CFRG D. Boneh
Internet-Draft Stanford University
Intended status: Informational S. Gorbunov
Expires: 14 March 2021 University of Waterloo
R. Wahby
Stanford University
H. Wee
NTT Research and ENS, Paris
Z. Zhang
Algorand
10 September 2020
BLS Signatures
draft-irtf-cfrg-bls-signature-04
Abstract
BLS is a digital signature scheme with aggregation properties. Given
set of signatures (signature_1, ..., signature_n) anyone can produce
an aggregated signature. Aggregation can also be done on secret keys
and public keys. Furthermore, the BLS signature scheme is
deterministic, non-malleable, and efficient. Its simplicity and
cryptographic properties allows it to be useful in a variety of use-
cases, specifically when minimal storage space or bandwidth are
required.
Status of This Memo
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Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Comparison with ECDSA . . . . . . . . . . . . . . . . . . 4
1.2. Organization of this document . . . . . . . . . . . . . . 4
1.3. Terminology and definitions . . . . . . . . . . . . . . . 5
1.4. API . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.5. Requirements . . . . . . . . . . . . . . . . . . . . . . 7
2. Core operations . . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Variants . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. Parameters . . . . . . . . . . . . . . . . . . . . . . . 7
2.3. KeyGen . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.4. SkToPk . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.5. KeyValidate . . . . . . . . . . . . . . . . . . . . . . . 11
2.6. CoreSign . . . . . . . . . . . . . . . . . . . . . . . . 11
2.7. CoreVerify . . . . . . . . . . . . . . . . . . . . . . . 12
2.8. Aggregate . . . . . . . . . . . . . . . . . . . . . . . . 12
2.9. CoreAggregateVerify . . . . . . . . . . . . . . . . . . . 13
3. BLS Signatures . . . . . . . . . . . . . . . . . . . . . . . 14
3.1. Basic scheme . . . . . . . . . . . . . . . . . . . . . . 14
3.1.1. AggregateVerify . . . . . . . . . . . . . . . . . . . 15
3.2. Message augmentation . . . . . . . . . . . . . . . . . . 15
3.2.1. Sign . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2.2. Verify . . . . . . . . . . . . . . . . . . . . . . . 16
3.2.3. AggregateVerify . . . . . . . . . . . . . . . . . . . 16
3.3. Proof of possession . . . . . . . . . . . . . . . . . . . 17
3.3.1. Parameters . . . . . . . . . . . . . . . . . . . . . 18
3.3.2. PopProve . . . . . . . . . . . . . . . . . . . . . . 18
3.3.3. PopVerify . . . . . . . . . . . . . . . . . . . . . . 19
3.3.4. FastAggregateVerify . . . . . . . . . . . . . . . . . 19
4. Ciphersuites . . . . . . . . . . . . . . . . . . . . . . . . 20
4.1. Ciphersuite format . . . . . . . . . . . . . . . . . . . 20
4.2. Ciphersuites for BLS12-381 . . . . . . . . . . . . . . . 22
4.2.1. Basic . . . . . . . . . . . . . . . . . . . . . . . . 22
4.2.2. Message augmentation . . . . . . . . . . . . . . . . 22
4.2.3. Proof of possession . . . . . . . . . . . . . . . . . 23
5. Security Considerations . . . . . . . . . . . . . . . . . . . 24
5.1. Validating public keys . . . . . . . . . . . . . . . . . 24
5.2. Skipping membership check . . . . . . . . . . . . . . . . 24
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5.3. Side channel attacks . . . . . . . . . . . . . . . . . . 25
5.4. Randomness considerations . . . . . . . . . . . . . . . . 25
5.5. Implementing hash_to_point and hash_pubkey_to_point . . . 25
6. Implementation Status . . . . . . . . . . . . . . . . . . . . 25
7. Related Standards . . . . . . . . . . . . . . . . . . . . . . 26
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
9. Normative References . . . . . . . . . . . . . . . . . . . . 26
10. Informative References . . . . . . . . . . . . . . . . . . . 27
Appendix A. BLS12-381 . . . . . . . . . . . . . . . . . . . . . 28
Appendix B. Test Vectors . . . . . . . . . . . . . . . . . . . . 29
Appendix C. Security analyses . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
1. Introduction
A signature scheme is a fundamental cryptographic primitive that is
used to protect authenticity and integrity of communication. Only
the holder of a secret key can sign messages, but anyone can verify
the signature using the associated public key.
Signature schemes are used in point-to-point secure communication
protocols, PKI, remote connections, etc. Designing efficient and
secure digital signature is very important for these applications.
This document describes the BLS signature scheme. The scheme enjoys
a variety of important efficiency properties:
1. The public key and the signatures are encoded as single group
elements.
2. Verification requires 2 pairing operations.
3. A collection of signatures (signature_1, ..., signature_n) can be
aggregated into a single signature. Moreover, the aggregate
signature can be verified using only n+1 pairings (as opposed to
2n pairings, when verifying n signatures separately).
Given the above properties, the scheme enables many interesting
applications. The immediate applications include
* Authentication and integrity for Public Key Infrastructure (PKI)
and blockchains.
- The usage is similar to classical digital signatures, such as
ECDSA.
* Aggregating signature chains for PKI and Secure Border Gateway
Protocol (SBGP).
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- Concretely, in a PKI signature chain of depth n, we have n
signatures by n certificate authorities on n distinct
certificates. Similarly, in SBGP, each router receives a list
of n signatures attesting to a path of length n in the network.
