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Simple at-rest encryption with key rotation for Sequel models
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README.md

Build Status

AttrVault

Simple encryption-at-rest plugin for Sequel.

N.B.: AttrVault 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 plaintext. Passwords do not fall in that category.

Philosophy

Sensitive data should be encrypted at rest. Data breaches are common, and while preventing the breach in the first place is preferable, defense in depth is a wise strategy.

AttrVault encrypts your data in-application, so your encryption keys are never sent to the database. It includes an HMAC, so you can be confident the data was not tampered with.

AttrVault is also designed with key rotation in mind. It relies on a keyring of keys, always using the newest one for encryption, and supporting decryption with any available key. It tracks which data was encrypted with which key, making it easy to age out keys when needed (e.g., on an employee departure).

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": "PV8+EHgJlHfsVVVstJHgEo+3OCSn4iJDzqJs55U650Q=",
  "2": "0HyJ15am4haRsCyiFCxDdlKwl3G5yPNKTUbadpaIfPI="
}

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, used for encryption and verification: see below.

Encryption and verification

The encryption mechanism in AttrVault is borrowed from another encryption library, Fernet. The encrypted payload format is slightly different: AttrVault drops the TTL (almost never useful for data at rest) and the Base64 encoding (since most modern databases can deal with binary data natively).

The key should be 32 bytes of random data, base64-encoded. A simple way to generate that is:

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

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

Usage

N.B.: AttrVault depends on the Sequel::Model#before_save hook. If you use this in your model, be sure to call super!

First generate a key as above.

General schema changes

AttrVault needs some small changes to your database schema. It requires a key identifier column for each model that uses encrypted fields, and a binary data column for each field.

Here is a sample Sequel migration for adding encrypted fields to Postgres, where binary data is stored in bytea columns:

Sequel.migration do
  change do
    alter_table(:diary_entries) do
      add_column :key_id, :integer
      add_column :secret_stuff, :bytea
    end
  end
end

Encrypted fields

AttrVault needs some configuration in models as well. A vault_keyring attribute specifies a keyring in JSON (see the expected format above). Then, for each field to be encrypted, include a vault_attr attribute with its desired attribute name. You can optionally specify the name of the encrypted column as well (by default, it will be the field name suffixed with _encrypted):

class DiaryEntry < Sequel::Model
  vault_keyring ENV['ATTR_VAULT_KEYRING']
  vault_attr :body, encrypted_field: :secret_stuff
end

AttrVault will generate getters and setters for any vault_attrs specified.

Lookups

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

DiaryEntry.where(body: '@SwiftOnSecurity is dreamy')

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

AttrVault includes a way to mitigate this. Another small schema change:

Sequel.migration do
  change do
    alter_table(:diary_entries) do
      add_column :secret_digest, :bytea
    end
  end
end

Another small model definition change:

class DiaryEntry < Sequel::Model
  vault_keyring ENV['ATTR_VAULT_KEYRING']
  vault_attr :body, encrypted_field: :secret_stuff,
    digest_field: :secret_digest
end

To be continued...

(storing digests is implemented, easy lookup by digest is not)

Migrating unencrypted data

If you have plaintext data that you'd like to start encrypting, doing so in one shot can require a maintenance window if your data volume is large enough. To avoid this, AttrVault supports online migration via an "encrypt-on-write" mechanism: models will be read as normal, but their fields will be encrypted whenever the models are saved. To enable this behavior, just specify where the unencrypted data is coming from:

class DiaryEntry < Sequel::Model
  vault_keyring ENV['ATTR_VAULT_KEYRING']
  vault_attr :body, encrypted_field: :secret_stuff,
    migrate_from_field: :please_no_snooping
end

It's safe to use the same name as the name of the encrypted attribute.

Key rotation

Because AttrVault 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. Any keys that are no longer in use can be removed from the keyring.

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

DiaryEntry.where(key_id: 123).empty?

If this is true, the key with that id can be safely removed.

For a large dataset, you may want to index the key_id column.

Contributing

Patches are warmly welcome.

To run tests locally, you'll need a DATABASE_URL environment variable pointing to a database AttrVault may use for testing. E.g.,

$ createdb attr_vault_test
$ DATABASE_URL=postgres:///attr_vault_test bundle exec rspec

Please follow the project's general coding style and open issues for any significant behavior or API changes.

A pull request is understood to mean you are offering your code to the project under the MIT License.

License

Copyright (c) 2014-2017 AttrVault Contributors

MIT License. See LICENSE for full text.

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