I know I'll get bashed for this, but here goes: For a project I'm doing, I have a single very important 2-bit integer that I need to encrypt (the "security level" of the program) as fast as possible, while ensuring integrity. However, it is using the same encryption password as certain data at rest. If I encrypt the data with AES-CBC without a difficult key derivation procedure, that would make it easier to bruteforce the key for the data at rest, which is not too good. However, if I do use a difficult key derivation procedure, that would make the program a lot slower.
Is there a way to encrypt less than one byte of data quickly and with integrity verification, without reducing the security of the key derivation? My current thoughts are that the easiest way to do this would be to combine an unpredictable single-byte encryption method with a hashing algorithm with a high collision rate.