draft-irtf-cfrg-bls-signature-04.txt

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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 14 March 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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   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 Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   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,
              <https://github.com/zkcrypto/pairing/blob/master/src/
              bls12_381/README.md#serialization>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.



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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, <https://link.springer.com/
              chapter/10.1007%2F3-540-39200-9_26>.

   [BNN07]    Bellare, M., Namprempre, C., and G. Neven, "Unrestricted
              aggregate signatures", July 2007,
              <https://link.springer.com/
              chapter/10.1007%2F978-3-540-73420-8_37>.

   [RY07]     Ristenpart, T. and S. Yilek, "The Power of Proofs-of-
              Possession: Securing Multiparty Signatures against Rogue-
              Key Attacks", May 2007, <https://link.springer.com/
              chapter/10.1007%2F978-3-540-72540-4_13>.

   [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,
              <https://www.usenix.org/system/files/conference/
              usenixsecurity12/sec12-final228.pdf>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <https://www.rfc-editor.org/info/rfc5869>.

   [ADR02]    An, J. H., Dodis, Y., and T. Rabin, "On the Security of
              Joint Signature and Encryption", April 2002,
              <https://doi.org/10.1007/3-540-46035-7_6>.

   [LOSSW06]  Lu, S., Ostrovsky, R., Sahai, A., Shacham, H., and B.
              Waters, "Sequential Aggregate Signatures and
              Multisignatures Without Random Oracles", May 2006,
              <https://link.springer.com/chapter/10.1007/11761679_28>.

   [Bowe19]   Bowe, S., "Faster subgroup checks for BLS12-381", July
              2019, <https://eprint.iacr.org/2019/814>.

   [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, <https://tools.ietf.org/html/draft-irtf-cfrg-hash-
              to-curve-09>.





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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,
              <https://www.rfc-editor.org/info/rfc8017>.

   [FIPS180-4]
              National Institute of Standards and Technology (NIST),
              "FIPS Publication 180-4: Secure Hash Standard", August
              2015, <https://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.180-4.pdf>.

   [BLS01]    Boneh, D., Lynn, B., and H. Shacham, "Short signatures
              from the Weil pairing", December 2001,
              <https://www.iacr.org/archive/asiacrypt2001/22480516.pdf>.

   [Bol03]    Boldyreva, A., "Threshold Signatures, Multisignatures and
              Blind Signatures Based on the Gap-Diffie-Hellman-Group
              Signature Scheme", January 2003,
              <https://link.springer.com/
              chapter/10.1007%2F3-540-36288-6_3>.

   [BDN18]    Boneh, D., Drijvers, M., and G. Neven, "Compact multi-
              signatures for shorter blockchains", December 2018,
              <https://link.springer.com/
              chapter/10.1007/978-3-030-03329-3_15>.

   [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, <https://tools.ietf.org/html/draft-irtf-cfrg-
              pairing-friendly-curves-07>.

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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