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This repository contains our implementation of the KLEIN-64 cipher [1] (optimised for high speed) on the AVR ATTiny45 microcontroller. The ATTiny45 is an 8-bit processor that has 32 registers, 512 bytes SRAM and 4096 bytes Flash memory. As it uses 16-bit addresses, addressing memory requires two 8-bit registers. It is worth mentioning that only register 30 and 31 can be used to load from Flash memory.

The KLEIN cipher is a lightweight block cipher that shows many similarities with the Rijndael (AES) cipher. KLEIN consists of roughly the same five operations: AddRoundKey, SubNibbles, RotateNibbles, MixNibbles and KeySchedule. The main difference with AES is the fact that KLEIN operates on nibbles rather than bytes.

AddRoundKey This operation consists of xoring the round key into the state.
SubNibbles In this step, the state is passed through the sbox, one nibble at a time.
RotateNibbles This operation rotates the state to the left by four nibbles.
MixNibbles This step consists of a series of permutations that can be represented as two matrix multiplications over a finite field. It operates on pairwise concatenations of nibbles as elements of an input vector, effectively mimicking the Rijndael MixColumn operation on bytes.
KeySchedule This operation computes the next round key by performing several shifts, sbox lookups and xor operations.

To run our implementation of the KLEIN cipher with a different input, change line 6 to 13 and line 28 to 35 according to the key and plaintext values. The initial key and plaintext are loaded into registers using ldi instructions, and the resulting ciphertext ends up in register 16 to 23. During the execution of the program we keep the round key in register 0 to 7, while the state is alternately in register 8 to 15 and register 16 to 23 depending on the parity of the round.

[1] Gong, Z., Nikova, S., & Law, Y. W. (2012). KLEIN: a new family of lightweight block ciphers. In RFID. Security and Privacy (pp. 1-18). Springer Berlin Heidelberg.