diff --git a/services/signing-service/README.md b/services/signing-service/README.md
new file mode 100644
index 0000000..49d4499
--- /dev/null
+++ b/services/signing-service/README.md
@@ -0,0 +1,113 @@
+# signing-service
+
+HTTP service that implements `POST /api/v1/sign/dsse` per
+[ADR-0005](../../docs/adr/0005-signing-service-and-transparency-log-api.md). It
+canonicalises an in-toto v1 Statement, DSSE-signs it with the key named by
+`key_ref`, and returns the envelope plus a stable `content_hash` that the
+admin-api and other clients use as the single attestation reference.
+
+## Status
+
+- **Signing is real**: Ed25519 PAE + envelope + content_hash. Verified
+  end-to-end against the admin-api `attest.HTTPSigner` client via a Go
+  interop test that re-verifies the signature with the server's public key.
+- **In-memory keyring**: `-backend=memory`. Ephemeral keys are generated at
+  startup unless `-keys-from-env` is set; stable dev keys can be supplied as
+  hex-encoded 64-byte Ed25519 private keys via `KEY_<slug>` env vars.
+- **OpenBao Transit backend**: stub (`-backend=openbao` returns
+  `ErrNotWired`). Separate subtask.
+- **Pocket ID JWKS verifier**: stub. Dev uses the static `-bearers` table.
+  Separate subtask.
+- **OPA policy decider**: stub. Dev uses AllowAll. Separate subtask.
+- **Deployment manifest**: separate subtask.
+
+## Layout
+
+```
+services/signing-service/
+├── README.md
+├── go.mod
+├── cmd/signing-service/main.go     — HTTP entry (bind + router + graceful shutdown)
+└── internal/
+    ├── canonical/canonical.go      — stable JSON serialisation (byte-for-byte
+    │                                  compatible with admin-api/internal/attest/hash.go)
+    ├── signer/signer.go            — Signer interface, InMemory (Ed25519), OpenBao stub,
+    │                                  DSSE PAE and SignStatement orchestration
+    ├── signer/signer_test.go       — signature verification, PAE format exactness, errors
+    ├── policy/policy.go            — Decider interface, AllowAll, OPA stub
+    ├── token/validator.go          — bearer validator, Static + PocketID stub
+    └── httpapi/
+        ├── sign.go                 — POST /api/v1/sign/dsse handler + /healthz
+        └── sign_test.go            — happy path, auth, policy, unknown key, bad statement
+```
+
+## Local dev
+
+```bash
+cd services/signing-service
+go vet ./... && go test ./...
+go run ./cmd/signing-service \
+  -listen=:8081 \
+  -backend=memory \
+  -keys=trust-api/rate-change-signer-2026-q4 \
+  -bearers=dev-token=agent-admin-api
+```
+
+Smoke-test it with curl:
+
+```bash
+curl -sS -X POST http://localhost:8081/api/v1/sign/dsse \
+  -H "Authorization: Bearer dev-token" \
+  -H "Content-Type: application/json" \
+  -d '{
+    "key_ref":"trust-api/rate-change-signer-2026-q4",
+    "statement":{
+      "_type":"https://in-toto.io/Statement/v1",
+      "subject":[{"name":"per_diem_rate/PDR-1","digest":{"sha256":"aaaa"}}],
+      "predicateType":"https://basicstack.de/attestations/rate_change/v1",
+      "predicate":{"change_kind":"create","rate_id":"PDR-1"}
+    }
+  }' | jq .
+```
+
+Pointing the admin-api at this instance:
+
+```bash
+# Terminal 1
+cd services/signing-service && go run ./cmd/signing-service -listen=:8081
+# Terminal 2 (admin-api still needs a tlog, so this will log "attestation failed"
+# until services/transparency-log is up — rows will still be created and marked
+# attestation-pending as designed):
+cd services/admin-api && go run ./cmd/admin-api -store=memory \
+  -signer-url=http://localhost:8081 \
+  -tlog-url=http://localhost:9099 \
+  -bearer=dev-token \
+  -signer-key=trust-api/rate-change-signer-2026-q4
+```
+
+## Keys
+
+### Generate one hex-encoded Ed25519 private key for dev
+
+```bash
+go run ./cmd/signing-service -gen-dev-key
+# → prints 128 hex chars (64 bytes raw private key)
+```
+
+Supply it via env:
+
+```bash
+# For key_ref "trust-api/rate-change-signer-2026-q4" the env name is
+# KEY_TRUST_API_RATE_CHANGE_SIGNER_2026_Q4 (upper-case, / - . → _)
+export KEY_TRUST_API_RATE_CHANGE_SIGNER_2026_Q4=<hex>
+go run ./cmd/signing-service -backend=memory -keys-from-env \
+  -keys=trust-api/rate-change-signer-2026-q4
+```
+
+### Prod
+
+Prod wiring in the OpenBao-Transit subtask: Signer backend becomes
+`signer.OpenBao` with HTTP client to the cluster-internal OpenBao, policy
+backend becomes `policy.OPA` with the Rego bundle under `policies/signing/`
+(not yet created), token validator becomes `token.PocketID` with JWKS from
+`https://auth.basicstack.de/.well-known/jwks.json`.
