JWE

Contents

• Primer
• Usage

Primer

A JSON Web Encryption (JWE) represents encrypted content. It provides confidentiality and optionally integrity protection for this content JWE consists of:

1. A JSON-based header
2. An encrypted content encryption key
3. An initialization vector
4. Some additional authenticated data
5. The ciphertext
6. An authentication tag

To understand a JWE, it is good to understand the way it encrypts its payload.

As you may have noticed, a JWE contains two encrypted parts. The encrypted content encryption key and the ciphertext. When you construct a JWE, you have to provide the public key of the JWE’s recipient. During initialization, a shared content encryption key is created. The provided payload is then encrypted using this key, yielding the ciphertext. The shared content encryption key is then encrypted using the recipient’s public key and included in the JWE.

The recipient can use his matching private key to decrypt the encrypted content encryption key. Using this decrypted key he can decrypt the ciphertext and read the JWE payload.

Click here for a visualization of the JWE encryption and decryption flow.

  Provided By You         Computed Internally                     JWE                               Recipient


 +-------------+                                            +---------------+                                +-------------+
 |             |                                            |               |                                |             |
 |   Payload   +------------------^------------------------->  Ciphertext   +---------------------------^----+   Payload   |
 |             |                  |                         |               |                           |    |             |
 +-------------+                  |                         +---------------+                           |    +-------------+
                         encrypts |                                                            decrypts |
                                  |                                                                     |
                                  |                                                                     |
                    +-------------+--------------+     +----------------------------+     +-------------+--------------+
                    |                            |     |                            |     |                            |
                    |           Shared           +--^-->         Encrypted          +--^-->           Shared           |
                    |   Content Encryption Key   |  |  |   Content Encryption Key   |  |  |   Content Encryption Key   |
                    |                            |  |  |                            |  |  |                            |
                    +----------------------------+  |  +----------------------------+  |  +----------------------------+
                                                    |                                  |
                                                    |                                  |
                                                    |                                  |
+----------------+              encrypts            |                                  |  decrypts  +-----------------+
|                |                                  |                                  |            |                 |
|   Public Key   +----------------------------------+                                  +------------+   Private Key   |
|                |                                                                                  |                 |
+----------------+                                                                                  +-----------------+

Don’t worry though, most of this logic and the associated values are constructed under the hood. You’ll not have to worry about them too much. Nonetheless, they are described in more detail in the following sections.

Header

The JWE Header is a JSON object specifying the encryption algorithms applied to the payload and the content encryption key. Optionally it can contain additional properties of the JWE.

Example

The following header specifies two algorithms. The JWE’s payload is encrypted using the AES_256_CBC_HMAC_SHA_512 algorithm, yielding the ciphertext. The shared key, with which this payload encryption is performed is encrypted using the RSAES-PKCS1-v1_5 algorithm and included in the JWE.

{ "alg": "RSA1_5", "enc": "A256CBC-HS512" }

A detailed list describing possible header parameters can be found here.

Content Encryption Key

The payload is encrypted using a shared content encryption key that is randomly generated when initializing a JWE. The recipient of the JWE has to know this key so that he or she can decrypt the ciphertext. This means that this content encryption key has to be encrypted as well and included in the JWE.

The public key used to encrypt this shared content encryption key has to be provided by you. The recipient can use the matching private key to decrypt the content encryption key and subsequently the ciphertext, yielding the payload.

Initialization Vector

If the content encryption algorithm uses an initialization vector, it’s value is included in the JWE. If it does not use an initialization vector, the value is empty.

Additional Authenticated Data

An additional value that is integrity protected by the authenticated encryption operation. Since JOSESwift implements compact serialization, the JWE header is used as additional authenticated data. This means that the header is not encrypted but integrity protected.

Ciphertext

The ciphertext resulting from the authenticated encryption of the plaintext and the additional authenticated data.

Authentication Tag

The output of the authenticated encryption that ensures the integrity of the ciphertext and the additional authenticated data. If the encryption algorithm does not use an authentication tag, the value is empty.

Serialization

JOSESwift implements compact serialization for JWE. Compact serialization represents a JWE as the following concatenation:

base64url(header) + "." +
base64url(encrypted content encryption key) + "." +
base64url(initialization vector) + "." +
base64url(ciphertext) + "." +
base64url(authentication tag) + "." +

Note that the additional authenticated data is not explicitly included in the compact serialization. This is because the header is used as additional authenticated data.

