🔑 JSON Web Token signing and verification (HMAC, RSA, ECDSA) using BoringSSL.

Major Releases

The table below shows a list of JWTKit major releases alongside their compatible Swift versions.

Version	Swift	SPM
4.0	5.2+	from: "4.0.0"
3.0	4.0+	from: "3.0.0"
2.0	3.1+	from: "2.0.0"
1.0	3.1+	from: "1.0.0"

Use the SPM string to easily include the dependendency in your Package.swift file.

.package(url: "https://github.com/vapor/jwt-kit.git", from: "4.0.0")

Note: Prior to version 4.0, this package was included in vapor/jwt.git.

Supported Platforms

JWTKit supports the following platforms:

• Ubuntu 16.04, 18.04, 20.04
• macOS 10.15, 11
• CentOS 8
• Amazon Linux 2

Overview

JWTKit provides APIs for signing and verifying JSON Web Tokens (RFC7519). It supports the following features:

• Verifying (parsing)
• Signing (serializing)
• RSA (RS256, RS384, RS512)
• ECDSA (ES256, ES384, ES512)
• HMAC (HS256, HS384, HS512)
• Claims (aud, exp, iss, etc)
• JSON Web Keys (JWK, JWKS)

This package ships a private copy of BoringSSL for cryptography.

Vapor

If you are using Vapor, check out the JWT package which makes it easier to configure and use JWTKit in your project.

Getting Started

To start verifying or signing JWT tokens, you will need an instance of JWTSigners.

import JWTKit

// Signs and verifies JWTs
let signers = JWTSigners()

Let’s add a simple HS256 signer for testing. HMAC signers can sign and verify tokens.

// Add HMAC with SHA-256 signer.
signers.use(.hs256(key: "secret"))

For this example, we’ll use the very secure key secret.

Verifying

Let’s try to verify the following example JWT.

let jwt = """
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJ2YXBvciIsImV4cCI6NjQwOTIyMTEyMDAsImFkbWluIjp0cnVlfQ.lS5lpwfRNSZDvpGQk6x5JI1g40gkYCOWqbc3J_ghowo
"""

You can inspect the contents of this token by visiting jwt.io and pasting the token in the debugger. Set the key in the “Verify Signature” section to secret.

We need to create a struct conforming to JWTPayload that represents the JWT’s structure. We’ll use JWTKit’s included claims to handle common fields like sub and exp.

// JWT payload structure.
struct TestPayload: JWTPayload, Equatable {
    // Maps the longer Swift property names to the
    // shortened keys used in the JWT payload.
    enum CodingKeys: String, CodingKey {
        case subject = "sub"
        case expiration = "exp"
        case isAdmin = "admin"
    }

    // The "sub" (subject) claim identifies the principal that is the
    // subject of the JWT.
    var subject: SubjectClaim

    // The "exp" (expiration time) claim identifies the expiration time on
    // or after which the JWT MUST NOT be accepted for processing.
    var expiration: ExpirationClaim

    // Custom data.
    // If true, the user is an admin.
    var isAdmin: Bool

    // Run any additional verification logic beyond
    // signature verification here.
    // Since we have an ExpirationClaim, we will
    // call its verify method.
    func verify(using signer: JWTSigner) throws {
        try self.expiration.verifyNotExpired()
    }
}

Now that we have a JWTPayload, we can use JWTSigners to parse and verify the JWT.

// Parses the JWT and verifies its signature.
let payload = try signers.verify(jwt, as: TestPayload.self)
print(payload)

If everything worked, you should see the payload printed:

TestPayload(
    subject: "vapor", 
    expiration: 4001-01-01 00:00:00 +0000, 
    isAdmin: true
)

Signing

We can also generate JWTs, also known as signing. To demonstrate this, let’s use the TestPayload from the previous section.

// Create a new instance of our JWTPayload
let payload = TestPayload(
    subject: "vapor",
    expiration: .init(value: .distantFuture),
    isAdmin: true
)

Then, pass the payload to JWTSigners.sign.

// Sign the payload, returning a JWT.
let jwt = try signers.sign(payload)
print(jwt)

You should see a JWT printed. This can be fed back into the verify method to access the payload.

JWK

A JSON Web Key (JWK) is a JavaScript Object Notation (JSON) data structure that represents a cryptographic key (RFC7517). These are commonly used to supply clients with keys for verifying JWTs.

For example, Apple hosts their Sign in with Apple JWKS at the following URL.

