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Ed25519: Add key-blinding function #18

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Ed25519: Add key-blinding function #18

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24 changes: 24 additions & 0 deletions Chaos.NaCl/Ed25519.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -143,5 +143,29 @@ public static void KeyExchange(ArraySegment<byte> sharedKey, ArraySegment<byte>
FieldOperations.fe_tobytes(sharedKey.Array, sharedKey.Offset, ref sharedMontgomeryX);
MontgomeryCurve25519.KeyExchangeOutputHashNaCl(sharedKey.Array, sharedKey.Offset);
}

public static bool CalculateBlindedPublicKey(byte[] publicKey, byte[] blindingFator, out byte[] output)
{
if (publicKey is null)
throw new ArgumentNullException("publicKey.Array");
if (publicKey.Length != PublicKeySizeInBytes)
throw new ArgumentException("publicKey.Count != 32");

output = new byte[PublicKeySizeInBytes];

byte[] zeros = new byte[PublicKeySizeInBytes];
byte[] pkCopy = new byte[PublicKeySizeInBytes];
Array.Copy(publicKey, pkCopy, PublicKeySizeInBytes);
pkCopy[31] ^= (1 << 7);

if (GroupOperations.ge_frombytes_negate_vartime(out var A, pkCopy, 0) != 0)
return false;

/* There isn't a regular ge_scalarmult -- we have to do tweak*A + zero*B. */
GroupOperations.ge_double_scalarmult_vartime(out var Aprime, blindingFator, ref A, zeros);
GroupOperations.ge_tobytes(output,0, ref Aprime);

return true;
}
}
}