Convert all valid subkeys when issuing a forwarding key

Author: wussler

openpgp/forwarding.go

@@ -11,66 +11,154 @@ import (
 	"github.com/ProtonMail/go-crypto/openpgp/packet"
 )
 
-func (e *Entity) NewForwardingEntity(name, comment, email string, config *packet.Config) (forwardeeKey *Entity, proxyParam []byte, err error) {
-	encryptionSubKey, ok := e.EncryptionKey(config.Now())
-	if !ok {
-		return nil, nil, errors.InvalidArgumentError("no valid encryption key found")
-	}
-
-	if encryptionSubKey.PublicKey.Version != 4 {
-		return nil, nil, errors.InvalidArgumentError("unsupported encryption subkey version")
-	}
+// ForwardingInstance represents a single forwarding instance (mapping IDs to a Proxy Param)
+type ForwardingInstance struct {
+	ForwarderKeyId uint64
+	ForwardeeKeyId uint64
+	ProxyParameter []byte
+}
 
-	if encryptionSubKey.PrivateKey.PubKeyAlgo != packet.PubKeyAlgoECDH {
-		return nil, nil, errors.InvalidArgumentError("encryption subkey is not algorithm 18 (ECDH)")
+// NewForwardingEntity generates a new forwardee key and derives the proxy parameters from the entity e.
+// If strict, it will return an error if encryption-capable non-revoked subkeys with a wrong algorithm are found,
+// instead of ignoring them
+func (e *Entity) NewForwardingEntity(
+	name, comment, email string, config *packet.Config, strict bool,
+) (
+	forwardeeKey *Entity, instances []ForwardingInstance, err error,
+) {
+	if e.PrimaryKey.Version != 4 {
+		return nil, nil, errors.InvalidArgumentError("unsupported key version")
 	}
 
-	ecdhKey, ok := encryptionSubKey.PrivateKey.PrivateKey.(*ecdh.PrivateKey)
-	if !ok {
-		return nil, nil, errors.InvalidArgumentError("encryption subkey is not type ECDH")
+	now := config.Now()
+	i := e.PrimaryIdentity()
+	if e.PrimaryKey.KeyExpired(i.SelfSignature, now) || // primary key has expired
+		i.SelfSignature == nil || // user ID has no self-signature
+		i.SelfSignature.SigExpired(now) || // user ID self-signature has expired
+		e.Revoked(now) || // primary key has been revoked
+		i.Revoked(now) { // user ID has been revoked
+		return nil, nil, errors.InvalidArgumentError("primary key is expired")
 	}
 
+	// Generate a new Primary key for the forwardee
 	config.Algorithm = packet.PubKeyAlgoEdDSA
 	config.Curve = packet.Curve25519
+	keyLifetimeSecs := config.KeyLifetime()
 
-	forwardeeKey, err = NewEntity(name, comment, email, config)
+	forwardeePrimaryPrivRaw, err := newSigner(config)
 	if err != nil {
 		return nil, nil, err
 	}
 
-	forwardeeEcdhKey, ok := forwardeeKey.Subkeys[0].PrivateKey.PrivateKey.(*ecdh.PrivateKey)
-	if !ok {
-		return nil, nil, goerrors.New("wrong forwarding sub key generation")
+	primary := packet.NewSignerPrivateKey(now, forwardeePrimaryPrivRaw)
+
+	forwardeeKey = &Entity{
+		PrimaryKey: &primary.PublicKey,
+		PrivateKey: primary,
+		Identities: make(map[string]*Identity),
+		Subkeys:    []Subkey{},
 	}
 
-	proxyParam, err = ecdh.DeriveProxyParam(ecdhKey, forwardeeEcdhKey)
+	err = forwardeeKey.addUserId(name, comment, email, config, now, keyLifetimeSecs)
 	if err != nil {
 		return nil, nil, err
 	}
 