In both settings, using the BLS signature scheme would allow us
to aggregate the n signatures into a single signature.
* consensus protocols for blockchains.
- There, BLS signatures are used for authenticating transactions
as well as votes during the consensus protocol, and the use of
aggregation significantly reduces the bandwidth and storage
requirements.
1.1. Comparison with ECDSA
The following comparison assumes BLS signatures with curve BLS12-381,
targeting 128 bits security.
For 128 bits security, ECDSA takes 37 and 79 micro-seconds to sign
and verify a signature on a typical laptop. In comparison, for the
same level of security, BLS takes 370 and 2700 micro-seconds to sign
and verify a signature.
In terms of sizes, ECDSA uses 32 bytes for public keys and 64 bytes
for signatures; while BLS uses 96 bytes for public keys, and 48 bytes
for signatures. Alternatively, BLS can also be instantiated with 48
bytes of public keys and 96 bytes of signatures. BLS also allows for
signature aggregation. In other words, a single signature is
sufficient to authenticate multiple messages and public keys.
1.2. Organization of this document
This document is organized as follows:
* The remainder of this section defines terminology and the high-
level API.
* Section 2 defines primitive operations used in the BLS signature
scheme. These operations MUST NOT be used alone.
* Section 3 defines three BLS Signature schemes giving slightly
different security and performance properties.
* Section 4 defines the format for a ciphersuites and gives
recommended ciphersuites.
* The appendices give test vectors, etc.
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1.3. Terminology and definitions
The following terminology is used through this document:
* SK: The secret key for the signature scheme.
* PK: The public key for the signature scheme.
* message: The input to be signed by the signature scheme.
* signature: The digital signature output.
* aggregation: Given a list of signatures for a list of messages and
public keys, an aggregation algorithm generates one signature that
authenticates the same list of messages and public keys.
* rogue key attack: An attack in which a specially crafted public
key (the "rogue" key) is used to forge an aggregated signature.
Section 3 specifies methods for securing against rogue key
attacks.
The following notation and primitives are used:
* a || b denotes the concatenation of octet strings a and b.
* A pairing-friendly elliptic curve defines the following primitives
(see [I-D.irtf-cfrg-pairing-friendly-curves] for detailed
discussion):
- E1, E2: elliptic curve groups defined over finite fields. This
document assumes that E1 has a more compact representation than
E2, i.e., because E1 is defined over a smaller field than E2.
- G1, G2: subgroups of E1 and E2 (respectively) having prime
order r.
- P1, P2: distinguished points that generate G1 and G2,
respectively.
- GT: a subgroup, of prime order r, of the multiplicative group
of a field extension.
- e : G1 x G2 -> GT: a non-degenerate bilinear map.
* For the above pairing-friendly curve, this document writes
operations in E1 and E2 in additive notation, i.e., P + Q denotes
point addition and x * P denotes scalar multiplication.
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Operations in GT are written in multiplicative notation, i.e., a *
b is field multiplication.
* For each of E1 and E2 defined by the above pairing-friendly curve,
we assume that the pairing-friendly elliptic curve definition
provides several primitives, described below.
Note that these primitives are named generically. When referring
to one of these primitives for a specific group, this document
appends the name of the group, e.g., point_to_octets_E1,
subgroup_check_E2, etc.
- point_to_octets(P) -> ostr: returns the canonical
representation of the point P as an octet string. This
operation is also known as serialization.
- octets_to_point(ostr) -> P: returns the point P corresponding
to the canonical representation ostr, or INVALID if ostr is not
a valid output of point_to_octets. This operation is also
known as deserialization.
- subgroup_check(P) -> VALID or INVALID: returns VALID when the
point P is an element of the subgroup of order r, and INVALID
otherwise. This function can always be implemented by checking
that r * P is equal to the identity element. In some cases,
faster checks may also exist, e.g., [Bowe19].
* I2OSP and OS2IP are the functions defined in [RFC8017], Section 4.
* hash_to_point(ostr) -> P: a cryptographic hash function that takes
as input an arbitrary octet string and returns a point on an
elliptic curve. Functions of this kind are defined in
[I-D.irtf-cfrg-hash-to-curve]. Each of the ciphersuites in
Section 4 specifies the hash_to_point algorithm to be used.
1.4. API
The BLS signature scheme defines the following API:
* KeyGen(IKM) -> SK: a key generation algorithm that takes as input
an octet string comprising secret keying material, and outputs a
secret key SK.
* SkToPk(SK) -> PK: an algorithm that takes as input a secret key
and outputs the corresponding public key.
* Sign(SK, message) -> signature: a signing algorithm that generates
a deterministic signature given a secret key SK and a message.
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* Verify(PK, message, signature) -> VALID or INVALID: a verification
algorithm that outputs VALID if signature is a valid signature of
message under public key PK, and INVALID otherwise.
* Aggregate((signature_1, ..., signature_n)) -> signature: an
aggregation algorithm that aggregates a collection of signatures
into a single signature.
* AggregateVerify((PK_1, ..., PK_n), (message_1, ..., message_n),
signature) -> VALID or INVALID: an aggregate verification
algorithm that outputs VALID if signature is a valid aggregated
signature for a collection of public keys and messages, and
outputs INVALID otherwise.
1.5. Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Core operations
This section defines core operations used by the schemes defined in
Section 3. These operations MUST NOT be used except as described in
that section.
2.1. Variants
Each core operation has two variants that trade off signature and
public key size:
1. Minimal-signature-size: signatures are points in G1, public keys
are points in G2. (Recall from Section 1.3 that E1 has a more
compact representation than E2.)
