x/crypto/ssh: Add Fingerprint method to PublicKey

Implement a standards-compliant fingerprint format method (RFC 4716 section 4)

Fixes golang/go#12292

Change-Id: I96562197a96c491ccb0b7b6f20435b2a9be30c20

ssh/certs.go

@@ -6,6 +6,7 @@ package ssh
 
 import (
 	"bytes"
+	"crypto/md5"
 	"errors"
 	"fmt"
 	"io"
@@ -470,6 +471,13 @@ func (c *Certificate) Verify(data []byte, sig *Signature) error {
 	return c.Key.Verify(data, sig)
 }
 
+// Fingerprint returns the key's fingerprint. It is part of the
+// PublicKey interface.
+func (c *Certificate) Fingerprint() string {
+	md5sum := md5.Sum(c.Key.Marshal())
+	return rfc4716hex(md5sum[:])
+}
+
 func parseSignatureBody(in []byte) (out *Signature, rest []byte, ok bool) {
 	format, in, ok := parseString(in)
 	if !ok {

ssh/keys.go

@@ -10,6 +10,7 @@ import (
 	"crypto/dsa"
 	"crypto/ecdsa"
 	"crypto/elliptic"
+	"crypto/md5"
 	"crypto/rsa"
 	"crypto/x509"
 	"encoding/asn1"
@@ -200,6 +201,10 @@ type PublicKey interface {
 	// Verify that sig is a signature on the given data using this
 	// key. This function will hash the data appropriately first.
 	Verify(data []byte, sig *Signature) error
+
+	// Fingerprint returns the user presentation of the key's
+	// fingerprint as described by RFC 4716 section 4.
+	Fingerprint() string
 }
 
 // A Signer can create signatures that verify against a public key.
@@ -217,6 +222,10 @@ type rsaPublicKey rsa.PublicKey
 func (r *rsaPublicKey) Type() string {
 	return "ssh-rsa"
 }
+func (r *rsaPublicKey) Fingerprint() string {
+	md5sum := md5.Sum(r.Marshal())
+	return rfc4716hex(md5sum[:])
+}
 
 // parseRSA parses an RSA key according to RFC 4253, section 6.6.
 func parseRSA(in []byte) (out PublicKey, rest []byte, err error) {
@@ -273,6 +282,11 @@ func (r *dsaPublicKey) Type() string {
 	return "ssh-dss"
 }
 
+func (r *dsaPublicKey) Fingerprint() string {
+	md5sum := md5.Sum(r.Marshal())
+	return rfc4716hex(md5sum[:])
+}
+
 // parseDSA parses an DSA key according to RFC 4253, section 6.6.
 func parseDSA(in []byte) (out PublicKey, rest []byte, err error) {
 	var w struct {
@@ -381,6 +395,11 @@ func (key *ecdsaPublicKey) nistID() string {
 	panic("ssh: unsupported ecdsa key size")
 }
 
+func (r *ecdsaPublicKey) Fingerprint() string {
+	md5sum := md5.Sum(r.Marshal())
+	return rfc4716hex(md5sum[:])
+}
+
 func supportedEllipticCurve(curve elliptic.Curve) bool {
 	return curve == elliptic.P256() || curve == elliptic.P384() || curve == elliptic.P521()
 }
@@ -644,3 +663,14 @@ func ParseDSAPrivateKey(der []byte) (*dsa.PrivateKey, error) {
 		X: k.Pub,
 	}, nil
 }
+
+func rfc4716hex(data []byte) string {
+	var fingerprint string
+	for i := 0; i < len(data); i++ {
+		fingerprint = fmt.Sprintf("%s%0.2x", fingerprint, data[i])
+		if i != len(data)-1 {
+			fingerprint = fingerprint + ":"
+		}
+	}
+	return fingerprint
+}

ssh/keys_test.go

@@ -304,3 +304,15 @@ func TestInvalidEntry(t *testing.T) {
 		t.Errorf("got valid entry for %q", authInvalid)
 	}
 }
+
+func TestFingerprint(t *testing.T) {
+	dsa := testPublicKeys["dsa"]
+	if dsa.Fingerprint() != "c9:67:63:e7:b5:34:5c:72:e3:7d:41:1b:cc:cd:89:28" {
+		t.Errorf("got invalid fingerprint %s", dsa.Fingerprint())
+	}
+
+	rsa := testPublicKeys["rsa"]
+	if rsa.Fingerprint() != "5c:00:08:96:4e:02:63:6c:ab:15:1c:b6:ea:6c:59:37" {
+		t.Errorf("got invalid fingerprint %s", rsa.Fingerprint())
+	}
+}