A journey to scripting Firefox Sync / Lockwise: understanding BrowserID

Migrating from unmaintained browserid-crypto (jwcrypto) to a generic implementation

August 8, 2021

Picture credit: Val

This article is part of a series about scripting Firefox Sync / Lockwise.

  1. A journey to scripting Firefox Sync / Lockwise: existing clients
  2. A journey to scripting Firefox Sync / Lockwise: figuring the protocol
  3. A journey to scripting Firefox Sync / Lockwise: understanding BrowserID
  4. A journey to scripting Firefox Sync / Lockwise: hybrid OAuth
  5. A journey to scripting Firefox Sync / Lockwise: complete OAuth

In the previous post we made a script that is able to fetch and decrypt collections from Firefox Sync, including Lockwise passwords. But one thing was still bugging me. When running the code, the console was showing a deprecation warning.

[DEP0005] DeprecationWarning: Buffer() is deprecated due to security and usability issues. Please use the Buffer.alloc(), Buffer.allocUnsafe(), or Buffer.from() methods instead.

By running node --trace-warnings, I could see that it was coming from the browserid-crypto package. It was trivial to fix by migrating from the deprecated new Buffer() constructor to Buffer.from(), so I made another PR for this.

Like the first PR, it’s reviewed and approved by Ryan (who’s name I saw countless times when researching about Firefox Accounts and Sync protocols). He also notes that this library is unmaintained:

FWIW, it would be best to consider this library unmaintained at this point, but I’m happy to take small fixes like this all the same.

While there’s nothing wrong with using an unmaintained library if it gets the job done, I took this as a challenge to implement the protocol using only native (or more generic, well-maintained) modules, and this is going to be the topic of this blog post!

Isolating the browserid-crypto code

Let’s start from the script we previously built and extract the part that use the browserid-crypto package.

const { promisify } = require('util')
const jwcrypto = require('browserid-crypto')

const kp = await promisify(jwcrypto.generateKeypair)({ algorithm: 'DS', keysize: 256 })

// Also works with RSA.
// const kp = await promisify(jwcrypto.generateKeypair)({ algorithm: 'RS', keysize: 256 })

// Time interval in milliseconds until the certificate will expire, up to a
// maximum of 24 hours as documented in <https://github.com/mozilla/fxa/blob/f6bc0268a9be12407456fa42494243f336d81a38/packages/fxa-auth-server/docs/api.md#request-body-32>.
const duration = 1000 * 60 * 60 * 24

const { cert } = await client.certificateSign(creds.sessionToken, kp.publicKey.toSimpleObject(), duration)

// Generate an "identity assertion" which is a JWT as documented in
// <https://github.com/mozilla/id-specs/blob/prod/browserid/index.md#identity-assertion>.
const signedObject = await promisify(jwcrypto.assertion.sign)(
  {},
  {
    audience: tokenServerUrl,
    issuer: authServerUrl,
    expiresAt: Date.now() + duration
  },
  kp.secretKey
)

// Certs are separated by a `~` as documented in <https://github.com/mozilla/id-specs/blob/prod/browserid/index.md#backed-identity-assertion>.
const backedAssertion = [cert, signedObject].join('~')

There are 3 steps here:

  1. First, we generate a DSA or RSA keypair, which would be trivial to do with the Node.js crypto module.
  2. Then we encode the public key in a way that is compatible with Firefox Accounts’ /certificate/sign endpoint.
  3. Finally, we sign a JWT using our private key, and bundle it with the certificate to make a “backed identity assertion”.

Generating the keypair

This is the easy part. The main difference is that while jwcrypto.generateKeyPair takes an internal keysize parameter, which they map to RSA, and DSA key sizes, we need to explicitly give the RSA key size (usually synonymous of the modulus length).

In our case, a BrowserID RSA key of “256” corresponds to a 2048-bit RSA key and similarly, a BrowserID DSA key of “256” corresponds to a 2048-bit DSA key. They also both specify SHA-256 as the JWT hash algorithm, which will be useful for later.

const { promisify } = require('util')
const crypto = require('crypto')

// With RSA
const kp = await promisify(crypto.generateKeyPair)('rsa', {
  modulusLength: 2048
})

// With DSA
const kp = await promisify(crypto.generateKeyPair)('dsa', {
  modulusLength: 2048,
  divisorLength: 256
})

Note: for DSA, the BrowserID key has a divisor length of 256 bits (the q parameter), and this is especially important as browserid-crypto and the TokenServer don’t accept any other divisor length for a key size of 2048.