2. Minimal-pubkey-size: public keys are points in G1, signatures are
points in G2.
Implementations using signature aggregation SHOULD use this
approach, since the size of (PK_1, ..., PK_n, signature) is
dominated by the public keys even for small n.
2.2. Parameters
The core operations in this section depend on several parameters:
* A signature variant, either minimal-signature-size or minimal-
pubkey-size. These are defined in Section 2.1.
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* A pairing-friendly elliptic curve, plus associated functionality
given in Section 1.3.
* H, a hash function that MUST be a secure cryptographic hash
function, e.g., SHA-256 [FIPS180-4]. For security, H MUST output
at least ceil(log2(r)) bits, where r is the order of the subgroups
G1 and G2 defined by the pairing-friendly elliptic curve.
* hash_to_point, a function whose interface is described in
Section 1.3. When the signature variant is minimal-signature-
size, this function MUST output a point in G1. When the signature
variant is minimal-pubkey size, this function MUST output a point
in G2. For security, this function MUST be either a random oracle
encoding or a nonuniform encoding, as defined in
[I-D.irtf-cfrg-hash-to-curve].
In addition, the following primitives are determined by the above
parameters:
* P, an elliptic curve point. When the signature variant is
minimal-signature-size, P is the distinguished point P2 that
generates the group G2 (see Section 1.3). When the signature
variant is minimal-pubkey-size, P is the distinguished point P1
that generates the group G1.
* r, the order of the subgroups G1 and G2 defined by the pairing-
friendly curve.
* pairing, a function that invokes the function e of Section 1.3,
with argument order depending on signature variant:
- For minimal-signature-size:
pairing(U, V) := e(U, V)
- For minimal-pubkey-size:
pairing(U, V) := e(V, U)
* point_to_pubkey and point_to_signature, functions that invoke the
appropriate serialization routine (Section 1.3) depending on
signature variant:
- For minimal-signature-size:
point_to_pubkey(P) := point_to_octets_E2(P)
point_to_signature(P) := point_to_octets_E1(P)
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- For minimal-pubkey-size:
point_to_pubkey(P) := point_to_octets_E1(P)
point_to_signature(P) := point_to_octets_E2(P)
* pubkey_to_point and signature_to_point, functions that invoke the
appropriate deserialization routine (Section 1.3) depending on
signature variant:
- For minimal-signature-size:
pubkey_to_point(ostr) := octets_to_point_E2(ostr)
signature_to_point(ostr) := octets_to_point_E1(ostr)
- For minimal-pubkey-size:
pubkey_to_point(ostr) := octets_to_point_E1(ostr)
signature_to_point(ostr) := octets_to_point_E2(ostr)
* pubkey_subgroup_check and signature_subgroup_check, functions that
invoke the appropriate subgroup check routine (Section 1.3)
depending on signature variant:
- For minimal-signature-size:
pubkey_subgroup_check(P) := subgroup_check_E2(P)
signature_subgroup_check(P) := subgroup_check_E1(P)
- For minimal-pubkey-size:
pubkey_subgroup_check(P) := subgroup_check_E1(P)
signature_subgroup_check(P) := subgroup_check_E2(P)
2.3. KeyGen
The KeyGen procedure described in this section generates a secret key
SK deterministically from a secret octet string IKM. SK is
guaranteed to be nonzero, as required by KeyValidate (Section 2.5).
KeyGen uses HKDF [RFC5869] instantiated with the hash function H.
For security, IKM MUST be infeasible to guess, e.g., generated by a
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trusted source of randomness. IKM MUST be at least 32 bytes long,
but it MAY be longer.
KeyGen takes an optional parameter, key_info. This parameter MAY be
used to derive multiple independent keys from the same IKM. By
default, key_info is the empty string.
SK = KeyGen(IKM)
Inputs:
- IKM, a secret octet string. See requirements above.
Outputs:
- SK, a uniformly random integer such that 1 <= SK < r.
Parameters:
- key_info, an optional octet string.
If key_info is not supplied, it defaults to the empty string.
Definitions:
- HKDF-Extract is as defined in RFC5869, instantiated with hash H.
- HKDF-Expand is as defined in RFC5869, instantiated with hash H.
- I2OSP and OS2IP are as defined in RFC8017, Section 4.
- L is the integer given by ceil((3 * ceil(log2(r))) / 16).
- "BLS-SIG-KEYGEN-SALT-" is an ASCII string comprising 20 octets.
Procedure:
1. salt = "BLS-SIG-KEYGEN-SALT-"
2. SK = 0
3. while SK == 0:
4. salt = H(salt)
5. PRK = HKDF-Extract(salt, IKM || I2OSP(0, 1))
6. OKM = HKDF-Expand(PRK, key_info || I2OSP(L, 2), L)
7. SK = OS2IP(OKM) mod r
8. return SK
KeyGen is the RECOMMENDED way of generating secret keys, but its use
is not required for compatibility, and implementations MAY use a
different KeyGen procedure. For security, such an alternative KeyGen
procedure MUST output SK that is statistically close to uniformly
random in the range 1 <= SK < r.
2.4. SkToPk
The SkToPk algorithm takes a secret key SK and outputs the
corresponding public key PK. Section 2.3 discusses requirements for
SK.
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PK = SkToPk(SK)
Inputs:
- SK, a secret integer such that 1 <= SK < r.
Outputs:
- PK, a public key encoded as an octet string.
