A Block-Cipher Cryptographic System utilizing a unique user supplied key to control the cryptographic function and including means for modifying an input data block prior to performing a key-controlled transformation operation thereon. Said means includes means for extracting a segment of data utilized in a prior cryptographic transformation of the system and combining said segment with the input data block. The means for combining comprises a transformation which is a discrete valued function which in the preferred embodiment is an exclusive-or. In the preferred embodiment of the invention a block of ciphertext data is combined with the just succeeding input data block by an exclusive-or operation prior to the key-controlled transformation operation which produces said ciphertext blocks.
A means for detecting and cryptographically transforming short blocks of data is also disclosed wherein an input short data block is combined in a mathematically invertible operation with a block of data which is passed through said key-controlled block cipher cryptographic subsystem prior to said combination. The so transformed short block is then output as a short block in cryptographically modified form.