diff --git a/services/signing-service/cmd/signing-service/main.go b/services/signing-service/cmd/signing-service/main.go
new file mode 100644
index 0000000..8181908
--- /dev/null
+++ b/services/signing-service/cmd/signing-service/main.go
@@ -0,0 +1,160 @@
+// Command signing-service exposes POST /api/v1/sign/dsse per ADR-0005.
+//
+// Dev mode is self-contained: in-memory Ed25519 keyring, AllowAll policy,
+// static bearer-to-subject table. Prod wiring (OpenBao Transit, OPA, Pocket
+// ID JWKS) is in follow-up subtasks; the interfaces are already in place so
+// swapping backends does not touch the HTTP layer.
+package main
+
+import (
+	"context"
+	"crypto/ed25519"
+	"crypto/rand"
+	"encoding/hex"
+	"flag"
+	"fmt"
+	"log/slog"
+	"net/http"
+	"os"
+	"os/signal"
+	"strings"
+	"syscall"
+	"time"
+
+	"basicstack.de/proofedmesh/services/signing-service/internal/httpapi"
+	"basicstack.de/proofedmesh/services/signing-service/internal/policy"
+	"basicstack.de/proofedmesh/services/signing-service/internal/signer"
+	"basicstack.de/proofedmesh/services/signing-service/internal/token"
+)
+
+func main() {
+	listen := flag.String("listen", ":8081", "HTTP listen address")
+	backend := flag.String("backend", "memory", "signing backend: memory | openbao")
+	keyRefs := flag.String("keys", "trust-api/rate-change-signer-2026-q4",
+		"comma-separated list of key_refs; for memory backend each gets an ephemeral Ed25519 key")
+	keysFromEnv := flag.Bool("keys-from-env", false,
+		"when true (memory backend), read KEY_<slug>=<hex> env for each key_ref instead of generating")
+	bearers := flag.String("bearers", "dev-token=agent-admin-api",
+		"comma-separated bearer=subject pairs for the static token validator")
+	genDevKeyAndExit := flag.Bool("gen-dev-key", false, "generate one dev Ed25519 key and exit (prints hex)")
+	flag.Parse()
+
+	logger := slog.New(slog.NewJSONHandler(os.Stdout, nil))
+
+	if *genDevKeyAndExit {
+		_, k, err := ed25519.GenerateKey(rand.Reader)
+		if err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(1)
+		}
+		fmt.Println(hex.EncodeToString(k))
+		return
+	}
+
+	var sgn signer.Signer
+	switch *backend {
+	case "memory":
+		mem := signer.NewInMemory()
+		for _, kr := range splitCSV(*keyRefs) {
+			var priv ed25519.PrivateKey
+			if *keysFromEnv {
+				envName := envNameForKey(kr)
+				raw := os.Getenv(envName)
+				if raw == "" {
+					logger.Error("missing env for key", "env", envName)
+					os.Exit(1)
+				}
+				p, err := signer.DecodeHexEd25519Key(raw)
+				if err != nil {
+					logger.Error("bad env key", "env", envName, "err", err)
+					os.Exit(1)
+				}
+				priv = p
+			} else {
+				_, p, err := ed25519.GenerateKey(rand.Reader)
+				if err != nil {
+					logger.Error("gen key", "err", err)
+					os.Exit(1)
+				}
+				priv = p
+				logger.Warn("generated ephemeral dev key — not safe for prod", "key_ref", kr)
+			}
+			mem.AddKey(kr, priv)
+		}
+		sgn = mem
+	case "openbao":
+		logger.Error("openbao backend not wired yet; use -backend=memory for dev")
+		os.Exit(1)
+	default:
+		fmt.Fprintln(os.Stderr, "unknown -backend; expected memory|openbao")
+		os.Exit(2)
+	}
+
+	srv := httpapi.Server{
