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+/* 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/. */
+
+const ECDH_PARAMS = {
+ name: "ECDH",
+ namedCurve: "P-256",
+};
+const AES_PARAMS = {
+ name: "AES-GCM",
+ length: 256,
+};
+const AES_TAG_LEN = 128;
+const AES_GCM_IV_SIZE = 12;
+const UTF8_ENCODER = new TextEncoder();
+const UTF8_DECODER = new TextDecoder();
+
+class JWCrypto {
+ /**
+ * Encrypts the given data into a JWE using AES-256-GCM content encryption.
+ *
+ * This function implements a very small subset of the JWE encryption standard
+ * from https://tools.ietf.org/html/rfc7516. The only supported content encryption
+ * algorithm is enc="A256GCM" [1] and the only supported key encryption algorithm
+ * is alg="ECDH-ES" [2].
+ *
+ * @param {Object} key Peer Public JWK.
+ * @param {ArrayBuffer} data
+ *
+ * [1] https://tools.ietf.org/html/rfc7518#section-5.3
+ * [2] https://tools.ietf.org/html/rfc7518#section-4.6
+ *
+ * @returns {Promise<String>}
+ */
+ async generateJWE(key, data) {
+ // Generate an ephemeral key to use just for this encryption.
+ // The public component gets embedded in the JWE header.
+ const epk = await crypto.subtle.generateKey(ECDH_PARAMS, true, [
+ "deriveKey",
+ ]);
+ const ownPublicJWK = await crypto.subtle.exportKey("jwk", epk.publicKey);
+ // Remove properties added by our WebCrypto implementation but that aren't typically
+ // used with JWE in the wild. This saves space in the resulting JWE, and makes it easier
+ // to re-import the resulting JWK.
+ delete ownPublicJWK.key_ops;
+ delete ownPublicJWK.ext;
+ let header = { alg: "ECDH-ES", enc: "A256GCM", epk: ownPublicJWK };
+ // Import the peer's public key.
+ const peerPublicKey = await crypto.subtle.importKey(
+ "jwk",
+ key,
+ ECDH_PARAMS,
+ false,
+ ["deriveKey"]
+ );
+ if (key.hasOwnProperty("kid")) {
+ header.kid = key.kid;
+ }
+ // Do ECDH agreement to get the content encryption key.
+ const contentKey = await deriveECDHSharedAESKey(
+ epk.privateKey,
+ peerPublicKey,
+ ["encrypt"]
+ );
+ // Encrypt with AES-GCM using the generated key.
+ // Note that the IV is generated randomly, which *in general* is not safe to do with AES-GCM because
+ // it's too short to guarantee uniqueness. But we know that the AES-GCM key itself is unique and will
+ // only be used for this single encryption, making a random IV safe to use for this particular use-case.
+ let iv = crypto.getRandomValues(new Uint8Array(AES_GCM_IV_SIZE));
+ // Yes, additionalData is the byte representation of the base64 representation of the stringified header.
+ const additionalData = UTF8_ENCODER.encode(
+ ChromeUtils.base64URLEncode(UTF8_ENCODER.encode(JSON.stringify(header)), {
+ pad: false,
+ })
+ );
+ const encrypted = await crypto.subtle.encrypt(
+ {
+ name: "AES-GCM",
+ iv,
+ additionalData,
+ tagLength: AES_TAG_LEN,
+ },
+ contentKey,
+ data
+ );
+ // JWE needs the authentication tag as a separate string.
+ const tagIdx = encrypted.byteLength - ((AES_TAG_LEN + 7) >> 3);
+ let ciphertext = encrypted.slice(0, tagIdx);
+ let tag = encrypted.slice(tagIdx);
+ // JWE serialization in compact format.
+ header = UTF8_ENCODER.encode(JSON.stringify(header));
+ header = ChromeUtils.base64URLEncode(header, { pad: false });
+ tag = ChromeUtils.base64URLEncode(tag, { pad: false });
+ ciphertext = ChromeUtils.base64URLEncode(ciphertext, { pad: false });
+ iv = ChromeUtils.base64URLEncode(iv, { pad: false });
+ return `${header}..${iv}.${ciphertext}.${tag}`; // No CEK
+ }
+
+ /**
+ * Decrypts the given JWE using AES-256-GCM content encryption into a byte array.
+ * This function does the opposite of `JWCrypto.generateJWE`.
