API.md

Introduction

iron is a cryptographic utility for sealing a JSON object using symmetric key encryption with message integrity verification. Or in other words, it lets you encrypt an object, send it around (in cookies, authentication credentials, etc.), then receive it back and decrypt it. The algorithm ensures that the message was not tampered with, and also provides a simple mechanism for password rotation.

Note: the wire protocol has not changed since 1.x (the version increments reflected a change in the internal error format used by the module and by the node API as well as other node API changes).

iron provides methods for encrypting an object, generating a message authentication code (MAC), and serializing both into a cookie / URI / HTTP header friendly format. Sealed objects are useful in cases where state has to reside on other applications not under your control, without exposing the details of this state to those application.

For example, sealed objects allow you to encrypt the permissions granted to the authenticated user, store those permissions using a cookie, without worrying about someone modifying (or even knowing) what those permissions are. Any modification to the encrypted data will invalidate its integrity.

The seal process follows these general steps:

• generate encryption salt saltE
• derive an encryption key keyE using saltE and a password
• generate an integrity salt saltI
• derive an integrity (HMAC) key keyI using saltI and the password
• generate a random initialization vector iv
• encrypt the serialized object string using keyE and iv
• mac the encrypted object along with saltE and iv
• concatenate saltE, saltI, iv, and the encrypted object into a URI-friendly string

Example

To seal an object:

const obj = {
    a: 1,
    b: 2,
    c: [3, 4, 5],
    d: {
        e: 'f'
    }
};

const password = 'some_not_random_password_that_is_at_least_32_characters';

try {
    const sealed = await Iron.seal(obj, password, Iron.defaults);
} catch (err) {
    console.log(err.message);
}

The result sealed object is a string which can be sent via cookies, URI query parameter, or an HTTP header attribute. To unseal the string:

try {
    const unsealed = await Iron.unseal(sealed, password, Iron.defaults);
} catch (err) {
    console.log(err.message);
}

Security Considerations

The greatest sources of security risks are usually found not in iron but in the policies and procedures surrounding its use. Implementers are strongly encouraged to assess how this module addresses their security requirements. This section includes an incomplete list of security considerations that must be reviewed and understood before using iron.

Plaintext Storage of Credentials

The iron password is only used to derive keys and is never sent or shared. However, in order to generate (and regenerate) the keys used to encrypt the object and compute the request MAC, the server must have access to the password in plaintext form. This is in contrast, for example, to modern operating systems, which store only a one-way hash of user credentials.

If an attacker were to gain access to the password - or worse, to the server's database of all such password - he or she would be able to encrypt and decrypt any sealed object. Accordingly, it is critical that servers protect these passwords from unauthorized access.

Frequently Asked Questions

Where is the protocol specification?

If you are looking for some prose explaining how all this works, there isn't any. iron is being developed as an open source project instead of a standard. In other words, the code is the specification. Not sure about something? Open an issue!

Is it done?

Yep.

How come the defaults must be manually passed and not automatically applied?

Because you should know what you are doing and explicitly set it. The options matter a lot to the security properties of the implementation. While reasonable defaults are provided, you still need to explicitly state you want to use them.

Acknowledgements

Special thanks to Adam Barth for his infinite patience, and always insightful feedback and advice.

Methods

await seal(object, password, options)

Seriealizes, encrypts, and signs objects into an iron protocol string where:

• object - the data being sealed. Can be any JavaScript value that is serializable via JSON.stringify(). Note: JSON.stringify will not keep object properties which values are undefined. So if you pass an object like {id: undefined} then it will be serialized as {}.

• password - one of:

  • a password string used to generate a key using the pbkdf2 algorithm.

  • a key buffer used as-is (after validating sufficient length based on the algorithm used).

  • an object with:

    • id - a password identifier (must consist of only letters, numbers, and _).
    • secret - a password string or key buffer used for both encryption and integrity.

  • an object with:

    • id - a password identifier (must consist of only letters, numbers, and _).
    • encryption - a password string or key buffer used for encryption.
    • integrity - a password string or key buffer used for integrity.