+		Signer:    sgn,
+		Policy:    policy.AllowAll{},
+		Validator: parseBearers(*bearers),
+		Logger:    logger,
+		Now:       time.Now,
+	}
+	mux := http.NewServeMux()
+	srv.Mount(mux)
+
+	hsrv := &http.Server{
+		Addr:              *listen,
+		Handler:           mux,
+		ReadHeaderTimeout: 5 * time.Second,
+	}
+	go func() {
+		logger.Warn("signing-service: AllowAll policy and static bearers are DEV ONLY")
+		logger.Info("signing-service listening", "addr", *listen, "backend", *backend)
+		if err := hsrv.ListenAndServe(); err != nil && err != http.ErrServerClosed {
+			logger.Error("http serve", "err", err)
+		}
+	}()
+	stop := make(chan os.Signal, 1)
+	signal.Notify(stop, syscall.SIGINT, syscall.SIGTERM)
+	<-stop
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	_ = hsrv.Shutdown(ctx)
+}
+
+func splitCSV(s string) []string {
+	if s == "" {
+		return nil
+	}
+	raw := strings.Split(s, ",")
+	out := make([]string, 0, len(raw))
+	for _, r := range raw {
+		r = strings.TrimSpace(r)
+		if r != "" {
+			out = append(out, r)
+		}
+	}
+	return out
+}
+
+func envNameForKey(keyRef string) string {
+	n := "KEY_" + strings.ToUpper(keyRef)
+	for _, c := range []string{"/", "-", "."} {
+		n = strings.ReplaceAll(n, c, "_")
+	}
+	return n
+}
+
+func parseBearers(s string) token.Validator {
+	t := token.Static{}
+	for _, pair := range splitCSV(s) {
+		eq := strings.IndexByte(pair, '=')
+		if eq <= 0 {
+			continue
+		}
+		t[pair[:eq]] = pair[eq+1:]
+	}
+	return t
+}
+
+// Compile-time import check for the signer package that main does not use
+// directly but needs for backend selection.
+var _ = signer.NewInMemory
diff --git a/services/signing-service/go.mod b/services/signing-service/go.mod
new file mode 100644
index 0000000..31659ac
--- /dev/null
+++ b/services/signing-service/go.mod
@@ -0,0 +1,3 @@
+module basicstack.de/proofedmesh/services/signing-service
+
+go 1.23
diff --git a/services/signing-service/internal/canonical/canonical.go b/services/signing-service/internal/canonical/canonical.go
new file mode 100644
index 0000000..f91ffe4
--- /dev/null
+++ b/services/signing-service/internal/canonical/canonical.go
@@ -0,0 +1,95 @@
+// Package canonical implements the stable JSON serialisation ProofedMesh uses
+// for hashing attestation envelopes and statements. It MUST stay byte-for-byte
+// compatible with services/admin-api/internal/attest/hash.go — the admin-api
+// hashes envelopes locally to verify the signing-service's response, and if
+// the two canonicalisations diverge every signing call rejects itself.
+//
+// This is not a full RFC 8785 implementation: all our hashed inputs are
+// strings, nulls, maps, and arrays of those, so Go's encoding/json gives
+// stable output for the leaves. We only need to force deterministic object
+// key ordering, which is what writeSorted does.
+package canonical
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"fmt"
+	"sort"
+)
+
+// SHA256Hex returns the hex-encoded SHA-256 of b.
+func SHA256Hex(b []byte) string {
+	sum := sha256.Sum256(b)
+	return hex.EncodeToString(sum[:])
+}
+
+// Marshal returns the canonical JSON encoding of v (sorted object keys).
