demo: fix UnusedBit panic on signing error

Author: Vaisman

demo/encode.go

@@ -406,8 +406,9 @@ func CreateCertificate(config *CAConfig, issuanceLog *MerkleTree, cosigners []*C
 				}
 			})
 			if certConfig.UnusedBit {
-				if sig, err := certSig.Bytes(); err == nil && len(sig) == 0 || sig[len(sig)-1]&1 != 0 {
-					certSig.SetError(errors.New("last bit in signature with not zero, unable to encode as unused"))
+				sig, err := certSig.Bytes()
+				if err == nil && (len(sig) == 0 || sig[len(sig)-1]&1 != 0) {
+					certSig.SetError(errors.New("last bit in signature is not zero, unable to encode as unused"))
 					return
 				}
 			}

demo/encode_unusedbit_test.go

@@ -0,0 +1,85 @@
+package main
+
+import (
+	"crypto"
+	"crypto/ed25519"
+	"errors"
+	"io"
+	"testing"
+	"time"
+)
+
+type failingSigner struct{}
+
+func (failingSigner) Public() crypto.PublicKey {
+	return ed25519.PublicKey(make([]byte, ed25519.PublicKeySize))
+}
+
+func (failingSigner) Sign(io.Reader, []byte, crypto.SignerOpts) ([]byte, error) {
+	return nil, errors.New("simulated signing failure")
+}
+
+func TestCreateCertificateUnusedBitCosignerError(t *testing.T) {
+	issuer, ok := TrustAnchorIDFromString("32473.1")
+	if !ok {
+		t.Fatalf("could not make issuer trust anchor ID")
+	}
+	cosignerID, ok := TrustAnchorIDFromString("32473.2")
+	if !ok {
+		t.Fatalf("could not make cosigner trust anchor ID")
+	}
+
+	publicKey := []byte{
+		0x30, 0x59, 0x30, 0x13, 0x06, 0x07, 0x2a, 0x86, 0x48, 0xce, 0x3d, 0x02,
+		0x01, 0x06, 0x08, 0x2a, 0x86, 0x48, 0xce, 0x3d, 0x03, 0x01, 0x07, 0x03,
+		0x42, 0x00, 0x04, 0xe6, 0x2b, 0x69, 0xe2, 0xbf, 0x65, 0x9f, 0x97, 0xbe,
+		0x2f, 0x1e, 0x0d, 0x94, 0x8a, 0x4c, 0xd5, 0x97, 0x6b, 0xb7, 0xa9, 0x1e,
+		0x0d, 0x46, 0xfb, 0xdd, 0xa9, 0xa9, 0x1e, 0x9d, 0xdc, 0xba, 0x5a, 0x01,
+		0xe7, 0xd6, 0x97, 0xa8, 0x0a, 0x18, 0xf9, 0xc3, 0xc4, 0xa3, 0x1e, 0x56,
+		0xe2, 0x7c, 0x83, 0x48, 0xdb, 0x16, 0x1a, 0x1c, 0xf5, 0x1d, 0x7e, 0xf1,
+		0x94, 0x2d, 0x4b, 0xcf, 0x72, 0x22, 0xc1,
+	}
+
+	entry := &EntryConfig{
+		PublicKey: publicKey,
+		CertConfigBase: CertConfigBase{
+			NotBefore: time.Unix(1577836800, 0),
+			NotAfter:  time.Unix(1609459199, 0),
+		},
+	}
+
+	logEntry, err := MarshalTBSCertificateLogEntry(VersionPlants04, issuer, entry)
+	if err != nil {
+		t.Fatalf("MarshalTBSCertificateLogEntry: %s", err)
+	}
+	tree := NewMerkleTree([][]byte{logEntry})
+
+	config := &CAConfig{
+		Version:   VersionPlants04,
+		ID:        issuer,
+		LogNumber: 1,
+	}
+
+	failingCosigner := &Cosigner{
+		Version:            VersionPlants04,
+		ID:                 cosignerID,
+		SignatureAlgorithm: SignatureAlgorithmEd25519,
+		Signer:             failingSigner{},
+		SignerOpts:         crypto.Hash(0),
+	}
+
+	certConfig := &CertificateConfig{
+		Cosigners: []TrustAnchorID{cosignerID},
+		UnusedBit: true,
+	}
+
+	defer func() {
+		if r := recover(); r != nil {
+			t.Fatalf("CreateCertificate panicked instead of returning an error: %v", r)
+		}
+	}()
+
+	if _, err := CreateCertificate(config, tree, []*Cosigner{failingCosigner}, entry, certConfig, 0, 0, 1); err == nil {
+		t.Fatalf("CreateCertificate succeeded, want an error from the failing cosigner")
+	}
+}