-	kdf := ecdh.KDF{
-		Version: ecdh.KDFVersionForwarding,
-		Hash: ecdhKey.KDF.Hash,
-		Cipher: ecdhKey.KDF.Cipher,
-		ReplacementFingerprint: encryptionSubKey.PublicKey.Fingerprint,
+	// Init empty instances
+	instances = []ForwardingInstance{}
+
+	// Handle all forwarder subkeys
+	for _, forwarderSubKey := range e.Subkeys {
+		// Filter flags
+		if !forwarderSubKey.Sig.FlagsValid || forwarderSubKey.Sig.FlagCertify || forwarderSubKey.Sig.FlagSign ||
+			forwarderSubKey.Sig.FlagAuthenticate || forwarderSubKey.Sig.FlagGroupKey {
+			continue
+		}
+
+		// Filter expiration & revokal
+		if forwarderSubKey.PublicKey.KeyExpired(forwarderSubKey.Sig, now) ||
+			forwarderSubKey.Sig.SigExpired(now) ||
+			forwarderSubKey.Revoked(now) {
+			continue
+		}
+
+		if forwarderSubKey.PublicKey.PubKeyAlgo != packet.PubKeyAlgoECDH {
+			if strict {
+				return nil, nil, errors.InvalidArgumentError("encryption subkey is not algorithm 18 (ECDH)")
+			} else {
+				continue
+			}
+		}
+
+		forwarderEcdhKey, ok := forwarderSubKey.PrivateKey.PrivateKey.(*ecdh.PrivateKey)
+		if !ok {
+			return nil, nil, errors.InvalidArgumentError("malformed key")
+		}
+
+		err = forwardeeKey.addEncryptionSubkey(config, now, 0)
+		if err != nil {
+			return nil, nil, err
+		}
+
+		forwardeeSubKey := forwardeeKey.Subkeys[len(forwardeeKey.Subkeys) - 1]
+
+		forwardeeEcdhKey, ok := forwardeeSubKey.PrivateKey.PrivateKey.(*ecdh.PrivateKey)
+		if !ok {
+			return nil, nil, goerrors.New("wrong forwarding sub key generation")
+		}
+
+		instance := ForwardingInstance{
+			ForwarderKeyId: forwarderSubKey.PublicKey.KeyId,
+		}
+
+		instance.ProxyParameter, err = ecdh.DeriveProxyParam(forwarderEcdhKey, forwardeeEcdhKey)
+		if err != nil {
+			return nil, nil, err
+		}
+
+		kdf := ecdh.KDF{
+			Version:                ecdh.KDFVersionForwarding,
+			Hash:                   forwarderEcdhKey.KDF.Hash,
+			Cipher:                 forwarderEcdhKey.KDF.Cipher,
+		}
+
+		// If deriving a forwarding key from a forwarding key
+		if forwarderSubKey.Sig.FlagForward {
+			if forwarderEcdhKey.KDF.Version != ecdh.KDFVersionForwarding {
+				return nil, nil, goerrors.New("malformed forwarder key")
+			}
+			kdf.ReplacementFingerprint = forwarderEcdhKey.KDF.ReplacementFingerprint
+		} else {
+			kdf.ReplacementFingerprint = forwarderSubKey.PublicKey.Fingerprint
+		}
+
+		err = forwardeeSubKey.PublicKey.ReplaceKDF(kdf)
+		if err != nil {
+			return nil, nil, err
+		}
+
+		// Set ID after changing the KDF
+		instance.ForwardeeKeyId = forwardeeSubKey.PublicKey.KeyId
+
+		// 0x04 - This key may be used to encrypt communications.
+		forwardeeSubKey.Sig.FlagEncryptCommunications = false
+
+		// 0x08 - This key may be used to encrypt storage.
+		forwardeeSubKey.Sig.FlagEncryptStorage = false
+
+		// 0x10 - The private component of this key may have been split by a secret-sharing mechanism.
+		forwardeeSubKey.Sig.FlagSplitKey = true
+
+		// 0x40 - This key may be used for forwarded communications.
+		forwardeeSubKey.Sig.FlagForward = true
+
+		// Append each valid instance to the list
+		instances = append(instances, instance)
 	}
 
-	err = forwardeeKey.Subkeys[0].PublicKey.ReplaceKDF(kdf)
-	if err != nil {
-		return nil, nil, err
+	if len(instances) == 0 {
+		return nil, nil, errors.InvalidArgumentError("no valid subkey found")
 	}
 
-	// 0x04 - This key may be used to encrypt communications.
-	forwardeeKey.Subkeys[0].Sig.FlagEncryptCommunications = false
-
-	// 0x08 - This key may be used to encrypt storage.
-	forwardeeKey.Subkeys[0].Sig.FlagEncryptStorage = false
-
-	// 0x10 - The private component of this key may have been split by a secret-sharing mechanism.
-	forwardeeKey.Subkeys[0].Sig.FlagSplitKey = true
-
-	// 0x40 - This key may be used for forwarded communications.
-	forwardeeKey.Subkeys[0].Sig.FlagForward = true
-
-	return forwardeeKey, proxyParam, nil
+	return forwardeeKey, instances, nil
 }

openpgp/forwarding_test.go

@@ -78,11 +78,23 @@ func TestForwardingFull(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	charlesEntity, proxyParam, err := bobEntity.NewForwardingEntity("charles", "", "[email protected]", keyConfig)
+	charlesEntity, instances, err := bobEntity.NewForwardingEntity("charles", "", "[email protected]", keyConfig, true)
 	if err != nil {
 		t.Fatal(err)
 	}
 
