git.stella-ops.org/docs/ci/dsse-build-flow.md

Build-Time DSSE Attestation Walkthrough

Status: Draft — aligns with the November 2025 advisory “Embed in-toto attestations (DSSE-wrapped) into .NET 10/C# builds.”
Owners: Attestor Guild · DevOps Guild · Docs Guild.

This guide shows how to emit signed, in-toto compliant DSSE envelopes for every container build step (scan → package → push) using Stella Ops Authority keys. The same primitives power our Signer/Attestor services, but this walkthrough targets developer pipelines (GitHub/GitLab, dotnet builds, container scanners).

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1. Concepts refresher

Term	Meaning
In-toto Statement	JSON document describing what happened (predicate) to which artifact (subject).
DSSE	Dead Simple Signing Envelope: wraps the statement, base64 payload, and signatures.
Authority Signer	Stella Ops client that signs data via file-based keys, HSM/KMS, or keyless Fulcio certs.
PAE	Pre-Authentication Encoding: canonical “DSSEv1 ” byte layout that is signed.

Requirements:

1. .NET 10 SDK (preview) for C# helper code.
2. Authority key material (dev: file-based Ed25519; prod: Authority/KMS signer).
3. Artifact digest (e.g., pkg:docker/registry/app@sha256:...) per step.

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2. Helpers (drop-in library)

Create src/StellaOps.Attestation with:

public sealed record InTotoStatement(
    string _type,
    IReadOnlyList<Subject> subject,
    string predicateType,
    object predicate);

public sealed record Subject(string name, IReadOnlyDictionary<string,string> digest);

public sealed record DsseEnvelope(
    string payloadType,
    string payload,
    IReadOnlyList<Signature> signatures);

public sealed record Signature(string keyid, string sig);

public interface IAuthoritySigner
{
    Task<string> GetKeyIdAsync(CancellationToken ct = default);
    Task<byte[]> SignAsync(ReadOnlyMemory<byte> pae, CancellationToken ct = default);
}

DSSE helper:

public static class DsseHelper
{
    public static async Task<DsseEnvelope> WrapAsync(
        InTotoStatement statement,
        IAuthoritySigner signer,
        string payloadType = "application/vnd.in-toto+json",
        CancellationToken ct = default)
    {
        var payloadBytes = JsonSerializer.SerializeToUtf8Bytes(statement,
            new JsonSerializerOptions { DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull });
        var pae = PreAuthEncode(payloadType, payloadBytes);

        var signature = await signer.SignAsync(pae, ct).ConfigureAwait(false);
        var keyId = await signer.GetKeyIdAsync(ct).ConfigureAwait(false);

        return new DsseEnvelope(
            payloadType,
            Convert.ToBase64String(payloadBytes),
            new[] { new Signature(keyId, Convert.ToBase64String(signature)) });
    }

    private static byte[] PreAuthEncode(string payloadType, byte[] payload)
    {
        static byte[] Cat(params byte[][] parts)
        {
            var len = parts.Sum(p => p.Length);
            var buf = new byte[len];
            var offset = 0;
            foreach (var part in parts)
            {
                Buffer.BlockCopy(part, 0, buf, offset, part.Length);
                offset += part.Length;
            }
            return buf;
        }

        var header = Encoding.UTF8.GetBytes("DSSEv1");
        var pt = Encoding.UTF8.GetBytes(payloadType);
        var lenPt = Encoding.UTF8.GetBytes(pt.Length.ToString(CultureInfo.InvariantCulture));
        var lenPayload = Encoding.UTF8.GetBytes(payload.Length.ToString(CultureInfo.InvariantCulture));
        var space = Encoding.UTF8.GetBytes(" ");

        return Cat(header, space, lenPt, space, pt, space, lenPayload, space, payload);
    }
}

Authority signer examples:

/dev (file Ed25519):

public sealed class FileEd25519Signer : IAuthoritySigner, IDisposable
{
    private readonly Ed25519 _ed;
    private readonly string _keyId;

    public FileEd25519Signer(byte[] privateKeySeed, string keyId)
    {
        _ed = new Ed25519(privateKeySeed);
        _keyId = keyId;
    }

    public Task<string> GetKeyIdAsync(CancellationToken ct) => Task.FromResult(_keyId);

    public Task<byte[]> SignAsync(ReadOnlyMemory<byte> pae, CancellationToken ct)
        => Task.FromResult(_ed.Sign(pae.Span.ToArray()));

    public void Dispose() => _ed.Dispose();
}

Prod (Authority KMS):

Reuse the existing StellaOps.Signer.KmsSigner adapter—wrap it behind IAuthoritySigner.

