AES

Software implementation of the Advanced Encryption Standard (AES) algorithm. This library supports encryption and decryption in 128-bit, 192-bit, and 256-bit key schedules and can use either ECB or CBC mode.

Setup

Resolve dependencies from shard.yml:

crystal deps

Run project unit tests to verify functionality:

crystal spec

Generate project documentation:

crystal doc

You can use this library in projects by passing a byte array. This means you may have to call an appropriate conversion function in the language.

Example:

# Create a new AES utility instance, providing the key and arguments
u = AES::Utils.new(key, AES::Mode::CBC, AES::Process::Encrypt)

# Pass the plaintext and resulting cipher text
ciphertext = u.process(plaintext)

An example is located in sample/encrypt_aes.cr which can be run by invoking the crystal interpreter on the file.

To run the included files with ASCII characters representing hexadecimal values, the following conversion is required:

p = [] of Int32
plaintext.split("").each_slice(2) {|x| p << x.join("").to_i(16)}

Implementation Decisions

• FiniteField stores everything as a UInt8 (byte). Ideally, every computation would be performed on the byte level, but literals in Crystal are of type Int32 and the type system strictly prevents interleaving of different integer types. In production, readability would be sacrificed for memory efficiency and UInt8 casts would occur throughout the code.
• The AES specification defines functions as using words, but for the majority of the time, operations (like SBox substitutions), are conducted on the byte-level. As a general rule, everything is treated as an array of bytes, rather than words.
• The FiniteField struct defines arithmetic operations for Galois Field Multiplication in GF(2^8)
• AES helper functions often serve as double-purpose for encryption and decryption processes