crypto: Handle message that is encrypted, but the signature is invalid

One case of this would be not having the public key of the signer. If
the verification of the signatures fails, then use the signatures from
the error, and try to redecrypt without verification.

I have no tests yet, and this probably deserves tests.

Fixes #1157

1 files changed, 6 insertions, 3 deletions

diff --git a/alot/crypto.py b/alot/crypto.py
index 6e3e8fa6..e8595d90 100644
--- a/alot/crypto.py
+++ b/alot/crypto.py
@@ -202,12 +202,15 @@ def decrypt_verify(encrypted):
     """
     ctx = gpg.core.Context()
     try:
-        (plaintext, _, verify_result) = ctx.decrypt(encrypted, verify=True)
+        plaintext, _, verify_result = ctx.decrypt(encrypted, verify=True)
+        sigs = verify_result.signatures
     except gpg.errors.GPGMEError as e:
         raise GPGProblem(str(e), code=e.getcode())
-    # what if the signature is bad?
+    except gpg.errors.BadSignatures as e:
+        plaintext, _, _ = ctx.decrypt(encrypted, verify=False)
+        sigs = e.result.signatures
 
-    return verify_result.signatures, plaintext
+    return sigs, plaintext
 
 
 def validate_key(key, sign=False, encrypt=False):