Simon (cipher)

Simon is a family of lightweight block ciphers that the United States National Security Agency released publicly in June 2013. The agency began work on it and on its sister cipher Speck in 2011, because it expected that some federal agencies would need a cipher that runs well on a wide range of Internet of Things devices while keeping an acceptable level of security. Simon is tuned for hardware implementations, whereas Speck is tuned for software. Structurally it is a balanced Feistel cipher working on n-bit words, so the block is 2n bits long, and the key is two, three or four words long, which gives variants such as Simon64/128, a 64-bit block with a 128-bit key. Its key schedule includes a round counter generated by a linear feedback shift register, which the designers say blocks slide and rotational attacks. As of 2018 no successful attack on any full-round variant was known, though reduced-round versions have been broken by differential cryptanalysis, and critics have pointed to its small security margin. Attempts to standardise it through ISO first failed, and it was later standardised as part of the RFID air interface standard ISO 29167-21.