Rendezvous/src/FinalFactory.Rendezvous.Server/Sessions/EphemeralCapabilityIssuer.cs

using System.Buffers.Binary;
using System.Security.Cryptography;
using System.Text;
using FinalFactory.Rendezvous.Server.State;

namespace FinalFactory.Rendezvous.Server.Sessions;

internal interface ISessionCapabilityService
{
    string CreateDerivationSalt();
    string DeriveCapability(
        string purpose,
        string ownerSubject,
        string idempotencyKey,
        string requestFingerprint,
        string derivationSalt);
    Guid DeriveGuid(
        string purpose,
        string ownerSubject,
        string idempotencyKey,
        string requestFingerprint,
        string derivationSalt);
    string DeriveOpaqueIdentifier(string purpose, string value);
    bool TryFingerprint(string? capability, out SecretFingerprint fingerprint);
}

internal sealed class EphemeralCapabilityIssuer : ISessionCapabilityService, IDisposable
{
    private readonly byte[] _key = RandomNumberGenerator.GetBytes(32);
    private bool _disposed;

    public string CreateDerivationSalt()
    {
        ObjectDisposedException.ThrowIf(_disposed, this);
        byte[] salt = RandomNumberGenerator.GetBytes(32);
        try
        {
            return Encode(salt);
        }
        finally
        {
            CryptographicOperations.ZeroMemory(salt);
        }
    }

    public string DeriveCapability(
        string purpose,
        string ownerSubject,
        string idempotencyKey,
        string requestFingerprint,
        string derivationSalt)
    {
        byte[] digest = Derive(
            purpose,
            ownerSubject,
            idempotencyKey,
            requestFingerprint,
            derivationSalt);
        try
        {
            return Encode(digest);
        }
        finally
        {
            CryptographicOperations.ZeroMemory(digest);
        }
    }

    public Guid DeriveGuid(
        string purpose,
        string ownerSubject,
        string idempotencyKey,
        string requestFingerprint,
        string derivationSalt)
    {
        byte[] digest = Derive(
            purpose,
            ownerSubject,
            idempotencyKey,
            requestFingerprint,
            derivationSalt);
        try
        {
            Span<byte> guidBytes = digest.AsSpan(0, 16);
            guidBytes[7] = (byte)((guidBytes[7] & 0x0f) | 0x80);
            guidBytes[8] = (byte)((guidBytes[8] & 0x3f) | 0x80);
            return new Guid(guidBytes);
        }
        finally
        {
            CryptographicOperations.ZeroMemory(digest);
        }
    }

    public string DeriveOpaqueIdentifier(string purpose, string value)
    {
        byte[] digest = Derive(purpose, value);
        try
        {
            return Encode(digest);
        }
        finally
        {
            CryptographicOperations.ZeroMemory(digest);
        }
    }

    public bool TryFingerprint(string? capability, out SecretFingerprint fingerprint)
    {
        fingerprint = default;
        if (_disposed
            || capability is null
            || capability.Length != 43
            || capability.Any(static character =>
                character is not (>= 'A' and <= 'Z')
                and not (>= 'a' and <= 'z')
                and not (>= '0' and <= '9')
                and not '-'
                and not '_'))
        {
            return false;
        }

        byte[] digest = Derive("fingerprint", capability);
        try
        {
            fingerprint = new SecretFingerprint(Encode(digest));
            return true;
        }
        finally
        {
            CryptographicOperations.ZeroMemory(digest);
        }
    }

    public void Dispose()
    {
        if (_disposed)
        {
            return;
        }

        _disposed = true;
        CryptographicOperations.ZeroMemory(_key);
    }

    public override string ToString() => "[EphemeralCapabilityIssuer: key and capabilities redacted]";

    private byte[] Derive(params string[] segments)
    {
        ObjectDisposedException.ThrowIf(_disposed, this);
        using IncrementalHash hmac = IncrementalHash.CreateHMAC(HashAlgorithmName.SHA256, _key);
        Span<byte> length = stackalloc byte[sizeof(int)];
        foreach (string segment in segments)
        {
            ArgumentException.ThrowIfNullOrEmpty(segment);
            byte[] encoded = Encoding.UTF8.GetBytes(segment);
            BinaryPrimitives.WriteInt32BigEndian(length, encoded.Length);
            hmac.AppendData(length);
            hmac.AppendData(encoded);
            CryptographicOperations.ZeroMemory(encoded);
        }

        return hmac.GetHashAndReset();
    }

    private static string Encode(ReadOnlySpan<byte> bytes) => Convert
        .ToBase64String(bytes)
        .TrimEnd('=')
        .Replace('+', '-')
        .Replace('/', '_');
}