crypto/x509: CertificateRequest signature verification

This implements a method for x509.CertificateRequest to prevent
certain attacks and to allow a CA/RA to properly check the validity
of the binding between an end entity and a key pair, to prove that
it has possession of (i.e., is able to use) the private key
corresponding to the public key for which a certificate is requested.

RFC 2986 section 3 states:

"A certification authority fulfills the request by authenticating the
requesting entity and verifying the entity's signature, and, if the
request is valid, constructing an X.509 certificate from the
distinguished name and public key, the issuer name, and the
certification authority's choice of serial number, validity period,
and signature algorithm."

Change-Id: I37795c3b1dfdfdd455d870e499b63885eb9bda4f
Reviewed-on: https://go-review.googlesource.com/7371Reviewed-by: Adam Langley <agl@golang.org>

src/crypto/x509/x509.go

@@ -619,6 +619,12 @@ func (c *Certificate) CheckSignatureFrom(parent *Certificate) (err error) {
 // CheckSignature verifies that signature is a valid signature over signed from
 // c's public key.
 func (c *Certificate) CheckSignature(algo SignatureAlgorithm, signed, signature []byte) (err error) {
+	return checkSignature(algo, signed, signature, c.PublicKey)
+}
+
+// CheckSignature verifies that signature is a valid signature over signed from
+// a crypto.PublicKey.
+func checkSignature(algo SignatureAlgorithm, signed, signature []byte, publicKey crypto.PublicKey) (err error) {
 	var hashType crypto.Hash
 
 	switch algo {
@@ -642,7 +648,7 @@ func (c *Certificate) CheckSignature(algo SignatureAlgorithm, signed, signature
 	h.Write(signed)
 	digest := h.Sum(nil)
 
-	switch pub := c.PublicKey.(type) {
+	switch pub := publicKey.(type) {
 	case *rsa.PublicKey:
 		return rsa.VerifyPKCS1v15(pub, hashType, digest, signature)
 	case *dsa.PublicKey:
@@ -1955,3 +1961,8 @@ func parseCertificateRequest(in *certificateRequest) (*CertificateRequest, error
 
 	return out, nil
 }
+
+// CheckSignature verifies that the signature on c is a valid signature
+func (c *CertificateRequest) CheckSignature() (err error) {
+	return checkSignature(c.SignatureAlgorithm, c.RawTBSCertificateRequest, c.Signature, c.PublicKey)
+}

src/crypto/x509/x509_test.go

@@ -904,6 +904,12 @@ func TestCreateCertificateRequest(t *testing.T) {
 			continue
 		}
 
+		err = out.CheckSignature()
+		if err != nil {
+			t.Errorf("%s: failed to check certificate request signature: %s", test.name, err)
+			continue
+		}
+
 		if out.Subject.CommonName != template.Subject.CommonName {
 			t.Errorf("%s: output subject common name and template subject common name don't match", test.name)
 		} else if len(out.Subject.Organization) != len(template.Subject.Organization) {