decrypt_keyctl

A passphrase caching script to be used in /etc/crypttab on Debian and Ubuntu. When there are multiple cryptsetup (either plain or LUKS) volumes with the same passphrase, it is an unnecessary task to input the passphrase more than once.

Just add this script as keyscript to your /etc/crypttab and it will cache the passphrase of all crypttab entries with the same identifier.

Either copy decrypt_keyctl into the default search path for keyscripts from cryptsetup /lib/cryptdisks/scripts/. So you can just write keyscript=decrypt_keyctl in /etc/crypttab, or use a random path of your choice and give the full path e.g keyscript=/sbin/decrypt_keyctl.

Requirements

• Debian cryptsetup package with /etc/crypttab handling and keyscript option
  • Tested with Debian Lenny, Squeeze and Sid
• Installed and working keyutils package (keyctl)
  • Needs CONFIG_KEYS=y in your kernel configuration

What For?

In old (pre 2.6.38) kernels, dm-crypt used to be single threaded. Thus every dm-crypt mapping only used a single core for crypto operations. To use the full power of your many-core processor it is was necessary to split the dm-crypt device. For Linux software raid arrays the easiest segmentation was to just put the dm-crypt layer below the software raid layer.

But with a 5 disk raid5 it is a rather daunting task to input the passphrase five times. This is what this keyscripts solve for you.

Usage

Best shown by example:

• 5 disks
• Linux software raid5

Layer:

  sda             sdb            sdc ... sde
+-----------+   +-----------+
| LUKS      |   | LUKS      |
| +-------+ |   | +-------+ |
| | RAID5 | |   | | RAID5 | |
| | ...   | |   | | ...   | |

Crypttab Entries:

<target>    <source>    <keyfile>        <options>
sda_crypt   /dev/sda2   main_data_raid   luks,discard,keyscript=decrypt_keyctl
sdb_crypt   /dev/sdb2   main_data_raid   luks,discard,keyscript=decrypt_keyctl
...
sde_crypt   /dev/sde2   main_data_raid   luks,discard,keyscript=decrypt_keyctl

How does it work

Crypttab Interface:

A keyscript is added to options including a keyfile definition as third parameter in the crypttab file. The keyscript is called with the keyfile as the first and only parameter. Additionally there are a few environment variables set but currently are not used by this keyscript (man 5 crypttab for exact description).

Keyscript:

decrypt_keyctl uses the Linux kernel keyring facility to securely cache passphrases between multiple invocations. The keyfile parameter from crypttab is used to find the same passphrase between multiple invocations. The term used to described the key in the user keyring is cryptsetup:$CRYPTTAB_KEY, unless $CRYPTTAB_KEY is empty or has the special value none, in which case the description is merely cryptsetup (thus allowing compatibility with other tools like gdm and systemd-ask-password(1).)

Currently the cache timeout is 60 seconds and not configurable (please report a bug if it is too low for you).

Problems

Passphrase is piped between processes and could end up in unsecured memory, thus later swapped to disk! => Use of cryptoswap recommend!

Hints

To remove all traces of this keyscript you may want to cleanup the keyring completely with the following command afterwards:

sudo keyctl clear @u

– Jonas Meurer jonas@freesources.org Mon, 27 Sep 2010 14:01:35 +0000

– Guilhem Moulin guilhem@debian.org Tue, 25 Dec 2018 01:12:24 +0100