Procedure:
1. xP = SK * P
2. PK = point_to_pubkey(xP)
3. return PK
2.5. KeyValidate
The KeyValidate algorithm ensures that a public key is valid. In
particular, it ensures that a public key represents a valid, non-
identity point that is in the correct subgroup. See Section 5.1 for
further discussion.
As an optimization, implementations MAY cache the result of
KeyValidate in order to avoid unnecessarily repeating validation for
known keys.
result = KeyValidate(PK)
Inputs:
- PK, a public key in the format output by SkToPk.
Outputs:
- result, either VALID or INVALID
Procedure:
1. xP = pubkey_to_point(PK)
2. If xP is INVALID, return INVALID
3. If xP is the identity element, return INVALID
4. If pubkey_subgroup_check(xP) is INVALID, return INVALID
5. return VALID
2.6. CoreSign
The CoreSign algorithm computes a signature from SK, a secret key,
and message, an octet string.
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signature = CoreSign(SK, message)
Inputs:
- SK, a secret key in the format output by KeyGen.
- message, an octet string.
Outputs:
- signature, an octet string.
Procedure:
1. Q = hash_to_point(message)
2. R = SK * Q
3. signature = point_to_signature(R)
4. return signature
2.7. CoreVerify
The CoreVerify algorithm checks that a signature is valid for the
octet string message under the public key PK.
result = CoreVerify(PK, message, signature)
Inputs:
- PK, a public key in the format output by SkToPk.
- message, an octet string.
- signature, an octet string in the format output by CoreSign.
Outputs:
- result, either VALID or INVALID.
Procedure:
1. R = signature_to_point(signature)
2. If R is INVALID, return INVALID
3. If signature_subgroup_check(R) is INVALID, return INVALID
4. If KeyValidate(PK) is INVALID, return INVALID
5. xP = pubkey_to_point(PK)
6. Q = hash_to_point(message)
7. C1 = pairing(Q, xP)
8. C2 = pairing(R, P)
9. If C1 == C2, return VALID, else return INVALID
2.8. Aggregate
The Aggregate algorithm aggregates multiple signatures into one.
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signature = Aggregate((signature_1, ..., signature_n))
Inputs:
- signature_1, ..., signature_n, octet strings output by
either CoreSign or Aggregate.
Outputs:
- signature, an octet string encoding a aggregated signature
that combines all inputs; or INVALID.
Precondition: n >= 1, otherwise return INVALID.
Procedure:
1. aggregate = signature_to_point(signature_1)
2. If aggregate is INVALID, return INVALID
3. for i in 2, ..., n:
4. next = signature_to_point(signature_i)
5. If next is INVALID, return INVALID
6. aggregate = aggregate + next
7. signature = point_to_signature(aggregate)
8. return signature
2.9. CoreAggregateVerify
The CoreAggregateVerify algorithm checks an aggregated signature over
several (PK, message) pairs.
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result = CoreAggregateVerify((PK_1, ..., PK_n),
(message_1, ... message_n),
signature)
Inputs:
- PK_1, ..., PK_n, public keys in the format output by SkToPk.
- message_1, ..., message_n, octet strings.
- signature, an octet string output by Aggregate.
Outputs:
- result, either VALID or INVALID.
Precondition: n >= 1, otherwise return INVALID.
Procedure:
1. R = signature_to_point(signature)
2. If R is INVALID, return INVALID
3. If signature_subgroup_check(R) is INVALID, return INVALID
4. C1 = 1 (the identity element in GT)
5. for i in 1, ..., n:
6. If KeyValidate(PK_i) is INVALID, return INVALID
7. xP = pubkey_to_point(PK_i)
8. Q = hash_to_point(message_i)
9. C1 = C1 * pairing(Q, xP)
10. C2 = pairing(R, P)
11. If C1 == C2, return VALID, else return INVALID
3. BLS Signatures
This section defines three signature schemes: basic, message
augmentation, and proof of possession. These schemes differ in the
ways that they defend against rogue key attacks (Section 1.3).
All of the schemes in this section are built on a set of core
operations defined in Section 2. Thus, defining a scheme requires
fixing a set of parameters as defined in Section 2.2.
All three schemes expose the KeyGen, SkToPk, and Aggregate operations
that are defined in Section 2. The sections below define the other
API functions (Section 1.4) for each scheme.
3.1. Basic scheme
In a basic scheme, rogue key attacks are handled by requiring all
messages signed by an aggregate signature to be distinct. This
requirement is enforced in the definition of AggregateVerify.
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The Sign and Verify functions are identical to CoreSign and
CoreVerify (Section 2), respectively. AggregateVerify is defined
below.
3.1.1. AggregateVerify
This function first ensures that all messages are distinct, and then
invokes CoreAggregateVerify.
result = AggregateVerify((PK_1, ..., PK_n),
(message_1, ..., message_n),
signature)
Inputs:
- PK_1, ..., PK_n, public keys in the format output by SkToPk.
- message_1, ..., message_n, octet strings.
- signature, an octet string output by Aggregate.
Outputs:
- result, either VALID or INVALID.
Precondition: n >= 1, otherwise return INVALID.
Procedure:
1. If any two input messages are equal, return INVALID.
2. return CoreAggregateVerify((PK_1, ..., PK_n),
(message_1, ..., message_n),
signature)
3.2. Message augmentation
In a message augmentation scheme, signatures are generated over the
concatenation of the public key and the message, ensuring that
messages signed by different public keys are distinct.
3.2.1. Sign
To match the API for Sign defined in Section 1.4, this function
recomputes the public key corresponding to the input SK.
