Tackpy implements TACK in python
Tackpy version 0.9.7 May 22 2012
Tackpy is written by Trevor Perrin and Moxie Marlinspike. It includes crypto code from Peter Pearson (ECDSA) and Bram Cohen (AES).
All code in tackpy has been dedicated to the public domain by its authors. See the LICENSE file for details.
Tackpy requires Python 2.6 or greater, or Python 3.
- Run “make install” or “python setup.py install”. This installs:
- The “tack” library for use by other Python programs (such as TLS Lite).
- The “tack” command-line tool.
To use the command-line tool without installation run “./tack.py”.
Tackpy tries to use OpenSSL for AES and ECDSA operations. If OpenSSL cannot be loaded, Tackpy will fall back to using slower python crypto code.
To use OpenSSL on Windows you need “libeay32.dll” on your path. On Red Hat systems you need to provide your own libcrypto as the system default does not include elliptic curve support.
Quick start with command-line tool
You will need to create one or more TACK keys to “pin” your hostnames to. You should use a different key for each hostname, unless those hostnames are closely related (such as aliases for the same host, or hosts sharing a TLS private key). Once you decide how many TACK keys you need, and the assignment of hostnames to keys, do the following:
- Create a TACK key:
- Run “tack genkey > KEY.pem” (replace “KEY” with a specific name)
- Back up the key file where it won’t be lost or stolen.
If a hostname is using TACK, each server at that hostname must have a TACK that signs the public key in the server’s certificate. To create and deploy these TACKs, do the following:
- Create a TACK for a certificate’s public key:
- Run “tack sign -k KEY.pem -c CERT > TACK.pem”.
- Deploy TACKs to a hostname
- Deploy TACKs to each server at the hostname.
- Apache: Set “SSLTackFile” to a TACK file.
- Once all servers are serving a TACK, activate pinning on each server.
- Apache: Set “SSLTackPinActivation On”.
- Test the site (if there are problems, see “Removing TACKs”).
- Whenever you change a server’s certificate, you must replace its TACK.
Disabling pin activation
If you wish to stop using TACK for a hostname but can tolerate a “waiting period” before the TACKs are removed, simply disable pin activation at all servers for that hostname (Apache: “SSLTackPinActivation Off”). Then wait for all existing client pins to become inactive.
The waiting period required is equal to the length of time that pin activation has been enabled for any servers at the hostname, or a maximum of 30 days. Once the waiting period is elapsed, all TACKs for the hostname can be safely removed.
(For example: If you start using a TACK for “example.com”, then decide to disable pin activation after one day, you can remove the TACK at the end of the second day.)
If you wish to abruptly stop publishing a TACK for a hostname, or abruptly change the hostname’s TACK key, or signal that a TACK key has been compromised, then you may publish a “break signature” from the TACK key as part of the TLS connection.
This break signature must remain published at the hostname until all pins between the hostname and the old TACK key have become inactive (30 days at most; this is exactly the same as the “waiting period” described in the previous section).
A break signature from a TACK key causes any client who encounters it to discard all pins involving the TACK key. Thus, once a break signature is published for a TACK key, all existing pins and TACKS for the TACK key cease providing security.
A server can have up to eight break signatures. However, keep in mind that break signatures add to TLS handshake overhead, so are best avoided.
- Create a break signature for a TACK:
- Run “tack break -k KEY.pem > TACK_Break_Sig.pem”
- Add the break signature to your web server.
- Apache: Set “SSLTACKBreakSigsFile” to a file with break signatures.
Revoking older generations of a TACK
If a server’s TLS key (not its TACK key) has been compromised and you are switching to a new TLS key, you may revoke the TACK for the old key by “-m <min_generation>” in the “sign” command. <min_generation> is a number from 0-255 that is larger than the generation of the TACK you wish to revoke.
Clients who encounter the new TACK will reject older generation TACKs from then on. Prior to publishing a new <min_generation> you should replace all your TACKs with this generation number (or higher) by signing with “-g <generation>”.
For example: By default TACKs have generation=0, so the first time you use this capability you will want to set “-m1” after pushing out a new set of TACKs signed with “-g1”. If you use it a second time, you will set “-m2”, and so on.
Security Consideration: This only provides protection if clients receive the new min_generation. For a more robust defense against TLS key compromise, consider using short-lived TACKs.
Every TACK contains a signature covering a TLS public key. The TLS key is contained in a certificate. By default the TACK is set to expire at the same time as the certificate, and must be replaced by an updated TACK at that point.
If you shorten the TACK’s expiration time, then a compromised TLS key will become unusable to an attacker once the TACK expires. For example, every day at midnight you could deploy a new TACK that expires within 48 hours.
A good way to handle short-lived TACKs is to generate a batch of them and store the TACKs on a secure system that distributes them to servers. This way, you do not have to use your TACK key to sign new TACKs frequently.
You can generate a batch of TACKs with the “-n NUM@INTERVAL” argument to “sign”, specifying the number of TACKs and the interval between their expiration times. The “-o” argument is taken as a filename prefix, and the “-e” time is used as the first expiration time. Example:
tack sign -k KEY.pem -c CERT -n 365@1d -e 2013-01-02Z -o T1
- produces 365 TACKs, one expiring at midnight (UTC) each day of 2013:
- T1_0000.pem T1_0001.pem T1_0002.pem … T1_0364.pem
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