ssh: support ECDSA private keys in OpenSSH format

This adds support for parsing OpenSSH ECDSA private keys. It
implements parsing for P-256, P-384, and P-521 keys.

Fixes golang/go#36722

Change-Id: I77c8e0a23ed6353f6667686cc79ec14661cb10db
GitHub-Last-Rev: 2324b920d080fc7ac35fbcf0a79e25161b6a7f82
GitHub-Pull-Request: golang/crypto#114
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/215540
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

2 files changed, 82 insertions, 0 deletions

diff --git a/ssh/keys.go b/ssh/keys.go
index 5377ec8..d63cbf6 100644
--- a/ssh/keys.go
+++ b/ssh/keys.go
@@ -1366,6 +1366,57 @@ func parseOpenSSHPrivateKey(key []byte, decrypt openSSHDecryptFunc) (crypto.Priv
 		pk := ed25519.PrivateKey(make([]byte, ed25519.PrivateKeySize))
 		copy(pk, key.Priv)
 		return &pk, nil
+	case KeyAlgoECDSA256, KeyAlgoECDSA384, KeyAlgoECDSA521:
+		key := struct {
+			Curve   string
+			Pub     []byte
+			D       *big.Int
+			Comment string
+			Pad     []byte `ssh:"rest"`
+		}{}
+
+		if err := Unmarshal(pk1.Rest, &key); err != nil {
+			return nil, err
+		}
+
+		if err := checkOpenSSHKeyPadding(key.Pad); err != nil {
+			return nil, err
+		}
+
+		var curve elliptic.Curve
+		switch key.Curve {
+		case "nistp256":
+			curve = elliptic.P256()
+		case "nistp384":
+			curve = elliptic.P384()
+		case "nistp521":
+			curve = elliptic.P521()
+		default:
+			return nil, errors.New("ssh: unhandled elliptic curve: " + key.Curve)
+		}
+
+		X, Y := elliptic.Unmarshal(curve, key.Pub)
+		if X == nil || Y == nil {
+			return nil, errors.New("ssh: failed to unmarshal public key")
+		}
+
+		if key.D.Cmp(curve.Params().N) >= 0 {
+			return nil, errors.New("ssh: scalar is out of range")
+		}
+
+		x, y := curve.ScalarBaseMult(key.D.Bytes())
+		if x.Cmp(X) != 0 || y.Cmp(Y) != 0 {
+			return nil, errors.New("ssh: public key does not match private key")
+		}
+
+		return &ecdsa.PrivateKey{
+			PublicKey: ecdsa.PublicKey{
+				Curve: curve,
+				X:     X,
+				Y:     Y,
+			},
+			D: key.D,
+		}, nil
 	default:
 		return nil, errors.New("ssh: unhandled key type")
 	}
diff --git a/ssh/testdata/keys.go b/ssh/testdata/keys.go
index 72cfaa0..a7da078 100644
--- a/ssh/testdata/keys.go
+++ b/ssh/testdata/keys.go
@@ -114,6 +114,37 @@ X1ViJuqqcQnJPVzpgSL826EC2xwOECTqoY8uvFpUdD7CtpksIxNVqRIhuNOlz0lqEAAABB
 ANkaHTTaPojClO0dKJ/Zjs7pWOCGliebBYprQ/Y4r9QLBkC/XaWMS26gFIrjgC7D2Rv+rZ
 wSD0v0RcmkITP1ZR0AAAAYcHF1ZXJuYUBMdWNreUh5ZHJvLmxvY2FsAQID
 -----END OPENSSH PRIVATE KEY-----`),
+	"p256-openssh-format": []byte(`-----BEGIN OPENSSH PRIVATE KEY-----
+b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAAaAAAABNlY2RzYS
+1zaGEyLW5pc3RwMjU2AAAACG5pc3RwMjU2AAAAQQSN5Ld/DFy8LJK0yrWg+Ryhq4/ifHry
+QyCQeT4UXSB+UGdRct7kWA0hARbTaSCh+8U/Gs5O+IkDNoTKVsgxKUMQAAAAsO3C7nPtwu
+5zAAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBI3kt38MXLwskrTK
+taD5HKGrj+J8evJDIJB5PhRdIH5QZ1Fy3uRYDSEBFtNpIKH7xT8azk74iQM2hMpWyDEpQx
+AAAAAhAIHB48R+goZaiXndfYTrwk4BT1+MeLPC2/dwe0J5d1QDAAAAE21hcmlhbm9AZW5k
+b3IubG9jYWwBAgME
+-----END OPENSSH PRIVATE KEY-----`),
+	"p384-openssh-format": []byte(`-----BEGIN OPENSSH PRIVATE KEY-----
+b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAAiAAAABNlY2RzYS
+1zaGEyLW5pc3RwMzg0AAAACG5pc3RwMzg0AAAAYQTZb2VzEPs2NN/i1qHddKTVfwoIq3Tf
+PeQ/kcWBvuCVJfIygvpm9MeusawEPuLSEXwiNDew+YHZ9xHIvFjCmZsLuEOzuh9t9KotwM
+57H+7N+RDFzhM2j8hAaOuT5XDLKfUAAADgn/Sny5/0p8sAAAATZWNkc2Etc2hhMi1uaXN0
+cDM4NAAAAAhuaXN0cDM4NAAAAGEE2W9lcxD7NjTf4tah3XSk1X8KCKt03z3kP5HFgb7glS
+XyMoL6ZvTHrrGsBD7i0hF8IjQ3sPmB2fcRyLxYwpmbC7hDs7ofbfSqLcDOex/uzfkQxc4T
+No/IQGjrk+Vwyyn1AAAAMQDg0hwGKB/9Eq+e2FeTspi8QHW5xTD6prqsHDFx4cKk0ccgFV
+61dhFhD/8SEbYlHzEAAAATbWFyaWFub0BlbmRvci5sb2NhbAECAwQ=
+-----END OPENSSH PRIVATE KEY-----`),
+	"p521-openssh-format": []byte(`-----BEGIN OPENSSH PRIVATE KEY-----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+-----END OPENSSH PRIVATE KEY-----`),
 	"user": []byte(`-----BEGIN EC PRIVATE KEY-----
 MHcCAQEEILYCAeq8f7V4vSSypRw7pxy8yz3V5W4qg8kSC3zJhqpQoAoGCCqGSM49
 AwEHoUQDQgAEYcO2xNKiRUYOLEHM7VYAp57HNyKbOdYtHD83Z4hzNPVC4tM5mdGD