Rijndaex

Pure-Elixir implementation of the Rijndael (AES) cipher, written as an exercise for an Intro to Cryptography class taught by Luis Kowada at Universidade Federal Fluminense.

Features

• AES block encryption and decryption
• Multiple block cipher strategies: :naive, :ecb, :cbc (more planned)
• CLI for encrypting/decrypting files from the command line

Block cipher strategies

Strategy	Description
:naive	Single 16-byte block, no chaining. Useful for testing.
:ecb	Electronic Codebook — blocks encrypted independently. Simple but does not hide repeated plaintext patterns.
:cbc	Cipher Block Chaining — each block is XOR-ed with the previous ciphertext block before encryption, hiding repetition.

More strategies (e.g. CTR, GCM) are planned for future releases.

Usage

Key requirements

The key must be a binary of exactly 128, 192, or 256 bits (16, 24, or 32 bytes). Passing any other size raises a MatchError at runtime.

Key size	Bytes	AES variant	Rounds
128 bits	16	AES-128	10
192 bits	24	AES-192	12
256 bits	32	AES-256	14

# Valid 128-bit key (exactly 16 bytes)
key   = <<0x54, 0x68, 0x61, 0x74, 0x73, 0x20, 0x6D, 0x79,
          0x20, 0x4B, 0x75, 0x6E, 0x67, 0x20, 0x46, 0x75>>
input = <<0x54, 0x77, 0x6F, 0x20, 0x4F, 0x6E, 0x65, 0x20,
          0x4E, 0x69, 0x6E, 0x65, 0x20, 0x54, 0x77, 0x6F>>

# Encrypt (defaults to :ecb)
ciphertext = Rijndaex.cypher_blocks(key, input)

# Encrypt with CBC
ciphertext = Rijndaex.cypher_blocks(key, input, :cbc)

# Decrypt
plaintext = Rijndaex.uncypher_blocks(key, ciphertext, :ecb)

# Invalid key — raises MatchError at runtime
Rijndaex.cypher_blocks(<<1, 2, 3>>, input)
# ** (MatchError) no match of right hand side value: {:error, "Invalid key size: '24'. Valid options are [128, 192, 256]"}

CLI

The rijndaex escript exposes encryption and decryption from the command line.

Build

mix escript.build

Options

Run ./rijndaex --help to print usage information directly from the binary.

Flag	Description
--mode	cypher (default) or uncypher
--key	Encryption key as a raw string — must be 16, 24, or 32 bytes (AES-128/192/256)
--key-file	Path to a file containing the raw key bytes
--key-size	Generate a random key of this bit size: 128, 192, or 256
--input	Plaintext/ciphertext as a string
--input-file	Path to a file to encrypt or decrypt
--strategy	ecb (default), cbc, or naive
--output-file	Destination file for the result (default: output.txt)
--help	Print usage information and exit

If no key option is provided, a random 128-bit key is generated automatically.

Examples

# Encrypt a file with ECB (default strategy)
./rijndaex --input-file plain.txt --key "mysecretkey12345" --output-file cipher.bin

# Decrypt with CBC
./rijndaex --mode uncypher --strategy cbc --input-file cipher.bin --key "mysecretkey12345" --output-file plain.txt

# Encrypt with a randomly generated 128-bit key
./rijndaex --input "Hello, World!" --key-size 128

References

• https://blog.0x7d0.dev/education/how-aes-is-implemented/
• https://www.samiam.org/key-schedule.html — especially useful for examples and test cases