When designing secret key ciphers, one approach which has not been very often considered so far, is the construction of block ciphers out of stream ciphers. It nevertheless would combine the advantage of both sides without their respective drawbacks. Only few attempts have been presented yet, and we propose here a completely new approach to block cipher design. By using strong Boolean functions and non-linear feedback shift register (NLFSR), we build a family of block ciphers called crossing over system (COS) ciphers, with arbitrary block size, and a key size of 128, 256 or 192 bits. It is based on a new design called crossing-over, using the internal state of NLFSRs as the constituent of the ciphering output blocks. Two versions have been developed: one considers any kind of plaintext, and the other one is devoted to plaintext without redundancy (e.g. compressed).
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