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package instance
import (
"bytes"
"context"
"crypto"
"crypto/ed25519"
"crypto/sha256"
"encoding/base64"
"encoding/binary"
"fmt"
"net/http"
"git.sigsum.org/sigsum-go/pkg/log"
"git.sigsum.org/sigsum-go/pkg/types"
)
// 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) {
log.Debug("handling get-checkpoint request")
sth, err := i.Stateman.ToCosignTreeHead(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)
}
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