If you want to do it right, take your user ID, put it with leading zeros, then encrypt it with something like AES and encode the result with base58. To return the identifier, simply decode, decrypt, and the int()result.
So for encryption:
>>> from Crypto.Cipher import AES
>>> import base64
>>> obj = AES.new('yoursecretkeyABC')
>>> x = base64.encodestring(obj.encrypt("%016d"%1))
>>> x
'tXDxMg1YGb1i0V29yCCBWg==\n'
and decryption
>>> int(obj.decrypt(base64.decodestring(x)))
1
If you can live with weak cryptography, you can also just flash the identifier with the key:
>>> key = [33, 53, 2, 42]
>>> id = "%04d" % 1
>>> x = ''.join([chr(a^ord(b)) for a, b in zip(key, id)])
>>> x
'\x11\x052\x1b'
>>> int(''.join([chr(a^ord(b)) for a, b in zip(key, x)]))
1
, OTP . , , .