4 files changed, 87 insertions, 18 deletions
diff --git a/issues/add-checkpoint-support.md b/issues/add-checkpoint-support.md
deleted file mode 100644
index f1d7951..0000000
--- a/issues/add-checkpoint-support.md
+++ /dev/null
@@ -1,18 +0,0 @@
-**Title:** Add checkpoint support </br>
-**Date:** 2021-12-09 </br>
-
-# Summary
-Add experimental checkpoint support.
-
-# Description
-Sigsum collaborated on a common
-	[checkpoint format](https://github.com/google/trillian-examples/tree/master/formats/log)
-a while back.  A checkpoint is basically a cosigned tree head.
-
-The current decision is to add experimental support for checkpoints.  There is
-no formal decision to adopt the above checkpoint yet, hence _experimental_.
-
-To keep it simple:
-1. Don't add any timestamp extension.
-2. Only serve the most recent tree head as a checkpoint.  This allows us to
-experiment with external feeders and distributors that are not part of the log.
diff --git a/pkg/instance/experimental_endpoint.go b/pkg/instance/experimental_endpoint.go
new file mode 100644
index 0000000..2986a27
--- /dev/null
+++ b/pkg/instance/experimental_endpoint.go
@@ -0,0 +1,85 @@
+package instance
+
+import (
+	"bytes"
+	"context"
+	"crypto"
+	"crypto/ed25519"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/binary"
+	"fmt"
+	"net/http"
+
+	"git.sigsum.org/sigsum-log-go/pkg/types"
+	"github.com/golang/glog"
+)
+
+// algEd25519 identifies a checkpoint signature algorithm
+const algEd25519 byte = 1
+
+// getCheckpoint is an experimental endpoint that is not part of the official
+// Sigsum API.  Documentation can be found in the transparency-dev repo.
+func getCheckpoint(ctx context.Context, i *Instance, w http.ResponseWriter, r *http.Request) (int, error) {
+	glog.V(3).Info("handling get-checkpoint request")
+	sth, err := i.Stateman.ToSign(ctx)
+	if err != nil {
+		return http.StatusInternalServerError, err
+	}
+	if err := i.signWriteNote(w, sth); err != nil {
+		return http.StatusInternalServerError, err
+	}
+	return http.StatusOK, nil
+}
+
+// signWriteNote signs and writes a checkpoint which uses "sigsum.org:<prefix>"
+// as origin string.  Origin string is also used as ID in the note signature.
+// This means that a sigsum log's prefix (say, "glass-frog"), must be unique.
+func (i *Instance) signWriteNote(w http.ResponseWriter, sth *types.SignedTreeHead) error {
+	origin := fmt.Sprintf("sigsum.org:%s", i.Prefix)
+	msg := fmt.Sprintf("%s\n%d\n%s\n",
+		origin,
+		sth.TreeSize,
+		base64.StdEncoding.EncodeToString(sth.RootHash[:]),
+	)
+	sig, err := noteSign(i.Signer, origin, msg)
+	if err != nil {
+		return err
+	}
+
+	fmt.Fprintf(w, "%s\n\u2014 %s %s\n", msg, origin, sig)
+	return nil
+}
+
+// noteSign returns a note signature for the provided origin and message
+func noteSign(signer crypto.Signer, origin, msg string) (string, error) {
+	sig, err := signer.Sign(nil, []byte(msg), crypto.Hash(0))
+	if err != nil {
+		return "", err
+	}
+
+	var hbuf [4]byte
+	binary.BigEndian.PutUint32(hbuf[:], noteKeyHash(origin, notePubKeyEd25519(signer)))
+	sig = append(hbuf[:], sig...)
+	return base64.StdEncoding.EncodeToString(sig), nil
+}
+
+// See:
+// https://cs.opensource.google/go/x/mod/+/refs/tags/v0.5.1:sumdb/note/note.go;l=336
+func notePubKeyEd25519(signer crypto.Signer) []byte {
+	return bytes.Join([][]byte{
+		[]byte{algEd25519},
+		signer.Public().(ed25519.PublicKey),
+	}, nil)
+}
+
+// Source:
+// https://cs.opensource.google/go/x/mod/+/refs/tags/v0.5.1:sumdb/note/note.go;l=222
+func noteKeyHash(name string, key []byte) uint32 {
+	h := sha256.New()
+	h.Write([]byte(name))
+	h.Write([]byte("\n"))
+	h.Write(key)
+	sum := h.Sum(nil)
+	return binary.BigEndian.Uint32(sum)
+}
diff --git a/pkg/instance/instance.go b/pkg/instance/instance.go
index bda553d..4dff31a 100644
--- a/pkg/instance/instance.go
+++ b/pkg/instance/instance.go
@@ -55,6 +55,7 @@ func (i *Instance) Handlers() []Handler {
 		Handler{Instance: i, Handler: getTreeHeadLatest, Endpoint: types.EndpointGetTreeHeadLatest, Method: http.MethodGet},
 		Handler{Instance: i, Handler: getTreeHeadToSign, Endpoint: types.EndpointGetTreeHeadToSign, Method: http.MethodGet},
 		Handler{Instance: i, Handler: getTreeHeadCosigned, Endpoint: types.EndpointGetTreeHeadCosigned, Method: http.MethodGet},
+		Handler{Instance: i, Handler: getCheckpoint, Endpoint: types.Endpoint("get-checkpoint"), Method: http.MethodGet},
 		Handler{Instance: i, Handler: getConsistencyProof, Endpoint: types.EndpointGetConsistencyProof, Method: http.MethodPost},
 		Handler{Instance: i, Handler: getInclusionProof, Endpoint: types.EndpointGetInclusionProof, Method: http.MethodPost},
 		Handler{Instance: i, Handler: getLeaves, Endpoint: types.EndpointGetLeaves, Method: http.MethodPost},
diff --git a/pkg/instance/instance_test.go b/pkg/instance/instance_test.go
index 9eeee5b..6ca7baf 100644
--- a/pkg/instance/instance_test.go
+++ b/pkg/instance/instance_test.go
@@ -19,6 +19,7 @@ func TestHandlers(t *testing.T) {
 		types.EndpointGetConsistencyProof: false,
 		types.EndpointGetInclusionProof:   false,
 		types.EndpointGetLeaves:           false,
+		types.Endpoint("get-checkpoint"):  false,
 	}
 	i := &Instance{
 		Config: testConfig,