Implementations MAY instead implement an interface that takes the
public key as an input.
Note that the point P and the point_to_pubkey function are defined in
Section 2.2.
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signature = Sign(SK, message)
Inputs:
- SK, a secret key in the format output by KeyGen.
- message, an octet string.
Outputs:
- signature, an octet string.
Procedure:
1. PK = SkToPk(SK)
2. return CoreSign(SK, PK || message)
3.2.2. Verify
result = Verify(PK, message, signature)
Inputs:
- PK, a public key in the format output by SkToPk.
- message, an octet string.
- signature, an octet string in the format output by CoreSign.
Outputs:
- result, either VALID or INVALID.
Procedure:
1. return CoreVerify(PK, PK || message, signature)
3.2.3. AggregateVerify
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result = AggregateVerify((PK_1, ..., PK_n),
(message_1, ..., message_n),
signature)
Inputs:
- PK_1, ..., PK_n, public keys in the format output by SkToPk.
- message_1, ..., message_n, octet strings.
- signature, an octet string output by Aggregate.
Outputs:
- result, either VALID or INVALID.
Precondition: n >= 1, otherwise return INVALID.
Procedure:
1. for i in 1, ..., n:
2. mprime_i = PK_i || message_i
3. return CoreAggregateVerify((PK_1, ..., PK_n),
(mprime_1, ..., mprime_n),
signature)
3.3. Proof of possession
A proof of possession scheme uses a separate public key validation
step, called a proof of possession, to defend against rogue key
attacks. This enables an optimization to aggregate signature
verification for the case that all signatures are on the same
message.
The Sign, Verify, and AggregateVerify functions are identical to
CoreSign, CoreVerify, and CoreAggregateVerify (Section 2),
respectively. In addition, a proof of possession scheme defines
three functions beyond the standard API (Section 1.4):
* PopProve(SK) -> proof: an algorithm that generates a proof of
possession for the public key corresponding to secret key SK.
* PopVerify(PK, proof) -> VALID or INVALID: an algorithm that
outputs VALID if proof is valid for PK, and INVALID otherwise.
* FastAggregateVerify((PK_1, ..., PK_n), message, signature) ->
VALID or INVALID: a verification algorithm for the aggregate of
multiple signatures on the same message. This function is faster
than AggregateVerify.
All public keys used by Verify, AggregateVerify, and
FastAggregateVerify MUST be accompanied by a proof of possession, and
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the result of evaluating PopVerify on the public key and proof MUST
be VALID.
3.3.1. Parameters
In addition to the parameters required to instantiate the core
operations (Section 2.2), a proof of possession scheme requires one
further parameter:
* hash_pubkey_to_point(PK) -> P: a cryptographic hash function that
takes as input a public key and outputs a point in the same
subgroup as the hash_to_point algorithm used to instantiate the
core operations.
For security, this function MUST be domain separated from the
hash_to_point function. In addition, this function MUST be either
a random oracle encoding or a nonuniform encoding, as defined in
[I-D.irtf-cfrg-hash-to-curve].
The RECOMMENDED way of instantiating hash_pubkey_to_point is to
use the same hash-to-curve function as hash_to_point, with a
different domain separation tag (see
[I-D.irtf-cfrg-hash-to-curve], Section 3.1). Section 4.1
discusses the RECOMMENDED way to construct the domain separation
tag.
3.3.2. PopProve
This function recomputes the public key coresponding to the input SK.
Implementations MAY instead implement an interface that takes the
public key as input.
Note that the point P and the point_to_pubkey and point_to_signature
functions are defined in Section 2.2. The hash_pubkey_to_point
function is defined in Section 3.3.1.
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proof = PopProve(SK)
Inputs:
- SK, a secret key in the format output by KeyGen.
Outputs:
- proof, an octet string.
Procedure:
1. PK = SkToPk(SK)
2. Q = hash_pubkey_to_point(PK)
3. R = SK * Q
4. proof = point_to_signature(R)
5. return proof
3.3.3. PopVerify
PopVerify uses several functions defined in Section 2. The
hash_pubkey_to_point function is defined in Section 3.3.1.
As an optimization, implementations MAY cache the result of PopVerify
in order to avoid unnecessarily repeating validation for known keys.
result = PopVerify(PK, proof)
Inputs:
- PK, a public key in the format output by SkToPk.
- proof, an octet string in the format output by PopProve.
Outputs:
- result, either VALID or INVALID
Procedure:
1. R = signature_to_point(proof)
2. If R is INVALID, return INVALID
3. If signature_subgroup_check(R) is INVALID, return INVALID
4. If KeyValidate(PK) is INVALID, return INVALID
5. xP = pubkey_to_point(PK)
6. Q = hash_pubkey_to_point(PK)
7. C1 = pairing(Q, xP)
8. C2 = pairing(R, P)
9. If C1 == C2, return VALID, else return INVALID
3.3.4. FastAggregateVerify
FastAggregateVerify uses several functions defined in Section 2.
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result = FastAggregateVerify((PK_1, ..., PK_n), message, signature)
Inputs:
- PK_1, ..., PK_n, public keys in the format output by SkToPk.
- message, an octet string.
- signature, an octet string output by Aggregate.
Outputs:
- result, either VALID or INVALID.
Precondition: n >= 1, otherwise return INVALID.