+func Marshal(v any) ([]byte, error) {
+	first, err := json.Marshal(v)
+	if err != nil {
+		return nil, err
+	}
+	var any_ any
+	if err := json.Unmarshal(first, &any_); err != nil {
+		return nil, err
+	}
+	var buf bytes.Buffer
+	if err := writeSorted(&buf, any_); err != nil {
+		return nil, err
+	}
+	return buf.Bytes(), nil
+}
+
+func writeSorted(buf *bytes.Buffer, v any) error {
+	switch x := v.(type) {
+	case nil:
+		buf.WriteString("null")
+	case bool:
+		if x {
+			buf.WriteString("true")
+		} else {
+			buf.WriteString("false")
+		}
+	case float64:
+		b, _ := json.Marshal(x)
+		buf.Write(b)
+	case string:
+		b, _ := json.Marshal(x)
+		buf.Write(b)
+	case []any:
+		buf.WriteByte('[')
+		for i, el := range x {
+			if i > 0 {
+				buf.WriteByte(',')
+			}
+			if err := writeSorted(buf, el); err != nil {
+				return err
+			}
+		}
+		buf.WriteByte(']')
+	case map[string]any:
+		keys := make([]string, 0, len(x))
+		for k := range x {
+			keys = append(keys, k)
+		}
+		sort.Strings(keys)
+		buf.WriteByte('{')
+		for i, k := range keys {
+			if i > 0 {
+				buf.WriteByte(',')
+			}
+			kb, _ := json.Marshal(k)
+			buf.Write(kb)
+			buf.WriteByte(':')
+			if err := writeSorted(buf, x[k]); err != nil {
+				return err
+			}
+		}
+		buf.WriteByte('}')
+	default:
+		return fmt.Errorf("canonical.writeSorted: unsupported type %T", v)
+	}
+	return nil
+}
diff --git a/services/signing-service/internal/httpapi/sign.go b/services/signing-service/internal/httpapi/sign.go
new file mode 100644
index 0000000..8cdc15a
--- /dev/null
+++ b/services/signing-service/internal/httpapi/sign.go
@@ -0,0 +1,136 @@
+// Package httpapi exposes the signing-service's single endpoint:
+// POST /api/v1/sign/dsse per ADR-0005.
+package httpapi
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"log/slog"
+	"net/http"
+	"time"
+
+	"basicstack.de/proofedmesh/services/signing-service/internal/policy"
+	"basicstack.de/proofedmesh/services/signing-service/internal/signer"
+	"basicstack.de/proofedmesh/services/signing-service/internal/token"
+)
+
+type Server struct {
+	Signer    signer.Signer
+	Policy    policy.Decider
+	Validator token.Validator
+	Logger    *slog.Logger
+	Now       func() time.Time
+}
+
+func (s Server) Mount(mux *http.ServeMux) {
+	mux.HandleFunc("POST /api/v1/sign/dsse", s.handleSign)
+	mux.HandleFunc("GET /healthz", s.handleHealth)
+}
+
+type signRequest struct {
+	KeyRef    string          `json:"key_ref"`
+	Statement signer.Statement `json:"statement"`
+}
+
+type signResponse struct {
+	Envelope    signer.Envelope `json:"envelope"`
+	ContentHash string          `json:"content_hash"`
+	SignedAt    time.Time       `json:"signed_at"`
+}
+
+type errResponse struct {
+	Error errBody `json:"error"`
+}
+
+type errBody struct {
+	Code    string `json:"code"`
+	Message string `json:"message"`
+}
+
+func (s Server) handleHealth(w http.ResponseWriter, _ *http.Request) {
+	w.WriteHeader(http.StatusOK)
+	_, _ = w.Write([]byte(`{"status":"ok"}`))
+}
+
+func (s Server) handleSign(w http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
+
+	bearer, err := token.FromAuthHeader(r)
+	if err != nil {
+		writeErr(w, http.StatusUnauthorized, "unauthorized", "missing or malformed Authorization header")
+		return
+	}
+	subject, err := s.Validator.Validate(ctx, bearer)
+	if err != nil {
+		writeErr(w, http.StatusUnauthorized, "unauthorized", err.Error())
+		return
+	}
+
+	var req signRequest
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeErr(w, http.StatusBadRequest, "bad_request", fmt.Sprintf("decode body: %v", err))
+		return
+	}
+	if req.KeyRef == "" {
+		writeErr(w, http.StatusBadRequest, "bad_request", "key_ref is required")
+		return
+	}
+	if req.Statement == nil {
+		writeErr(w, http.StatusBadRequest, "bad_request", "statement is required")
+		return
+	}
+	if t, _ := req.Statement["_type"].(string); t != "https://in-toto.io/Statement/v1" {
+		writeErr(w, http.StatusBadRequest, "bad_request", `statement._type must be "https://in-toto.io/Statement/v1"`)
+		return
+	}
+	if pt, _ := req.Statement["predicateType"].(string); pt == "" {