Encoding the key

Where it gets a bit more tricky, especially for DSA, is when we want to encode the key as JSON for the Firefox Accounts API to sign:

const { cert } = await client.certificateSign(creds.sessionToken, kp.publicKey.toSimpleObject(), duration)

The keys from browserid-crypto conveniently include a toSimpleObject function that formats the key in the BrowserID JSON format. I couldn’t find documentation for it, but from looking at the actual JSON objects, it is very similar to (but not compatible with) the JWK format.

BrowserID vs. JWK and base conversion

BrowserID was introduced in 2011, well before the JWK specification was proposed in 2015. They both encode the low-level key parameters in a JSON object, and there is just a couple of differences, especially:

  1. JWK doesn’t support DSA keys.
  2. To specify the key type, JWK has a kty property (set to RSA for RSA keys),while BrowserID uses an algorithm property that can be RS or DS.
  3. JWK encodes the key parameters as Base64URL, while BrowserID uses decimal (base 10) for RSA and hexadecimal (base 16) for DSA.

RSA

The RSA parameters are n (modulus) and e (exponent), as well as a fuckton of other parameters for private keys (which we don’t need here). You can see them on an existing key with openssl rsa -in rsa-private-key.pem -text -noout.

We can take the earlier code to generate a RSA keypair, and export the public key as a JWK like this:

const { promisify } = require('util')
const crypto = require('crypto')

const kp = await promisify(crypto.generateKeyPair)('rsa', {
  modulusLength: 2048
})

console.log(kp.publicKey.export({ format: 'jwk' }))
{
  "kty": "RSA",
  "n": "3M852Cy7DIH1wYJVgRxQfDYPa26fC4KR4uYmHeGV7rTtiQ2-IdypkOQd6Clp01-J4L9e28w-3hR06ZWKRMIbfyajcer1bd_9luBKkRiFlYxa-CBNTlOJBmtej7MbouQJdqcxRIHufk7R4HBWYzR8H1WUDzJfIZJLxz2eymTNXu7CPFyDoNZXQ9SRu7tzPzhUsDrkdpNSs2x8tRrllJRiO-BOC2Ce3W5vCE9eB91VFuIOHOuL5y-Fr6K-vCfvpLBzoF2uk399ZGxZ8rLXHk01QDoin3BVXQzGBKNXoVNrNe-tKflp5QJ5wMifvL4tPfCCrps8rrfbE1NDPE2x1QmCfQ",
  "e": "AQAB"
}

On the other hand, a BrowserID RSA public key looks like this:

const jwcrypto = require('browserid-crypto')

require('browserid-crypto/lib/algs/rs')

jwcrypto.generateKeypair({ algorithm: 'RS', keysize: 256 }, (err, { publicKey }) => {
  console.log(publicKey.toSimpleObject())
})
{
  "algorithm": "RS",
  "n": "24561144013955114361783231655761853176741812326893374232205401875943449227620158204608340216900927757193227109312970662811636219675773452185909191206484694392560433664701055247500397746104758184735693308844235833317883872067955852418577691056051019648528118784214798195301896767050575864274186910237901534713406182369363255235410257674380032656581487055343920363852506722639241918085307849979198768941882638020102729524988683333585179817471524571511030397962907590237048329319430881173155778553010801560573247170682531231684185163187096747308113243183139470492492221024173487301503496674419087411376160055924262029047",
  "e": "65537"
}

It is easy to convert a native JWK to the BrowserID format, as we can leverage BigInt to output a large base 10 string. But the constructor doesn’t accept Base64 directly, so we need to do an intermediate conversion through hexadecimal.

const { promisify } = require('util')
const crypto = require('crypto')

function base64to10 (data) {
  return BigInt('0x' + Buffer.from(data, 'base64').toString('hex')).toString(10)
}

const kp = await promisify(crypto.generateKeyPair)('rsa', {
  modulusLength: 2048
})

const jwk = kp.publicKey.export({ format: 'jwk' })

const publicKey = {
  algorithm: jwk.algorithm.slice(0, 2),
  n: base64to10(jwk.n),
  e: base64to10(jwk.e)
}

DSA

The DSA parameters are p (larger prime modulus), q (smaller prime modulus), g (generator), y (public group element), as well as x (private exponent) for private keys.

