Merge pull request #266 from ethereum/JustinDrake-patch-10
First pass cleanup of bls_verify.md
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Danny Ryan
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### BLS Verification
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# BLS signature verification
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**Warning: This document is pending academic review and should not yet be considered secure.**
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See https://z.cash/blog/new-snark-curve/ for BLS-12-381 parameters. `q` is the field modulus.
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## Table of contents
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<!-- TOC -->
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We represent coordinates as defined in https://github.com/zkcrypto/pairing/tree/master/src/bls12_381/.
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- [BLS signature verification](#bls-signature-verification)
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- [Table of contents](#table-of-contents)
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- [Curve parameters](#curve-parameters)
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- [Point representations](#point-representations)
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- [G1 points](#g1-points)
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- [G2 points](#g2-points)
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- [Helpers](#helpers)
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- [`hash_to_G2`](#hash_to_g2)
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- [`modular_squareroot`](#modular_squareroot)
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- [Signature verification](#signature-verification)
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- [`bls_verify`](#bls_verify)
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- [`bls_verify_multiple`](#bls_verify_multiple)
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Specifically, a point in G1 as a 384-bit integer `z`, which we decompose into:
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<!-- /TOC -->
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* `x = z % 2**381` (must be `< q`)
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* `highflag = z // 2**382`
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* `lowflag = (z % 2**382) // 2**381`
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## Curve parameters
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If `highflag == 3`, the point is the point at infinity and we require `lowflag = x = 0`. Otherwise, we require `highflag == 2`, in which case the point is `(x, y)` where `y` is the valid coordinate such that `(y * 2) // q == lowflag`.
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The BLS12-381 curve parameters are defined [here](https://z.cash/blog/new-snark-curve).
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We represent a point in G2 as a pair of 384-bit integers `(z1, z2)` that are each decomposed into `x1`, `highflag1`, `lowflag1`, `x2`, `highflag2`, `lowflag2` as above, where `x1` and `x2` must both be `< q`. We require `lowflag2 == highflag2 == 0`. If `highflag1 == 3`, the point is the point at infinity and we require `lowflag1 == x1 == x2 == 0`. Otherwise, we require `highflag == 2`, in which case the point is `(x1 * i + x2, y)` where `y` is the valid coordinate such that the imaginary part of `y` satisfies `(y_im * 2) // q == lowflag1`.
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## Point representations
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`BLSVerify(pubkey: uint384, msg: bytes32, sig: [uint384], domain: uint64)` is done as follows:
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We represent points in the groups G1 and G2 following [zkcrypto/pairing](https://github.com/zkcrypto/pairing/tree/master/src/bls12_381). We denote by `q` the field modulus and by `i` the imaginary unit.
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* Verify that `pubkey` is a valid G1 point and `sig` is a valid G2 point.
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* Convert `msg` to a G2 point using `hash_to_G2` defined below.
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* Do the pairing check: verify `e(pubkey, hash_to_G2(msg, domain)) == e(G1, sig)` (where `e` is the BLS pairing function)
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### G1 points
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Here is the `hash_to_G2` definition:
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A point in G1 is represented as a 384-bit integer `z` decomposed as a 381-bit integer `x` and three 1-bit flags in the top bits:
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* `x = z % 2**381`
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* `a_flag = (z % 2**382) // 2**381`
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* `b_flag = (z % 2**383) // 2**382`
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* `c_flag = (z % 2**384) // 2**383`
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Respecting bit ordering, `z` is decomposed as `(c_flag, b_flag, a_flag, x)`.
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We require:
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* `x < q`
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* `c_flag == 1`
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* if `b_flag == 1` then `a_flag == x == 0` and `z` represents the point at infinity
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* if `b_flag == 0` then `z` represents the point `(x, y)` where `y` is the valid coordinate such that `(y * 2) // q == a_flag`
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### G2 points
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A point in G2 is represented as a pair of 384-bit integers `(z1, z2)`. We decompose `z1` as above into `x1`, `a_flag1`, `b_flag1`, `c_flag1` and `z2` into `x2`, `a_flag2`, `b_flag2`, `c_flag2`.