Procedure:
1. aggregate = pubkey_to_point(PK_1)
2. for i in 2, ..., n:
3. next = pubkey_to_point(PK_i)
4. aggregate = aggregate + next
5. PK = point_to_pubkey(aggregate)
6. return CoreVerify(PK, message, signature)
4. Ciphersuites
This section defines the format for a BLS ciphersuite. It also gives
concrete ciphersuites based on the BLS12-381 pairing-friendly
elliptic curve [I-D.irtf-cfrg-pairing-friendly-curves].
4.1. Ciphersuite format
A ciphersuite specifies all parameters from Section 2.2, a scheme
from Section 3, and any parameters the scheme requires. In
particular, a ciphersuite comprises:
* ID: the ciphersuite ID, an ASCII string. The REQUIRED format for
this string is
"BLS_SIG_" || H2C_SUITE_ID || SC_TAG || "_"
- Strings in double quotes are ASCII-encoded literals.
- H2C_SUITE_ID is the suite ID of the hash-to-curve suite used to
define the hash_to_point and hash_pubkey_to_point functions.
- SC_TAG is a string indicating the scheme and, optionally,
additional information. The first three characters of this
string MUST chosen as follows:
o "NUL" if SC is basic,
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o "AUG" if SC is message-augmentation, or
o "POP" if SC is proof-of-possession.
o Other values MUST NOT be used.
SC_TAG MAY be used to encode other information about the
ciphersuite, for example, a version number. When used in this
way, SC_TAG MUST contain only ASCII characters between 0x21 and
0x7e (inclusive), except that it MUST NOT contain underscore
(0x5f).
The RECOMMENDED way to add user-defined information to SC_TAG
is to append a colon (':', ASCII 0x3a) and then the
informational string. For example, "NUL:version=2" is an
appropriate SC_TAG value.
Note that hash-to-curve suite IDs always include a trailing
underscore, so no field separator is needed between H2C_SUITE_ID
and SC_TAG.
* SC: the scheme, one of basic, message-augmentation, or proof-of-
possession.
* SV: the signature variant, either minimal-signature-size or
minimal-pubkey-size.
* EC: a pairing-friendly elliptic curve, plus all associated
functionality (Section 1.3).
* H: a cryptographic hash function.
* hash_to_point: a hash from arbitrary strings to elliptic curve
points. hash_to_point MUST be defined in terms of a hash-to-curve
suite [I-D.irtf-cfrg-hash-to-curve].
The RECOMMENDED hash-to-curve domain separation tag is the
ciphersuite ID string defined above.
* hash_pubkey_to_point (only specified when SC is proof-of-
possession): a hash from serialized public keys to elliptic curve
points. hash_pubkey_to_point MUST be defined in terms of a hash-
to-curve suite [I-D.irtf-cfrg-hash-to-curve].
The hash-to-curve domain separation tag MUST be distinct from the
domain separation tag used for hash_to_point. It is RECOMMENDED
that the domain separation tag be constructed similarly to the
ciphersuite ID string, namely:
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"BLS_POP_" || H2C_SUITE_ID || SC_TAG || "_"
4.2. Ciphersuites for BLS12-381
The following ciphersuites are all built on the BLS12-381 elliptic
curve. The required primitives for this curve are given in
Appendix A.
These ciphersuites use the hash-to-curve suites BLS12381G1_XMD:SHA-
256_SSWU_RO_ and BLS12381G2_XMD:SHA-256_SSWU_RO_ defined in
[I-D.irtf-cfrg-hash-to-curve], Section 8.7. Each ciphersuite defines
a unique hash_to_point function by specifying a domain separation tag
(see [@I-D.irtf-cfrg-hash-to-curve, Section 3.1).
4.2.1. Basic
BLS_SIG_BLS12381G1_XMD:SHA-256_SSWU_RO_NUL_ is defined as follows:
* SC: basic
* SV: minimal-signature-size
* EC: BLS12-381, as defined in Appendix A.
* H: SHA-256
* hash_to_point: BLS12381G1_XMD:SHA-256_SSWU_RO_ with the ASCII-
encoded domain separation tag
BLS_SIG_BLS12381G1_XMD:SHA-256_SSWU_RO_NUL_
BLS_SIG_BLS12381G2_XMD:SHA-256_SSWU_RO_NUL_ is identical to
BLS_SIG_BLS12381G1_XMD:SHA-256_SSWU_RO_NUL_, except for the following
parameters:
* SV: minimal-pubkey-size
* hash_to_point: BLS12381G2_XMD:SHA-256_SSWU_RO_ with the ASCII-
encoded domain separation tag
BLS_SIG_BLS12381G2_XMD:SHA-256_SSWU_RO_NUL_
4.2.2. Message augmentation
BLS_SIG_BLS12381G1_XMD:SHA-256_SSWU_RO_AUG_ is defined as follows:
* SC: message-augmentation
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* SV: minimal-signature-size
* EC: BLS12-381, as defined in Appendix A.
* H: SHA-256
* hash_to_point: BLS12381G1_XMD:SHA-256_SSWU_RO_ with the ASCII-
encoded domain separation tag
BLS_SIG_BLS12381G1_XMD:SHA-256_SSWU_RO_AUG_
BLS_SIG_BLS12381G2_XMD:SHA-256_SSWU_RO_AUG_ is identical to
BLS_SIG_BLS12381G1_XMD:SHA-256_SSWU_RO_AUG_, except for the following
parameters:
* SV: minimal-pubkey-size
* hash_to_point: BLS12381G2_XMD:SHA-256_SSWU_RO_ with the ASCII-
encoded domain separation tag
BLS_SIG_BLS12381G2_XMD:SHA-256_SSWU_RO_AUG_
4.2.3. Proof of possession
BLS_SIG_BLS12381G1_XMD:SHA-256_SSWU_RO_POP_ is defined as follows:
* SC: proof-of-possession
* SV: minimal-signature-size
* EC: BLS12-381, as defined in Appendix A.