Since JWK doesn’t support DSA, we cannot do kp.publicKey.export({ format: 'jwk' }) with a DSA key generated by the crypto module, as that would fail with [ERR_CRYPTO_JWK_UNSUPPORTED_KEY_TYPE]: Unsupported JWK Key Type.

With browserid-crypto, we can replace RS by DS in the previous example to generate the following key, and get a better idea of what we’re trying to reproduce:

const jwcrypto = require('browserid-crypto')

require('browserid-crypto/lib/algs/rs')

jwcrypto.generateKeypair({ algorithm: 'DS', keysize: 256 }, (err, { publicKey }) => {
  console.log(publicKey.toSimpleObject())
})
{
  "algorithm": "DS",
  "y": "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",
  "p": "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",
  "q": "b1e370f6472c8754ccd75e99666ec8ef1fd748b748bbbc08503d82ce8055ab3b",
  "g": "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"
}

Exporting the key as a JWK was really handy earlier to access the low-level key parameters, but we cannot do that with DSA. We can only export the key as DER (binary) or PEM (Base64 encoded DER).

This means that we’ll need to use the OpenSSL CLI to dump the key parameters as we saw earlier. Let’s start by generating the PEM private key.

const { promisify } = require('util')
const crypto = require('crypto')

const kp = await promisify(crypto.generateKeyPair)('dsa', {
  modulusLength: 2048,
  divisorLength: 256
})

const privateKey = kp.privateKey.export({ format: 'pem', type: 'pkcs8' })

Then, we can invoke the openssl command, piping it the private key.

const cp = require('child_process')

const sub = cp.spawn('openssl', ['dsa', '-in', '-', '-text', '-noout'])

sub.stdin.write(privateKey)

The child process will now emit data events on the stdout stream and we can use that to parse the OpenSSL output. For context, here’s what a typical output looks like:

