[jwk] Change ecdh importer/exporter

Changes

@@ -4,6 +4,11 @@ Changes
 v3 has many incompatibilities with v2. To see the full list of differences between
 v2 and v3, please read the Changes-v3.md file (https://github.com/lestrrat-go/jwx/blob/develop/v3/Changes-v3.md)
 
+v3.0.0-alpha3
+  * [jwk] Importing/Exporting from jwk.Key to ecdh.PrivateKey/ecdh.PublicKey should now work. Previously these keys were not properly
+  recognized by the exporter/importer. Note that keys that use X25519 and
+  P256/P384/P521 behave differently: X25519 keys can only be exported to/imported from OKP keys, which P256/P384/P521 can be exported to either ecdsa or ecdh keys.
+
 v3.0.0-alpha2 25 Feb 2025
   * Update to work with go1.24
   * Update tests to work with latest latchset/jose

jwk/convert.go

@@ -4,13 +4,16 @@ import (
 	"crypto/ecdh"
 	"crypto/ecdsa"
 	"crypto/ed25519"
+	"crypto/elliptic"
 	"crypto/rsa"
 	"errors"
 	"fmt"
+	"math/big"
 	"reflect"
 	"sync"
 
 	"github.com/lestrrat-go/blackmagic"
+	"github.com/lestrrat-go/jwx/v3/internal/ecutil"
 	"github.com/lestrrat-go/jwx/v3/jwa"
 )
 
@@ -119,20 +122,113 @@ func init() {
 	}
 	{
 		f := KeyImportFunc(okpPrivateKeyToJWK)
-		for _, k := range []interface{}{ed25519.PrivateKey(nil), ecdh.PrivateKey{}, &ecdh.PrivateKey{}} {
+		for _, k := range []interface{}{ed25519.PrivateKey(nil)} {
+			RegisterKeyImporter(k, f)
+		}
+	}
+	{
+		f := KeyImportFunc(ecdhPrivateKeyToJWK)
+		for _, k := range []interface{}{ecdh.PrivateKey{}, &ecdh.PrivateKey{}} {
 			RegisterKeyImporter(k, f)
 		}
 	}
 	{
 		f := KeyImportFunc(okpPublicKeyToJWK)
-		for _, k := range []interface{}{ed25519.PublicKey(nil), ecdh.PublicKey{}, &ecdh.PublicKey{}} {
+		for _, k := range []interface{}{ed25519.PublicKey(nil)} {
+			RegisterKeyImporter(k, f)
+		}
+	}
+	{
+		f := KeyImportFunc(ecdhPublicKeyToJWK)
+		for _, k := range []interface{}{ecdh.PublicKey{}, &ecdh.PublicKey{}} {
 			RegisterKeyImporter(k, f)
 		}
 	}
-
 	RegisterKeyImporter([]byte(nil), KeyImportFunc(bytesToKey))
 }
 