---

3. Emitting attestations per step

Subject helper:

static Subject ImageSubject(string imageDigest) => new(
    name: imageDigest,
    digest: new Dictionary<string,string>{{"sha256", imageDigest.Replace("sha256:", "", StringComparison.Ordinal)}});

3.1 Scan

var scanStmt = new InTotoStatement(
    _type: "https://in-toto.io/Statement/v1",
    subject: new[]{ ImageSubject(imageDigest) },
    predicateType: "https://stella.ops/predicates/scanner-evidence/v1",
    predicate: new {
        scanner = "StellaOps.Scanner 0.9.0",
        findingsSha256 = scanResultsHash,
        startedAt = startedIso,
        finishedAt = finishedIso,
        rulePack = "lattice:default@2025-11-01"
    });

var scanEnvelope = await DsseHelper.WrapAsync(scanStmt, signer);
await File.WriteAllTextAsync("artifacts/attest-scan.dsse.json", JsonSerializer.Serialize(scanEnvelope));

3.2 Package (SLSA provenance)

var pkgStmt = new InTotoStatement(
    "https://in-toto.io/Statement/v1",
    new[]{ ImageSubject(imageDigest) },
    "https://slsa.dev/provenance/v1",
    new {
        builder = new { id = "stella://builder/dockerfile" },
        buildType = "dockerfile/v1",
        invocation = new { configSource = repoUrl, entryPoint = dockerfilePath },
        materials = new[] { new { uri = repoUrl, digest = new { git = gitSha } } }
    });

var pkgEnvelope = await DsseHelper.WrapAsync(pkgStmt, signer);
await File.WriteAllTextAsync("artifacts/attest-package.dsse.json", JsonSerializer.Serialize(pkgEnvelope));

3.3 Push

var pushStmt = new InTotoStatement(
    "https://in-toto.io/Statement/v1",
    new[]{ ImageSubject(imageDigest) },
    "https://stella.ops/predicates/push/v1",
    new { registry = registryUrl, repository = repoName, tags, pushedAt = DateTimeOffset.UtcNow });

var pushEnvelope = await DsseHelper.WrapAsync(pushStmt, signer);
await File.WriteAllTextAsync("artifacts/attest-push.dsse.json", JsonSerializer.Serialize(pushEnvelope));

---

4. CI integration

4.1 GitLab example

.attest-template: &attest
  image: mcr.microsoft.com/dotnet/sdk:10.0-preview
  before_script:
    - dotnet build src/StellaOps.Attestation/StellaOps.Attestation.csproj
  variables:
    AUTHORITY_KEY_FILE: "$CI_PROJECT_DIR/secrets/ed25519.key"
    IMAGE_DIGEST: "$CI_REGISTRY_IMAGE@${CI_COMMIT_SHA}"

attest:scan:
  stage: scan
  script:
    - dotnet run --project tools/StellaOps.Attestor.Tool -- step scan --subject "$IMAGE_DIGEST" --out artifacts/attest-scan.dsse.json
  artifacts:
    paths: [artifacts/attest-scan.dsse.json]

attest:package:
  stage: package
  script:
    - dotnet run --project tools/StellaOps.Attestor.Tool -- step package --subject "$IMAGE_DIGEST" --out artifacts/attest-package.dsse.json

attest:push:
  stage: push
  script:
    - dotnet run --project tools/StellaOps.Attestor.Tool -- step push --subject "$IMAGE_DIGEST" --registry "$CI_REGISTRY" --tags "$CI_COMMIT_REF_NAME"

4.2 GitHub Actions snippet

jobs:
  attest:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-dotnet@v4
        with: { dotnet-version: '10.0.x' }
      - name: Build attestor helpers
        run: dotnet build src/StellaOps.Attestation/StellaOps.Attestation.csproj
      - name: Emit scan attestation
        run: dotnet run --project tools/StellaOps.Attestor.Tool -- step scan --subject "${{ env.IMAGE_DIGEST }}" --out artifacts/attest-scan.dsse.json
        env:
          AUTHORITY_KEY_REF: ${{ secrets.AUTHORITY_KEY_REF }}

---

5. Verification

• stella attest verify --file artifacts/attest-scan.dsse.json (CLI planned under DSSE-CLI-401-021).
• Manual validation:
  1. Base64 decode payload → ensure _type = https://in-toto.io/Statement/v1, subject[].digest.sha256 matches artifact.
  2. Recompute PAE and verify signature with the Authority public key.
  3. Attach envelope to Rekor (optional) via existing Attestor API.

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6. Storage conventions

Store DSSE files next to build outputs:

artifacts/
  attest-scan.dsse.json
  attest-package.dsse.json
  attest-push.dsse.json

Include the SHA-256 digest of each envelope in promotion manifests (docs/release/promotion-attestations.md) so downstream verifiers can trace chain of custody.

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7. References

• In-toto Statement v1
• DSSE specification
• docs/modules/signer/architecture.md
• docs/modules/attestor/architecture.md
• docs/release/promotion-attestations.md

Keep this file updated alongside DSSE-LIB-401-020 and DSSE-CLI-401-021. When the bench repo publishes sample attestations, link them here.