Support NIST curve P256 (#769)

* Support NIST curve P256

This change adds support for NIST curve P256. When you use `nebula-cert ca`
or `nebula-cert keygen`, you can specify `-curve P256` to enable it. The
curve to use is based on the curve defined in your CA certificate.

Internally, we use ECDSA P256 to sign certificates, and ECDH P256 to do
Noise handshakes. P256 is not supported natively in Noise Protocol, so
we define `DHP256` in the `noiseutil` package to implement support for
it.

You cannot have a mixed network of Curve25519 and P256 certificates,
since the Noise protocol will only attempt to parse using the Curve
defined in the host's certificate.

* verify the curves match in VerifyPrivateKey

This would have failed anyways once we tried to actually use the bytes
in the private key, but its better to detect the issue up front with
a better error message.

* add cert.Curve argument to Sign method

* fix mismerge

* use crypto/ecdh

This is the preferred method for doing ECDH functions now, and also has
a boringcrypto specific codepath.

* remove other ecdh uses of crypto/elliptic

use crypto/ecdh instead

cmd/nebula-cert/sign.go

@@ -1,7 +1,7 @@
 package main
 
 import (
-	"crypto/ed25519"
+	"crypto/ecdh"
 	"crypto/rand"
 	"flag"
 	"fmt"
@@ -78,10 +78,11 @@ func signCert(args []string, out io.Writer, errOut io.Writer, pr PasswordReader)
 		return fmt.Errorf("error while reading ca-key: %s", err)
 	}
 
-	var caKey ed25519.PrivateKey
+	var curve cert.Curve
+	var caKey []byte
 
 	// naively attempt to decode the private key as though it is not encrypted
-	caKey, _, err = cert.UnmarshalEd25519PrivateKey(rawCAKey)
+	caKey, _, curve, err = cert.UnmarshalSigningPrivateKey(rawCAKey)
 	if err == cert.ErrPrivateKeyEncrypted {
 		// ask for a passphrase until we get one
 		var passphrase []byte
@@ -103,7 +104,7 @@ func signCert(args []string, out io.Writer, errOut io.Writer, pr PasswordReader)
 			return fmt.Errorf("cannot open encrypted ca-key without passphrase")
 		}
 
-		caKey, _, err = cert.DecryptAndUnmarshalEd25519PrivateKey(passphrase, rawCAKey)
+		curve, caKey, _, err = cert.DecryptAndUnmarshalSigningPrivateKey(passphrase, rawCAKey)
 		if err != nil {
 			return fmt.Errorf("error while parsing encrypted ca-key: %s", err)
 		}
@@ -121,7 +122,7 @@ func signCert(args []string, out io.Writer, errOut io.Writer, pr PasswordReader)
 		return fmt.Errorf("error while parsing ca-crt: %s", err)
 	}
 
-	if err := caCert.VerifyPrivateKey(caKey); err != nil {
+	if err := caCert.VerifyPrivateKey(curve, caKey); err != nil {
 		return fmt.Errorf("refusing to sign, root certificate does not match private key")
 	}
 
@@ -181,12 +182,16 @@ func signCert(args []string, out io.Writer, errOut io.Writer, pr PasswordReader)
 		if err != nil {
 			return fmt.Errorf("error while reading in-pub: %s", err)
 		}
-		pub, _, err = cert.UnmarshalX25519PublicKey(rawPub)
+		var pubCurve cert.Curve
+		pub, _, pubCurve, err = cert.UnmarshalPublicKey(rawPub)
 		if err != nil {
 			return fmt.Errorf("error while parsing in-pub: %s", err)
 		}
+		if pubCurve != curve {
+			return fmt.Errorf("curve of in-pub does not match ca")
+		}
 	} else {
-		pub, rawPriv = x25519Keypair()
+		pub, rawPriv = newKeypair(curve)
 	}
 
 	nc := cert.NebulaCertificate{
@@ -200,6 +205,7 @@ func signCert(args []string, out io.Writer, errOut io.Writer, pr PasswordReader)
 			PublicKey: pub,
 			IsCA:      false,
 			Issuer:    issuer,
+			Curve:     curve,
 		},
 	}
 
@@ -219,7 +225,7 @@ func signCert(args []string, out io.Writer, errOut io.Writer, pr PasswordReader)
 		return fmt.Errorf("refusing to overwrite existing cert: %s", *sf.outCertPath)
 	}
 
-	err = nc.Sign(caKey)
+	err = nc.Sign(curve, caKey)
 	if err != nil {
 		return fmt.Errorf("error while signing: %s", err)
 	}
@@ -229,7 +235,7 @@ func signCert(args []string, out io.Writer, errOut io.Writer, pr PasswordReader)
 			return fmt.Errorf("refusing to overwrite existing key: %s", *sf.outKeyPath)
 		}
 
-		err = ioutil.WriteFile(*sf.outKeyPath, cert.MarshalX25519PrivateKey(rawPriv), 0600)
+		err = ioutil.WriteFile(*sf.outKeyPath, cert.MarshalPrivateKey(curve, rawPriv), 0600)
 		if err != nil {
 			return fmt.Errorf("error while writing out-key: %s", err)
 		}
@@ -260,6 +266,17 @@ func signCert(args []string, out io.Writer, errOut io.Writer, pr PasswordReader)
 	return nil
 }
 
+func newKeypair(curve cert.Curve) ([]byte, []byte) {
+	switch curve {
+	case cert.Curve_CURVE25519:
+		return x25519Keypair()
+	case cert.Curve_P256:
+		return p256Keypair()
+	default:
+		return nil, nil
+	}
+}
+
 func x25519Keypair() ([]byte, []byte) {
 	privkey := make([]byte, 32)
 	if _, err := io.ReadFull(rand.Reader, privkey); err != nil {
@@ -274,6 +291,15 @@ func x25519Keypair() ([]byte, []byte) {
 	return pubkey, privkey
 }
 
+func p256Keypair() ([]byte, []byte) {
+	privkey, err := ecdh.P256().GenerateKey(rand.Reader)
+	if err != nil {
+		panic(err)
+	}
+	pubkey := privkey.PublicKey()
+	return pubkey.Bytes(), privkey.Bytes()
+}
+
 func signSummary() string {
 	return "sign <flags>: create and sign a certificate"
 }