Re: Randomnes in LibreOffice Encryption

Thank's for your fast reply.

> of course, but if you use an *actual* random function to derive the key 
> from the salt and the password then you'll have trouble decrypting the 
> resulting ciphertext...

Why should that be problematic? Randomly generated values ​​(for 
example, salt or initialization vector) are known because they are 
included in the unencrypted manifest.xml file. So the decryption 
algorithm (which is complete deterministic) has access to the values and 
there is no randomness in the decryption process.

> both: the method specified in ODF applies the HMAC-SHA1 function to 
> generate the key from the salt, not to generate the salt.

Ah, by that you mean the PRF, which used in the F()-function defined in 
RFC2898 on page 10, is the HMAC-SHA-1 function.

But how is the salt generated? The PBKDF2 key derivation method needs a 
Salt which is randomly generated:

> In password-based encryption, the party encrypting a message can gain 
> assurance that these benefits are realized simply by selecting a large 
> and sufficiently random salt when deriving an encryption key from a 
> password.
(at bottom on Page 5)


> > I looked at the referenced RFC 2898: "PKCS #5: Password-Based 
> > Cryptography Specification Version 2.0" 
> > (https://www.ietf.org/rfc/rfc2898.txt) how this will be made. In RFC 
> > 2898 at the end of page 6 and start of page 7 is written the 
> > following:
> >
> > > If a random number generator or pseudorandom generator is not 
> > > available,
>
> LibreOffice requires one.
>
> > > available, a deterministic alternative for generating the salt (or 
> > > the random part of it) is to apply a password-based key derivation 
> > > function to the password and the message M to be processed. For 
> > > instance, the salt could be computed with a key derivation function 
> > > as S = KDF (P, M). This approach is not recommended if the message M 
> > > is known to belong to a small message space (e.g., "Yes" or "No"), 
> > > however, since then there will only be a small number of possible 
> > > salts.
>
> > My question: Which method is implemented in LibreOffice? Does 
> > LibreOffice use a PRNG or the method specified in the ODF standard 
> > with the HMAC-SHA-1() function over the plaintext and (password)/(hash 
> > of the password))? The second one is a little bit insecure.
>
> both: the method specified in ODF applies the HMAC-SHA1 function to
> generate the key from the salt, not to generate the salt.

Or more precisely, LibreOffice uses a PRNG() to create a random salt. Is 
that correct?
If yes, does LibreOffice use a PRNG provided by the operating system?
If not, how is the salt generated?

Thanks many for your help.

Greetings

Steve