ProofedMesh/services/signing-service/internal/signer/signer.go

// Package signer produces DSSE envelopes for a given statement + key_ref.
// Current state:
//   - DSSE PAE and signing are real.
//   - InMemory uses an Ed25519 keyring loaded from flags/env — suitable for
//     dev, CI, and local integration tests.
//   - OpenBao Transit is a stub (ErrNotWired) and will land in its own
//     subtask; the interface is already in place so the HTTP layer stays
//     unchanged when it does.
package signer

import (
	"context"
	"crypto/ed25519"
	"encoding/base64"
	"encoding/hex"
	"errors"
	"fmt"
	"strings"
	"sync"

	"basicstack.de/proofedmesh/services/signing-service/internal/canonical"
)

// ErrUnknownKey means the Signer does not know the supplied key_ref.
var ErrUnknownKey = errors.New("signer: unknown key_ref")

// ErrNotWired means the backend is a placeholder.
var ErrNotWired = errors.New("signer: backend not wired yet")

// Statement is the in-toto v1 Statement the client submits. We keep the type
// loosely coupled to avoid pulling the admin-api's attest package here.
type Statement map[string]any

// EnvelopeSignature mirrors the DSSE spec.
type EnvelopeSignature struct {
	KeyID string `json:"keyid"`
	Sig   string `json:"sig"`
}

// Envelope is the DSSE envelope.
type Envelope struct {
	PayloadType string              `json:"payloadType"`
	Payload     string              `json:"payload"`
	Signatures  []EnvelopeSignature `json:"signatures"`
}

// Signer wraps the backend-dependent step of signing a PAE with a given key.
type Signer interface {
	Sign(ctx context.Context, keyRef string, pae []byte) (keyid string, sig []byte, err error)
}

// InMemory holds Ed25519 keys indexed by key_ref.
type InMemory struct {
	mu   sync.RWMutex
	keys map[string]ed25519.PrivateKey
}

func NewInMemory() *InMemory { return &InMemory{keys: map[string]ed25519.PrivateKey{}} }

// AddKey registers a key; priv must be an Ed25519 private key.
func (m *InMemory) AddKey(keyRef string, priv ed25519.PrivateKey) {
	m.mu.Lock()
	defer m.mu.Unlock()
	m.keys[keyRef] = priv
}

// Keys returns the registered key_refs in a stable order (for /healthz).
func (m *InMemory) Keys() []string {
	m.mu.RLock()
	defer m.mu.RUnlock()
	out := make([]string, 0, len(m.keys))
	for k := range m.keys {
		out = append(out, k)
	}
	return out
}

func (m *InMemory) Sign(_ context.Context, keyRef string, pae []byte) (string, []byte, error) {
	m.mu.RLock()
	priv, ok := m.keys[keyRef]
	m.mu.RUnlock()
	if !ok {
		return "", nil, fmt.Errorf("%w: %q", ErrUnknownKey, keyRef)
	}
	return keyRef, ed25519.Sign(priv, pae), nil
}

// OpenBao is a stub implementation that returns ErrNotWired. The real wiring
// (vault/openbao HTTP transit API) lives in a follow-up subtask.
type OpenBao struct{}

func (OpenBao) Sign(_ context.Context, keyRef string, pae []byte) (string, []byte, error) {
	return "", nil, ErrNotWired
}

// Signed is what the HTTP layer returns from the signing service.
type Signed struct {
	Envelope    Envelope
	ContentHash string
}

// SignStatement runs the full DSSE pipeline:
//
//  1. canonicalise statement
//  2. base64url(no pad) encode as payload
//  3. build PAE
//  4. hand PAE to Signer backend
//  5. assemble Envelope
//  6. canonicalise Envelope and hash → content_hash
func SignStatement(ctx context.Context, s Signer, keyRef string, stmt Statement) (Signed, error) {
	payloadBytes, err := canonical.Marshal(stmt)
	if err != nil {
		return Signed{}, fmt.Errorf("canonicalise statement: %w", err)
	}
	payloadB64 := base64.RawURLEncoding.EncodeToString(payloadBytes)
	payloadType := "application/vnd.in-toto+json"
	pae := encodePAE(payloadType, payloadBytes)

	keyid, sig, err := s.Sign(ctx, keyRef, pae)
	if err != nil {
		return Signed{}, err
	}
	sigB64 := base64.RawURLEncoding.EncodeToString(sig)

	env := Envelope{
		PayloadType: payloadType,
		Payload:     payloadB64,
		Signatures:  []EnvelopeSignature{{KeyID: keyid, Sig: sigB64}},
	}
	envBytes, err := canonical.Marshal(env)
	if err != nil {
		return Signed{}, fmt.Errorf("canonicalise envelope: %w", err)
	}
	return Signed{
		Envelope:    env,
		ContentHash: "sha256:" + canonical.SHA256Hex(envBytes),
	}, nil
}

// encodePAE implements the DSSE Pre-Authentication Encoding:
//
//	DSSEv1 <len(type)> <type> <len(payload)> <payload>
func encodePAE(payloadType string, payload []byte) []byte {
	var b strings.Builder
	fmt.Fprintf(&b, "DSSEv1 %d ", len(payloadType))
	b.WriteString(payloadType)
	fmt.Fprintf(&b, " %d ", len(payload))
	b.Write(payload)
	return []byte(b.String())
}

// DecodeHexEd25519Key parses a 64-byte hex-encoded Ed25519 private key as
// produced by `openssl genpkey -algorithm Ed25519` + rawkey extraction, or
// the dev convenience generator in cmd/.
func DecodeHexEd25519Key(h string) (ed25519.PrivateKey, error) {
	raw, err := hex.DecodeString(h)
	if err != nil {
		return nil, fmt.Errorf("decode hex: %w", err)
	}
	if len(raw) != ed25519.PrivateKeySize {
		return nil, fmt.Errorf("expected %d-byte Ed25519 private key, got %d", ed25519.PrivateKeySize, len(raw))
	}
	return ed25519.PrivateKey(raw), nil
}