First draft of PasswordSafe format v3

Hi,

First, thanks to all for interesting comments and discussion on this, both
on and off-list.

Following is my proposal for the new format. Comments and constructive
criticism welcome.

	Cheers,

		Rony

The format described below has two goals:
1. To fix a minor design flaw in previous versions of the PasswordSafe
database format.
2. To replace the underlying cryptographic functions with more advanced
versions.

Meeting these goals is impossible without breaking compatibility: The new
format will NOT be compatible with existing implementations. An
implementation supporting this format SHALL be capable of importing from
and exporting to the previous (2.x) format.

A V3 format PasswordSafe will be structured as follows:

SALT|H(P')|B1|B2|IV|HDR|R1|R2|...|Rn

Where:

SALT is a 256 bit random value, generated at file creation time.

P' is the "stretched key" of the user's passphrase and the SALT, as defined
by the hash-function-based key stretching algorithm in
http://www.schneier.com/paper-low-entropy.pdf (Section 4.1), with SHA-256
as the hash function, and 2048 iterations (i.e., t = 11).

H(P') is SHA-256(P'), and is used to verify that the user has the correct
passphrase.

B1 and B2 are two 128-bit blocks encrypted with Twofish using P' as the
key, in ECB mode. These blocks contain the 256 bit random key K that is used
to encrypt the actual records. (This has the property that there is no known
or guessable
information on the plaintext encrypted with the passphrase-derived key that
allows an attacker to mount an attack that bypasses the key stretching
algorithm.)

IV is the 128-bit random Initial Value for CBC mode.

All the following records are encrypted using Twofish in CBC mode, with K
as the encryption key.

HDR: The database header, containing the version number, non-default user
preferences, and perhaps other housekeeping information.

R1..Rn: The actual password entries, in the format described in the V2
document, with possible additional fields.

END OF DRAFT





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