Requirements

• Python 2.7 or Python 3.5+

• cryptography

• asn1crypto

Example

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" <alice@foo.com>',
    'From: "Bob" <bob@bar.com>',
    'Subject: A message from python',
    '',
    'Now you see me.'
]

with open('alice-public-key.pem', 'rb') as pem:
    print(smime.encrypt('\n'.join(message), pem.read()))

Output:

To: "Alice" <alice@foo.com>
From: "Bob" <bob@bar.com>
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

<base64-enveloped-data>

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-----
MIIBdgYJKoZIhvcNAQcDoIIBZzCCAWMCAQAxgb4wgbsCAQAwJjASMRAwDgYDVQQD
EwdDYXJsUlNBAhBGNGvHgABWvBHTbi7EELOwMAsGCSqGSIb3DQEBAQSBgCVAQwNg
LmJ5ESYxOM1YbOLz2gvzWY1Fk+LZZiylYe7+o1/e/MjtzNwhnu+8vziFwHbXEH1Y
jndIbUxiLyXb3omtNDunRICQin5bdo6BI7oE0MufUSqMjk0YUk8UQeNCiUfK89PR
RfDclb1/sM3XZ7mUJa2OzpnuQIWec3MuJ3k4MIGcBgkqhkiG9w0BBwEwHQYJYIZI
AWUDBAEqBBCVZVOt2lxSzmd+Ti1M372xgHDR0+ToLk1MJeTTtmJdnnNNH6631PN0
i3NJeJBKDDs4onI8xywqFtJP0of6GPoTGV/7D2vkgO2+jhCBTrzjYczbdOhh6Z5X
o0i/81NPSoaLhrfwKMQvT7sXX7c9YdbTjyglyGqhXUN8h+mIRlP9IStD
-----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.