Use RFC 7093 truncated SHA256 hash for Subject Key Identifier (#7179)

- Adds a feature flag to gate rollout for SHA256 Subject Key Identifiers
for end-entity certificates.
- The ceremony tool will now use the RFC 7093 section 2 option 1 method
for generating Subject Key Identifiers for future root CA, intermediate
CA, and cross-sign ceremonies.

- - - -

[RFC 7093 section 2 option
1](https://datatracker.ietf.org/doc/html/rfc7093#section-2) provides a
method for generating a truncated SHA256 hash for the Subject Key
Identifier field in accordance with Baseline Requirement [section
7.1.2.11.4 Subject Key
Identifier](https://github.com/cabforum/servercert/blob/90a98dc7c1131eaab01af411968aa7330d315b9b/docs/BR.md#712114-subject-key-identifier).

> [RFC5280] specifies two examples for generating key identifiers from
>    public keys.  Four additional mechanisms are as follows:
> 
>    1) The keyIdentifier is composed of the leftmost 160-bits of the
>       SHA-256 hash of the value of the BIT STRING subjectPublicKey
>       (excluding the tag, length, and number of unused bits).

The related [RFC 5280 section
4.2.1.2](https://datatracker.ietf.org/doc/html/rfc5280#section-4.2.1.2)
states:
>   For CA certificates, subject key identifiers SHOULD be derived from
>   the public key or a method that generates unique values.  Two common
>   methods for generating key identifiers from the public key are:
>   ...
>   Other methods of generating unique numbers are also acceptable.

cmd/ceremony/cert.go

@@ -207,8 +207,13 @@ func generateSKID(pk []byte) ([]byte, error) {
 	if _, err := asn1.Unmarshal(pk, &pkixPublicKey); err != nil {
 		return nil, err
 	}
+
+	// RFC 7093 Section 2 Additional Methods for Generating Key Identifiers: The
+	// keyIdentifier [may be] composed of the leftmost 160-bits of the SHA-256
+	// hash of the value of the BIT STRING subjectPublicKey (excluding the tag,
+	// length, and number of unused bits).
 	skid := sha256.Sum256(pkixPublicKey.BitString.Bytes)
-	return skid[:], nil
+	return skid[0:20:20], nil
 }
 
 // makeTemplate generates the certificate template for use in x509.CreateCertificate

cmd/ceremony/cert_test.go

@@ -579,3 +579,10 @@ func TestLoadCert(t *testing.T) {
 	_, err = loadCert("../../test/test-root.pubkey.pem")
 	test.AssertError(t, err, "should have failed when trying to parse a public key")
 }
+
+func TestGenerateSKID(t *testing.T) {
+	sha256skid, err := generateSKID(samplePubkey())
+	test.AssertNotError(t, err, "Error generating SKID")
+	test.AssertEquals(t, len(sha256skid), 20)
+	test.AssertEquals(t, cap(sha256skid), 20)
+}

features/features.go

@@ -81,6 +81,11 @@ const (
 	// successful validations slower by serializing the DCV and CAA work, but
 	// makes unsuccessful validations easier by not doing CAA work at all.
 	CAAAfterValidation
+
+	// SHA256SubjectKeyIdentifier enables the generation and use of an RFC 7093
+	// compliant truncated SHA256 Subject Key Identifier in end-entity
+	// certificates.
+	SHA256SubjectKeyIdentifier
 )
 
 // List of features and their default value, protected by fMu
@@ -104,6 +109,7 @@ var features = map[FeatureFlag]bool{
 	AllowNoCommonName:                 false,
 	LeaseCRLShards:                    false,
 	CAAAfterValidation:                false,
+	SHA256SubjectKeyIdentifier:        false,
 
