/// <summary>
/// Compute an EC Diffie-Hellman secret in constant time.
/// </summary>
/// <param name="resultOutput">A 32-byte array which will be populated by an ECDH secret computed from the point and scalar.</param>
/// <param name="publicKey">A secp256k1_pubkey containing an initialized public key.</param>
/// <param name="privateKey">A 32-byte scalar with which to multiply the point.</param>
/// <param name="hashFunction">Pointer to a hash function. If null, sha256 is used.</param>
/// <param name="data">Arbitrary data that is passed through.</param>
/// <returns>True if exponentiation was successful, false if scalar was invalid (zero or overflow).</returns>
public bool Ecdh(Span <byte> resultOutput, Span <byte> publicKey, Span <byte> privateKey, EcdhHashFunction hashFunction, IntPtr data)
{
if (resultOutput.Length < SECRET_LENGTH)
{
throw new ArgumentException($"{nameof(resultOutput)} must be {SECRET_LENGTH} bytes");
}
if (publicKey.Length < PUBKEY_LENGTH)
{
throw new ArgumentException($"{nameof(publicKey)} must be {PUBKEY_LENGTH} bytes");
}
if (privateKey.Length < PRIVKEY_LENGTH)
{
throw new ArgumentException($"{nameof(privateKey)} must be {PRIVKEY_LENGTH} bytes");
}
int outputLength = resultOutput.Length;
secp256k1_ecdh_hash_function hashFunctionPtr = (void *output, void *x, void *y, IntPtr d) =>
{
var outputSpan = new Span <byte>(output, outputLength);
var xSpan = new Span <byte>(x, 32);
var ySpan = new Span <byte>(y, 32);
return(hashFunction(outputSpan, xSpan, ySpan, d));
};
GCHandle gch = GCHandle.Alloc(hashFunctionPtr);
try
{
var fp = Marshal.GetFunctionPointerForDelegate(hashFunctionPtr);
fixed(byte *resPtr = &MemoryMarshal.GetReference(resultOutput),
pubPtr = &MemoryMarshal.GetReference(publicKey),
privPtr = &MemoryMarshal.GetReference(privateKey))
{
return(secp256k1_ecdh.Value(_ctx, resPtr, pubPtr, privPtr, fp, data) == 1);
}
}
finally
{
gch.Free();
}
}
/// <summary>
/// Compute an EC Diffie-Hellman secret in constant time.
/// </summary>
/// <param name="resultOutput">A 32-byte array which will be populated by an ECDH secret computed from the point and scalar.</param>
/// <param name="publicKey">A secp256k1_pubkey containing an initialized public key.</param>
/// <param name="privateKey">A 32-byte scalar with which to multiply the point.</param>
/// <param name="hashFunction">Pointer to a hash function. If null, sha256 is used.</param>
/// <param name="data">Arbitrary data that is passed through.</param>
/// <returns>True if exponentiation was successful, false if scalar was invalid (zero or overflow).</returns>
public bool Ecdh(ReadOnlySpan <byte> publicKey, ReadOnlySpan <byte> privateKey, EcdhHashFunction hashFunction, IntPtr data, out byte[] resultOutput)
{
resultOutput = new byte[SECRET_LENGTH];
return(Ecdh(publicKey, privateKey, hashFunction, data, resultOutput));
}
