[Cryptography-dev] Low level API for Symmetric Encryption

[Donald Stufft]

So to kick things off I'd like to get AES-GCM exposed and figured it could be a good way to start the ball rolling for figuring out how we want to expose symmetric ciphers at the low level API.

I'm thinking cryptography.primitives.aes which has classes named like AES128GCM, AES256CBC, etc. The obvious naming scheme being AlgorithmKeysizeMode.

classes look something like

class AES128GCM:

    # Information about the Cipher
    authenticated = True
    block_size = 128 

    def __init__(self, key, iv, …)

    def update(self, plaintext)  # Updates an internal buffer as well as returns the encrypted chunk of data

    def finalize(self)  # Updates the internal buffer witth finalized data and returns the same finalized data


Some open questions:

A lot of these are going to be block ciphers, do we want to do padding for people or expect them to hand us chunks of the correct block size?

How do we decrypt vs encrypt. I think that:
    
    cipher = AES128GCM(key, iv)
    enciphered = cipher.update(plaintext) + cipher.finalize()

    cipher = AES128GCM(key, iv)
    plaintext = cipher.update(enciphered) + cipher.finalize()

Makes a decent API here, but we need a way to make a decryption vs encryption cipher. Possibly something like encrypt=True, or decrypt=True (specifying both being an error)?

Some ciphers (AES-GCM included) are authenticated and thus return (and require giving) a MAC in order to authenticate it, some authenticated ciphers also support the ability to pass along unencrypted but still authenticated data as well. I can't think of a decent way of doing this besides just adding functions (or __init__ args) to pass this data in, does anyone else have any ideas?

Any other thoughts? I'm just spitballing here so let's see what we can come up with!


-----------------
Donald Stufft
PGP: 0x6E3CBCE93372DCFA // 7C6B 7C5D 5E2B 6356 A926 F04F 6E3C BCE9 3372 DCFA

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