CS 361 Final Project: AES implementation
Authors: Mario Sergio Fuentes Juarez and Vishal Gullapalli
Link to repository in GitHub: https://github.com/msf1013/cs361-final-project-aes
The scripts in this repository implement the AES encryption and decryption procedures defined in https://nvlpubs.nist.gov/nistpubs/fips/nist.fips.197.pdf
Our implementation specifically uses the ECB mode, and conforms to the standard algorithms in FIPS 197.
Our code almost directly implements the algorithms defined in the AES reference spec, reading 16 bytes of input data at a time and passing that data to the conventional encryption/decryption pipeline outlined in the reference spec. 10 rounds are run for 128 bits long keys, whereas 14 rounds are run for 256 bits long keys, and each round consists of the following operations over the input block:
- Substitute bytes: Applying a byte-by-byte transformation using a pre-computed look-up table.
- Shift rows: By performing a cyclical shift as defined in the spec.
- Mix columns: Using pre-computed look-up tables for Galois Field multiplication.
- Add round key: Adding the round key to the state data block via XOR operations.
(It is worth noting that inverse operations where defined for decryption).
All of the resulting data blocks are then concatenated to produce the output cipher/plain text.
As suggested by our previous notes, we made use of a number of pre-computed look-up tables for the following operations:
- Bytes substitution
- Inverse bytes substitution
- Round constant word array used for key expansion
- Multiplication by 2, 3, 9, 11, 13, and 14 in the Galois Field.
Finally, we handle arbitrary file sizes (that is, not necessarily having a number of bytes multiple of 16), by using Zero Length padding: 0x00 bytes are appended to the input message to make its length a multiple of 16, and the very last byte's value is the number of bytes that were added for padding. Following this technique, for example, a message whose length is equal to 10 mod 16 will have 6 padding bytes (5 zeros and a final byte with value 6); whereas a message whose length is multiple of 16 will have 16 padding bytes (15 zeros and a final byte with value 16).
a) The program was written in Python 3.6, and tested in Windows and Ubuntu platforms. All libraries are standard, so no special imports or installation are needed.
The program can be executed in the following way:
> python aes.py --keysize <keysize> --keyfile <keyfile> --inputfile <inputfile> --outputfile <outputfile> --mode <mode>b) The program has 5 arguments which must be supplied via command line:
keysize: An integer, either 128 or 256, representing the key size in bits.keyfile: Name of file containing the key.inputfile: Name of file containing input plaintext/ciphertext to process, depending on whether mode is encrypt/decrypt.outputfile: Name of file where the output plaintext/ciphertext will be produced, depending on whether mode is encrypt/decrypt.mode: A string, either 'encrypt' or 'decrypt'.
Sample call:
> python aes.py --keysize 128 --keyfile key --inputfile plaintext --outputfile ciphertext --mode encrypt