ffff ff firma - ffff encrypted mailing list manager ff - a firma cai mas nao quebra ff Index ----- 1 - The concept 2 - Everyone has the private key X The server has the private key 3 - Why bash 4 - Development Guidelines 5 - Setup 6 - Tips 7 - Design and features 8 - Caveats 9 - Contact The concept ----------- In portuguese, "firma" means for signature, signature and trust checking and is a slang for strength and enterprise. All this together just can be one thing: an encrypted mailing list manager :P In the streets, the expression "a firma cai mas nao quebra" (the enterprise falls but does not breaks) is commonly used to talk about the strength of a group that operates in secrecy. To our list manager, this concept says that your encrypted mailing list can stops to work -- if the server goes offline or abducted by aliens -- but the system can never be broken. With firma, we are trying to follow this philosophy. Firma is based on the gpgmailalias.pl perl script, hosted at http://www.rediris.es/app/pgplist/index.en.html, but completely rewritten in bash. Firma works as a command line and MTA pipe alias tool that receives an email message in its input, grab the encrypted/signed message, re-encrypt and send it to each subscriber. In the server you just need the script, a keyring with the list keypair and two config files. When mail arrives, it is redirected by the MTA to the script and it process the message if its properly encrypted and signed. No temporary files are written during a message processing and no default message archive, so no messages -- encrypted or not -- rest on your system in all steps of the process. Everyone has the private key X The server has the private key ------------------------------------------------------------- When using a encrypted mailing list software, one must choose between keeping the private key in the server or send it to each of the subscribers. We'll not consider the case where every subscribers encrypt the message to all recipients cause this has none automation in the process we are looking for. For the first we have GPG Mailman, http://medien.informatik.uni-ulm.de/~stefan/gpg-mailman.xhtml and Crypt-ML, http://www.synacklabs.net/projects/crypt-ml/ For the second option we have the NAH6 Mailman patch, http://mail.python.org/pipermail/mailman-coders/2003-June/000506.html For the firsts releases of Firma, we choose to use just the first option. In the future the code should contain support for an one-keypair list, but this is not the main behavior we want in an encrypted mailing list. This is a question of centralized versus decentralized vulnerability. An one-keypair list is more or less just like a mail alias: someone send an encrypted email to the list address and the manager just forwards that encrypted email to the lists subscribers, optionally removing some headers and performing some security auditing. Every user has the list private key, so if someone lost it to the world then one must regenerate a keypair and send again to every subscriber. In the case of a keypair stored in the server, where subscribers has just the list pubkey, the list admin just need to remove the correspondent pubkey from the list's keyring with in case some user has its keypair compromised. These two approaches has a similar external hole of some private key turned public. Designing Firma, we decided for the centralized model, for three main reasons: 1 - a server can be safer than any user's ordinary computer. 2 - automation: when some user is removed from the list, we just remove their key from the list keyring; in the decentralized approach, the list admin needs to regenerate a new list keypair, otherwise the unsubscribed user can still decrypt list messages if he can sniff the mail server traffic. 3 - use a public-key and all info stored onto it as the subscriber info. Why Bash -------- You may ask why we choose bash. Its really strange a crypto project using shell scripting language. But bash has many advantages: - Bash is found in almost all unix-like systems - Small dependencies: firma needs just tools like sed, awk, grep, cut and gpg itself. Look at the file "GUIDELINES" to see a complete list of all unix commands needed to run firma. - You can easily put all the tools, scripts and config files in a read-only media to protect against cracks such as rootkits. - Keeping your encrypted list manager out from a huge and sometimes bugged mail software prevents insecure use of your mailing list by an excess of unwanted functions and routines. - Firma has a total KISS design, and bash helps to keep it simple. - Firma adopted the style suggested in the Advanced Bash-Scripting Guide, http://www.tldp.org/LDP/abs/html/scrstyle.html Development Guidelines ---------------------- As a security project we made a restricted set of guidelines / design policy, attempting to get a clean, bug-free and high quality code. We choose a slow development rather than sit-and-code. These rules are detailed in the file GUIDELINES. Setup ----- Note for Debian users: you'll need the "expect" package to run firma. Firma installation is quite simple: 1 - Create a folder to store lists; by default firma use /usr/local/etc/lists but you can use anything, just edit firma and change FIRMA_LIST_PATH variable. 