Encrypt session key to multiple keys in keyring. (#59)

`EncryptSessionKey` now creates encrypted key packet for each valid key
in keyring.

Co-authored-by: Aron Wussler <aron@wussler.it>

crypto/sessionkey_test.go

@@ -44,6 +44,21 @@ func TestAsymmetricKeyPacket(t *testing.T) {
 	assert.Exactly(t, testSessionKey, outputSymmetricKey)
 }
 
+func TestMultipleAsymmetricKeyPacket(t *testing.T) {
+	keyPacket, err := keyRingTestMultiple.EncryptSessionKey(testSessionKey)
+	if err != nil {
+		t.Fatal("Expected no error while generating key packet, got:", err)
+	}
+
+	// Password defined in keyring_test
+	outputSymmetricKey, err := keyRingTestMultiple.DecryptSessionKey(keyPacket)
+	if err != nil {
+		t.Fatal("Expected no error while decrypting key packet, got:", err)
+	}
+
+	assert.Exactly(t, testSessionKey, outputSymmetricKey)
+}
+
 func TestSymmetricKeyPacket(t *testing.T) {
 	password := []byte("I like encryption")
 
@@ -90,7 +105,8 @@ func TestDataPacketEncryption(t *testing.T) {
 	assert.Exactly(t, message.GetString(), decrypted.GetString())
 
 	// Encrypt session key
-	keyPacket, err := keyRingTestPublic.EncryptSessionKey(testSessionKey)
+	assert.Exactly(t, 3, len(keyRingTestMultiple.entities))
+	keyPacket, err := keyRingTestMultiple.EncryptSessionKey(testSessionKey)
 	if err != nil {
 		t.Fatal("Unable to encrypt key packet, got:", err)
 	}
@@ -108,6 +124,9 @@ func TestDataPacketEncryption(t *testing.T) {
 	if err != nil {
 		t.Fatal("Unable to unarmor pgp message, got:", err)
 	}
+	ids, ok := pgpMessage.getEncryptionKeyIDs()
+	assert.True(t, ok)
+	assert.Exactly(t, 3, len(ids))
 
 	// Test if final decryption succeeds
 	finalMessage, err := keyRingTestPrivate.Decrypt(pgpMessage, nil, 0)