Swift PASETO

A Swift implementation of PASETO.

⚠️ WARNING: IMPLEMENTATION IS A PRE-RELEASE.

Paseto is everything you love about JOSE (JWT, JWE, JWS) without any of the many design deficits that plague the JOSE standards.

What is Paseto?
- Key Differences between Paseto and JWT
Installation
- Requirements
- Dependencies
Overview of the Swift library
Supported Paseto Versions

What is Paseto?

Paseto (Platform-Agnostic SEcurity TOkens) is a specification for secure stateless tokens.

Key Differences between Paseto and JWT

Unlike JSON Web Tokens (JWT), which gives developers more than enough rope with which to hang themselves, Paseto only allows secure operations. JWT gives you "algorithm agility", Paseto gives you "versioned protocols". It's incredibly unlikely that you'll be able to use Paseto in an insecure way.

Caution: Neither JWT nor Paseto were designed for stateless session management. Paseto is suitable for tamper-proof cookies, but cannot prevent replay attacks by itself.

Installation

Using Swift Package Manager, add the following to your Package.swift.

dependencies: [
    .package(
        url: "https://github.com/aidantwoods/swift-paseto.git",
        .upToNextMajor(from: "0.2.0")
    )
]

Requirements

Swift 4.1 and above.

Dependencies

The following are automatically resolved when using Swift Package Manager.

Swift-Sodium

Swift-Sodium provides the public key cryptography implementations, which are used in Version 2 public Paseto tokens, and is deferred to for various tasks such as: constant-time comparisons, constant-time encoding, and random number generation.
Clibsodium

Clibsodium is required by Swift-Sodium.

Clibsodium is used directly to provide the secret key cryptography implementations, which are used in Version 2 local Paseto tokens.

Overview of the Swift library

The Paseto Swift library is designed with the aim of using the Swift compiler to catch as many usage errors as possible.

At some point, you the user will have to decide which key to use when using Paseto. As soon as you do this you effectively lock in two things: (i) the version of Paseto tokens that you may use, (ii) the type of payload you either want to check or produce (i.e. encrypted if using local tokens, or signed if using public tokens).

The Paseto Swift library passes this information via type arguments (generics) so entire classes of misuse examples aren't possible (e.g. creating a version 2 key and accidentally attempting to produce a version 1 token, or trying to decrypt a signed token). In-fact, the functions that would enable you to even attempt these examples just don't exist.

Okay, so what does all that look like?

When creating a key, simply append the key type name to the version. Let's say we want to generate a new version 2 symmetric key:

let symmetricKey = Version2.SymmetricKey()

Okay, now let's create a token:

let token = Token(claims: [
    "data":    "this is a signed message",
    "expires": "2019-01-01T00:00:00+00:00",
])

Now encrypt it:

guard let message = try? token.encrypt(with: symmetricKey) else { /* respond to failure */ }

Then to get the encrypted token as a string, simply:

let pasetoToken = message.asString

Or even as data:

let pasetoTokenData = message.asData

message is of type Message<Version2.Local>. This means that it has a specialised decrypt(with: Version2.SymmetricKey) method, which can be used to retrieve the original token (when given a key). i.e. we can do:

guard let try? decryptedToken = message.decrypt(with: symmetricKey) else { /* respond to failure */ }

Let's say we want to generate a new version 2 secret (private) key:

let secretKey = Version2.AsymmetricSecretKey()

Now, if we wish produce a token which can be verified by others, we can do the following:

guard let signedMessage = try? token.sign(with: secretKey) else { /* respond to failure */ }

signedMessage is of type Message<Version2.Public>. This means that it has a specialised verify(with: Version2.AsymmetricPublicKey) method, which can be used to verify the contents and produce a verified token.

To do this we need to export the public key from our secretKey.

let publicKey = secretKey.publicKey

publicKey is of type Version2.AsymmetricPublicKey, so we may use:

guard let try? verifiedToken = signedMessage.verify(with: publicKey) else { /* respond to failure */ }

to reproduce the original token from the signedMessage.

Lastly, let's suppose that we do not start with any objects. How do we create messages and keys from strings or data?

Let's use the example from Paseto's readme:

The Paseto token is as follows (as a string/data)

v2.local.QAxIpVe-ECVNI1z4xQbm_qQYomyT3h8FtV8bxkz8pBJWkT8f7HtlOpbroPDEZUKop_vaglyp76CzYy375cHmKCW8e1CCkV0Lflu4GTDyXMqQdpZMM1E6OaoQW27gaRSvWBrR3IgbFIa0AkuUFw.UGFyYWdvbiBJbml0aWF0aXZlIEVudGVycHJpc2Vz

And the symmetric key, given in hex is:

707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f

To produce a token, use the following:

let rawToken = "v2.local.QAxIpVe-ECVNI1z4xQbm_qQYomyT3h8FtV8bxkz8pBJWkT8f7HtlOpbroPDEZUKop_vaglyp76CzYy375cHmKCW8e1CCkV0Lflu4GTDyXMqQdpZMM1E6OaoQW27gaRSvWBrR3IgbFIa0AkuUFw.UGFyYWdvbiBJbml0aWF0aXZlIEVudGVycHJpc2Vz"

guard let key = try? Version2.SymmetricKey(
    hex: "707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f"
) else {
    /* respond to failure */
}

guard let message = try? Message<Version2.Local>(rawToken) else {
    /* respond to failure */
}

guard let token = try? message.decrypt(with: key) else {
    /* respond to failure */
}

// the following will succeed
assert(token.claims == ["data": "this is a signed message", "exp": "2039-01-01T00:00:00+00:00"])
assert(token.footer == "Paragon Initiative Enterprises")

Keys can also be created using url safe base64 (with no padding) using init(encoded: String) or with the raw key material as data by using init(material: Data).

If you need to determine the type of a received raw token, you can use the helper function Util.header(of: String) -> Header? to retrieve a Header corresponding to the given token. This only checks that the given string is of a valid format, and does not guarantee anything about the contents.

For example, using rawToken from above:

guard let header = Util.header(of: rawToken) else { /* this isn't a valid Paseto token */ }

A Header is of the following structure:

struct Header {
    let version: Version
    let purpose: Purpose
}

where version is .v2, and purpose is either .Public (a signed message) or .Local (an encrypted message).

As Version and Purpose are enums, it is recommended that you use an explicitly exhaustive (i.e. no default) switch-case construct to select different code paths. Making this explicitly exhaustive ensures that if, say additional versions are added then the Swift compiler will inform you when you have not considered all possibilities.

If you attempt to create a message using a raw token which produces a header that does not correspond to the message's type arguments then the initialiser will fail.

Supported Paseto Versions

Version 2

Version 2 (the recommended version by the specification) is fully supported.

davidperrenoud/swift-paseto