+ * The only supported content encryption algorithm is enc="A256GCM" [1]
+ * and the only supported key encryption algorithm is alg="ECDH-ES" [2].
+ *
+ * @param {"ECDH-ES"} algorithm
+ * @param {CryptoKey} key Local private key
+ *
+ * [1] https://tools.ietf.org/html/rfc7518#section-5.3
+ * [2] https://tools.ietf.org/html/rfc7518#section-4.6
+ *
+ * @returns {Promise<Uint8Array>}
+ */
+ async decryptJWE(jwe, key) {
+ let [header, cek, iv, ciphertext, authTag] = jwe.split(".");
+ const additionalData = UTF8_ENCODER.encode(header);
+ header = JSON.parse(
+ UTF8_DECODER.decode(
+ ChromeUtils.base64URLDecode(header, { padding: "reject" })
+ )
+ );
+ if (!!cek.length || header.enc !== "A256GCM" || header.alg !== "ECDH-ES") {
+ throw new Error("Unknown algorithm.");
+ }
+ if ("apu" in header || "apv" in header) {
+ throw new Error("apu and apv header values are not supported.");
+ }
+ const peerPublicKey = await crypto.subtle.importKey(
+ "jwk",
+ header.epk,
+ ECDH_PARAMS,
+ false,
+ ["deriveKey"]
+ );
+ // Do ECDH agreement to get the content encryption key.
+ const contentKey = await deriveECDHSharedAESKey(key, peerPublicKey, [
+ "decrypt",
+ ]);
+ iv = ChromeUtils.base64URLDecode(iv, { padding: "reject" });
+ ciphertext = new Uint8Array(
+ ChromeUtils.base64URLDecode(ciphertext, { padding: "reject" })
+ );
+ authTag = new Uint8Array(
+ ChromeUtils.base64URLDecode(authTag, { padding: "reject" })
+ );
+ const bundle = new Uint8Array([...ciphertext, ...authTag]);
+
+ const decrypted = await crypto.subtle.decrypt(
+ {
+ name: "AES-GCM",
+ iv,
+ tagLength: AES_TAG_LEN,
+ additionalData,
+ },
+ contentKey,
+ bundle
+ );
+ return new Uint8Array(decrypted);
+ }
+}
+
+/**
+ * Do an ECDH agreement between a public and private key,
+ * returning the derived encryption key as specced by
+ * JWA RFC.
+ * The raw ECDH secret is derived into a key using
+ * Concat KDF, as defined in Section 5.8.1 of [NIST.800-56A].
+ * @param {CryptoKey} privateKey
+ * @param {CryptoKey} publicKey
+ * @param {String[]} keyUsages See `SubtleCrypto.deriveKey` 5th paramater documentation.
+ * @returns {Promise<CryptoKey>}
+ */
+async function deriveECDHSharedAESKey(privateKey, publicKey, keyUsages) {
+ const params = { ...ECDH_PARAMS, ...{ public: publicKey } };
+ const sharedKey = await crypto.subtle.deriveKey(
+ params,
+ privateKey,
+ AES_PARAMS,
+ true,
+ keyUsages
+ );
+ // This is the NIST Concat KDF specialized to a specific set of parameters,
+ // which basically turn it into a single application of SHA256.
+ // The details are from the JWA RFC.
+ let sharedKeyBytes = await crypto.subtle.exportKey("raw", sharedKey);
+ sharedKeyBytes = new Uint8Array(sharedKeyBytes);
+ const info = [
+ "\x00\x00\x00\x07A256GCM", // 7-byte algorithm identifier
+ "\x00\x00\x00\x00", // empty PartyUInfo
+ "\x00\x00\x00\x00", // empty PartyVInfo
+ "\x00\x00\x01\x00", // keylen == 256
+ ].join("");
+ const pkcs = `\x00\x00\x00\x01${String.fromCharCode.apply(
+ null,
+ sharedKeyBytes
+ )}${info}`;
+ const pkcsBuf = Uint8Array.from(
+ Array.prototype.map.call(pkcs, c => c.charCodeAt(0))
+ );
+ const derivedKeyBytes = await crypto.subtle.digest(
+ {
+ name: "SHA-256",
+ },
+ pkcsBuf
+ );
+ return crypto.subtle.importKey(
+ "raw",
+ derivedKeyBytes,
+ AES_PARAMS,
+ false,
+ keyUsages
+ );
+}
+
+export const jwcrypto = new JWCrypto();