GET https://appleid.apple.com/auth/keys

You can add this JSON Web Key Set (JWKS) to your JWTSigners.

import Foundation
import JWTKit

// Download the JWKS.
// This could be done asynchronously if needed.
let jwksData = try Data(
    contentsOf: URL(string: "https://appleid.apple.com/auth/keys")!
)

// Decode the downloaded JSON.
let jwks = try JSONDecoder().decode(JWKS.self, from: jwksData)

// Create signers and add JWKS.
let signers = JWTSigners()
try signers.use(jwks: jwks)

You can now pass JWTs from Apple to the verify method. The key identifier (kid) in the JWT header will be used to automatically select the correct key for verification.

Note: As of writing, JWK only supports RSA keys.

HMAC

HMAC is the simplest JWT signing algorithm. It uses a single key that can both sign and verify tokens. The key can be any length.

• hs256: HMAC with SHA-256
• hs384: HMAC with SHA-384
• hs512: HMAC with SHA-512

// Add HMAC with SHA-256 signer.
signers.use(.hs256(key: "secret"))

RSA

RSA is the most commonly used JWT signing algorithm. It supports distinct public and private keys. This means that a public key can be distributed for verifying JWTs are authentic while the private key that generates them is kept secret.

To create an RSA signer, first initialize an RSAKey. This can be done by passing in the components.

// Initialize an RSA key with components.
let key = RSAKey(
    modulus: "...",
    exponent: "...",
    // Only included in private keys.
    privateExponent: "..."
)

You can also choose to load a PEM file:

let rsaPublicKey = """
-----BEGIN PUBLIC KEY-----
MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC0cOtPjzABybjzm3fCg1aCYwnx
PmjXpbCkecAWLj/CcDWEcuTZkYDiSG0zgglbbbhcV0vJQDWSv60tnlA3cjSYutAv
7FPo5Cq8FkvrdDzeacwRSxYuIq1LtYnd6I30qNaNthntjvbqyMmBulJ1mzLI+Xg/
aX4rbSL49Z3dAQn8vQIDAQAB
-----END PUBLIC KEY-----
"""

// Initialize an RSA key with public pem.
let key = RSAKey.public(pem: rsaPublicKey)

Use .private for loading private RSA pem keys. These start with:

-----BEGIN RSA PRIVATE KEY-----

Use .certificate for loading X.509 certificates. These start with:

-----BEGIN CERTIFICATE-----

Once you have the RSAKey, you can use it to create an RSA signer.

• rs256: RSA with SHA-256
• rs384: RSA with SHA-384
• rs512: RSA with SHA-512

// Add RSA with SHA-256 signer.
try signers.use(.rs256(key: .public(pem: rsaPublicKey)))

ECDSA

ECDSA is a more modern algorithm that is similar to RSA. It is considered to be more secure for a given key length than RSA¹. However, you should do your own research before deciding.

Like RSA, you can load ECDSA keys using PEM files:

let ecdsaPublicKey = """
-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE2adMrdG7aUfZH57aeKFFM01dPnkx
C18ScRb4Z6poMBgJtYlVtd9ly63URv57ZW0Ncs1LiZB7WATb3svu+1c7HQ==
-----END PUBLIC KEY-----
"""

// Initialize an ECDSA key with public pem.
let key = ECDSAKey.public(pem: ecdsaPublicKey)

Use .private for loading private ECDSA pem keys. These start with:

-----BEGIN PRIVATE KEY-----

Once you have the ECDSAKey, you can use it to create an ECDSA signer.

• es256: ECDSA with SHA-256
• es384: ECDSA with SHA-384
• es512: ECDSA with SHA-512

// Add ECDSA with SHA-256 signer.
try signers.use(.es256(key: .public(pem: ecdsaPublicKey)))

Claims

JWTKit includes several helpers for implementing common JWT claims.

Claim	Type	Verify Method
aud	AudienceClaim	verifyIntendedAudience(includes:)
exp	ExpirationClaim	verifyNotExpired(currentDate:)
jti	IDClaim	n/a
iat	IssuedAtClaim	n/a
iss	IssuerClaim	n/a
locale	LocaleClaim	n/a
nbf	NotBeforeClaim	verifyNotBefore(currentDate:)
sub	SubjectClaim	n/a

All claims should be verified in the JWTPayload.verify method. If the claim has a special verify method, you can use that. Otherwise, access the value of the claim using value and check that it is valid.

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This package was originally authored by the wonderful @siemensikkema.

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1: https://sectigostore.com/blog/ecdsa-vs-rsa-everything-you-need-to-know/