+func ecdhPrivateKeyToJWK(src interface{}) (Key, error) {
+	var raw *ecdh.PrivateKey
+	switch src := src.(type) {
+	case *ecdh.PrivateKey:
+		raw = src
+	case ecdh.PrivateKey:
+		raw = &src
+	default:
+		return nil, fmt.Errorf(`cannot convert key type '%T' to ECDH jwk.Key`, src)
+	}
+
+	switch raw.Curve() {
+	case ecdh.X25519():
+		return okpPrivateKeyToJWK(raw)
+	case ecdh.P256():
+		return ecdhPrivateKeyToECJWK(raw, elliptic.P256())
+	case ecdh.P384():
+		return ecdhPrivateKeyToECJWK(raw, elliptic.P384())
+	case ecdh.P521():
+		return ecdhPrivateKeyToECJWK(raw, elliptic.P521())
+	default:
+		return nil, fmt.Errorf(`unsupported curve %s`, raw.Curve())
+	}
+}
+
+func ecdhPrivateKeyToECJWK(raw *ecdh.PrivateKey, crv elliptic.Curve) (Key, error) {
+	pub := raw.PublicKey()
+	rawpub := pub.Bytes()
+
+	size := ecutil.CalculateKeySize(crv)
+	var x, y, d big.Int
+	x.SetBytes(rawpub[1 : 1+size])
+	y.SetBytes(rawpub[1+size:])
+	d.SetBytes(raw.Bytes())
+
+	var ecdsaPriv ecdsa.PrivateKey
+	ecdsaPriv.Curve = crv
+	ecdsaPriv.D = &d
+	ecdsaPriv.X = &x
+	ecdsaPriv.Y = &y
+	return ecdsaPrivateKeyToJWK(&ecdsaPriv)
+}
+
+func ecdhPublicKeyToJWK(src interface{}) (Key, error) {
+	var raw *ecdh.PublicKey
+	switch src := src.(type) {
+	case *ecdh.PublicKey:
+		raw = src
+	case ecdh.PublicKey:
+		raw = &src
+	default:
+		return nil, fmt.Errorf(`cannot convert key type '%T' to ECDH jwk.Key`, src)
+	}
+
+	switch raw.Curve() {
+	case ecdh.X25519():
+		return okpPublicKeyToJWK(raw)
+	case ecdh.P256():
+		return ecdhPublicKeyToECJWK(raw, elliptic.P256())
+	case ecdh.P384():
+		return ecdhPublicKeyToECJWK(raw, elliptic.P384())
+	case ecdh.P521():
+		return ecdhPublicKeyToECJWK(raw, elliptic.P521())
+	default:
+		return nil, fmt.Errorf(`unsupported curve %s`, raw.Curve())
+	}
+}
+
+func ecdhPublicKeyToECJWK(raw *ecdh.PublicKey, crv elliptic.Curve) (Key, error) {
+	rawbytes := raw.Bytes()
+	size := ecutil.CalculateKeySize(crv)
+	var x, y big.Int
+
+	x.SetBytes(rawbytes[1 : 1+size])
+	y.SetBytes(rawbytes[1+size:])
+	var ecdsaPriv ecdsa.PublicKey
+	ecdsaPriv.Curve = crv
+	ecdsaPriv.X = &x
+	ecdsaPriv.Y = &y
+	return ecdsaPublicKeyToJWK(&ecdsaPriv)
+}
+
 // These may seem a bit repetitive and redandunt, but the problem is that
 // each key type has its own Import method -- for example, Import(*ecdsa.PrivateKey)
 // vs Import(*rsa.PrivateKey), and therefore they can't just be bundled into
@@ -277,21 +373,21 @@ func Export(key Key, dst interface{}) error {
 	muKeyExporters.RLock()
 	exporters, ok := keyExporters[key.KeyType()]
 	muKeyExporters.RUnlock()
-	if ok {
-		for _, conv := range exporters {
-			v, err := conv.Export(key, dst)
-			if err != nil {
-				if errors.Is(err, ContinueError()) {
-					continue
-				}
-				return fmt.Errorf(`jwk.Export: failed to export jwk.Key to raw format: %w`, err)
-			}
-
-			if err := blackmagic.AssignIfCompatible(dst, v); err != nil {
-				return fmt.Errorf(`jwk.Export: failed to assign key: %w`, err)
+	if !ok {
+		return fmt.Errorf(`jwk.Export: no exporters registered for key type '%T'`, key)
+	}
+	for _, conv := range exporters {
+		v, err := conv.Export(key, dst)
+		if err != nil {
+			if errors.Is(err, ContinueError()) {
+				continue
 			}
-			return nil
+			return fmt.Errorf(`jwk.Export: failed to export jwk.Key to raw format: %w`, err)
+		}
+		if err := blackmagic.AssignIfCompatible(dst, v); err != nil {
+			return fmt.Errorf(`jwk.Export: failed to assign key: %w`, err)
 		}
+		return nil
 	}
-	return fmt.Errorf(`jwk.Export: failed to find exporter for key type '%T'`, key)
+	return fmt.Errorf(`jwk.Export: no suitable exporter found for key type '%T'`, key)
 }

jwk/ecdsa.go

@@ -2,10 +2,12 @@ package jwk
 
 import (
 	"crypto"
+	"crypto/ecdh"
 	"crypto/ecdsa"
 	"crypto/elliptic"
 	"fmt"
 	"math/big"
+	"reflect"
 
 	"github.com/lestrrat-go/jwx/v3/internal/base64"
 	"github.com/lestrrat-go/jwx/v3/internal/ecutil"
@@ -102,13 +104,59 @@ func buildECDSAPublicKey(alg jwa.EllipticCurveAlgorithm, xbuf, ybuf []byte) (*ec
 	return &ecdsa.PublicKey{Curve: crv, X: &x, Y: &y}, nil
 }
 
+func buildECDHPublicKey(alg jwa.EllipticCurveAlgorithm, xbuf, ybuf []byte) (*ecdh.PublicKey, error) {
+	var ecdhcrv ecdh.Curve
+	switch alg {
+	case jwa.X25519():
+		ecdhcrv = ecdh.X25519()
+	case jwa.P256():
+		ecdhcrv = ecdh.P256()
+	case jwa.P384():
+		ecdhcrv = ecdh.P384()
+	case jwa.P521():
+		ecdhcrv = ecdh.P521()
+	default:
+		return nil, fmt.Errorf(`jwk: unsupported ECDH curve %s`, alg)
+	}
+
+	return ecdhcrv.NewPublicKey(append([]byte{0x04}, append(xbuf, ybuf...)...))
+}
+
+func buildECDHPrivateKey(alg jwa.EllipticCurveAlgorithm, dbuf []byte) (*ecdh.PrivateKey, error) {
+	var ecdhcrv ecdh.Curve
+	switch alg {
+	case jwa.X25519():
+		ecdhcrv = ecdh.X25519()
+	case jwa.P256():
+		ecdhcrv = ecdh.P256()
+	case jwa.P384():
+		ecdhcrv = ecdh.P384()
+	case jwa.P521():
+		ecdhcrv = ecdh.P521()
+	default:
+		return nil, fmt.Errorf(`jwk: unsupported ECDH curve %s`, alg)
+	}
+
+	return ecdhcrv.NewPrivateKey(dbuf)
+}
+
 func ecdsaJWKToRaw(keyif Key, hint interface{}) (interface{}, error) {
+	var isECDH bool
 	switch k := keyif.(type) {
 	case *ecdsaPublicKey:
 		switch hint.(type) {
-		case ecdsa.PublicKey, *ecdsa.PublicKey, interface{}:
+		case ecdsa.PublicKey, *ecdsa.PublicKey:
+		case ecdh.PublicKey, *ecdh.PublicKey:
+			isECDH = true
 		default:
-			return nil, fmt.Errorf(`invalid destination object type %T: %w`, hint, ContinueError())
+			rv := reflect.ValueOf(hint)
+			//nolint:revive
+			if rv.Kind() == reflect.Ptr && rv.Elem().Kind() == reflect.Interface {
+				// pointer to an interface value, presumably they want us to dynamically
+				// create an object of the right type
+			} else {
+				return nil, fmt.Errorf(`invalid destination object type %T: %w`, hint, ContinueError())
+			}
 		}
 
