Rijn

A small Ruby library and CLI for AES-GCM encryption.

In 1998, two Belgian cryptographers entered a contest run by the US government. Their cipher was called Rijndael. It won. The US government renamed it AES, the Advanced Encryption Standard, and it now protects most of the internet.

Rijn takes its name from Rijndael (say it "Rain-dahl" or "Rhine-dahl" — Rijndael's creators said either works). It does not reimplement the cipher. It uses AES through Ruby's own OpenSSL bindings, tested and fast, and wraps it in an API you can actually enjoy using.

Install the gem:

gem install rijn

Generate a key, encrypt something, decrypt it back:

$ rijn keygen
RQ+WWoEXHK+2c0adDvepnxoyaRsF7cvaG9QUTXKPxFk=

$ rijn encrypt -v "Hello, Rijn!" -k "RQ+WWoEXHK+2c0adDvepnxoyaRsF7cvaG9QUTXKPxFk="
tUFnikYpDHYbpaYeyFzaNHAeut5CAjPLdBhUB8mAacFQAgzyrWpKnA==

$ rijn decrypt -v "tUFnikYpDHYbpaYeyFzaNHAeut5CAjPLdBhUB8mAacFQAgzyrWpKnA==" -k "RQ+WWoEXHK+2c0adDvepnxoyaRsF7cvaG9QUTXKPxFk="
Hello, Rijn!

That's the whole library. Three commands, three methods, one cipher.

• AES-GCM at 128, 192, or 256 bits. You choose the size. Rijn picks a safe default.
• A fresh IV every time. You never generate or track it yourself.
• Tamper detection built in. A changed or truncated ciphertext fails to decrypt. It never silently returns garbage.
• A three-method Ruby API. generate_key, encrypt, decrypt. Nothing else to learn.

require "rijn"

key = Rijn.generate_key          # 256 bits by default
key = Rijn.generate_key(128)     # or 128, or 192

encrypted = Rijn.encrypt("Hello, Rijn!", key)
plaintext = Rijn.decrypt(encrypted, key)
# => "Hello, Rijn!"

rijn keygen                                    # generate a key
rijn keygen --bits 128                         # or -b, choose a size

rijn encrypt --value "Hello, Rijn!" --key "<key>"   # or -v / -k
rijn decrypt --value "<encrypted-value>" --key "<key>"

rijn encrypt                                   # skip a flag, get prompted for it
rijn decrypt

rijn version                                   # print the installed version
rijn help                                      # list every command
rijn help encrypt                              # help for one command

Rijn keys are Base64-encoded strings, 128, 192, or 256 bits. Rijn.generate_key defaults to 256.

Always generate keys with Rijn.generate_key. Never hand-write or edit key material.

Keep keys secret. Anyone holding a key can decrypt anything encrypted with it. Do not commit keys to source control, and do not hardcode them in application code. Store them in an environment variable, a secrets manager, or another secure configuration system.

AES-GCM gives you two guarantees:

• Confidentiality — no one reads an encrypted value without the key.
• Integrity — any change to an encrypted value is caught at decryption time.

Rijn generates a fresh IV for every encryption call and stores it alongside the ciphertext. The IV is not secret; it does not need to be. Decryption fails if the value was tampered with, or if the key is wrong.

Rijn does not manage key storage, rotation, distribution, or access control. That responsibility stays with your application. Rijn is a cipher, not a secrets manager.

bundle install              # install dependencies
bundle exec rspec           # run the test suite
gem build rijn.gemspec      # build the gem
gem install ./rijn-0.4.0.gem  # install your local build
./bin/rijn                  # run the CLI from source

Ruby 3.4 or newer.

See CHANGELOG.md for release history.

MIT. See LICENSE.md.