+		writeErr(w, http.StatusBadRequest, "bad_request", "statement.predicateType is required")
+		return
+	}
+
+	dec, err := s.Policy.Allow(ctx, subject, req.KeyRef)
+	if err != nil {
+		writeErr(w, http.StatusInternalServerError, "policy_eval_error", err.Error())
+		return
+	}
+	if !dec.Allow {
+		writeErr(w, http.StatusForbidden, "signing_denied", dec.Reason)
+		return
+	}
+
+	signed, err := signer.SignStatement(ctx, s.Signer, req.KeyRef, req.Statement)
+	if err != nil {
+		if errors.Is(err, signer.ErrUnknownKey) {
+			writeErr(w, http.StatusBadRequest, "unknown_key", err.Error())
+			return
+		}
+		if errors.Is(err, signer.ErrNotWired) {
+			writeErr(w, http.StatusServiceUnavailable, "signer_unavailable", err.Error())
+			return
+		}
+		writeErr(w, http.StatusInternalServerError, "sign_failed", err.Error())
+		return
+	}
+
+	if s.Logger != nil {
+		s.Logger.Info("signed", "subject", subject, "key_ref", req.KeyRef, "content_hash", signed.ContentHash)
+	}
+
+	now := time.Now().UTC()
+	if s.Now != nil {
+		now = s.Now()
+	}
+	w.Header().Set("Content-Type", "application/json")
+	_ = json.NewEncoder(w).Encode(signResponse{
+		Envelope:    signed.Envelope,
+		ContentHash: signed.ContentHash,
+		SignedAt:    now,
+	})
+}
+
+func writeErr(w http.ResponseWriter, status int, code, message string) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	_ = json.NewEncoder(w).Encode(errResponse{Error: errBody{Code: code, Message: message}})
+}
diff --git a/services/signing-service/internal/httpapi/sign_test.go b/services/signing-service/internal/httpapi/sign_test.go
new file mode 100644
index 0000000..fbc2de9
--- /dev/null
+++ b/services/signing-service/internal/httpapi/sign_test.go
@@ -0,0 +1,218 @@
+package httpapi
+
+import (
+	"bytes"
+	"context"
+	"crypto/ed25519"
+	"crypto/rand"
+	"encoding/base64"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+
+	"basicstack.de/proofedmesh/services/signing-service/internal/policy"
+	"basicstack.de/proofedmesh/services/signing-service/internal/signer"
+	"basicstack.de/proofedmesh/services/signing-service/internal/token"
+)
+
+func mustGenKey(t *testing.T) ed25519.PrivateKey {
+	t.Helper()
+	_, k, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return k
+}
+
+func newTestServer(t *testing.T) (*httptest.Server, ed25519.PublicKey) {
+	t.Helper()
+	priv := mustGenKey(t)
+	s := signer.NewInMemory()
+	s.AddKey("trust-api/test-key", priv)
+	srv := Server{
+		Signer:    s,
+		Policy:    policy.AllowAll{},
+		Validator: token.Static{"dev-token": "agent-admin-api"},
+		Now:       func() time.Time { return time.Date(2026, 10, 25, 18, 30, 0, 0, time.UTC) },
+	}
+	mux := http.NewServeMux()
+	srv.Mount(mux)
+	return httptest.NewServer(mux), priv.Public().(ed25519.PublicKey)
+}
+
+func post(url, bearer, body string) (*http.Response, error) {
+	req, _ := http.NewRequest("POST", url, bytes.NewBufferString(body))
+	req.Header.Set("Content-Type", "application/json")
+	if bearer != "" {
+		req.Header.Set("Authorization", "Bearer "+bearer)
+	}
+	return http.DefaultClient.Do(req)
+}
+
+func TestSign_HappyPath_VerifiesEndToEnd(t *testing.T) {
+	srv, pub := newTestServer(t)
+	defer srv.Close()
+	body := `{
+      "key_ref":"trust-api/test-key",
+      "statement":{
+        "_type":"https://in-toto.io/Statement/v1",
+        "subject":[{"name":"per_diem_rate/PDR-1","digest":{"sha256":"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"}}],
+        "predicateType":"https://basicstack.de/attestations/rate_change/v1",
+        "predicate":{"change_kind":"create","rate_id":"PDR-1"}
+      }
+    }`
+	resp, err := post(srv.URL+"/api/v1/sign/dsse", "dev-token", body)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("status %d", resp.StatusCode)
+	}
+	var out signResponse
+	if err := json.NewDecoder(resp.Body).Decode(&out); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if !strings.HasPrefix(out.ContentHash, "sha256:") || len(out.ContentHash) != 71 {
+		t.Fatalf("content_hash shape: %q", out.ContentHash)
+	}
+
+	// Independently verify the signature with the server's public key.