Example

Given the following header and payload:

// Header
{ "alg": "RSA1_5", "enc": "A256CBC-HS512" }

// Payload
"Trumpets of Mexico 🏜"

We get the following values, encoded as base64url strings:

// base64url(header)
"eyJhbGciOiJSU0ExXzUiLCJlbmMiOiJBMjU2Q0JDLUhTNTEyIn0"

// base64url(encrypted content encryption key)
"b5Zbb...NglvQ"

// base64url(initialization vector)
"3vgaWb4EQbk_W1PHiBZBlg"

// base64url(ciphertext)
"5Hjsko9pPFpmkaNDLBYj9fUZp52FGVIpJo-jdAut3wk"

// base64url(authentication tag)
"hZXvBYxZEV4x7ACGhx3ky5pzNuf1X6UsmI3iEcYjHmM"

Which yields the following compact serialization (with line breaks added for readability):

"""
eyJhbGciOiJSU0ExXzUiLCJlbmMiOiJBMjU2Q0JDLUhTNTEyIn0.
b5Zbb...NglvQ.
3vgaWb4EQbk_W1PHiBZBlg.
5Hjsko9pPFpmkaNDLBYj9fUZp52FGVIpJo-jdAut3wk.
hZXvBYxZEV4x7ACGhx3ky5pzNuf1X6UsmI3iEcYjHmM
"""

Usage

This section describes how the above concepts are implemented and used in JOSESwift.

A JWE in JOSESwift is an immutable struct:

struct JWE {
    let header: JWEHeader
    let encryptedKey: Data
    let initializationVector: Data
    let ciphertext: Data
    let authenticationTag: Data
}

In JOSESwift you can construct a new JWE from some arbitrary payload or parse an existing compact serialized JWE to decrypt its payload.

Constructing a New JWE

You need three parts to construct a new JWE instance in JOSESwift.

1. A JWEHeader instance
2. A Payload instance
3. An Encrypter instance which computes the needed encryptions and its associated values.

These three parts are described in more detail in the following sections.

Header

A JWEHeader is a struct, holding a dictionary of parameters:

struct JWEHeader {
    let parameters: [String: Any]
}

For convenient use, a JWEHeader has an initializer that lets you provide the value of its two encryption algorithms:

init(algorithm: AsymmetricKeyAlgorithm, encryptionAlgorithm: SymmetricKeyAlgorithm) 

The algorithm value describes the algorithm used to encrypt the shared content encryption key. The encryptionAlgorithm value describes the algorithm used to encrypt the payload using that shared content encryption key.

To instantiate a JWEHeader you specify the algorithms that it should carry:

let header = JWEHeader(algorithm: .RSA1_5, encryptionAlgorithm: .A256CBCHS512)

💡 A JWE Header should also support optional additional parameters. This is currently not supported. See issue #60.

Payload

Payload has an initializer that lets you specify the data that your JWE should encrypt:

init(_ payload: Data)

To instantiate Payload you provide it with the data that it should carry:

let message = "Trumpets of Mexico 🏜"

let data = message.data(using: .utf8)!

let payload = Payload(data)

Encrypter

An Encrypter handles all cryptographic functionality needed to compute a JWE. It then passes all cryptographic computations to the respective libraries providing the cryptographic primitives:

• The generation of the shared content encryption key
• The encryption of the shared content encryption key
• The encryption of the payload, yielding the ciphertext, initialization vector, and authentication tag

Encrypter provides an initializer which lets you specify the desired encryption algorithms and the public key of the receiver:

init<KeyType>(keyEncryptionAlgorithm: AsymmetricKeyAlgorithm, keyEncryptionKey kek: KeyType, contentEncyptionAlgorithm: SymmetricKeyAlgorithm)

Note that the KeyType depends on the underlying crypto implementation. On iOS it’s SecKey per default. The initializer will return nil if you specify the wrong key type.

Make sure to check that you get a valid Encrypter instance after calling the initializer:

let publicKey: SecKey = /* ... */

guard let encrypter = Encrypter(keyEncryptionAlgorithm: .RSA1_5, keyEncryptionKey: publicKey, contentEncyptionAlgorithm: .A256CBCHS512) else {
    // Wrong key type.
}

// You have a valid encrypter.

As a framework user, you don’t need to worry about calling the encrypter’s encrypt function. It happens automatically during the initialization of a JWE.