BBS.get_random_scalars operation (which is used to calculate the secret_prover_blind value).¶
One natural use case for the blind signatures extension of the BBS scheme is key binding. In the context of BBS Signatures, key binding guarantees that only entities in control of a specific private key can compute BBS proofs. This can be achieved by committing to the private key prior to issuance, resulting in a BBS signature that includes that key as one of the signed messages. Creating a BBS proof from that signature will then require knowledge of that key (similar to any signed message). The Prover MUST NOT disclose that key as part of a proof generation procedure. Note also that the secret_prover_blind value returned by the Commit operation defined in Section 4.1.1 (see Section 6.1), has a similar property, i.e., it's knowledge is required to generate a proof from a blind signature. Many applications however, requiring key binding, mandate that the same private key is used among multiple signatures, whereas the secret_prover_blind is uniquely generated for each blind signature issuance request. In those cases, a commitment to a private key must be used, as described above.¶
This document uses the BBS_BLS12381G1_XOF:SHAKE-256_SSWU_RO_ and BBS_BLS12381G1_XMD:SHA-256_SSWU_RO_ defined in Section 7.2.1 and Section 7.2.2 correspondingly, of [I-D.irtf-cfrg-bbs-signatures].¶
TBD¶
This document does not make any requests of IANA.¶