+	payload, err := base64.RawURLEncoding.DecodeString(out.Envelope.Payload)
+	if err != nil {
+		t.Fatalf("decode payload: %v", err)
+	}
+	pae := encodePAEForTest("application/vnd.in-toto+json", payload)
+	sig, err := base64.RawURLEncoding.DecodeString(out.Envelope.Signatures[0].Sig)
+	if err != nil {
+		t.Fatalf("decode sig: %v", err)
+	}
+	if !ed25519.Verify(pub, pae, sig) {
+		t.Fatal("returned signature does not verify against server public key")
+	}
+}
+
+func encodePAEForTest(payloadType string, payload []byte) []byte {
+	// Mirror of signer.encodePAE — kept local so this test does not depend on
+	// an unexported helper and so a drift between client and server would
+	// surface here.
+	s := "DSSEv1 "
+	s += itoa(len(payloadType)) + " " + payloadType + " " + itoa(len(payload)) + " "
+	return append([]byte(s), payload...)
+}
+
+func itoa(n int) string {
+	if n == 0 {
+		return "0"
+	}
+	var buf [20]byte
+	i := len(buf)
+	for n > 0 {
+		i--
+		buf[i] = byte('0' + n%10)
+		n /= 10
+	}
+	return string(buf[i:])
+}
+
+func TestSign_RejectsMissingBearer(t *testing.T) {
+	srv, _ := newTestServer(t)
+	defer srv.Close()
+	resp, _ := post(srv.URL+"/api/v1/sign/dsse", "", "{}")
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusUnauthorized {
+		t.Fatalf("expected 401, got %d", resp.StatusCode)
+	}
+}
+
+func TestSign_RejectsUnknownBearer(t *testing.T) {
+	srv, _ := newTestServer(t)
+	defer srv.Close()
+	resp, _ := post(srv.URL+"/api/v1/sign/dsse", "bogus", "{}")
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusUnauthorized {
+		t.Fatalf("expected 401, got %d", resp.StatusCode)
+	}
+}
+
+func TestSign_RejectsUnknownKeyRef(t *testing.T) {
+	srv, _ := newTestServer(t)
+	defer srv.Close()
+	body := `{"key_ref":"nope","statement":{"_type":"https://in-toto.io/Statement/v1","predicateType":"x"}}`
+	resp, _ := post(srv.URL+"/api/v1/sign/dsse", "dev-token", body)
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusBadRequest {
+		t.Fatalf("expected 400, got %d", resp.StatusCode)
+	}
+	var out errResponse
+	_ = json.NewDecoder(resp.Body).Decode(&out)
+	if out.Error.Code != "unknown_key" {
+		t.Errorf("expected code=unknown_key, got %q (%s)", out.Error.Code, out.Error.Message)
+	}
+}
+
+type denyAll struct{}
+
+func (denyAll) Allow(_ context.Context, _, _ string) (policy.Decision, error) {
+	return policy.Decision{Allow: false, Reason: "no such key allowed for this client"}, nil
+}
+
+func TestSign_RejectsOnPolicyDeny(t *testing.T) {
+	priv := mustGenKey(t)
+	s := signer.NewInMemory()
+	s.AddKey("k", priv)
+	srv := Server{
+		Signer:    s,
+		Policy:    denyAll{},
+		Validator: token.Static{"dev-token": "agent-x"},
+	}
+	mux := http.NewServeMux()
+	srv.Mount(mux)
+	hs := httptest.NewServer(mux)
+	defer hs.Close()
+	body := `{"key_ref":"k","statement":{"_type":"https://in-toto.io/Statement/v1","predicateType":"p"}}`
+	resp, _ := post(hs.URL+"/api/v1/sign/dsse", "dev-token", body)
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusForbidden {
+		t.Fatalf("expected 403, got %d", resp.StatusCode)
+	}
+	var out errResponse
+	_ = json.NewDecoder(resp.Body).Decode(&out)
+	if out.Error.Code != "signing_denied" {
+		t.Errorf("expected signing_denied, got %q", out.Error.Code)
+	}
+}
+
+func TestSign_RejectsBadStatement(t *testing.T) {
+	srv, _ := newTestServer(t)
+	defer srv.Close()
+	bad := []string{
+		`{"key_ref":"trust-api/test-key","statement":{}}`,
+		`{"key_ref":"trust-api/test-key","statement":{"_type":"wrong"}}`,
+		`{"key_ref":"trust-api/test-key","statement":{"_type":"https://in-toto.io/Statement/v1"}}`,
+		`{"key_ref":"","statement":{"_type":"https://in-toto.io/Statement/v1","predicateType":"p"}}`,
+	}
+	for i, b := range bad {
+		resp, _ := post(srv.URL+"/api/v1/sign/dsse", "dev-token", b)
+		if resp.StatusCode != http.StatusBadRequest {
+			t.Errorf("case %d: expected 400, got %d", i, resp.StatusCode)
+		}
+		resp.Body.Close()
+	}
+}
+
+func TestHealth(t *testing.T) {
+	srv, _ := newTestServer(t)
+	defer srv.Close()
+	resp, err := http.Get(srv.URL + "/healthz")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != 200 {
+		t.Fatalf("status %d", resp.StatusCode)
+	}
+}
diff --git a/services/signing-service/internal/policy/policy.go b/services/signing-service/internal/policy/policy.go
new file mode 100644
index 0000000..9554138
--- /dev/null
+++ b/services/signing-service/internal/policy/policy.go
@@ -0,0 +1,37 @@
+// Package policy decides whether a given subject (= agent / client identity
+// from the Pocket ID token) may sign a given key_ref. The in-prod backend is
+// OPA; the dev-mode backend is AllowAll. OPA wiring is a follow-up subtask.