 		k.mu.RLock()
@@ -118,12 +166,25 @@ func ecdsaJWKToRaw(keyif Key, hint interface{}) (interface{}, error) {
 		if !ok {
 			return nil, fmt.Errorf(`missing "crv" field`)
 		}
+
+		if isECDH {
+			return buildECDHPublicKey(crv, k.x, k.y)
+		}
 		return buildECDSAPublicKey(crv, k.x, k.y)
 	case *ecdsaPrivateKey:
 		switch hint.(type) {
-		case ecdsa.PrivateKey, *ecdsa.PrivateKey, interface{}:
+		case ecdsa.PrivateKey, *ecdsa.PrivateKey:
+		case ecdh.PrivateKey, *ecdh.PrivateKey:
+			isECDH = true
 		default:
-			return nil, fmt.Errorf(`invalid destination object type %T: %w`, hint, ContinueError())
+			rv := reflect.ValueOf(hint)
+			//nolint:revive
+			if rv.Kind() == reflect.Ptr && rv.Elem().Kind() == reflect.Interface {
+				// pointer to an interface value, presumably they want us to dynamically
+				// create an object of the right type
+			} else {
+				return nil, fmt.Errorf(`invalid destination object type %T: %w`, hint, ContinueError())
+			}
 		}
 
 		k.mu.RLock()
@@ -133,6 +194,10 @@ func ecdsaJWKToRaw(keyif Key, hint interface{}) (interface{}, error) {
 		if !ok {
 			return nil, fmt.Errorf(`missing "crv" field`)
 		}
+
+		if isECDH {
+			return buildECDHPrivateKey(crv, k.d)
+		}
 		pubk, err := buildECDSAPublicKey(crv, k.x, k.y)
 		if err != nil {
 			return nil, fmt.Errorf(`failed to build public key: %w`, err)

jwk/jwk_test.go

@@ -2004,3 +2004,44 @@ func TestParse_fail(t *testing.T) {
 		})
 	})
 }
+
+func TestGH1262(t *testing.T) {
+	t.Run("Updated Example test", func(t *testing.T) {
+		keyCli, err := ecdh.P384().GenerateKey(rand.Reader)
+		require.NoError(t, err, `ecdh.P384().GenerateKey should succeed`)
+
+		jwkCliPriv, err := jwk.Import(keyCli)
+		require.NoError(t, err, `jwk.Import should succeed`)
+		_ = jwkCliPriv
+
+		var rawCliPriv ecdh.PrivateKey
+		require.NoError(t, jwk.Export(jwkCliPriv, &rawCliPriv), `jwk.Export should succeed`)
+
+		pubCli := keyCli.PublicKey() // server is able to retrieve the pub key part of client
+
+		keySrv, err := ecdh.P384().GenerateKey(rand.Reader)
+		require.NoError(t, err, `ecdh.P384().GenerateKey should succeed`)
+
+		jwkSrv, err := jwk.Import(keySrv.PublicKey())
+		require.NoError(t, err, `jwk.Import should succeed`)
+		jwkBuf, err := json.Marshal(jwkSrv)
+
+		require.NoError(t, err, `json.Marshal should succeed`)
+
+		secretSrv, err := keySrv.ECDH(pubCli)
+		require.NoError(t, err, `keySrv.ECDH should succeed`)
+
+		_ = secretSrv // doing some non-standard encryption & response with encrypted data
+
+		// client
+		pubSrv := &ecdh.PublicKey{}
+		jwkCli, err := jwk.ParseKey(jwkBuf) // extract jwkBuf
+		require.NoError(t, err, `jwk.ParseKey should succeed`)
+
+		require.NoError(t, jwk.Export(jwkCli, pubSrv), `jwk.Export should succeed`)
+		secretCli, err := keyCli.ECDH(pubSrv)
+		require.NoError(t, err, `keyCli.ECDH should succeed`)
+
+		_ = secretCli // doing some non-standard encryption
+	})
+}