missions/tests/Azaion.Missions.E2E.Tests/Helpers/ForeignKeypair.cs

using System.Security.Cryptography;
using System.Text;
using System.Text.Json;
using System.Text.Json.Nodes;

namespace Azaion.Missions.E2E.Helpers;

/// <summary>
/// Test-only ECDSA P-256 signer used by NFT-SEC-02 to mint a token signed by
/// a keypair the JWKS endpoint never published. This is the ONE in-test
/// signing path allowed by the task spec — every other test mints via the
/// jwks-mock <c>POST /sign</c> endpoint.
/// </summary>
/// <remarks>
/// The private key lives entirely in the test process and is disposed with
/// the helper. The wire shape mirrors <c>JwksMock.TokenSigner</c> (JWS-compact
/// ES256) so the only thing that differs from a "real" mock-minted token is
/// the signing key — defeating any IssuerSigningKeyResolver that fails to
/// match <c>kid</c> against the published JWKS.
/// </remarks>
public sealed class ForeignKeypair : IDisposable
{
    private readonly ECDsa _ec;
    private readonly string _kid;

    public ForeignKeypair()
    {
        _ec = ECDsa.Create(ECCurve.NamedCurves.nistP256);
        // Deterministic kid that is clearly NOT what jwks-mock issues
        // (mock kids are base64url SHA-256 hashes; this label is plain ASCII).
        _kid = "foreign-keypair-not-in-jwks";
    }

    public string Mint(string issuer, string audience, string permissions, int expOffsetSeconds = 3600)
    {
        var nowUnix = DateTimeOffset.UtcNow.ToUnixTimeSeconds();
        var expUnix = nowUnix + expOffsetSeconds;

        var header = new JsonObject
        {
            ["alg"] = "ES256",
            ["kid"] = _kid,
            ["typ"] = "JWT"
        };
        var payload = new JsonObject
        {
            ["iss"] = issuer,
            ["aud"] = audience,
            ["iat"] = nowUnix,
            ["exp"] = expUnix,
            ["permissions"] = permissions
        };

        var headerSeg = Base64UrlEncode(JsonSerializer.SerializeToUtf8Bytes(header));
        var payloadSeg = Base64UrlEncode(JsonSerializer.SerializeToUtf8Bytes(payload));
        var signingInput = Encoding.ASCII.GetBytes($"{headerSeg}.{payloadSeg}");
        var signature = _ec.SignData(signingInput, HashAlgorithmName.SHA256,
            DSASignatureFormat.IeeeP1363FixedFieldConcatenation);
        var sigSeg = Base64UrlEncode(signature);
        return $"{headerSeg}.{payloadSeg}.{sigSeg}";
    }

    public void Dispose() => _ec.Dispose();

    private static string Base64UrlEncode(ReadOnlySpan<byte> bytes)
    {
        var b64 = Convert.ToBase64String(bytes);
        return b64.Replace('+', '-').Replace('/', '_').TrimEnd('=');
    }
}