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Specification for BLS Signatures over BLS12-381 v1

Initial draft: Apr 29, 2019. Current verison: May 1, 2019.

This is an initial specification for BLS signatures. The objective is to provide a specification which enable consistent implementations with respect to low-level encoding and algorithmic choices. Note that it does not cover aggregation or protection against rogue key attacks.

Preliminaries

Hash to curve

We follow WB19 paper, implementation. We will rely on the following subroutines:

  • hash_to_field is a generic construction that uses a cryptographic hash function to output field elements:
hash_to_field(msg, ctr, p, m, hash_fn, hash_reps)

Parameters:
  - msg is an octet string to be hashed.
  - ctr is an integer < 2^8 used to orthogonalize hash functions
  - p and m specify the field as GF(p^m)
  - hash_fn is a hash function, e.g., SHA256
  - hash_reps is the number of concatenated hash outputs
    used to produce an element of F_p

hash_to_field(msg, ctr, p, m, hash_fn, hash_reps) :=
    msg' = hash_fn(msg) || I2OSP(ctr, 1)
    for i in (1, ..., m):
        t = ""  // initialize to the empty string
        for j in (1, ..., hash_reps):
            t = t || hash_fn( msg' || I2OSP(i, 1) || I2OSP(j, 1) )
        e_i = OS2IP(t) mod p
    return (e_1, ..., e_m)

In the above, an element of the extension field GF(p^m) is represented by a vector V of elements of GF(p). GF(p^m) defines a primitive element α, and the vector V represents the element determined by the inner product of V with the vector (α^0, ..., α^{m-1}). For BLS12-381 G2, α = sqrt(-1).

  • Using the above, define Hp and Hp2 as:

    Hp(msg, ctr)  := hash_to_field(msg, ctr, p, 1, SHA256, 2)
    
    Hp2(msg, ctr) := hash_to_field(msg, ctr, p, 2, SHA256, 2)
    
  • Map1 and Map2 are the maps given in Section 4 of WB19.

  • hashtoG1(msg in {0,1}*) is construction #2 in WB19, instantiated with Hp (msg, 0) and Hp (msg, 1). In particular,

      hashtoG1(msg) := (Map1(Hp(msg, 0)) * Map1(Hp(msg, 1)))^{1-z}
    
  • hashtoG2(msg in {0,1}*) is construction #5 in WB19, instantiated with Hp2 (msg, 0) and Hp2 (msg, 1).

Basic signature in G1

  • key generation:

    • sk = x is 32 octets (256 bits)
    • compute x' = hash_to_field(x, 0, r, 1, SHA256, 2)
    • pk := x' * [P2]
  • sign(sk, msg in {0,1}*, ciphersuite in {0,1}^8)

    • derive x' from sk as in key generation
    • H = hashtoG1(ciphersuite || msg)
    • output x' * H

Basic signature in G2

As before, replace P2, hashtoG1 with P1, hashtoG2

TODOs

  • Specify how to represent curve points as octet strings.

  • Specify a variant where we sign the concatenation of the public key and the message. Here, the public key has a fixed length (which is determined by the ciphersuite), and we need to fix a representation of the public key as an octet string.

  • Generate test vectors for ciphersuite being all-zeroes.

  • To add a ciphersuite "look up" table. The ciphersuite string will tell us

    • which curve to use
    • whether signatures sit in G1 or in G2
    • which encoding algorithm to use, e.g. WB19 vs FT12
    • which data hashing algorithm to use, namely SHA256 vs SHA
    • whether and how we clear the co-factor
    • possibly which mechanism is used to prevent rogue-key attacks (message augmentation vs proof of posession)

    As a place-holder, we will use 0x01 for signatures in G1, 0x02 for signatures in G2.

  • To decide if we want separate ciphersuite strings for the signature scheme and hash-to-curve. Given that hash-to-curve is only used as an intermediate building, our motivating principle here is that only the final application (e.g. signatures or VRFs) should provide the ciphersuite string.

Design Rationale

  • We hash the randomness during key generation to mitigate any attacks arising from weak sources of randomness. This was also done in EdDSA spec.

  • The specification uses SHA256 as used in many existing implementations. If we decide to support SHA512 later, the change should be straight-forward.

  • For hashing to curves, we use indifferentiable hashing in order to be "future-proof", even though a weaker security notion (with a slightly more efficient instantiation) suffices for security for BLS signatures.

  • There will be no explicit pre-hash mode. If the signature algorithm gets as input the hash H(M) of a huge message, then we should think of this as signing H(M), and not signing M, pre-hashed. This has the advantage of allowing the application to use a different data hash algorithm.

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