GnuPG: do not incorrectly trim fingerprint in presence of exclamation marks for specfic subkey selection

README.rst

@@ -188,6 +188,22 @@ the example files and pgp key provided with the repository::
 
 This last step will decrypt ``example.yaml`` using the test private key.
 
+Encrypting with GnuPG subkeys
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If you want to encrypt with specific GnuPG subkeys, it does not suffice to provide the
+exact key ID of the subkey to SOPS, since GnuPG might use *another* subkey instead
+to encrypt the file key with. To force GnuPG to use a specific subkey, you need to
+append ``!`` to the key's fingerprint.
+
+.. code:: yaml
+
+    creation_rules:
+        - pgp: >-
+            85D77543B3D624B63CEA9E6DBC17301B491B3F21!,
+            E60892BB9BD89A69F759A1A0A3D652173B763E8F!
+
+Please note that this is only passed on correctly to GnuPG since SOPS 3.9.3.
 
 Encrypting using age
 ~~~~~~~~~~~~~~~~~~~~

pgp/keysource.go

@@ -634,7 +634,13 @@ func gnuPGHome(customPath string) string {
 // This is mostly used for compatibility reasons, as older versions of GnuPG
 // do not always like long IDs.
 func shortenFingerprint(fingerprint string) string {
-	if offset := len(fingerprint) - 16; offset > 0 {
+	offset := len(fingerprint) - 16
+	// If the fingerprint ends with '!', we must include '!' in the ID *and* the
+	// 16 hex digits before it. See https://github.com/getsops/sops/issues/1365.
+	if strings.HasSuffix(fingerprint, "!") {
+		offset -= 1
+	}
+	if offset > 0 {
 		fingerprint = fingerprint[offset:]
 	}
 	return fingerprint

pgp/keysource_test.go

@@ -697,10 +697,23 @@ func Test_gnuPGHome(t *testing.T) {
 }
 
 func Test_shortenFingerprint(t *testing.T) {
+	// Test with regular fingerprint
 	shortId := shortenFingerprint(mockFingerprint)
 	assert.Equal(t, "9732075EA221A7EA", shortId)
 
 	assert.Equal(t, shortId, shortenFingerprint(shortId))
+
+	// Test with forced subkey
+	shortId = shortenFingerprint(mockFingerprint + "!")
+	assert.Equal(t, "9732075EA221A7EA!", shortId)
+
+	assert.Equal(t, shortId, shortenFingerprint(shortId))
+
+	// Make sure that too short IDs are kept
+	for _, tooShort := range []string{"012345679abcdef", "012345679abcdef!", "123", "123!"} {
+		shortId = shortenFingerprint(tooShort)
+		assert.Equal(t, tooShort, shortId)
+	}
 }
 
 // TODO(hidde): previous tests kept around for now.