+package policy
+
+import (
+	"context"
+	"errors"
+)
+
+// Decision carries both the boolean verdict and (on deny) a human-readable
+// reason that the HTTP layer surfaces in the error response.
+type Decision struct {
+	Allow  bool
+	Reason string
+}
+
+// Decider abstracts the policy backend.
+type Decider interface {
+	Allow(ctx context.Context, subject, keyRef string) (Decision, error)
+}
+
+// AllowAll permits everything. Dev / CI only. Logs loudly in main.
+type AllowAll struct{}
+
+func (AllowAll) Allow(_ context.Context, _, _ string) (Decision, error) {
+	return Decision{Allow: true}, nil
+}
+
+// OPA is a stub for the real Rego-backed policy evaluator.
+type OPA struct{}
+
+var errNotWired = errors.New("policy.OPA: not wired yet (follow-up subtask)")
+
+func (OPA) Allow(_ context.Context, _, _ string) (Decision, error) {
+	return Decision{}, errNotWired
+}
diff --git a/services/signing-service/internal/signer/signer.go b/services/signing-service/internal/signer/signer.go
new file mode 100644
index 0000000..aad7bbb
--- /dev/null
+++ b/services/signing-service/internal/signer/signer.go
@@ -0,0 +1,164 @@
+// 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
+}
diff --git a/services/signing-service/internal/signer/signer_test.go b/services/signing-service/internal/signer/signer_test.go
new file mode 100644
index 0000000..28d6a04
--- /dev/null
+++ b/services/signing-service/internal/signer/signer_test.go
@@ -0,0 +1,99 @@
+package signer
+
+import (
+	"context"
+	"crypto/ed25519"
+	"crypto/rand"
+	"encoding/base64"
+	"errors"
+	"testing"
+
+	"basicstack.de/proofedmesh/services/signing-service/internal/canonical"
+)
+
+func newKey(t *testing.T) ed25519.PrivateKey {
+	t.Helper()
+	_, priv, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return priv
+}
+
+func TestInMemory_SignRoundtrip(t *testing.T) {
+	priv := newKey(t)
+	m := NewInMemory()
+	m.AddKey("test-key", priv)
+
+	stmt := Statement{
+		"_type":         "https://in-toto.io/Statement/v1",
+		"predicateType": "https://basicstack.de/attestations/rate_change/v1",
+		"subject":       []any{map[string]any{"name": "per_diem_rate/PDR-1", "digest": map[string]any{"sha256": "deadbeef"}}},
+		"predicate":     map[string]any{"change_kind": "create"},
+	}
+	s, err := SignStatement(context.Background(), m, "test-key", stmt)
+	if err != nil {
+		t.Fatalf("sign: %v", err)
+	}
+	if s.Envelope.PayloadType != "application/vnd.in-toto+json" {
+		t.Errorf("payload type: %q", s.Envelope.PayloadType)
+	}
+	if len(s.Envelope.Signatures) != 1 {
+		t.Fatalf("signatures: %d", len(s.Envelope.Signatures))
+	}
+	if s.Envelope.Signatures[0].KeyID != "test-key" {
+		t.Errorf("keyid: %q", s.Envelope.Signatures[0].KeyID)
+	}
+
+	// Verify the signature verifies against the public key.
