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// Copyright (C) 2023 Vaughn Nugent
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as
// published by the Free Software Foundation, either version 3 of the
// License, or (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program. If not, see <https://www.gnu.org/licenses/>.
using System;
using System.Security.Cryptography;
using System.Runtime.InteropServices;
using VNLib.Hashing;
using VNLib.Utils;
using VNLib.Utils.Memory;
using VNLib.Utils.Extensions;
using static NVault.Crypto.Secp256k1.LibSecp256k1;
namespace NVault.Crypto.Secp256k1
{
public static unsafe class ContextExtensions
{
/// <summary>
/// Creates a new <see cref="Secp256k1Context"/> from the current managed library
/// </summary>
/// <param name="Lib"></param>
/// <returns>The new <see cref="Secp256k1Context"/> object from the library</returns>
/// <exception cref="CryptographicException"></exception>
public static Secp256k1Context CreateContext(this LibSecp256k1 Lib)
{
//Protect for released lib
Lib.SafeLibHandle.ThrowIfClosed();
//Create new context
IntPtr context = Lib._create(1);
if (context == IntPtr.Zero)
{
throw new CryptographicException("Failed to create the new Secp256k1 context");
}
return new Secp256k1Context(Lib, context);
}
/// <summary>
/// Signs a 32byte message digest with the specified secret key on the current context and writes the signature to the specified buffer
/// </summary>
/// <param name="context"></param>
/// <param name="secretKey">The 32byte secret key used to sign messages from the user</param>
/// <param name="digest">The 32byte message digest to compute the signature of</param>
/// <param name="signature">The buffer to write the signature output to, must be at-least 64 bytes</param>
/// <returns>The number of bytes written to the signature buffer, or less than 1 if the operation failed</returns>
public static ERRNO SchnorSignDigest(this in Secp256k1Context context, ReadOnlySpan<byte> secretKey, ReadOnlySpan<byte> digest, Span<byte> signature)
{
//Check the signature buffer size
if (signature.Length < SignatureSize)
{
return ERRNO.E_FAIL;
}
//Message digest must be exactly 32 bytes long
if (digest.Length != (int)HashAlg.SHA256)
{
return ERRNO.E_FAIL;
}
//Stack allocated keypair
KeyPair keyPair = new();
//Randomize the context and create the keypair
if (!context.CreateKeyPair(&keyPair, secretKey))
{
return ERRNO.E_FAIL;
}
//Create the random nonce
byte* random = stackalloc byte[RandomBufferSize];
//Fill the buffer with random bytes
context.Lib.GetRandomBytes(new Span<byte>(random, RandomBufferSize));
try
{
fixed (byte* sigPtr = &MemoryMarshal.GetReference(signature),
digestPtr = &MemoryMarshal.GetReference(digest))
{
//Sign the message hash and write the output to the signature buffer
if (context.Lib._signHash(context.Context, sigPtr, digestPtr, &keyPair, random) != 1)
{
return ERRNO.E_FAIL;
}
}
}
finally
{
//Erase entropy
MemoryUtil.InitializeBlock(random, RandomBufferSize);
//Clear the keypair, contains the secret key, even if its stack allocated
MemoryUtil.ZeroStruct(&keyPair);
}
//Signature size is always 64 bytes
return SignatureSize;
}
/// <summary>
/// Generates an x-only Schnor encoded public key from the specified secret key on the
/// current context and writes it to the specified buffer.
/// </summary>
/// <param name="context"></param>
/// <param name="secretKey">The 32byte secret key used to derrive the public key from</param>
/// <param name="pubKeyBuffer">The buffer to write the x-only Schnor encoded public key</param>
/// <returns>The number of bytes written to the output buffer, or 0 if the operation failed</returns>
/// <exception cref="CryptographicException"></exception>
public static ERRNO GeneratePubKeyFromSecret(this in Secp256k1Context context, ReadOnlySpan<byte> secretKey, Span<byte> pubKeyBuffer)
{
if (secretKey.Length != SecretKeySize)
{
throw new CryptographicException($"Your secret key must be exactly {SecretKeySize} bytes long");
}
if (pubKeyBuffer.Length < XOnlyPublicKeySize)
{
throw new CryptographicException($"Your public key buffer must be at least {XOnlyPublicKeySize} bytes long");
}
//Protect for released lib
context.Lib.SafeLibHandle.ThrowIfClosed();
//Stack allocated keypair and x-only public key
XOnlyPubKey xOnlyPubKey = new();
KeyPair keyPair = new();
try
{
//Init context and keypair
if (!context.CreateKeyPair(&keyPair, secretKey))
{
return ERRNO.E_FAIL;
}
//X-only public key from the keypair
if (context.Lib._createXonly(context.Context, &xOnlyPubKey, 0, &keyPair) != 1)
{
return ERRNO.E_FAIL;
}
fixed (byte* pubBuffer = &MemoryMarshal.GetReference(pubKeyBuffer))
{
//Serialize the public key to the buffer as an X-only public key without leading status byte
if (context.Lib._serializeXonly(context.Context, pubBuffer, &xOnlyPubKey) != 1)
{
return ERRNO.E_FAIL;
}
}
}
finally
{
//Clear the keypair, contains the secret key, even if its stack allocated
MemoryUtil.ZeroStruct(&keyPair);
}
//PubKey length is constant
return XOnlyPublicKeySize;
}
/// <summary>
/// Verifies that a given secret key is valid using the current context
/// </summary>
/// <param name="context"></param>
/// <param name="secretKey">The secret key to verify</param>
/// <returns>A boolean value that indicates if the secret key is valid or not</returns>
/// <exception cref="CryptographicException"></exception>
public static bool VerifySecretKey(this in Secp256k1Context context, ReadOnlySpan<byte> secretKey)
{
if (secretKey.Length != SecretKeySize)
{
throw new CryptographicException($"Your secret key must be exactly {SecretKeySize} bytes long");
}
context.Lib.SafeLibHandle.ThrowIfClosed();
//Get sec key ref and verify
fixed(byte* ptr = &MemoryMarshal.GetReference(secretKey))
{
return context.Lib._secKeyVerify.Invoke(context.Context, ptr) == 1;
}
}
}
}
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