* H: SHA-256
* hash_to_point: BLS12381G1_XMD:SHA-256_SSWU_RO_ with the ASCII-
encoded domain separation tag
BLS_SIG_BLS12381G1_XMD:SHA-256_SSWU_RO_POP_
* hash_pubkey_to_point: BLS12381G1_XMD:SHA-256_SSWU_RO_ with the
ASCII-encoded domain separation tag
BLS_POP_BLS12381G1_XMD:SHA-256_SSWU_RO_POP_
BLS_SIG_BLS12381G2_XMD:SHA-256_SSWU_RO_POP_ is identical to
BLS_SIG_BLS12381G1_XMD:SHA-256_SSWU_RO_POP_, except for the following
parameters:
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* SV: minimal-pubkey-size
* hash_to_point: BLS12381G2_XMD:SHA-256_SSWU_RO_ with the ASCII-
encoded domain separation tag
BLS_SIG_BLS12381G2_XMD:SHA-256_SSWU_RO_POP_
* hash_pubkey_to_point: BLS12381G2_XMD:SHA-256_SSWU_RO_ with the
ASCII-encoded domain separation tag
BLS_POP_BLS12381G2_XMD:SHA-256_SSWU_RO_POP_
5. Security Considerations
5.1. Validating public keys
All algorithms in Section 2 and Section 3 that operate on public keys
require first validating those keys. For the basic and message
augmentation schemes, the use of KeyValidate is REQUIRED. For the
proof of possession scheme, each public key MUST be accompanied by a
proof of possession, and use of PopVerify is REQUIRED.
KeyValidate requires all public keys to represent valid, non-identity
points in the correct subgroup. A valid point and subgroup
membership are required to ensure that the pairing operation is
defined (Section 5.2).
A non-identity point is required because the identity public key has
the property that the corresponding secret key is equal to zero,
which means that the identity point is the unique valid signature for
every message under this key. A malicious signer could take
advantage of this fact to equivocate about which message he signed.
While non-equivocation is not a required property for a signature
scheme, equivocation is infeasible for BLS signatures under any
nonzero secret key because it would require finding colliding inputs
to the hash_to_point function, which is assumed to be collision
resistant. Prohibiting SK == 0 eliminates the exceptional case,
which may help to prevent equivocation-related security issues in
protocols that use BLS signatures.
5.2. Skipping membership check
Some existing implementations skip the signature_subgroup_check
invocation in CoreVerify (Section 2.7), whose purpose is ensuring
that the signature is an element of a prime-order subgroup. This
check is REQUIRED of conforming implementations, for two reasons.
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1. For most pairing-friendly elliptic curves used in practice, the
pairing operation e (Section 1.3) is undefined when its input
points are not in the prime-order subgroups of E1 and E2. The
resulting behavior is unpredictable, and may enable forgeries.
2. Even if the pairing operation behaves properly on inputs that are
outside the correct subgroups, skipping the subgroup check breaks
the strong unforgeability property [ADR02].
5.3. Side channel attacks
Implementations of the signing algorithm SHOULD protect the secret
key from side-channel attacks. One method for protecting against
certain side-channel attacks is ensuring that the implementation
executes exactly the same sequence of instructions and performs
exactly the same memory accesses, for any value of the secret key.
In other words, implementations on the underlying pairing-friendly
elliptic curve SHOULD run in constant time.
5.4. Randomness considerations
BLS signatures are deterministic. This protects against attacks
arising from signing with bad randomness, for example, the nonce
reuse attack on ECDSA [HDWH12].
As discussed in Section 2.3, the IKM input to KeyGen MUST be
infeasible to guess and MUST be kept secret. One possibility is to
generate IKM from a trusted source of randomness. Guidelines on
constructing such a source are outside the scope of this document.
5.5. Implementing hash_to_point and hash_pubkey_to_point
The security analysis models hash_to_point and hash_pubkey_to_point
as random oracles. It is crucial that these functions are
implemented using a cryptographically secure hash function. For this
purpose, implementations MUST meet the requirements of
[I-D.irtf-cfrg-hash-to-curve].
In addition, ciphersuites MUST specify unique domain separation tags
for hash_to_point and hash_pubkey_to_point. The domain separation
tag format used in Section 4 is the RECOMMENDED one.
6. Implementation Status
This section will be removed in the final version of the draft.
There are currently several implementations of BLS signatures using
the BLS12-381 curve.
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* Algorand: bls_sigs_ref (https://github.com/kwantam/bls_sigs_ref).
* Chia: spec (https://github.com/Chia-Network/bls-
signatures/blob/master/SPEC.md) python/C++ (https://github.com/
Chia-Network/bls-signatures). Here, they are swapping G1 and G2
so that the public keys are small, and the benefits of avoiding a
membership check during signature verification would even be more
substantial. The current implementation does not seem to
implement the membership check. Chia uses the Fouque-Tibouchi
hashing to the curve, which can be done in constant time.
* Dfinity: go (https://github.com/dfinity/go-dfinity-crypto) BLS
(https://github.com/dfinity/bls). The current implementations do
not seem to implement the membership check.
* Ethereum 2.0: spec (https://github.com/ethereum/eth2.0-
specs/blob/master/specs/bls_signature.md).
