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Python S/MIME Toolkit

Project description

This library implements a S/MIME handler. It supports only S/MIME messages encryption using a public RSA key, in AES128-CBC, AES192-CBC or AES256-CBC modes.

The ASN.1 implementation does not depend on pyasn1, as it showed too broken for creating and reading CMS (Cryptographic Message Syntax). Instead, the asn1crypto was used in this project.

This implementation does not use the deprecated pycrypto anymore; instead it was switched to the more modern cryptography library. It is not ‘pure python’ anymore (because of the latter dependency), but at least works.


  • Python 2.7 or Python 3.5+

  • cryptography

  • asn1crypto


The code below loads Alice’s public key in PEM format and uses it to encrypt the e-mail in S/MIME format:

import sys
import smime

message = [
    'To: "Alice" <>',
    'From: "Bob" <>',
    'Subject: A message from python',
    'Now you see me.'

with open('alice-public-key.pem', 'rb') as pem:


To: "Alice" <>
From: "Bob" <>
Subject: A message from python
MIME-Version: 1.0
Content-Type: application/pkcs7-mime; smime-type=enveloped-data; name=smime.p7m
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7m


The same can be decrypted using OpenSSL from the command line:

$ openssl smime -decrypt -in smime.p7m -inkey alice-private-key.pem

OpenSSL expects that the smime.p7m file above should be in DER or PEM format. The latter should be enclosed in —–BEGIN PKCS7—– and —–END PKCS7—– and the content should be in base64 encoding, just like the output of the command above. Example:

-----BEGIN PKCS7-----
-----END PKCS7-----

Remember that the above formatting serves only for the purpose of testing the encryption with OpenSSL. Do not make such enclosing in e-mails.


This software is licensed under the Apache License 2.0. See the LICENSE file in the top distribution directory for the full license text.


This software follows Semantic Versioning

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