New clause `shared <id> algorithm <alg> subkey <info>' in a keygen
block enables `cgdconfig -C' to reuse a key between different params
files, so you can, e.g., use a single password for multiple disks.
This is better than simply caching the password itself because:
- Hashing the password is expensive, so it should only be done once.
Suppose your budget is time t before you get bored, and you
calibrate password hash parameters to unlock n disks before you get
bored waiting for `cgdconfig -C'.
. With n password hashings the adversary's cost goes up only by a
factor of t/n.
. With one password hashing and n subkeys the adversary's cost goes
up by a factor of n.
And if you ever add a disk, rehashing it will make `cgdconfig -C'
go over budget, whereas another subkey adds negligible cost to you.
- Subkeys work for other types of keygen blocks, like shell_cmd,
which could be used to get a key from a hardware token that needs a
button press.
The <info> parameter must be different for each params file;
everything else in the keygen block must be the same. With this
clause, the keygen block determines a shared key used only to derive
keys; the actual key used by cgdconfig is derived from the shared key
by the specified algorithm.
The only supported algorithm is hkdf-hmac-sha256, which uses
HKDF-Expand of RFC 5869 instantiated with SHA-256.
Example:
algorithm aes-cbc;
iv-method encblkno1;
keylength 128;
verify_method none;
keygen pkcs5_pbkdf2/sha1 {
iterations 39361;
salt AAAAgMoHiYonye6KogdYJAobCHE=;
shared "pw" algorithm hkdf-hmac-sha256
subkey AAAAgFlw0BMQ5gY+haYkZ6JC+yY=;
};
The key used for this disk will be derived by
HKDF-HMAC-SHA256_k(WXDQExDmBj6FpiRnokL7Jg==),
where k is the outcome of PBKDF2-SHA1 with the given parameters.
Note that <info> encodes a four-byte prefix giving the big-endian
length in bits of the info argument to HKDF, just like all other bit
strings in cgdconfig parameters files.
If you have multiple disks configured using the same keygen block
except for the info parameter, `cgdconfig -C' will only prompt once
for your passphrase, generate a shared key k with PBKDF2 as usual,
and then reuse it for each of the disks.
This provides an extra level of side-channel and cracking resistance
compared to the pre-existing pkcs5_pbkdf2/sha1 method used for
password-based disk encryption.
Several new keygen parameters are supported:
memory (integer, in kilobytes)
parallelism (integer, usually the number of CPU cores)
version (integer, usually 19...)
We do our best to calibrate these automatically when the paramsfile
is initially generated.
lgtm riastradh@
o added new features:
o -G: generate a new paramsfile that produces the same
key as the old paramsfile,
o ffs verify_method,
o multiple keygen methods that are xor'ed together
(for n-factor authentication), and
o calibrating the iteration count of PKCS#5 PBKDF2 to
the current machine's speed.
o changed paramsfile format to allow for the new features.
o replaced open-coded parser with yacc grammar.
o lots of supporting changes.
o updated documentation to reflect new features and new
paramsfile format.
riastradh