 	StoreLintingCertificateInsteadOfPrecertificate: false,
 }

issuance/cert.go

@@ -7,6 +7,7 @@ import (
 	"crypto/rand"
 	"crypto/rsa"
 	"crypto/sha1"
+	"crypto/sha256"
 	"crypto/x509"
 	"crypto/x509/pkix"
 	"encoding/asn1"
@@ -21,6 +22,7 @@ import (
 	ctx509 "github.com/google/certificate-transparency-go/x509"
 	"github.com/jmhodges/clock"
 
+	"github.com/letsencrypt/boulder/features"
 	"github.com/letsencrypt/boulder/precert"
 )
 
@@ -211,8 +213,18 @@ func generateSKID(pk crypto.PublicKey) ([]byte, error) {
 	if _, err := asn1.Unmarshal(pkBytes, &pkixPublicKey); err != nil {
 		return nil, err
 	}
-	skid := sha1.Sum(pkixPublicKey.BitString.Bytes)
-	return skid[:], nil
+
+	if features.Enabled(features.SHA256SubjectKeyIdentifier) {
+		// RFC 7093 Section 2 Additional Methods for Generating Key Identifiers:
+		// The keyIdentifier [may be] composed of the leftmost 160-bits of the
+		// SHA-256 hash of the value of the BIT STRING subjectPublicKey
+		// (excluding the tag, length, and number of unused bits).
+		skid := sha256.Sum256(pkixPublicKey.BitString.Bytes)
+		return skid[0:20:20], nil
+	} else {
+		skid := sha1.Sum(pkixPublicKey.BitString.Bytes)
+		return skid[:], nil
+	}
 }
 
 // IssuanceRequest describes a certificate issuance request

issuance/cert_test.go

@@ -18,6 +18,7 @@ import (
 	"github.com/jmhodges/clock"
 
 	"github.com/letsencrypt/boulder/ctpolicy/loglist"
+	"github.com/letsencrypt/boulder/features"
 	"github.com/letsencrypt/boulder/linter"
 	"github.com/letsencrypt/boulder/test"
 )
@@ -764,3 +765,25 @@ func TestMismatchedProfiles(t *testing.T) {
 	test.AssertError(t, err, "preparing final cert issuance")
 	test.AssertContains(t, err.Error(), "precert does not correspond to linted final cert")
 }
+
+func TestGenerateSKID(t *testing.T) {
+	key, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	test.AssertNotError(t, err, "Error generating key")
+
+	_ = features.Set(map[string]bool{"SHA256SubjectKeyIdentifier": true})
+	defer features.Reset()
+	// RFC 7093 section 2 method 1 allows us to use 160 of the leftmost bits for
+	// the Subject Key Identifier. This is the same amount of bits as the
+	// related SHA1 hash.
+	sha256skid, err := generateSKID(key.Public())
+	test.AssertNotError(t, err, "Error generating SKID")
+	test.AssertEquals(t, len(sha256skid), 20)
+	test.AssertEquals(t, cap(sha256skid), 20)
+	features.Reset()
+
+	_ = features.Set(map[string]bool{"SHA256SubjectKeyIdentifier": false})
+	sha1skid, err := generateSKID(key.Public())
+	test.AssertNotError(t, err, "Error generating SKID")
+	test.AssertEquals(t, len(sha1skid), 20)
+	test.AssertEquals(t, cap(sha1skid), 20)
+}

test/config-next/ca-a.json

@@ -113,7 +113,8 @@
 		"ctLogListFile": "test/ct-test-srv/log_list.json",
 		"features": {
 			"ECDSAForAll": true,
-			"AllowNoCommonName": true
+			"AllowNoCommonName": true,
+			"SHA256SubjectKeyIdentifier": true
 		}
 	},
 	"pa": {

test/config-next/ca-b.json

@@ -113,7 +113,8 @@
 		"ctLogListFile": "test/ct-test-srv/log_list.json",
 		"features": {
 			"ECDSAForAll": true,
-			"AllowNoCommonName": true
+			"AllowNoCommonName": true,
+			"SHA256SubjectKeyIdentifier": true
 		}
 	},
 	"pa": {