Churning (cipher)


Churning is an encryption function used to scramble downstream user data of the ATM passive optical network system defined by the ITU G.983.1 standard.
The standard states that churning "offers a low level of protection for data confidentiality". Cryptanalysis had shown that "the churning cipher is robustly weak".

Algorithm

Churning uses 24 bits of the key, designated X1..X8 and P1..P16.
Ten static K bits are generated from the key:
K1 = + + +
K2 = + + +
K3 = +
K4 = +
K5 = +
K6 = +
K7 = +
K8 = +
K9 = +
K10 = +
The churning transforms eight bits into eight bits:
= TransformNibble
= TransformNibble

Cryptanalysis

The cryptanalysis had shown the cipher to be effectively broken in more than one way:
Due to extreme weakness of the churning cipher, PON systems frequently use the "triple churning" technique, where the three churning operations are combined with two XORs with adjacent data in the stream.

Patents

holds patents on triple churning.