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BLS12-381 serialization: fix edge case

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Mamy Ratsimbazafy 2023-10-10 21:49:06 +02:00
parent 6489053da9
commit 4dd0a02f1a
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1 changed files with 5 additions and 3 deletions
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constantine/serialization/codecs_bls12_381.nim
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@ -129,9 +129,11 @@ func serialize_g1_compressed*(dst: var array[48, byte], g1Point: G1P): CttCodecE
# The curve equation has 2 solutions for y² = x³ + 4 with y unknown and x known
# The lexicographically largest will have bit 381 set to 1
# (and bit 383 for the compressed representation)
# The solutions are {y, p-y} hence the lexicographyically largest is greater than p/2
# so with exact integers, as p is odd, greater or equal (p+1)/2
let lexicographicallyLargest = byte(g1Point.y.toBig() >= Fp[BLS12_381].getPrimePlus1div2())
# The solutions are {y, p-y}.
# The field contains [0, p-1] hence lexicographically largest
# are numbers greater or equal (p-1)/2
# https://github.com/zkcrypto/bls12_381/blob/0.7.0/src/fp.rs#L271-L277
let lexicographicallyLargest = byte(g1Point.y.toBig() >= Fp[BLS12_381].getPrimeMinus1div2())
dst[0] = dst[0] or (0b10000000 or (lexicographicallyLargest shl 5))
return cttCodecEcc_Success