Output of openssl dsa -in dsa-private-key.pem -text -noout
Private-Key: (2048 bit)
priv:
    4b:66:fe:d5:68:c2:7e:3d:4a:fc:c0:45:10:01:91:
    fe:d7:83:be:39:0b:79:f3:0f:a1:c3:63:0e:8a:8f:
    63:db
pub:
    7e:50:55:ea:62:b8:70:0f:89:ca:f9:ad:41:21:05:
    8d:2c:71:e3:14:a5:1c:70:7d:a6:68:97:10:2f:93:
    f3:82:ee:98:25:7c:6a:42:71:9a:e0:b0:bf:c2:76:
    18:df:fe:68:63:ba:a8:a0:4d:10:9f:5a:da:c6:e3:
    c9:94:23:4a:d5:8e:00:ac:6b:f8:40:06:10:d1:6a:
    09:17:7e:73:8e:10:5b:5a:a0:dc:7a:c7:7d:cb:96:
    3b:8d:d8:d5:27:05:e0:0f:d8:e3:04:24:c3:ef:49:
    0d:56:54:54:3a:cd:c8:bf:36:03:2e:e7:8f:21:a2:
    8e:14:f9:17:57:85:7f:83:73:01:bc:90:aa:01:d1:
    4b:cb:84:c0:99:ee:2a:d2:3d:d7:30:97:51:89:fd:
    ef:b8:7a:ea:5e:5f:17:37:53:ce:43:b5:05:64:b9:
    09:c8:3f:07:eb:c4:9b:77:a5:6b:d2:d3:d0:ed:3e:
    47:1d:54:7d:f1:a1:ef:66:25:a6:fc:61:1b:cb:ae:
    60:f9:3b:7d:58:f3:e4:19:3b:09:4d:3f:87:c6:97:
    95:9b:78:02:55:fc:d8:74:86:06:50:8b:78:23:63:
    c6:b2:46:96:48:88:93:c6:32:d4:88:33:c7:44:f1:
    b9:73:b7:1a:72:0c:1e:55:40:7c:f3:cf:7a:fe:06:
    b7
P:
    00:bc:a3:68:a5:2b:1d:b5:c6:8a:4e:70:0d:78:4b:
    17:83:37:8f:d4:3a:9c:27:e7:08:b5:6d:9a:91:b4:
    8e:22:81:7e:ee:10:8c:08:45:c3:a1:f5:95:b3:9c:
    71:83:49:c2:dc:58:67:d3:c4:5c:1a:db:2e:c6:a4:
    18:4a:8a:15:b8:3b:b8:94:29:b4:43:79:e3:32:11:
    98:26:6e:65:01:11:f0:b9:cf:a2:e5:dc:4b:f8:4c:
    31:27:ff:75:cf:b8:b4:13:b0:f5:e8:da:ab:76:7b:
    ba:7d:ca:9b:fd:c1:29:89:77:6e:ee:95:33:3c:64:
    94:5e:4d:5b:0b:f4:b8:4f:91:54:8c:40:35:75:11:
    06:1e:7f:ed:ae:17:9e:ce:9b:8d:e1:79:75:7c:fd:
    d2:60:3f:89:10:6e:95:04:67:5b:08:31:71:ea:13:
    76:78:28:cd:cb:03:2b:66:19:3e:39:12:98:86:d3:
    90:d6:43:72:6e:32:bf:27:c6:76:f4:ab:04:e6:54:
    f3:41:ca:52:60:7e:74:1c:26:b3:e9:4c:0e:94:88:
    bd:7d:3e:af:a0:0d:50:58:89:a5:7a:d2:9d:4c:27:
    0f:2c:c2:6e:98:2e:a8:6d:22:97:19:2a:7c:ae:0c:
    b8:d3:1e:46:f9:e5:62:b4:91:2c:43:a2:02:1d:30:
    6f:4f
Q:
    00:b0:60:bd:58:73:4e:5a:37:e5:4e:a3:15:2a:a7:
    d9:dd:e2:b6:c2:f9:3d:37:4b:9d:43:33:9b:25:9c:
    bc:97:67
G:
    02:80:e7:af:91:ef:92:ef:51:67:2e:84:a8:e4:f1:
    c5:e0:c1:98:c2:c9:59:e0:89:3f:71:3f:99:fd:ee:
    cf:fa:db:6e:6f:bc:8b:5b:d0:06:35:0d:c2:19:96:
    c1:be:18:43:ed:76:52:70:4d:d3:8f:71:e2:b4:d0:
    a6:1e:ed:0d:67:71:24:dd:f1:86:06:99:f4:39:a8:
    45:d1:ac:5b:55:af:f3:89:0d:44:87:e9:36:ac:02:
    a6:fc:5d:27:56:96:92:d5:5e:35:a8:62:5f:63:9c:
    bf:da:ff:8e:c0:a0:28:7a:9c:41:2a:2c:bb:c6:80:
    7c:7b:86:58:4e:af:95:2c:06:51:5f:15:81:cc:8f:
    c1:9b:72:fa:82:71:65:81:ee:9e:99:f7:04:f9:1e:
    90:e4:ea:88:0e:44:b1:78:0e:67:8b:b6:61:7b:94:
    27:f1:7f:a6:7f:7b:59:21:73:71:92:a6:5f:98:67:
    a3:b7:e4:b2:dd:e7:55:f3:22:ac:de:44:1a:54:71:
    e3:33:ce:22:ac:38:93:e1:6b:9b:96:43:ce:4c:8c:
    87:a3:86:97:a1:1c:b6:7c:cc:d8:ab:7d:82:a2:0f:
    f5:7a:75:a5:f1:bc:e7:04:94:ae:83:98:98:70:5d:
    89:b0:54:8b:84:bf:ec:b1:eb:bb:fc:55:98:d0:ca:
    b4

With the following code, we can parse it into an object:

const readline = require('readline')

const rl = readline.createInterface({ input: sub.stdout })

const params = {}
let currentParam

rl.on('line', line => {
  // Continuation of an existing parameter, append to it.
  if (line.startsWith(' ') && currentParam) {
    params[currentParam] += line.trim()
    return
  }

