Merge pull request #340 from lukpueh/pad-pgp-eddsa-sig

Optionally pad OpenPGP EdDSA signature parameters

securesystemslib/gpg/eddsa.py

@@ -37,6 +37,8 @@
 # EdDSA Point Format (see RFC4880-bis8 13.3.)
 ED25519_PUBLIC_KEY_LENGTH = 33
 ED25519_PUBLIC_KEY_PREFIX = 0x40
+# EdDSA signature byte length (see RFC 8032 5.1.6. (6))
+ED25519_SIG_LENGTH = 64
 
 
 
@@ -140,6 +142,12 @@ def get_signature_params(data):
   ptr += 2
   s = data[ptr:ptr + s_length]
 
+  # Left-zero-pad 'r' and 's' values that are shorter than required by RFC 8032
+  # (5.1.6.), to make up for omitted leading zeros in RFC 4880 (3.2.) MPIs.
+  # This is especially important for 's', which is little-endian.
+  r = r.rjust(ED25519_SIG_LENGTH // 2, b"\x00")
+  s = s.rjust(ED25519_SIG_LENGTH // 2, b"\x00")
+
   return r + s
 
 

tests/test_gpg.py

@@ -46,6 +46,7 @@
 from securesystemslib.gpg.rsa import create_pubkey as rsa_create_pubkey
 from securesystemslib.gpg.dsa import create_pubkey as dsa_create_pubkey
 from securesystemslib.gpg.eddsa import create_pubkey as eddsa_create_pubkey
+from securesystemslib.gpg.eddsa import ED25519_SIG_LENGTH
 from securesystemslib.gpg.common import (parse_pubkey_payload,
     parse_pubkey_bundle, get_pubkey_bundle, _assign_certified_key_info,
     _get_verified_subkeys, parse_signature_packet)
@@ -778,5 +779,25 @@ def test_gpg_sign_and_verify_object_with_specific_key(self):
     self.assertFalse(verify_signature(signature, key_data, wrong_data))
 
 
+  def test_verify_short_signature(self):
+    """Correctly verify a special-crafted short signature. """
+
+    test_data = b"hello"
+    signature_path = os.path.join(self.gnupg_home, "short.sig")
+
+    # Read special-crafted raw gpg signature that is one byte too short
+    with open(signature_path, "rb") as f:
+      signature_data = f.read()
+
+    # Check that the signature is padded upon parsing
+    # NOTE: The returned signature is a hex string and thus twice as long
+    signature = parse_signature_packet(signature_data)
+    self.assertTrue(len(signature["signature"]) == (ED25519_SIG_LENGTH * 2))
+
+    # Check that the signature can be successfully verified
+    key = export_pubkey(self.default_keyid, homedir=self.gnupg_home)
+    self.assertTrue(verify_signature(signature, key, test_data))
+
+
 if __name__ == "__main__":
   unittest.main()