Blowfish maximum key size

[jpboivin]

BouncyCastle 1.9 introduced validations of the Blowfish key size (https://github.com/bcgit/bc-csharp/blob/release/v2.0/crypto/src/crypto/engines/BlowfishEngine.cs#L444). More precisely, it must be between 32 and 448 bits, which is indeed the official upper limit. With that said, this is incompatible with OpenSSL's limit which is ((BF_ROUNDS + 2) * 4) or 576 bits (https://github.com/openssl/openssl/blob/openssl-3.0/crypto/bf/bf_skey.c#L31). Due to this difference, BouncyCastle is not interoperable with product/protocols based on OpenSSL, and using non-standard key sizes.

The change would be trivial to make, so the question is whether or not BC should support this non-standard key size.

[cipherboy]

@jpboivin Interesting, Wikipedia currently states:

When asked which Blowfish version is the correct one, Bruce Schneier answered: "The test vectors should be used to determine the one true Blowfish".

However, does not cite a source for this assertion. However, the OpenSSL code has persisted since the initial import, so there hasn't really been a discussion on it as far as I can tell there. NSS lacks a Blowfish implementation, but JDK maintains the original spec's limitation.

So, using these keys will cause broader interoperability problems with other existing implementations.

Do you have a protocol which requires these larger key sizes? Perhaps it is better to move to stronger, modern ciphers like XChaCha20 or AES, if no protocol requires it.

[jpboivin]

NSS lacks a Blowfish implementation, but JDK maintains the original spec's limitation.

Interesting to see that OpenSSL seems to be the black sheep, I'm not sure why they picked the key size limit based on rounds.

Do you have a protocol which requires these larger key sizes? Perhaps it is better to move to stronger, modern ciphers like XChaCha20 or AES, if no protocol requires it.

Yes, but it isn't a common / public protocol (it's the protocol used by an old game for which I wrote a server emulator to keep me up to date with my C#). A change to more modern ciphers is as such out of scope in this context, but I could always re-implement a non-standard Blowfish as I had done before switching to BC.

I would totally understand if BC prefers to stick with the more standard key limit :)

[cipherboy]

@jpboivin Hmm, if you're interested in proposing a patch, I'll run it by the others and see if there's objections.

It looks like Nettle (underpinning Gnutls) supports completely arbitrary key lengths as well...

[jpboivin]

@cipherboy As discussed, I've created a patch (https://github.com/bcgit/bc-csharp/pull/519) and added test vectors based on OpenSSL and Nettle.

[pdugre]

An example of use with keys over 448 bits is implementing encryption/decryption of SSH keys using a passphrase. It uses what they call brypt_pbkdf which is a modified version of PBKDF2 using a modified version of bcrypt as the primitive. This uses 512 bit keys internally for Blowfish(since they use a SHA512 hash of the password as the key).

This issue is currently blocking for us because of this particular case. Let me know if I can help!

[Recurse-blip]

An example of use with keys over 448 bits is implementing encryption/decryption of SSH keys using a passphrase. It uses what they call brypt_pbkdf which is a modified version of PBKDF2 using a modified version of bcrypt as the primitive. This uses 512 bit keys internally for Blowfish(since they use a SHA512 hash of the password as the key).

This issue is currently blocking for us because of this particular case. Let me know if I can help!

I think we should enforce 72 byte size (576 bits) just like the bcrypt Rust crate or make the maximum size configurable, I am also facing an issue using the bouncy castle implementation in a specific use case. I'll be forced to make a fork until this change is merged. https://github.com/bcgit/bc-csharp/pull/519

See this : https://docs.rs/bcrypt/latest/src/bcrypt/lib.rs.html#118