// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System;
using System.Diagnostics;

namespace System.Security.Cryptography
{
    internal static partial class AsymmetricAlgorithmHelpers
    {
        // Encodes a EC key as an uncompressed set of concatenated scalars,
        // optionally including the private key. To omit the private parameter,
        // "d" must have a length of zero.
        public static void EncodeToUncompressedAnsiX963Key(
            ReadOnlySpan<byte> x,
            ReadOnlySpan<byte> y,
            ReadOnlySpan<byte> d,
            Span<byte> destination)
        {
            const byte UncompressedKeyPrefix = 0x04;
            if (x.Length != y.Length || (d.Length > 0 && d.Length != y.Length))
                throw new CryptographicException(SR.Cryptography_NotValidPublicOrPrivateKey);

            int size = 1 + x.Length + y.Length + d.Length; // 0x04 || X || Y { || D }

            if (destination.Length < size)
            {
                Debug.Fail("destination.Length < size");
                throw new CryptographicException();
            }

            destination[0] = UncompressedKeyPrefix;
            x.CopyTo(destination.Slice(1));
            y.CopyTo(destination.Slice(1 + x.Length));
            d.CopyTo(destination.Slice(1 + x.Length + y.Length));
        }

        public static void DecodeFromUncompressedAnsiX963Key(
            ReadOnlySpan<byte> ansiKey,
            bool hasPrivateKey,
            out ECParameters ret)
        {
            ret = default;

            const byte UncompressedKeyPrefix = 0x04;
            if (ansiKey.Length < 1 || ansiKey[0] != UncompressedKeyPrefix)
                throw new CryptographicException(SR.Cryptography_NotValidPublicOrPrivateKey);

            int fieldCount = hasPrivateKey ? 3 : 2;
            int fieldSize = (ansiKey.Length - 1) / fieldCount;

            if (ansiKey.Length != 1 + fieldSize * fieldCount)
                throw new CryptographicException(SR.Cryptography_NotValidPublicOrPrivateKey);

            ret.Q = new ECPoint {
                X = ansiKey.Slice(1, fieldSize).ToArray(),
                Y = ansiKey.Slice(1 + fieldSize, fieldSize).ToArray()
            };

            if (hasPrivateKey)
            {
                ret.D = ansiKey.Slice(1 + fieldSize + fieldSize, fieldSize).ToArray();
            }
        }
    }
}