Key Agreement

Key Agreement is, much like Key Wrapping, a method to pass the CEK in the token using a Key-Encryption Key (KEK). Unlike Key Wrapping, which requires exchanging a symmetric KEK between the producer and the recipient, Key Agreement uses a mathematical property of Elliptic Curves to generate a shared secret, which can be used as a KEK.

How it works

Given Derive a method that computes a secret Z from a private key and a public key. The 2 equations:

Zprod = Derive(ProducerPrivateKey, RecipientPublicKey)

and

Zrec = Derive(RecipientPrivateKey, ProducerPublicKey)

Will both yield the same result, meaning:

Zprod = Zrec

Using this property, both party can exchange their (non-critical) public keys, and compute a secret only known to themselves, without ever exposing private information.

package main

import (
	"crypto/ecdh"
	"github.com/a-novel-kit/jwt/jwe/jwek"
)

func main() {
	// Public key shared by the recipient, ahead of the operation.
	var recipientPublicKey *ecdh.PublicKey

	// Private key used by the producer.
	var producerPrivateKey *ecdh.PrivateKey

	keyManager := jwek.NewECDHKeyAgrManager(
		&jwek.ECDHKeyAgrManagerConfig{
			ProducerKey: producerPrivateKey,
			RecipientKey: recipientPublicKey,
			// Required to authenticate the data when decoding.
			ProducerInfo: "Bob",
			RecipientInfo: "Alice",
		},
		jwek.ECDHESA128CBC,
	)
}

Available presets:

Preset	Target "enc"
jwek.ECDHESA128CBC	A128CBC
jwek.ECDHESA192CBC	A192CBC
jwek.ECDHESA256CBC	A256CBC
jwek.ECDHESA128GCM	A128GCM
jwek.ECDHESA192GCM	A192GCM
jwek.ECDHESA256GCM	A256GCM

WARNING

Unlike most Key Managers, the ECDH Key Agreement is not agnostic to the "enc" algorithm used.

Using a preset that mismatch the "enc" algorithm used by the encrypter will result in an error.