+	if len(instances) != 1 {
+		t.Fatalf("invalid number of instances, expected 1 got %d", len(instances))
+	}
+
+	if instances[0].ForwarderKeyId != bobEntity.Subkeys[0].PublicKey.KeyId {
+		t.Fatalf("invalid forwarder key ID, expected: %x, got: %x", bobEntity.Subkeys[0].PublicKey.KeyId, instances[0].ForwarderKeyId)
+	}
+
+	if instances[0].ForwardeeKeyId != charlesEntity.Subkeys[0].PublicKey.KeyId {
+		t.Fatalf("invalid forwardee key ID, expected: %x, got: %x", charlesEntity.Subkeys[0].PublicKey.KeyId, instances[0].ForwardeeKeyId)
+	}
+
 	// Encrypt message
 	buf := bytes.NewBuffer(nil)
 	w, err := Encrypt(buf, []*Entity{bobEntity}, nil, nil, nil)
@@ -114,6 +126,43 @@ func TestForwardingFull(t *testing.T) {
 	}
 
 	// Forward message
+
+	transformed := transformTestMessage(t, encrypted, instances[0])
+
+	// Decrypt forwarded message for Charles
+	m, err = ReadMessage(bytes.NewBuffer(transformed), EntityList([]*Entity{charlesEntity}), nil /* no prompt */, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	dec, err = ioutil.ReadAll(m.decrypted)
+
+	if bytes.Compare(dec, plaintext) != 0 {
+		t.Fatal("forwarded decrypted does not match original")
+	}
+
+	// Setup further forwarding
+	danielEntity, secondForwardInstances, err := charlesEntity.NewForwardingEntity("Daniel", "", "[email protected]", keyConfig, true)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	secondTransformed := transformTestMessage(t, transformed, secondForwardInstances[0])
+
+	// Decrypt forwarded message for Charles
+	m, err = ReadMessage(bytes.NewBuffer(secondTransformed), EntityList([]*Entity{danielEntity}), nil /* no prompt */, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	dec, err = ioutil.ReadAll(m.decrypted)
+
+	if bytes.Compare(dec, plaintext) != 0 {
+		t.Fatal("forwarded decrypted does not match original")
+	}
+}
+
+func transformTestMessage(t *testing.T, encrypted []byte, instance ForwardingInstance) []byte {
 	bytesReader := bytes.NewReader(encrypted)
 	packets := packet.NewReader(bytesReader)
 	splitPoint := int64(0)
@@ -131,9 +180,9 @@ Loop:
 		switch p := p.(type) {
 		case *packet.EncryptedKey:
 			err = p.ProxyTransform(
-				proxyParam,
-				charlesEntity.Subkeys[0].PublicKey.KeyId,
-				bobEntity.Subkeys[0].PublicKey.KeyId,
+				instance.ProxyParameter,
+				instance.ForwarderKeyId,
+				instance.ForwardeeKeyId,
 			)
 			if err != nil {
 				t.Fatalf("error transforming PKESK: %s", err)
@@ -152,15 +201,5 @@ Loop:
 	transformed := transformedEncryptedKey.Bytes()
 	transformed = append(transformed, encrypted[splitPoint:]...)
 
-	// Decrypt forwarded message for Charles
-	m, err = ReadMessage(bytes.NewBuffer(transformed), EntityList([]*Entity{charlesEntity}), nil /* no prompt */, nil)
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	dec, err = ioutil.ReadAll(m.decrypted)
-
-	if bytes.Compare(dec, plaintext) != 0 {
-		t.Fatal("forwarded decrypted does not match original")
-	}
+	return transformed
 }

openpgp/packet/encrypted_key.go

@@ -463,12 +463,12 @@ func SerializeEncryptedKeyWithHiddenOption(w io.Writer, pub *PublicKey, cipherFu
 	return SerializeEncryptedKeyAEADwithHiddenOption(w, pub, cipherFunc, config.AEAD() != nil, key, hidden, config)
 }
 
-func (e *EncryptedKey) ProxyTransform(proxyParam []byte, forwardeeKeyId, forwardingKeyId uint64) error {
+func (e *EncryptedKey) ProxyTransform(proxyParam []byte, forwarderKeyId, forwardeeKeyId uint64) error {
 	if e.Algo != PubKeyAlgoECDH {
 		return errors.InvalidArgumentError("invalid PKESK")
 	}
 
-	if e.KeyId != 0 && e.KeyId != forwardingKeyId {
+	if e.KeyId != 0 && e.KeyId != forwarderKeyId {
 		return errors.InvalidArgumentError("invalid key id in PKESK")
 	}