7. Related Standards
* Pairing-friendly curves, [I-D.irtf-cfrg-pairing-friendly-curves]
* Pairing-based Identity-Based Encryption IEEE 1363.3
(https://ieeexplore.ieee.org/document/6662370).
* Identity-Based Cryptography Standard rfc5901
(https://tools.ietf.org/html/rfc5091).
* Hashing to Elliptic Curves [I-D.irtf-cfrg-hash-to-curve], in order
to implement the hash function hash_to_point.
* EdDSA rfc8032 (https://tools.ietf.org/html/rfc8032).
8. IANA Considerations
TBD (consider to register ciphersuite identifiers for BLS signature
and underlying pairing curves)
9. Normative References
[ZCash] Electric Coin Company, "BLS12-381", July 2017,
.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
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10. Informative References
[BGLS03] Boneh, D., Gentry, C., Lynn, B., and H. Shacham,
"Aggregate and verifiably encrypted signatures from
bilinear maps", May 2003, .
[BNN07] Bellare, M., Namprempre, C., and G. Neven, "Unrestricted
aggregate signatures", July 2007,
.
[RY07] Ristenpart, T. and S. Yilek, "The Power of Proofs-of-
Possession: Securing Multiparty Signatures against Rogue-
Key Attacks", May 2007, .
[HDWH12] Heninger, N., Durumeric, Z., Wustrow, E., and J.A.
Halderman, "Mining your Ps and Qs: Detection of widespread
weak keys in network devices", August 2012,
.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
.
[ADR02] An, J. H., Dodis, Y., and T. Rabin, "On the Security of
Joint Signature and Encryption", April 2002,
.
[LOSSW06] Lu, S., Ostrovsky, R., Sahai, A., Shacham, H., and B.
Waters, "Sequential Aggregate Signatures and
Multisignatures Without Random Oracles", May 2006,
.
[Bowe19] Bowe, S., "Faster subgroup checks for BLS12-381", July
2019, .
[I-D.irtf-cfrg-hash-to-curve]
Faz-Hernandez, A., Scott, S., Sullivan, N., Wahby, R., and
C. Wood, "Hashing to Elliptic Curves", Work in Progress,
Internet-Draft, draft-irtf-cfrg-hash-to-curve-09, 29 June
2020, .
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[RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
"PKCS #1: RSA Cryptography Specifications Version 2.2",
RFC 8017, DOI 10.17487/RFC8017, November 2016,
.
[FIPS180-4]
National Institute of Standards and Technology (NIST),
"FIPS Publication 180-4: Secure Hash Standard", August
2015, .
[BLS01] Boneh, D., Lynn, B., and H. Shacham, "Short signatures
from the Weil pairing", December 2001,
.
[Bol03] Boldyreva, A., "Threshold Signatures, Multisignatures and
Blind Signatures Based on the Gap-Diffie-Hellman-Group
Signature Scheme", January 2003,
.
[BDN18] Boneh, D., Drijvers, M., and G. Neven, "Compact multi-
signatures for shorter blockchains", December 2018,
.
[I-D.irtf-cfrg-pairing-friendly-curves]
Sakemi, Y., Kobayashi, T., Saito, T., and R. Wahby,
"Pairing-Friendly Curves", Work in Progress, Internet-
Draft, draft-irtf-cfrg-pairing-friendly-curves-07, 18 June
2020, .
Appendix A. BLS12-381
The ciphersuites in Section 4 are based upon the BLS12-381 pairing-
friendly elliptic curve. The following defines the correspondence
between the primitives in Section 1.3 and the parameters given in
Section 4.2.2 of [I-D.irtf-cfrg-pairing-friendly-curves].
* E1, G1: the curve E and its order-r subgroup.
* E2, G2: the curve E' and its order-r subgroup.
* GT: the subgroup G_T.
* P1: the point BP.
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* P2: the point BP'.
* e: the optimal Ate pairing defined in Appendix A of
[I-D.irtf-cfrg-pairing-friendly-curves].
* point_to_octets and octets_to_point use the compressed
serialization formats for E1 and E2 defined by [ZCash].
* subgroup_check MAY use either the naive check described in
Section 1.3 or the optimized check given by [Bowe19].
Appendix B. Test Vectors
TBA: (i) test vectors for both variants of the signature scheme
(signatures in G2 instead of G1) , (ii) test vectors ensuring
membership checks, (iii) intermediate computations ctr, hm.
Appendix C. Security analyses
The security properties of the BLS signature scheme are proved in
[BLS01].
[BGLS03] prove the security of aggregate signatures over distinct
messages, as in the basic scheme of Section 3.1.
[BNN07] prove security of the message augmentation scheme of
Section 3.2.
[Bol03][LOSSW06][RY07] prove security of constructions related to the
proof of possession scheme of Section 3.3.
[BDN18] prove the security of another rogue key defense; this defense
is not standardized in this document.
Authors' Addresses
Dan Boneh
Stanford University
United States of America
Email: dabo@cs.stanford.edu
Sergey Gorbunov
University of Waterloo
Waterloo, ON
Canada
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Email: sgorbunov@uwaterloo.ca
Riad S. Wahby
Stanford University
United States of America
Email: rsw@cs.stanford.edu
Hoeteck Wee
NTT Research and ENS, Paris
Boston, MA,
United States of America
Email: wee@di.ens.fr
Zhenfei Zhang
Algorand
Boston, MA,
United States of America
Email: zhenfei@algorand.com
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