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We require:
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* `x1 < q` and `x2 < q`
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* `a_flag2 == b_flag2 == c_flag2 == 0`
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* `c_flag1 == 1`
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* if `b_flag1 == 1` then `a_flag1 == x1 == x2 == 0` and `(z1, z2)` represents the point at infinity
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* if `b_flag1 == 0` then `(z1, z2)` represents the point `(x1 * i + x2, y)` where `y` is the valid coordinate such that the imaginary part `y_im` of `y` satisfies `(y_im * 2) // q == a_flag1`
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## Helpers
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### `hash_to_G2`
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```python
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G2_cofactor = 305502333931268344200999753193121504214466019254188142667664032982267604182971884026507427359259977847832272839041616661285803823378372096355777062779109
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field_modulus = 4002409555221667393417789825735904156556882819939007885332058136124031650490837864442687629129015664037894272559787
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q = 4002409555221667393417789825735904156556882819939007885332058136124031650490837864442687629129015664037894272559787
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def hash_to_G2(m, domain):
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x1 = hash(bytes8(domain) + b'\x01' + m)
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x2 = hash(bytes8(domain) + b'\x02' + m)
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x_coord = FQ2([x1, x2]) # x1 + x2 * i
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def hash_to_G2(message: bytes32, domain: uint64) -> [uint384]:
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# Initial candidate x coordinate
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x_re = int.from_bytes(hash(bytes8(domain) + b'\x01' + message), 'big')
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x_im = int.from_bytes(hash(bytes8(domain) + b'\x02' + message), 'big')
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x_coordinate = Fq2([x_re, x_im]) # x = x_re + i * x_im
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# Test candidate y coordinates until a one is found
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while 1:
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x_cubed_plus_b2 = x_coord ** 3 + FQ2([4,4])
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y_coord = mod_sqrt(x_cubed_plus_b2)
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if y_coord is not None:
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break
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x_coord += FQ2([1, 0]) # Add one until we get a quadratic residue
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assert is_on_curve((x_coord, y_coord))
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return multiply((x_coord, y_coord), G2_cofactor)
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y_coordinate_squared = x_coordinate ** 3 + Fq2([4, 4]) # The curve is y^2 = x^3 + 4(i + 1)
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y_coordinate = modular_squareroot(y_coordinate_squared)
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if y_coordinate is not None: # Check if quadratic residue found
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return multiply_in_G2((x_coordinate, y_coordinate), G2_cofactor)
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x_coordinate += Fq2([1, 0]) # Add 1 and try again
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```
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Here is a sample implementation of `mod_sqrt`:
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### `modular_squareroot`
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`modular_squareroot(x)` returns a solution `y` to `y**2 % q == x`, and `None` if none exists. If there are two solutions the one with higher imaginary component is favored; if both solutions have equal imaginary component the one with higher real component is favored.
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```python
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qmod = field_modulus ** 2 - 1
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eighth_roots_of_unity = [FQ2([1,1]) ** ((qmod * k) // 8) for k in range(8)]
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Fq2_order = q ** 2 - 1
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eighth_roots_of_unity = [Fq2([1,1]) ** ((Fq2_order * k) // 8) for k in range(8)]
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def mod_sqrt(val):
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candidate_sqrt = val ** ((qmod + 8) // 16)
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check = candidate_sqrt ** 2 / val
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def modular_squareroot(value: int) -> int:
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candidate_squareroot = value ** ((Fq2_order + 8) // 16)
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check = candidate_squareroot ** 2 / value
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if check in eighth_roots_of_unity[::2]:
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return candidate_sqrt / eighth_roots_of_unity[eighth_roots_of_unity.index(check) // 2]
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x1 = candidate_squareroot / eighth_roots_of_unity[eighth_roots_of_unity.index(check) // 2]
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x2 = -x1
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return x1 if (x1.coeffs[1].n, x1.coeffs[0].n) > (x2.coeffs[1].n, x2.coeffs[0].n) else x2
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return None
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```
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`BLSMultiVerify(pubkeys: [uint384], msgs: [bytes32], sig: [uint384], domain: uint64)` is done as follows:
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## Signature verification
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* Verify that each element of `pubkeys` is a valid G1 point and `sig` is a valid G2 point.
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* Convert each element of `msg` to a G2 point using `hash_to_G2` defined above, using the specified `domain`.
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* Check that the length of `pubkeys` and `msgs` is the same, call the length `L`
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* Do the pairing check: verify `e(pubkeys[0], hash_to_G2(msgs[0], domain)) * ... * e(pubkeys[L-1], hash_to_G2(msgs[L-1], domain)) == e(G1, sig)`
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In the following `e` is the pairing function and `g` is the G1 generator with the following coordinates (see [here](https://github.com/zkcrypto/pairing/tree/master/src/bls12_381#g1)):
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```python
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g_x = 3685416753713387016781088315183077757961620795782546409894578378688607592378376318836054947676345821548104185464507
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g_y = 1339506544944476473020471379941921221584933875938349620426543736416511423956333506472724655353366534992391756441569
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g = Fq2(g_x, g_y)
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```
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### `bls_verify`
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Let `bls_verify(pubkey: uint384, message: bytes32, signature: [uint384], domain: uint64) -> bool`:
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* Verify that `pubkey` is a valid G1 point.
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* Verify that `signature` is a valid G2 point.
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* Verify that `e(pubkey, hash_to_G2(message, domain)) == e(g, signature)`.
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### `bls_verify_multiple`
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Let `bls_verify_multiple(pubkeys: [uint384], messages: [bytes32], signature: [uint384], domain: uint64) -> bool`:
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* Verify that each `pubkey` in `pubkeys` is a valid G1 point.
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* Verify that `signature` is a valid G2 point.
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* Verify that `len(pubkeys)` equals `len(messages)` and denote the length `L`.
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* Verify that `e(pubkeys[0], hash_to_G2(messages[0], domain)) * ... * e(pubkeys[L-1], hash_to_G2(messages[L-1], domain)) == e(g, signature)`.
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