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Diffstat (limited to '')
| -rw-r--r-- | services/crypto/modules/jwcrypto.sys.mjs | 214 |
1 files changed, 214 insertions, 0 deletions
diff --git a/services/crypto/modules/jwcrypto.sys.mjs b/services/crypto/modules/jwcrypto.sys.mjs new file mode 100644 index 0000000000..d1f37eaa58 --- /dev/null +++ b/services/crypto/modules/jwcrypto.sys.mjs @@ -0,0 +1,214 @@ +/* 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(); |