Strongbox

Strongbox provides Public Key Encryption for ActiveRecord. By using a public key sensitive information can be encrypted and stored automatically. Once stored a password is required to access the information.

Because the largest amount of data that can practically be encrypted with a public key is 245 byte, by default Strongbox uses a two layer approach. First it encrypts the attribute using symmetric encryption with a randomly generated key and initialization vector (IV) (which can just be through to as a second key), then it encrypts those with the public key.

Strongbox stores the encrypted attribute in a database column by the same name, i.e. if you tell Strongbox to encrypt “secret” then it will be store in “secret” in the database, just as the unencrypted attribute would be. If symmetric encryption is used (the default) two additional columns “secret_key” and “secret_iv” are needed as well.

The attribute is automatically encrypted simply by setting it:

user.secret = “Shhhhhhh…”

and decrypted by calling the “decrypt” method with the private key password.

plain_text = user.secret.decrypt ‘letmein’

Quick Start

In your model:

class User < ActiveRecord::Base
  encrypt_with_public_key :secret,
    :key_pair => File.join(RAILS_ROOT,'config','keypair.pem')
end

In your migrations:

class AddSecretColumnsToUser < ActiveRecord::Migration
  def self.up
    add_column :users, :secret, :binary
    add_column :users, :secret_key, :binary
    add_column :users, :secret_iv, :binary
  end
  def self.down
    remove_column :users, :secret
    remove_column :users, :secret_key
    remove_column :users, :secret_iv
  end  
end

Generate a key pair:

(Choose a strong password.)

openssl genrsa -des3 -out config/private.pem 2048
openssl rsa -in config/private.pem -out config/public.pem -outform PEM -pubout
cat config/private.pem  config/public.pem >> config/keypair.pem

In your views and forms you don’t need to do anything special to encrypt data. To decrypt call:

user.secret.decrypt 'password'

Gem installation (Rails 2.1+)

In config/environment.rb:

config.gem "strongbox"

Usage

encrypt_with_public_key sets up the attribute it’s called on for automatic encryption. It’s simplest form is:

class User < ActiveRecord::Base
  encrypt_with_public_key :secret,
    :key_pair => File.join(RAILS_ROOT,'config','keypair.pem')
end

Which will encrypt the attribute “secret”. The attribute will be encrypted using symmetric encryption with an automatically generated key and IV encrypted using the public key. This requires three columns in the database “secret”, “secret_key”, and “secret_iv” (see below).

Options to encrypt_with_public_key are:

:public_key – Path to the public key file. Overrides :keypair.

:private_key – Path to the private key file. Overrides :keypair.

:keypair – Path to a file containing both the public and private keys.

:symmetric :always/:never – Encrypt the date using symmetric encryption. The public key is used to encrypt an automatically generated key and IV. This allows for large amounts of data to be encrypted. The size of data that can be encrypted directly with the public is limit to key size (in bytes) – 11. So a 2048 key can encrypt 245 bytes. Defaults to :always

:symmetric_cipher – Cipher to use for symmetric encryption. Defaults to ‘aes-256-cbc’. Other ciphers support by OpenSSL may be used.

:base64 true/false – Use Base64 encoding to convert encrypted data to text. Use when binary save data storage is not available. Defaults to false

:padding – Method used to pad data encrypted with the public key. Defaults to RSA_PKCS1_PADDING. The default should be fine unless you are dealing with legacy data.

For example, encrypting a small attribute, providing only the public key for extra security, and Base64 encoding the encrypted data:

class User < ActiveRecord::Base
  validates_length_of :pin_code, :is => 4
  encrypt_with_public_key :pin_code, 
    :symmetric => :never,
    :base64 => true,
    :public_key => File.join(RAILS_ROOT,'config','public.pem')
end

Key Generation

Generate a key pair:

openssl genrsa -des3 -out config/private.pem 2048
Generating RSA private key, 2048 bit long modulus
......+++
.+++
e is 65537 (0x10001)
Enter pass phrase for config/private.pem:
Verifying - Enter pass phrase for config/private.pem:

and extract the the public key:

openssl rsa -in config/private.pem -out config/public.pem -outform PEM -pubout
Enter pass phrase for config/private.pem:
writing RSA key

If you are going to leave the private key installed it’s easiest to create a single key pair file:

cat config/private.pem  config/public.pem >> config/keypair.pem

Or, for added security, store the private key file else where, leaving only the public key.

Table Creation

In it’s default configuration Strongbox requires three columns, one the encrypted data, one for the encrypted symmetric key, and one for the encrypted symmetric IV. If symmetric encryption is disabled then only the columns for the data being encrypted is needed.

If your underlying database allows, use the binary column type. If you must store your data in text format be sure to enable Base64 encoding and to use the text column type. If you use a string column and encrypt anything greater than 186 bytes (245 bytes if you don’t enable Base64 encoding) your data will be lost.

Security Caveats

If you don’t encrypt your data, then an attacker only needs to steal that data to get your secrets.

If encrypt your data using symmetric encrypts and a stored key, then the attacker needs the data and the key stored on the server.

If you use public key encryption, the attacker needs the data, the private key, and the password. This means the attacker has to sniff the password somehow, so that’s what you need to protect against.

Authors

Spike Ilacqua

Thanks

Strongbox’s implementation drew inspiration from Thoughtbot’s Paperclip gem http://www.thoughtbot.com/projects/paperclip