Impenetrable Thoughts


Cryptlist is a simple mailing list script intended to be run via something such as procmail. It's goal is to create an optionally encrypted mailing list easily using any available computer and email address. Encryption is optional in that messages which are not encrypted will be distributed to the membership in the clear, while messages encrypted to the list key will be re-encrypted to each member separately.


  • A Unix-like operating system such as Mac OSX, Linux, FreeBSD, OpenBSD, etc.

  • A recent version of gpg, not gpg2

  • A recent version of Python 2.x

  • An outgoing smtp mail server

  • A valid incoming email address. This can be delivered directly to a user account or it could be an ISP supplied POP3 or IMAP mailbox. All you require is some way to have the mail passed through cryptlist as a filter, such as you can do if you process the email with procmail.

Security and Limitations

Cryptlist provides a moderate level of security. However, in the interest of ease of use the encrypted mailing list setup is not maximally secure. Specifically:

  • Cryptlist trusts all the keys on the list keyring

  • Every member receives a separately encrypted copy of the message, leaving open the possibility of weaknesses related to one cleartext being encrypted with multiple keys

  • Messages which are signed and encrypted when sent to the list will be re-encrypted to each member. The original signature will be lost.

  • The list could, but does not sign the re-encrypted message because it is not clear that it would provide additional security.


  1. Edit The list settings are all at the top of the file.

  2. Create the list keyring in the gpg directory 'gpg --homedir . --gen-key'. It is recommended that you note that this key is for a mailing list in the key comment. Note the key fingerprint since you will need that in a following step. Enter an empty passphrase for this key.

  3. Export the list public key 'gpg --homedir . --export --output list_key.key'

  4. Sign the key with your regular key so other may know that they are getting the correct key 'gpg --import list_key.key; gpg --sign-key'. Ensure that the fingerprints match at this step.

  5. Import the signature back into the list keyring 'gpg --export --armour --output list_key.asc' and then 'gpg --homedir . --import list_key.asc'. Keep list_key.asc around as that is the keyfile you will want to distribute (if you aren't putting the key on a keyserver).

  6. For each member of the list for which you have a public key you must add the key to the list keyring. As the list will trust every key on its keyring you should not configure gpg for the list to automatically retrieve keys from a keyserver. You import keys using the script.

  7. Finally add the cryptlist script to the mail filtering system. See the next section. Once this step is completed you should be able to send some test messages successfully.

Setting up as a filter

If you are using a POP3 or IMAP mailbox with fetchmail then you can easily set Cryptlist to be the script called on reception of any email.