PCBC mode

PCBC (Propagating Cipher Block Chaining) is a variant of CBC in which each block depends on both the previous plaintext and the previous ciphertext. The idea was that a single flipped bit would propagate through every subsequent block, which in theory should make undetected tampering harder.

In practice the combination has a known flaw: two adjacent ciphertext blocks can be swapped without changing how the rest of the message decrypts. An attacker can reorder blocks and still get a message that decrypts to plausible data. PCBC therefore provides no meaningful integrity.

Kerberos version 4 used PCBC, and it is one of the most cited examples of a mode that looks like it resists tampering without doing so. Error propagation is not the same thing as error detection.