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Use RFC 7093 SHA-512 truncated SKIs #551

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13 changes: 8 additions & 5 deletions ca/ca.go
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Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,7 @@ import (
"crypto/elliptic"
"crypto/rand"
"crypto/rsa"
"crypto/sha1"
"crypto/sha512"
"crypto/x509"
"crypto/x509/pkix"
"encoding/asn1"
Expand Down Expand Up @@ -76,7 +76,11 @@ func makeSerial() *big.Int {
return serial
}

// Taken from https://github.com/cloudflare/cfssl/blob/b94e044bb51ec8f5a7232c71b1ed05dbe4da96ce/signer/signer.go#L221-L244
// makeSubjectKeyID generates a Subject Key Identifier according to
// RFC 7093 Section 2 method 3: the leftmost 160 bits of the SHA-512 hash of
// the subjectPublicKey BIT STRING (excluding the tag, length, and number of
// unused bits). This differs from Boulder's RFC 7093 Section 2 method 1
// (truncated SHA-256) so ACME clients cannot overfit on Let's Encrypt.
func makeSubjectKeyID(key crypto.PublicKey) ([]byte, error) {
// Marshal the public key as ASN.1
pubAsDER, err := x509.MarshalPKIXPublicKey(key)
Expand All @@ -94,9 +98,8 @@ func makeSubjectKeyID(key crypto.PublicKey) ([]byte, error) {
return nil, err
}

// Hash it according to https://tools.ietf.org/html/rfc5280#section-4.2.1.2 Method #1:
ski := sha1.Sum(pubInfo.SubjectPublicKey.Bytes)
return ski[:], nil
ski := sha512.Sum512(pubInfo.SubjectPublicKey.Bytes)
return ski[:20], nil
}

// makeKey and makeRootCert are adapted from MiniCA:
Expand Down
28 changes: 28 additions & 0 deletions ca/ca_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,7 @@
"crypto/x509"
"crypto/x509/pkix"
"encoding/asn1"
"encoding/hex"
"log"
"net"
"os"
Expand All @@ -30,6 +31,33 @@
return New(logger, db, "", "ecdsa", 0, 1, map[string]Profile{"default": {}})
}

func TestMakeSubjectKeyID(t *testing.T) {
// RFC 7093 Section 3 example P-256 public key.
publicKeyBytes, err := hex.DecodeString(
"047f7f35a79794c950060b8029fc8f363a28f11159692d9d34e6ac948190434735" +
"f833b1a66652dc514337aff7f5c9c75d670c019d95a5d639b72744c64a9128bb",
)
if err != nil {
t.Fatal(err)
}
x, y := elliptic.Unmarshal(elliptic.P256(), publicKeyBytes)

Check failure on line 43 in ca/ca_test.go

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GitHub Actions / go-lint-checks 

SA1019: elliptic.Unmarshal has been deprecated since Go 1.21: for ECDH, use the crypto/ecdh package. This function accepts an encoding equivalent to that of the NewPublicKey methods in crypto/ecdh. (staticcheck)

@aarongable aarongable Sep 11, 2026

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See lint finding: use crypto/ecdh instead of elliptic.Unmarshal

if x == nil || y == nil {
t.Fatal("failed to parse RFC 7093 public key")
}

got, err := makeSubjectKeyID(&ecdsa.PublicKey{Curve: elliptic.P256(), X: x, Y: y})
if err != nil {
t.Fatal(err)
}
want, err := hex.DecodeString("907e7e9d05878a273d597f2aea91bdb6056245cb")
if err != nil {
t.Fatal(err)
}
if !bytes.Equal(got, want) {
t.Fatalf("unexpected subject key identifier: got %x, want %x", got, want)
}
}

func makeCertOrderWithExtensions(extensions []pkix.Extension) core.Order {
privateKey, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
if err != nil {
Expand Down
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