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Mon, 23 Jun 2008

Crypto processing in POHMELFS. OpenSSL vs GNU TLS.

If I did not miss something, GNU TLS (I never worked with it) supports very limited amount of ciphers and hashes, so it is not appropriate for filesystem data protection layer.
According to its documentation GNU TLS only supports AES, RC4 and 3DES ciphers and SHA1 and MD5 hashes. There is also only CBC chaining mode and several hash/cipher schemes.

So, POHMELFS server will use OpenSSL for data protection. Sooner or later OpenSSL will get hardware crypto support on Linux too (well, Linux crypto stack should first implement userspace API, which does not exist yet, although there is a work by Loc Ho from AMCC to add such support).

So far I decided to implement following protection scheme: checksumm or encryption will cover full transaction data, but will be applied by chunks:

• Transaction 'first-level' data, i.e. header and data immediately placed after transaction header. For all commands except page writing it will be finish.
• For write pages command, each header is generated dynamically and does not exist until data is really being sent, so crypto code will run over all pages and update checksum processing headers and data pages separately. Checkum update should be simple enough, since there are crypto helpers to update and finalize checksum, but encryption is more complex: I requires all chunks to be setup in advance in single scatterlist chain, with dynamic header generation it is too big overhead (it requires not only scatterlist allocation, but also header allocation just for encryption), so encryption will be done separately for headers and pages, and I will have to create some IV propagation scheme (like last bytes of previous unencrypted chunk will become IV for the next chunk, or something like that). I understand, that it may be not very secure approach though.
• Reading data back from server is simpler, since there are no transactions, and data will be encrypted/checksummed like in the first step above. It is possible, that it will force to increase network header structure a bit (32 or 16 bits to store size of the attached checksumm).

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