git.stella-ops.org/src/__Libraries/StellaOps.DeltaVerdict/Signing/DeltaSigningService.cs

using System.Security.Cryptography;
using System.Text;
using System.Text.Json;
using System.Text.Json.Serialization;
using StellaOps.DeltaVerdict.Models;
using StellaOps.DeltaVerdict.Serialization;

namespace StellaOps.DeltaVerdict.Signing;

public interface IDeltaSigningService
{
    Task<DeltaVerdict.Models.DeltaVerdict> SignAsync(
        DeltaVerdict.Models.DeltaVerdict delta,
        SigningOptions options,
        CancellationToken ct = default);

    Task<VerificationResult> VerifyAsync(
        DeltaVerdict.Models.DeltaVerdict delta,
        VerificationOptions options,
        CancellationToken ct = default);
}

public sealed class DeltaSigningService : IDeltaSigningService
{
    private static readonly JsonSerializerOptions JsonOptions = new(JsonSerializerDefaults.Web)
    {
        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
        WriteIndented = false
    };

    public Task<DeltaVerdict.Models.DeltaVerdict> SignAsync(
        DeltaVerdict.Models.DeltaVerdict delta,
        SigningOptions options,
        CancellationToken ct = default)
    {
        ArgumentNullException.ThrowIfNull(delta);
        ArgumentNullException.ThrowIfNull(options);
        ct.ThrowIfCancellationRequested();

        var withDigest = DeltaVerdictSerializer.WithDigest(delta);
        var payloadJson = DeltaVerdictSerializer.Serialize(withDigest with { Signature = null });
        var payloadBytes = Encoding.UTF8.GetBytes(payloadJson);
        var envelope = BuildEnvelope(payloadBytes, options);
        var envelopeJson = JsonSerializer.Serialize(envelope, JsonOptions);

        return Task.FromResult(withDigest with { Signature = envelopeJson });
    }

    public Task<VerificationResult> VerifyAsync(
        DeltaVerdict.Models.DeltaVerdict delta,
        VerificationOptions options,
        CancellationToken ct = default)
    {
        ArgumentNullException.ThrowIfNull(delta);
        ArgumentNullException.ThrowIfNull(options);
        ct.ThrowIfCancellationRequested();

        if (string.IsNullOrEmpty(delta.Signature))
        {
            return Task.FromResult(VerificationResult.Fail("Delta is not signed"));
        }

        DsseEnvelope? envelope;
        try
        {
            envelope = JsonSerializer.Deserialize<DsseEnvelope>(delta.Signature, JsonOptions);
        }
        catch (JsonException ex)
        {
            return Task.FromResult(VerificationResult.Fail($"Invalid signature envelope: {ex.Message}"));
        }

        if (envelope is null)
        {
            return Task.FromResult(VerificationResult.Fail("Signature envelope is empty"));
        }

        var payloadBytes = Convert.FromBase64String(envelope.Payload);
        var pae = BuildPae(envelope.PayloadType, payloadBytes);
        var expectedSig = ComputeSignature(pae, options);

        var matched = envelope.Signatures.Any(sig =>
            string.Equals(sig.KeyId, options.KeyId, StringComparison.Ordinal)
            && string.Equals(sig.Sig, expectedSig, StringComparison.Ordinal));

        if (!matched)
        {
            return Task.FromResult(VerificationResult.Fail("Signature verification failed"));
        }

        if (!string.IsNullOrEmpty(delta.DeltaDigest))
        {
            var computed = DeltaVerdictSerializer.ComputeDigest(delta);
            if (!string.Equals(computed, delta.DeltaDigest, StringComparison.OrdinalIgnoreCase))
            {
                return Task.FromResult(VerificationResult.Fail("Delta digest mismatch"));
            }
        }

        return Task.FromResult(VerificationResult.Success());
    }

    private static DsseEnvelope BuildEnvelope(byte[] payload, SigningOptions options)
    {
        var pae = BuildPae(options.PayloadType, payload);
        var signature = ComputeSignature(pae, options);

        return new DsseEnvelope(
            options.PayloadType,
            Convert.ToBase64String(payload),
            [new DsseSignature(options.KeyId, signature)]);
    }

    private static string ComputeSignature(byte[] pae, SigningOptions options)
    {
        return ComputeSignatureCore(pae, options.Algorithm, options.SecretBase64);
    }

    private static string ComputeSignature(byte[] pae, VerificationOptions options)
    {
        return ComputeSignatureCore(pae, options.Algorithm, options.SecretBase64);
    }

    private static string ComputeSignatureCore(byte[] pae, SigningAlgorithm algorithm, string? secretBase64)
    {
        return algorithm switch
        {
            SigningAlgorithm.HmacSha256 => ComputeHmac(pae, secretBase64),
            SigningAlgorithm.Sha256 => Convert.ToBase64String(SHA256.HashData(pae)),
            _ => throw new InvalidOperationException($"Unsupported signing algorithm: {algorithm}")
        };
    }

    private static string ComputeHmac(byte[] data, string? secretBase64)
    {
        if (string.IsNullOrWhiteSpace(secretBase64))
        {
            throw new InvalidOperationException("HMAC signing requires a base64 secret.");
        }

        var secret = Convert.FromBase64String(secretBase64);
        using var hmac = new HMACSHA256(secret);
        var sig = hmac.ComputeHash(data);
        return Convert.ToBase64String(sig);
    }

    private static byte[] BuildPae(string payloadType, byte[] payload)
    {
        var prefix = "DSSEv1";
        var typeBytes = Encoding.UTF8.GetBytes(payloadType);
        var prefixBytes = Encoding.UTF8.GetBytes(prefix);
        var lengthType = Encoding.UTF8.GetBytes(typeBytes.Length.ToString());
        var lengthPayload = Encoding.UTF8.GetBytes(payload.Length.ToString());

        using var stream = new MemoryStream();
        stream.Write(prefixBytes);
        stream.WriteByte((byte)' ');
        stream.Write(lengthType);
        stream.WriteByte((byte)' ');
        stream.Write(typeBytes);
        stream.WriteByte((byte)' ');
        stream.Write(lengthPayload);
        stream.WriteByte((byte)' ');
        stream.Write(payload);
        return stream.ToArray();
    }
}

public sealed record SigningOptions
{
    public required string KeyId { get; init; }
    public SigningAlgorithm Algorithm { get; init; } = SigningAlgorithm.HmacSha256;
    public string? SecretBase64 { get; init; }
    public string PayloadType { get; init; } = "application/vnd.stellaops.delta-verdict+json";
}

public sealed record VerificationOptions
{
    public required string KeyId { get; init; }
    public SigningAlgorithm Algorithm { get; init; } = SigningAlgorithm.HmacSha256;
    public string? SecretBase64 { get; init; }
}

public enum SigningAlgorithm
{
    HmacSha256,
    Sha256
}

public sealed record VerificationResult
{
    public required bool IsValid { get; init; }
    public string? Error { get; init; }

    public static VerificationResult Success() => new() { IsValid = true };
    public static VerificationResult Fail(string error) => new() { IsValid = false, Error = error };
}

public sealed record DsseEnvelope(
    string PayloadType,
    string Payload,
    IReadOnlyList<DsseSignature> Signatures);

public sealed record DsseSignature(string KeyId, string Sig);