Move GenerateSKID into Core for sharing across ca, ceremony, and lints (#8922)

Create a new core.GenerateSKID function, which computes the Subject Key
Identifier value in the way we want (a truncated hash). Use this to
replace the two existing helper functions in the CA and Ceremony tool,
and to prepare for additional usage inside our upcoming CP/CPS lints.

Author: aarongable

ca/ca.go

@@ -3,12 +3,8 @@ package ca
 import (
 	"bytes"
 	"context"
-	"crypto"
 	"crypto/rand"
-	"crypto/sha256"
 	"crypto/x509"
-	"crypto/x509/pkix"
-	"encoding/asn1"
 	"encoding/hex"
 	"errors"
 	"fmt"
@@ -251,7 +247,7 @@ func (ca *certificateAuthorityImpl) IssueCertificate(ctx context.Context, req *c
 		return nil, err
 	}
 
-	subjectKeyId, err := generateSKID(csr.PublicKey)
+	subjectKeyId, err := core.GenerateSKID(csr.PublicKey)
 	if err != nil {
 		return nil, fmt.Errorf("computing subject key ID: %w", err)
 	}
@@ -492,29 +488,6 @@ func (ca *certificateAuthorityImpl) generateSerialNumber() *big.Int {
 	return serialBigInt
 }
 
-// generateSKID computes the Subject Key Identifier using one of the methods in
-// 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).
-func generateSKID(pk crypto.PublicKey) ([]byte, error) {
-	pkBytes, err := x509.MarshalPKIXPublicKey(pk)
-	if err != nil {
-		return nil, err
-	}
-
-	var pkixPublicKey struct {
-		Algo      pkix.AlgorithmIdentifier
-		BitString asn1.BitString
-	}
-	if _, err := asn1.Unmarshal(pkBytes, &pkixPublicKey); err != nil {
-		return nil, err
-	}
-
-	skid := sha256.Sum256(pkixPublicKey.BitString.Bytes)
-	return skid[0:20:20], nil
-}
-
 // verifyTBSCertIsDeterministic verifies that x509.CreateCertificate signing
 // operation is deterministic and produced identical DER bytes between the given
 // lint certificate and leaf certificate. If the DER byte equality check fails

ca/ca_test.go

@@ -941,18 +941,6 @@ func TestNoteSignError(t *testing.T) {
 	test.AssertMetricWithLabelsEquals(t, metrics.signErrorCount, prometheus.Labels{"type": "HSM"}, 1)
 }
 
-func TestGenerateSKID(t *testing.T) {
-	t.Parallel()
-	key, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
-	test.AssertNotError(t, err, "Error generating key")
-
-	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()
-}
-
 func TestVerifyTBSCertIsDeterministic(t *testing.T) {
 	t.Parallel()
 

cmd/ceremony/cert.go

@@ -2,17 +2,17 @@ package main
 
 import (
 	"crypto"
-	"crypto/sha256"
 	"crypto/x509"
 	"crypto/x509/pkix"
-	"encoding/asn1"
 	"errors"
 	"fmt"
 	"io"
 	"math/big"
 	"regexp"
 	"slices"
 	"time"
+
+	"github.com/letsencrypt/boulder/core"
 )
 
 type policyInfoConfig struct {
@@ -187,25 +187,8 @@ var stringToKeyUsage = map[string]x509.KeyUsage{
 	"Cert Sign":         x509.KeyUsageCertSign,
 }
 
-func generateSKID(pk []byte) ([]byte, error) {
-	var pkixPublicKey struct {
-		Algo      pkix.AlgorithmIdentifier
-		BitString asn1.BitString
-	}
-	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[0:20:20], nil
-}
-
 // makeTemplate generates the certificate template for use in x509.CreateCertificate
-func makeTemplate(randReader io.Reader, profile *certProfile, pubKey []byte, tbcs *x509.Certificate, ct certType) (*x509.Certificate, error) {
+func makeTemplate(randReader io.Reader, profile *certProfile, pubKey crypto.PublicKey, tbcs *x509.Certificate, ct certType) (*x509.Certificate, error) {
 	// Handle "unrestricted" vs "restricted" subordinate CA profile specifics.
 	if ct == crossCert && tbcs == nil {
 		return nil, fmt.Errorf("toBeCrossSigned cert field was nil, but was required to gather EKUs for the lint cert")
@@ -220,7 +203,7 @@ func makeTemplate(randReader io.Reader, profile *certProfile, pubKey []byte, tbc
 		issuingCertificateURL = []string{profile.IssuerURL}
 	}
 
-	subjectKeyID, err := generateSKID(pubKey)
+	subjectKeyID, err := core.GenerateSKID(pubKey)
 	if err != nil {
 		return nil, err
 	}