• options - see Options.

Return value: iron sealed string.

Note: assigning the password used an id allows for password rotation to improve the security of your deployment. Passwords should be rotated over time to reduce the risk of compromised security. When providing a password id, the id is included with the iron protocol string and it must match the id used to unseal.

It is recommended to combine password id with the ttl option to generate iron protocol strings of limited time validity which also allow for rotating passwords without the need to keep all previous passwords around (only the number of passwords used within the ttl window).

await unseal(sealed, password, options)

Verifies, decrypts, and reconstruct an iron protocol string into an object where:

• sealed - the iron protocol string generated with seal().

• password - must match the password value passed to seal() and be one of:

  • a password string used to generate a key using the pbkdf2 algorithm.

  • a key buffer used as-is (after validating sufficient length based on the algorithm used).

  • an object with id as the key and value is one of:

    • a password string or key buffer used for both encryption and integrity.
    • an object with:
      • encryption - a password string or key buffer used for encryption.
      • integrity - a password string or key buffer used for integrity.

• options - see Options. Must match the options value passed to seal()

Return value: the verified, decripted object.

Note: In order to enable password rotation, the password argument can accept an object with more than one password, each keyed by its id. Together with the ttl option, the password object only needs to include the passwords used within the ttl window.

await generateKey(password, options)

Generates an key from the password where:

• password - a password string or buffer key.
• options - see Options.

Return value: an object with the following keys:

• key
• salt
• iv

await encrypt(password, options, data)

Encrypts data where:

• password - a password string or buffer key.
• options - see Options.
• 'data' - the string to encrypt.

Return value: an object with the following keys:

• encrypted
• key:
  • key
  • salt
  • iv

await decrypt(password, options, data)

Decrypts data where:

• password - a password string or buffer key.
• options - see Options.
• 'data' - the string to decrypt.

Return value: the decrypted string.

await hmacWithPassword(password, options, data)

Calculates an HMAC digest where:

• password - a password string or buffer key.
• options - see Options.
• 'data' - the string to calculate the HMAC over.

Return value: an object with the following keys:

• digest
• salt

Options

iron provides options for customizing the key deriviation algorithm used to generate encryption and integrity verification keys, as well as the algorithms and salt sizes used.iron methods take an options object with the following keys:

• encryption - (required) defines the options used by the encryption process.
• integrity - (required) defines the options used by the HMAC integrity verification process.

Each of these option objects support the following keys:

• algorithm - (required) the algorithm name ('aes-256-cbc' and 'aes-128-ctr' for encryption and 'sha256' for integrity are the only two supported at this time).
• iv - (optional) an initialization vector buffer. If no iv is provided, one is generated based on the algorithm ivBits configuration.

When the password argument passed is a string (used for key generation), the following options are used:

• salt - (optional) a pre-generated salt string (a random buffer used to ensure that two identical objects will generate a different encrypted result).
• saltBits - (required if salt is not provided, otherwise ignored) the size of the salt.
• iterations - (required) the number of iterations used to derive a key from the password. Defaults to 1 iteration. The number of ideal iterations to use is dependent on your application's performance requirements. More iterations means it takes longer to generate the key.
• minPasswordlength - (required) the minimum password string length required for key generation. Defaults to 32 characters.

The 'seal()' and 'unseal()' methods also support the following options:

• ttl - sealed object lifetime in milliseconds where 0 means forever. Defaults to 0.
• timestampSkewSec - number of seconds of permitted clock skew for incoming expirations. Defaults to 60 seconds.
• localtimeOffsetMsec - local clock time offset, expressed in number of milliseconds (positive or negative). Defaults to 0.

Defaults

iron includes a default options object which can be passed to the methods as shown above in the example. The default settings are:

var options = {
    encryption: {
        saltBits: 256,
        algorithm: 'aes-256-cbc',
        iterations: 1
    },
    integrity: {
        saltBits: 256,
        algorithm: 'sha256',
        iterations: 1
    },
    ttl: 0,
    timestampSkewSec: 60,
    localtimeOffsetMsec: 0
};