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attr_keyring: Simple encryption-at-rest with key rotation support for Ruby.

Tests Code Climate Gem Gem

N.B.: attr_keyring is not for encrypting passwords--for that, you should use something like bcrypt. It's meant for encrypting sensitive data you will need to access in plain text (e.g. storing OAuth token from users). Passwords do not fall in that category.

This library is heavily inspired by attr_vault, and can read encrypted messages if you encode them in base64 (e.g. Base64.strict_encode64(encrypted_by_attr_vault)).

Installation

Add this line to your application's Gemfile:

gem "attr_keyring"

And then execute:

$ bundle

Or install it yourself as:

$ gem install attr_keyring

Usage

Basic usage

gem "attr_keyring"
require "keyring"

keyring = Keyring.new(
  {"1" => "uDiMcWVNTuz//naQ88sOcN+E40CyBRGzGTT7OkoBS6M="},
  digest_salt: "<custom salt>"
)

# STEP 1: Encrypt message using latest encryption key.
encrypted, keyring_id, digest = keyring.encrypt("super secret")

puts "🔒 #{encrypted}"
puts "🔑 #{keyring_id}"
puts "🔎 #{digest}"

# STEP 2: Decrypted message using encryption key defined by keyring id.
decrypted = keyring.decrypt(encrypted, keyring_id)
puts "✉️ #{decrypted}"

Change encryption algorithm

You can choose between AES-128-CBC, AES-192-CBC and AES-256-CBC. By default, AES-128-CBC will be used.

To specify the encryption algorithm, set the encryption option. The following example uses AES-256-CBC.

keyring = Keyring.new(
  "1" => "uDiMcWVNTuz//naQ88sOcN+E40CyBRGzGTT7OkoBS6M=",
  encryptor: Keyring::Encryptor::AES::AES256CBC,
  digest_salt: "<custom salt>"
)

Configuration

As far as database schema goes:

  1. You'll need a column to track the key that was used for encryption; by default it's called keyring_id.
  2. Every encrypted column must follow the name encrypted_<column name>.
  3. Optionally, you can also have a <column name>_digest to help with searching (see Lookup section below).

As far as model configuration goes, they're pretty similar, as you can see below:

ActiveRecord

From Rails 5+, ActiveRecord models now inherit from ApplicationRecord instead. This is how you set it up:

class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true
  include AttrKeyring.active_record
end

Sequel

Sequel doesn't have an abstract model class (but it could), so you can set up the model class directly like the following:

class User < Sequel::Model
  include AttrKeyring.sequel
end

Defining encrypted attributes

To set up your model, you have to define the keyring (set of encryption keys) and the attributes that will be encrypted. Both ActiveRecord and Sequel have the same API, so the examples below work for both ORMs.

class User < ApplicationRecord
  attr_keyring ENV["USER_KEYRING"],
               digest_salt: "<custom salt>"
  attr_encrypt :twitter_oauth_token, :social_security_number
end

The code above will encrypt your columns with the current key. If you're updating a record, then the column will be migrated to the latest key available.

You can use the model as you would normally do.

user = User.create(
  email: "john@example.com"
)

user.email
#=> john@example.com

user.keyring_id
#=> 1

user.encrypted_email
#=> WG8Epo0ABz0Z1X5gX7kttc98w9Ei59B5uXGK36Zin9G0VqbxX3naOWOm4RI6w6Uu

If you want to store a hash, you can use the encoder: option.

class User < ApplicationRecord
  attr_keyring ENV["USER_KEYRING"],
               digest_salt: "<custom salt>"

  attr_encrypt :data, encoder: JSON
end

An encoder is just an object that responds to the methods dump(data) and parse(data), just like the JSON interface. Alternatively, you can use AttrKeyring::Encoders::JSON, which returns hashes with symbolized keys.

Encryption

By default, AES-128-CBC is the algorithm used for encryption. This algorithm uses 16 bytes keys, but you're required to use a key that's double the size because half of that keys will be used to generate the HMAC. The first 16 bytes will be used as the encryption key, and the last 16 bytes will be used to generate the HMAC.

Using random data base64-encoded is the recommended way. You can easily generate keys by using the following command:

$ dd if=/dev/urandom bs=32 count=1 2>/dev/null | openssl base64 -A
qUjOJFgZsZbTICsN0TMkKqUvSgObYxnkHDsazTqE5tM=

Include the result of this command in the value section of the key description in the keyring. Half this key is used for encryption, and half for the HMAC.

Key size

The key size depends on the algorithm being used. The key size should be double the size as half of it is used for HMAC computation.

