client-sessions


client-sessions is connect middleware that implements sessions in encrypted tamper-free cookies. For a complete introduction to encrypted client side sessions, refer to [Francois Marier's blog post on the subject][];

[Francois Marier's blog post on the subject]: https://hacks.mozilla.org/2012/12/using-secure-client-side-sessions-to-build-simple-and-scalable-node-js-applications-a-node-js-holiday-season-part-3/

NOTE: It is not recommended using both this middleware and connect's built-in session middleware.

Installation

Usage

Basic usage:

``js
var sessions = require("client-sessions");
app.use(sessions({
cookieName: 'mySession', // cookie name dictates the key name added to the request object
secret: 'blargadeeblargblarg', // should be a large unguessable string
duration: 24 60 60 * 1000, // how long the session will stay valid in ms
activeDuration: 1000 60 5 // if expiresIn < activeDuration, the session will be extended by activeDuration milliseconds
}));

app.use(function(req, res, next) {
if (req.mySession.seenyou) {
res.setHeader('X-Seen-You', 'true');
} else {
// setting a property will automatically cause a Set-Cookie response
// to be sent
req.mySession.seenyou = true;
res.setHeader('X-Seen-You', 'false');
}
});
`

You can control more specific cookie behavior during setup:

`js
app.use(sessions({
cookieName: 'mySession', // cookie name dictates the key name added to the request object
secret: 'blargadeeblargblarg', // should be a large unguessable string
duration: 24 60 60 * 1000, // how long the session will stay valid in ms
cookie: {
path: '/api', // cookie will only be sent to requests under '/api'
maxAge: 60000, // duration of the cookie in milliseconds, defaults to duration above
ephemeral: false, // when true, cookie expires when the browser closes
httpOnly: true, // when true, cookie is not accessible from javascript
secure: false // when true, cookie will only be sent over SSL. use key 'secureProxy' instead if you handle SSL not in your node process
}
}));
`

You can have multiple cookies:

`js
// a 1 week session
app.use(sessions({
cookieName: 'shopping_cart',
secret: 'first secret',
duration: 7 24 60 60 1000
}));

// a 2 hour encrypted session
app.use(sessions({
cookieName: 'authenticated',
secret: 'first secret',
duration: 2 60 60 * 1000
}));
`

In this example, there's a 2 hour authentication session, but shopping carts persist for a week.

Finally, you can use requestKey to force the name where information can be accessed on the request object.

`js
var sessions = require("client-sessions");
app.use(sessions({
cookieName: 'mySession',
requestKey: 'forcedSessionKey', // requestKey overrides cookieName for the key name added to the request object.
secret: 'blargadeeblargblarg', // should be a large unguessable string or Buffer
duration: 24 60 60 * 1000, // how long the session will stay valid in ms
}));

app.use(function(req, res, next) {
// requestKey forces the session information to be
// accessed via forcedSessionKey
if (req.forcedSessionKey.seenyou) {
res.setHeader('X-Seen-You', 'true');
}
next();
});
`

Cryptography

A pair of encryption and signature keys are derived from the secret option
via HMAC-SHA-256; the secret isn't used directly to encrypt or compute the
MAC.

The key-derivation function, in pseudocode:

`text
encKey := HMAC-SHA-256(secret, 'cookiesession-encryption');
sigKey := HMAC-SHA-256(secret, 'cookiesession-signature');
`

The AES-256-CBC cipher is used to encrypt the session contents, with an
HMAC-SHA-256 authentication tag (via Encrypt-then-Mac composition). A
random 128-bit Initialization Vector (IV) is generated for each encryption
operation (this is the AES block size regardless of the key size). The
CBC-mode input is padded with the usual PKCS#5 scheme.

In pseudocode, the encryption looks like the following, with || denoting
concatenation. The createdAt and duration parameters are decimal strings.

