Apparent issues with cipher quality

[FrozenBob]

I was testing an implementation of shuffling lists using pyffx, in an attempt to avoid holding the entire shuffled list in memory. One of my implementations is as follows:

def shuffle_generator(sequence):
  key = secrets.token_bytes()
  cipher = pyffx.Sequence(key, alphabet=sequence, length=1)
  for i in sequence:
    yield cipher.encrypt([i])[0]

In tests, this quickly proved to be a terrible shuffle. The result was always a rotation of the input - for example, list(shuffle_generator(range(5))) could produce [2, 3, 4, 0, 1] or [0, 1, 2, 3, 4], but never [0, 4, 2, 3, 1].

Am I using pyffx wrong in some way I'm not realizing?

I'm on pyffx 0.3.0 and Python 3.8.3.

[bschoening]

Likely your domain is too small. See the NIST FPE standards FF1 & FF3 and the requirement for a domain of > 1M. You have a domain of just 5 values.