Add support for encoding/decoding compressed EC keys #57
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huskcasaca
commented
Dec 13, 2024
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| internal fun ECParameterSpec.decodePoint(bytes: ByteArray): ECPoint { | ||
| val fieldSize = curveOrderSize() | ||
| return when (bytes[0].toInt()) { | ||
| 0x02, // compressed evenY | ||
| 0x03, // compressed oddY | ||
| -> { | ||
| check(bytes.size == fieldSize + 1) { "Wrong compressed key size ${bytes.size}" } | ||
| val p = (curve.field as ECFieldFp).p | ||
| val a = curve.a | ||
| val b = curve.b | ||
| val x = BigInteger(1, bytes.copyOfRange(1, bytes.size)) | ||
| var y = x.multiply(x).add(a).multiply(x).add(b).mod(p).modSqrt(p) | ||
| if (y.testBit(0) != (bytes[0].toInt() == 0x03)) { | ||
| y = p.subtract(y) | ||
| } | ||
| ECPoint(x, y) | ||
| } | ||
| 0x04, // uncompressed | ||
| -> { | ||
| check(bytes.size == fieldSize * 2 + 1) { "Wrong uncompressed key size ${bytes.size}" } | ||
| val x = bytes.copyOfRange(1, fieldSize + 1) | ||
| val y = bytes.copyOfRange(fieldSize + 1, fieldSize + 1 + fieldSize) | ||
| ECPoint(BigInteger(1, x), BigInteger(1, y)) | ||
| } | ||
| else -> error("Unsupported key type ${bytes[0].toInt()}") | ||
| } | ||
| } | ||
|
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||
| internal fun BigInteger.modSqrt(p: BigInteger): BigInteger { | ||
| check(p.testBit(0) && p.testBit(1)) { "Unsupported curve modulus" } // p ≡ 3 (mod 4) | ||
| return modPow(p.add(BigInteger.ONE).shiftRight(2), p) // Tonelli-Shanks | ||
| } | ||
|
|
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BC is added as a provider without some API access so it requires manually calculating y.
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This PR add support for
CompressedandUncompressed(default) format underRAWEC Public Key.