Fix exception causes all over the codebase

rsa/cli.py

@@ -58,10 +58,10 @@ def keygen() -> None:
 
     try:
         keysize = int(cli_args[0])
-    except ValueError:
+    except ValueError as ex:
         parser.print_help()
         print('Not a valid number: %s' % cli_args[0], file=sys.stderr)
-        raise SystemExit(1)
+        raise SystemExit(1) from ex
 
     print('Generating %i-bit key' % keysize, file=sys.stderr)
     (pub_key, priv_key) = rsa.newkeys(keysize)
@@ -280,8 +280,8 @@ def perform_operation(self, indata: bytes, pub_key: rsa.key.AbstractKey,
 
         try:
             rsa.verify(indata, signature, pub_key)
-        except rsa.VerificationError:
-            raise SystemExit('Verification failed.')
+        except rsa.VerificationError as ex:
+            raise SystemExit('Verification failed.') from ex
 
         print('Verification OK', file=sys.stderr)
 

rsa/key.py

@@ -123,10 +123,10 @@ def _assert_format_exists(file_format: str, methods: typing.Mapping[str, typing.
 
         try:
             return methods[file_format]
-        except KeyError:
+        except KeyError as ex:
             formats = ', '.join(sorted(methods.keys()))
             raise ValueError('Unsupported format: %r, try one of %s' % (file_format,
-                                                                        formats))
+                                                                        formats)) from ex
 
     def save_pkcs1(self, format: str = 'PEM') -> bytes:
         """Saves the key in PKCS#1 DER or PEM format.
@@ -675,7 +675,7 @@ def calculate_keys_custom_exponent(p: int, q: int, exponent: int) -> typing.Tupl
         raise rsa.common.NotRelativePrimeError(
             exponent, phi_n, ex.d,
             msg="e (%d) and phi_n (%d) are not relatively prime (divider=%i)" %
-                (exponent, phi_n, ex.d))
+                (exponent, phi_n, ex.d)) from ex
 
     if (exponent * d) % phi_n != 1:
         raise ValueError("e (%d) and d (%d) are not mult. inv. modulo "

rsa/pkcs1.py

@@ -261,8 +261,8 @@ def decrypt(crypto: bytes, priv_key: key.PrivateKey) -> bytes:
     # Find the 00 separator between the padding and the message
     try:
         sep_idx = cleartext.index(b'\x00', 2)
-    except ValueError:
-        raise DecryptionError('Decryption failed')
+    except ValueError as ex:
+        raise DecryptionError('Decryption failed') from ex
 
     return cleartext[sep_idx + 1:]
 

rsa/pkcs1_v2.py

@@ -49,12 +49,12 @@ def mgf1(seed: bytes, length: int, hasher: str = 'SHA-1') -> bytes:
 
     try:
         hash_length = pkcs1.HASH_METHODS[hasher]().digest_size
-    except KeyError:
+    except KeyError as ex:
         raise ValueError(
             'Invalid `hasher` specified. Please select one of: {hash_list}'.format(
                 hash_list=', '.join(sorted(pkcs1.HASH_METHODS.keys()))
             )
-        )
+        ) from ex
 
     # If l > 2^32(hLen), output "mask too long" and stop.
     if length > (2**32 * hash_length):