chore: update vendor

vendor/github.com/google/certificate-transparency-go/signatures.go

@@ -20,6 +20,7 @@ import (
 	"crypto/elliptic"
 	"crypto/rsa"
 	"crypto/sha256"
+	"encoding/base64"
 	"encoding/pem"
 	"flag"
 	"fmt"
@@ -42,6 +43,15 @@ func PublicKeyFromPEM(b []byte) (crypto.PublicKey, SHA256Hash, []byte, error) {
 	return k, sha256.Sum256(p.Bytes), rest, err
 }
 
+// PublicKeyFromB64 parses a base64-encoded public key.
+func PublicKeyFromB64(b64PubKey string) (crypto.PublicKey, error) {
+	der, err := base64.StdEncoding.DecodeString(b64PubKey)
+	if err != nil {
+		return nil, fmt.Errorf("error decoding public key: %s", err)
+	}
+	return x509.ParsePKIXPublicKey(der)
+}
+
 // SignatureVerifier can verify signatures on SCTs and STHs
 type SignatureVerifier struct {
 	pubKey crypto.PublicKey