+	pub := priv.Public().(ed25519.PublicKey)
+	payloadBytes, err := base64.RawURLEncoding.DecodeString(s.Envelope.Payload)
+	if err != nil {
+		t.Fatalf("decode payload: %v", err)
+	}
+	pae := encodePAE("application/vnd.in-toto+json", payloadBytes)
+	sig, err := base64.RawURLEncoding.DecodeString(s.Envelope.Signatures[0].Sig)
+	if err != nil {
+		t.Fatalf("decode sig: %v", err)
+	}
+	if !ed25519.Verify(pub, pae, sig) {
+		t.Fatal("signature does not verify against public key")
+	}
+
+	// content_hash should be reproducible from the envelope.
+	envBytes, _ := canonical.Marshal(s.Envelope)
+	expected := "sha256:" + canonical.SHA256Hex(envBytes)
+	if s.ContentHash != expected {
+		t.Errorf("content_hash mismatch:\n got: %s\nwant: %s", s.ContentHash, expected)
+	}
+}
+
+func TestInMemory_UnknownKeyRef(t *testing.T) {
+	m := NewInMemory()
+	_, _, err := m.Sign(context.Background(), "nope", []byte("x"))
+	if !errors.Is(err, ErrUnknownKey) {
+		t.Fatalf("expected ErrUnknownKey, got %v", err)
+	}
+}
+
+func TestOpenBao_StubReturnsNotWired(t *testing.T) {
+	_, _, err := OpenBao{}.Sign(context.Background(), "k", []byte("x"))
+	if !errors.Is(err, ErrNotWired) {
+		t.Fatalf("expected ErrNotWired, got %v", err)
+	}
+}
+
+func TestEncodePAE_FormatIsExact(t *testing.T) {
+	got := encodePAE("application/vnd.in-toto+json", []byte("hello"))
+	// "application/vnd.in-toto+json" is 28 bytes, "hello" is 5 bytes.
+	want := []byte("DSSEv1 28 application/vnd.in-toto+json 5 hello")
+	if string(got) != string(want) {
+		t.Errorf("PAE mismatch:\n got: %q\nwant: %q", got, want)
+	}
+}
+
+func TestDecodeHexEd25519Key_RejectsWrongLength(t *testing.T) {
+	if _, err := DecodeHexEd25519Key("deadbeef"); err == nil {
+		t.Fatal("expected error for short key")
+	}
+}
diff --git a/services/signing-service/internal/token/validator.go b/services/signing-service/internal/token/validator.go
new file mode 100644
index 0000000..5f312aa
--- /dev/null
+++ b/services/signing-service/internal/token/validator.go
@@ -0,0 +1,50 @@
+// Package token validates incoming Bearer tokens and extracts the subject
+// (client agent identity). Prod uses Pocket ID JWKS; dev uses a static table.
+package token
+
+import (
+	"context"
+	"errors"
+	"net/http"
+	"strings"
+)
+
+// ErrInvalid is returned when a bearer token cannot be parsed or verified.
+var ErrInvalid = errors.New("token: invalid or missing bearer token")
+
+// Validator turns a raw bearer string into the authenticated client subject
+// (the Pocket ID `sub` claim, i.e. the client/agent identity).
+type Validator interface {
+	Validate(ctx context.Context, bearer string) (subject string, err error)
+}
+
+// Static is a dev-mode validator that accepts a fixed set of bearers → subjects.
+type Static map[string]string
+
+func (s Static) Validate(_ context.Context, bearer string) (string, error) {
+	if bearer == "" {
+		return "", ErrInvalid
+	}
+	sub, ok := s[bearer]
+	if !ok {
+		return "", ErrInvalid
+	}
+	return sub, nil
+}
+
+// PocketID is a stub for a JWKS-based Pocket ID token verifier.
+type PocketID struct{}
+
+func (PocketID) Validate(_ context.Context, _ string) (string, error) {
+	return "", errors.New("token.PocketID: not wired yet (follow-up subtask)")
+}
+
+// FromAuthHeader extracts the bearer from an "Authorization: Bearer …" header.
+func FromAuthHeader(r *http.Request) (string, error) {
+	h := r.Header.Get("Authorization")
+	const prefix = "Bearer "
+	if !strings.HasPrefix(h, prefix) {
+		return "", ErrInvalid
+	}
+	return strings.TrimPrefix(h, prefix), nil
+}