  • aes-128-cbc: 16 bytes (encryption) + 16 bytes (HMAC).
  • aes-192-cbc: 24 bytes (encryption) + 24 bytes (HMAC).
  • aes-256-cbc: 32 bytes (encryption) + 32 bytes (HMAC).

About the encrypted message

Initialization vectors (IV) should be unpredictable and unique; ideally, they will be cryptographically random. They do not have to be secret: IVs are typically just added to ciphertext messages unencrypted. It may sound contradictory that something has to be unpredictable and unique, but does not have to be secret; it is important to remember that an attacker must not be able to predict ahead of time what a given IV will be.

With that in mind, attr_keyring uses base64(hmac(unencrypted iv + encrypted message) + unencrypted iv + encrypted message) as the final message. If you're planning to migrate from other encryption mechanisms or read encrypted values from the database without using attr_keyring, make sure you account for this. The HMAC is 32-bytes long and the IV is 16-bytes long.

Keyring

Keys are managed through a keyring--a short JSON document describing your encryption keys. The keyring must be a JSON object mapping numeric ids of the keys to the key values. A keyring must have at least one key. For example:

{
  "1": "uDiMcWVNTuz//naQ88sOcN+E40CyBRGzGTT7OkoBS6M=",
  "2": "VN8UXRVMNbIh9FWEFVde0q7GUA1SGOie1+FgAKlNYHc="
}

The id is used to track which key encrypted which piece of data; a key with a larger id is assumed to be newer. The value is the actual bytes of the encryption key.

Dynamically loading keyring

If you're using Rails 5.2+, you can use credentials to define your keyring. Your credentials.yml must be define like the following:

---
user_keyring:
  "1": "QSXyoiRDPoJmfkJUZ4hJeQ=="
  "2": "r6AfOeilPDJomFsiOXLdfQ=="

Then you can setup your model by using attr_keyring Rails.application.credentials.user_keyring.

Other possibilities (e.g. the keyring file is provided by configuration management):

  • attr_keyring YAML.load_file(keyring_file), digest_salt: "<custom salt>"
  • attr_keyring JSON.parse(File.read(keyring_file)), digest_salt: "<custom salt>".

Lookup

One tricky aspect of encryption is looking up records by known secret. E.g.,

User.where(email: "john@example.com")

is trivial with plain text fields, but impossible with the model defined as above.

If a column <attribute>_digest exists, then a SHA1 digest from the value will be saved. This will allow you to lookup by that value instead and add unique indexes. You don't have to use a hashing salt, but it's highly recommended; this way you can avoid leaking your users' info via rainbow tables.

User.where(email: User.keyring.digest("john@example.com")).first

Key Rotation

Because attr_keyring uses a keyring, with access to multiple keys at once, key rotation is fairly straightforward: if you add a key to the keyring with a higher id than any other key, that key will automatically be used for encryption when records are either created or updated. Any keys that are no longer in use can be safely removed from the keyring.

To check if an existing key with id 123 is still in use, run:

# For a large dataset, you may want to index the `keyring_id` column.
User.where(keyring_id: 123).empty?

You may not want to wait for records to be updated (e.g. key leaking). In that case, you can rollout a key rotation:

User.where(keyring_id: 1234).find_each do |user|
    user.keyring_rotate!
end

What if I don't use ActiveRecord/Sequel?

You can also leverage the encryption mechanism of attr_keyring totally decoupled from ActiveRecord/Sequel. First, make sure you load keyring instead. Then you can create a keyring to encrypt/decrypt strings, without even touching the database.

require "keyring"

keyring = Keyring.new(
  {"1" => "QSXyoiRDPoJmfkJUZ4hJeQ=="},
  digest_salt: "<custom salt>"
)

encrypted, keyring_id, digest = keyring.encrypt("super secret")

puts encrypted
#=> encrypted: +mOWmIWKMV01nCm076OBnzgPGhWAZqNs8Etaad/0s3I=

puts keyring_id
#=> 1

puts digest
#=> e24fe0dea7f9abe8cbb192702578715079689a3e

decrypted = keyring.decrypt(encrypted, keyring_id)

puts decrypted
#=> super secret

Exchange data with Node.js

If you use Node.js, you may be interested in https://github.com/fnando/keyring-node, which is able to read and write messages using the same format.

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/fnando/attr_keyring. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.

License

The gem is available as open source under the terms of the MIT License.

Icon

Icon made by Icongeek26 from Flaticon is licensed by Creative Commons BY 3.0.

Code of Conduct

Everyone interacting in the attr_keyring project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.