Cloak.AES.CTR
A Cloak.Cipher
which encrypts values with the AES cipher in CTR (stream) mode.
Configuration
In addition to the normal :default
and :tag
configuration options, this cipher takes a :keys
option to support using multiple AES keys at the same time.
config :cloak, Cloak.AES.CTR,
default: true,
tag: "AES",
keys: [
%{tag: <<1>>, key: :base64.decode("..."), default: true},
%{tag: <<2>>, key: :base64.decode("..."), default: false}
]
Key Configuration Options
A key may have the following attributes:
:tag
- The ID of the key. This is included in the ciphertext, and should be only a single byte. Seeencrypt/2
for more details.:key
- The AES key to use, in binary. If you store your keys in Base64 format you will need to decode them first.:default
- Boolean. Whether to use this key by default or not.
Upgrading to a New Key
To upgrade to a new key, simply add the key to the :keys
array, and set it as default: true
.
keys: [
%{tag: <<1>>, key: "old key", default: false},
%{tag: <<2>>, key: "new key", default: true}
]
You'll then likely need to recompile the Cloak dependency. After this, your new key will automatically be used for all new encyption, while the old key will be used to decrypt legacy values.
To migrate everything proactively to the new key, see the mix cloak.migrate
mix task defined in Mix.Tasks.Cloak.Migrate
.
Summary
Callback implementation for Cloak.Cipher.decrypt
. Decrypts a value encrypted with AES in CTR mode
Callback implementation for Cloak.Cipher.encrypt
. Encrypts a value using AES in CTR mode
Callback implementation for Cloak.Cipher.version
. Returns the tag of the current default key
Functions
Callback implementation for Cloak.Cipher.decrypt
. Decrypts a value encrypted with AES in CTR mode.
Uses the key tag to find the correct key for decryption, and the IV included in the header to decrypt the body of the ciphertext.
Parameters
ciphertext
- Binary ciphertext generated byencrypt/2
.
Examples
iex> encrypt("Hello") |> decrypt
"Hello"
Callback implementation for Cloak.Cipher.encrypt
. Encrypts a value using AES in CTR mode.
Generates a random IV for every encryption, and prepends the key tag and IV to the beginning of the ciphertext. The format can be diagrammed like this:
+----------------------------------+----------------------+
| HEADER | BODY |
+------------------+---------------+----------------------+
| Key Tag (1 byte) | IV (16 bytes) | Ciphertext (n bytes) |
+------------------+---------------+----------------------+
When this function is called through Cloak.encrypt/1
, the module's :tag
will be added, and the resulting binary will be in this format:
+---------------------------------------------------------+----------------------+
| HEADER | BODY |
+----------------------+------------------+---------------+----------------------+
| Module Tag (n bytes) | Key Tag (1 byte) | IV (16 bytes) | Ciphertext (n bytes) |
+----------------------+------------------+---------------+----------------------+
The header information allows Cloak to know enough about each ciphertext to ensure a successful decryption. See decrypt/1
for more details.
Important: Because a random IV is used for every encryption, encrypt/2
will not produce the same ciphertext twice for the same value.
Parameters
plaintext
- Any type of value to encrypt.key_tag
- Optional. The tag of the key to use for encryption.
Example
iex> encrypt("Hello") != "Hello"
true
iex> encrypt("Hello") != encrypt("Hello")
true