Generate larger, compliant serial numbers (#500)

UUIDs generate 16 byte serial numbers. Serial numbers can be no larger than 20 bytes. They also must be positive, giving us 159 bits to generate a serial number. Signed-off-by: Hayden Blauzvern <[email protected]>
sigstore · Apr 2, 2022 · 0b972e8 · 0b972e8
1 parent 5b27cd3
commit 0b972e8
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Showing 6 changed files with 60 additions and 16 deletions.
diff --git a/cmd/app/createca.go b/cmd/app/createca.go
@@ -24,13 +24,12 @@ import (
 	"crypto/x509/pkix"
 	"encoding/pem"
 	"fmt"
-	"math"
-	"math/big"
 	"os"
 	"path/filepath"
 	"time"
 
 	"github.com/ThalesIgnite/crypto11"
+	"github.com/sigstore/fulcio/pkg/ca/x509ca"
 	"github.com/sigstore/fulcio/pkg/log"
 	"github.com/spf13/cobra"
 	"github.com/spf13/viper"
@@ -106,8 +105,7 @@ func runCreateCACmd(cmd *cobra.Command, args []string) {
 
 	pubKey := privKey.Public()
 
-	// TODO: We could make it so this could be passed in by the user
-	serialNumber, err := rand.Int(rand.Reader, new(big.Int).SetInt64(math.MaxInt64))
+	serialNumber, err := x509ca.GenerateSerialNumber()
 	if err != nil {
 		log.Logger.Fatal(err)
 	}

diff --git a/go.mod b/go.mod
@@ -10,7 +10,6 @@ require (
 	github.com/fsnotify/fsnotify v1.5.1
 	github.com/go-chi/chi v4.1.2+incompatible
 	github.com/google/go-cmp v0.5.7
-	github.com/google/uuid v1.3.0
 	github.com/hashicorp/golang-lru v0.5.4
 	github.com/magiconair/properties v1.8.6
 	github.com/miekg/pkcs11 v1.1.1 // indirect

diff --git a/go.sum b/go.sum
@@ -543,7 +543,6 @@ github.com/google/uuid v1.0.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+
 github.com/google/uuid v1.1.1/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
 github.com/google/uuid v1.1.2/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
 github.com/google/uuid v1.2.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
-github.com/google/uuid v1.3.0 h1:t6JiXgmwXMjEs8VusXIJk2BXHsn+wx8BZdTaoZ5fu7I=
 github.com/google/uuid v1.3.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
 github.com/googleapis/gax-go/v2 v2.0.4/go.mod h1:0Wqv26UfaUD9n4G6kQubkQ+KchISgw+vpHVxEJEs9eg=
 github.com/googleapis/gax-go/v2 v2.0.5/go.mod h1:DWXyrwAJ9X0FpwwEdw+IPEYBICEFu5mhpdKc/us6bOk=

diff --git a/pkg/ca/ephemeralca/ephemeral.go b/pkg/ca/ephemeralca/ephemeral.go
@@ -19,8 +19,6 @@ import (
 	"crypto/rand"
 	"crypto/x509"
 	"crypto/x509/pkix"
-	"math"
-	"math/big"
 	"time"
 
 	"github.com/sigstore/fulcio/pkg/ca/x509ca"
@@ -42,8 +40,7 @@ func NewEphemeralCA() (*EphemeralCA, error) {
 
 	e.PrivKey = signer
 
-	// TODO: We could make it so this could be passed in by the user
-	serialNumber, err := rand.Int(rand.Reader, new(big.Int).SetInt64(math.MaxInt64))
+	serialNumber, err := x509ca.GenerateSerialNumber()
 	if err != nil {
 		return nil, err
 	}

diff --git a/pkg/ca/x509ca/common.go b/pkg/ca/x509ca/common.go
@@ -27,7 +27,7 @@ import (
 	"net/url"
 	"time"
 
-	"github.com/google/uuid"
+	"github.com/pkg/errors"
 	"github.com/sigstore/fulcio/pkg/ca"
 	"github.com/sigstore/fulcio/pkg/challenges"
 	"github.com/sigstore/sigstore/pkg/cryptoutils"
@@ -39,18 +39,18 @@ type X509CA struct {
 }
 
 func MakeX509(subject *challenges.ChallengeResult) (*x509.Certificate, error) {
-	// TODO: Track / increment serial nums instead, although unlikely we will create dupes, it could happen
-	uuid := uuid.New()
-	var serialNumber big.Int
-	serialNumber.SetBytes(uuid[:])
+	serialNumber, err := GenerateSerialNumber()
+	if err != nil {
+		return nil, err
+	}
 
 	skid, err := cryptoutils.SKID(subject.PublicKey)
 	if err != nil {
 		return nil, err
 	}
 
 	cert := &x509.Certificate{
-		SerialNumber: &serialNumber,
+		SerialNumber: serialNumber,
 		NotBefore:    time.Now(),
 		NotAfter:     time.Now().Add(time.Minute * 10),
 		SubjectKeyId: skid,
@@ -164,3 +164,16 @@ func IssuerExtension(issuer string) []pkix.Extension {
 		Value: []byte(issuer),
 	}}
 }
+
+// GenerateSerialNumber creates a compliant serial number as per RFC 5280 4.1.2.2.
+// Serial numbers must be positive, and can be no longer than 20 bytes.
+// The serial number is generated with 159 bits, so that the first bit will always
+// be 0, resulting in a positive serial number.
+func GenerateSerialNumber() (*big.Int, error) {
+	// Pick a random number from 0 to 2^159.
+	serial, err := rand.Int(rand.Reader, (&big.Int{}).Exp(big.NewInt(2), big.NewInt(159), nil))
+	if err != nil {
+		return nil, errors.Wrap(err, "error generating serial number")
+	}
+	return serial, nil
+}
diff --git a/pkg/ca/x509ca/common_test.go b/pkg/ca/x509ca/common_test.go
@@ -0,0 +1,38 @@
+// Copyright 2022 The Sigstore Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package x509ca
+
+import (
+	"math/big"
+	"testing"
+)
+
+func TestGenerateSerialNumber(t *testing.T) {
+	serialNumber, err := GenerateSerialNumber()
+	if err != nil {
+		t.Fatalf("unexpected error generating serial number: %v", err)
+	}
+	if serialNumber.Cmp(big.NewInt(0)) == -1 {
+		t.Fatalf("serial number is negative: %v", serialNumber)
+	}
+	if serialNumber.Cmp(big.NewInt(0)) == 0 {
+		t.Fatalf("serial number is 0: %v", serialNumber)
+	}
+	maxSerial := (&big.Int{}).Exp(big.NewInt(2), big.NewInt(159), nil)
+	// Serial number must be less than max serial number.
+	if serialNumber.Cmp(maxSerial) >= 0 {
+		t.Fatalf("serial number is too large: %v", serialNumber)
+	}
+}