`text
sessionText := cookieName || '=' || sessionJson
iv := secureRandom(16 bytes)
ciphertext := AES-256-CBC(encKey, iv, sessionText)
payload := iv || '.' || ciphertext || '.' || createdAt || '.' || duration
hmac := HMAC-SHA-256(sigKey, payload)
cookie := base64url(iv) || '.' ||
base64url(ciphertext) || '.' ||
createdAt || '.' ||
duration || '.' ||
base64url(hmac)
`

For decryption, a constant-time equality operation is used to verify the HMAC
output to avoid the plausible timing attack.

$3

The defaults are secure, but may not suit your requirements. Some example scenarios:
- You want to use randomly-generated keys instead of using the key-derivation
function used in this module.
- AES-256 is overkill for the type of data you store in the session (e.g. not
personally-identifiable or sensitive) and you'd like to trade-off decreasing
the security level for CPU economy.
- SHA-256 is maybe too weak for your application and you want to have more
MAC security by using SHA-512, which grows the size of your cookies slightly.

If the defaults don't suit your needs, you can customize client-sessions.
**Beware: Changing keys and/or algorithms will make previously-generated
Cookies invalid!**

#### Configuring Keys

To configure independent encryption and signature (HMAC) keys:

`js
app.use(sessions({
encryptionKey: loadFromKeyStore('session-encryption-key'),
signatureKey: loadFromKeyStore('session-signature-key'),
// ... other options discussed above ...
}));
`

#### Configuring Algorithms

To specify custom algorithms and keys:

`js
app.use(sessions({
// use WEAKER-than-default encryption:
encryptionAlgorithm: 'aes128',
encryptionKey: loadFromKeyStore('session-encryption-key'),
// use a SHORTER-than-default MAC:
signatureAlgorithm: 'sha256-drop128',
signatureKey: loadFromKeyStore('session-signature-key'),
// ... other options discussed above ...
}));
`

#### Encryption Algorithms

Supported CBC-mode encryptionAlgorithms (and key length requirements):

| Cipher | Key length |
| ------ | ---------- |
| aes128 | 16 bytes |
| aes192 | 24 bytes |
| aes256 | 32 bytes |

These key lengths are exactly as required by the Advanced Encryption
Standard.

#### Signature (HMAC) Algorithms

Supported HMAC signatureAlgorithms (and key length requirements):

| HMAC | Minimum Key Length | Maximum Key Length |
| -------------- | ------------------ | ------------------ |
| sha256 | 32 bytes | 64 bytes |
| sha256-drop128 | 32 bytes | 64 bytes |
| sha384 | 48 bytes | 128 bytes |
| sha384-drop192 | 48 bytes | 128 bytes |
| sha512 | 64 bytes | 128 bytes |
| sha512-drop256 | 64 bytes | 128 bytes |

The HMAC key length requirements are derived from RFC 2104 section
3. The maximum key length can
be exceeded, but it doesn't increase the security of the signature.

The -dropN algorithms discard the latter half of the HMAC output, which
provides some additional protection against SHA2 length-extension attacks on
top of HMAC. The same technique is used in the upcoming JSON Web Algorithms
AES_CBC_HMAC_SHA2 authenticated
cipher.

#### Generating Keys

One can easily generate both AES and HMAC-SHA2 keys via command line: openssl
rand -base64 32 for a 32-byte (256-bit) key. It's easy to then parse that
output into a Buffer:

`js
function loadKeyFromStore(name) {
var text = myConfig.keys[name];
return new Buffer(text, 'base64');
}
`

#### Key Constraints

If you specify encryptionKey or signatureKey, you must supply the other as
well.

The following constraints must be met or an Error will be thrown:

1. both keys must be Buffers.
2. the keys must be _different_.
3. the encryption key are _exactly_ the length required (see above).
4. the signature key has _at least_ the length required (see above).

Based on the above, please note that if you specify a secret _and_ a
signatureAlgorithm, you need to use sha256 or sha256-drop128`.

License

> This Source Code Form is subject to the terms of the Mozilla Public
> License, v. 2.0. If a copy of the MPL was not distributed with this
> file, You can obtain one at http://mozilla.org/MPL/2.0/.