2 - Copy firma script to whatever you like, e.g. /usr/local/bin and check that it has no write permission 3 - Create a list-wide config file (default is /etc/firma.conf) with the common definitions for all lists, GPG_BINARY= path to the GnuPG binary MAIL_AGENT= path to the mail transport agent to be used (e.g., sendmail) MAIL_AGENT_ARGS= command-line arguments to be passed to the command above LISTS_DIR= path to the mailing lists directory All those variables can be overwritten at each list's own config file; firma.conf should be chmoded as 600, chowned nobody.nobody or whatever user your MTA runs. If you run postfix, the user is specified by the main.cf parameter "default_privs". We suggest you to use MAIL_AGENT=/usr/sbin/sendmail MAIL_AGENT_ARGS=-t As optional parameters, you can also set FIRMA_USER= user that runs firma (usually the same as your MTA user); defaults to "nobody"; you can also specify this parameter in each mailing list config file if you plan to have one user per mailing list FIRMA_GROUP= group that runs firma (usually the same as your MTA group); defaults to "nobody"; you can also specify this parameter in each mailing list config file if you plan to have one group per mailing list LOG_TO_SYSLOG= set to '1' to log errors and warnings to syslog, else firma will print errors to STDERR LOGGER_BINARY= if logging to syslog, set the path to logger's binary SYSLOG_PRIORITY= if logging to syslog, set a priority for the error messages (defaults to 'user.err') USE_GPG_HIDDEN_RECIPIENT_OPTION= set to '1' to use GnuPG's --hidden-recipient option, available from version 1.4.0 onwards (try 'man gpg' for more information) REMOVE_THESE_HEADERS_ON_ALL_LISTS= headers that should be stripped from list messages on all lists running under firma (space separated case-insensitive entries) (may include regexps (e.g., X-.*) KEYSERVER= default keyserver to import/export keys (defaults to keyserver.noreply.org) For a list of all config parameters, type firma --help config 4 - Then create your lists with the command firma -c your-list Then firma will ask some questions and create a gpg keyring and a config file with the following variables: LIST_ADDRESS= list's email address LIST_ADMIN= list's administrators email addresses (space separated) LIST_HOMEDIR= list's GnuPG homedir, where the list's keyrings are located PASSPHRASE= passphrase for the list's private keyring Then a gpg keypair and a config file are automatically generated; the owner of the config file and keyring should be nobody.nobody (or the user your MTA run as) and its permissions must be 600. After that you can add some optional parameters on this list config file: SUBJECT_PREFIX= prefix to be included in the subject of list messages REMOVE_THESE_HEADERS= headers that should be stripped from list messages (space separated case-insensitive entries) (may include regexps (e.g., X-.*) REPLIES_SHOULD_GO_TO_LIST= set to '1' to add a Reply-To header containing the list address SILENTLY_DISCARD_INVALID_MESSAGES= set to '1' to silently discard invalid messages (message not signed/encrypted, sender not subscribed to the list, etc.) instead of sending bounces back to sender KEYSERVER= default keyserver to import/export keys (defaults to keyserver.noreply.org) REQUIRE_SIGNATURE= whether messages sent to the list should be (1) or don't need to be (0) signed to be processed; defaults to '1'; this doesn't affect the way email administration works, when signature is mandatory For a list of all config parameters, type firma --help config 5 - Create an alias to the list at your MTA; on sendmail or postfix, add this to your aliases file: your-list: "| /usr/local/bin/firma -p your-list" your-list-request: "| /usr/local/bin/firma -e your-list" and then run the command newaliases alternatively, you can use a virtual mailbox table if you want to easily host a lot of encrypted mailing lists. 6 - Admin tasks are performed through aliases like your-list-request@yourmachine or via command-line: firma -a your-list inside this command or encrypted in your mailing list request, use the following commands: sub keyserver|keyfile key-id subscribe key-id pubkey from file or keyserver (currently not implemented) unsub email-address unsubscribe all keys with email-address IDs (currently not implemented) use email-address uses the given address for message delivery instead of the primary address of a subscribed key 7 - To subscribe and unsubscribe manually the users and the list admins on, use a command line like gpg --homedir [path-to-your-list-keyring] --import < file and be sure that after this command the list keyring is owned by nobody.nobody. 8 - Send encrypted AND signed messages to your-list@yourmachine and look what happens :) Tips ---- - Use an encrypted swap memory - Use a read-only media to store firma and its needed apps - Use ramdisk to FIRMA_LIST_PATH so all keys and passwords vanishes if the server is shutdown - Use a big PASSPHRASE, 25+ chars with alpha-numeric and special ascii keys Design and features (OUTDATED) ------------------- Firma is simple but its simplicity doesn't reflect in lack of design. - Uses a gpg keyring to store both the keys and the subscribers options - Command line is simple to avoid admin tasks resting in some .bash_history - Non-pgp blocks in a message are discarded since we don't want to deal with unencrypted content - All unwanted email headers are striped as a privacy measure for who sends the message - Firma doesn't use any disk write when processing a message; no temp files that may rest in the system; everything goes in memory (but take care, sometimes it will use the swap and then is best to make it encrypted) - By default it doesn't archive messages in the server - By default it removes the Subject header and put it inside the encrypted message, as Subject are outside the PGP/MIME context - Messages appear to be sent To: Undisclosed Recipients Major features are: - Keyring support - Administration through email or command-line 8 - Caveats People that uses Sylpheed should config the list to accept messages that arent signed. Thats because currently firma depends that the encrypted and signed parts come in the same PGP block and Sylpheed splits the encrypted message and the signature in two separate blocks. Thats a firma and not Sylpheed issue. The PGP/MIME spec allows one to send the PGP/MIME encrypted message in two separate blocks. We hope to fix this soon :) 9 - Contact Contact: firma (@) sarava.org Messages should be encrypted with the list pubkey 0xD68AFEDC found at keyserver.noreply.org.