  // Definition of a new parameter.
  const split = line.split(':')

  if (split.length < 2) {
    return
  }

  currentParam = split[0]
  params[currentParam] = split[1].trim()
})

await new Promise((resolve, reject) => {
  sub.on('exit', code => {
    if (code > 0) {
      return reject(new Error(`OpenSSL failed with code ${code}`))
    }

    resolve()
  })
})

console.log(params)

Conveniently, OpenSSL returns the parameters in hexadecimal form already, so we just need to remove the : that it separates each byte with, and rename the properties to make a BrowserID JSON key.

const publicKey = {
  algorithm: 'DS',
  y: params.pub.replaceAll(':', ''),
  p: params.P.replaceAll(':', ''),
  q: params.Q.replaceAll(':', ''),
  g: params.G.replaceAll(':', '')
}

Signing the JWT

The last part to refactor is the JWT:

const signedObject = await promisify(jwcrypto.assertion.sign)(
  {},
  {
    audience: tokenServerUrl,
    issuer: authServerUrl,
    expiresAt: Date.now() + duration
  },
  kp.secretKey
)

It seems that this is a standard JWT, so we can use the njwt package for this (a simpler and more flexible alternative to jsonwebtoken).

Note: the main quirk is that Mozilla uses a milliseconds timestamp for the exp field, while JWT defines it as a standard timestamp (in seconds).

This means that with both libraries, we need to work around that in order to force the exp field to be in milliseconds. For njwt, this means doing jwt.setClaim('exp', expiresAt) instead of using jwt.setExpiration(expiresAt), and for jsonwebtoken it means including the exp claim as part of the payload instead of using the expiresIn parameter that is otherwise expressed as a duration in seconds.

As for the private key, while njwt documents it should be a (PEM) string or a (DER) buffer, since it just forwards it to Node.js crypto module, we can directly give it the KeyObject that’s in kp.privateKey.

const njwt = require('njwt')

const signedObject = njwt.create({ aud: tokenServerUrl, iss: authServerUrl }, kp.privateKey, 'RS256')
  .setClaim('exp', Date.now() + duration)
  .compact()

Here we specified the RS256 algorithm for a RSA key. This works perfectly, and with this code, we can effectively generate a BrowserID assertion that will be accepted by the TokenServer!

Hacking around DSA

But neither njwt nor jsonwebtoken support DSA signatures. In fact, it seems that most JWT libraries don’t support DSA whatsoever.

That being said, we can leverage the fact that njwt forwards the private key to the crypto.sign method to make it work with our DSA key. All we need to do is to trick it into thinking that it’s signing with the RS256 algorithm so that it follows the SHA-256 signature code path (which actually works perfectly with a DSA key), and force the alg header to be DS256 just at the time it is encoded in the JWT (otherwise the library will complain that the algorithm is unsupported).

const njwt = require('njwt')

const jwt = njwt.create({ aud: tokenServerUrl, iss: authServerUrl }, kp.privateKey, 'RS256')
  .setClaim('exp', Date.now() + duration)

jwt.header.compact = function compact () {
  const alg = this.alg
  this.alg = 'DS256'
  const header = njwt.JwtHeader.prototype.compact.call(this)
  this.alg = alg
  return header
}

const signedObject = jwt.compact()

This should work but it does not. I keep getting HTTP 401s with invalid-credentials error from the TokenServer. Why? It took some trial and error to figure as this was far from obvious.

It turns out that Node.js defaults the DSA signature encoding to DER, and BrowserID only supports the IEEE P1363 format.

Thankfully, Node.js allows us to wrap the private key in an object to specify extra options like a dsaEncoding: 'ieee-p1363'. In the previous code this would look like:

const jwt = njwt.create(
  { aud: tokenServerUrl, iss: authServerUrl },
  { key: kp.privateKey, dsaEncoding: 'ieee-p1363' },
  'RS256'
)

Congratulations! We now also have a working DSA JWT!

Fully native implementation

Now, let’s even remove the njwt dependency, and bake our own JWT in-house. Because why not. It’s actually pretty trivial, and to be honest, a simpler and cleaner solution for DSA than the hack we just made.

const crypto = require('crypto')

const header = { alg: 'DS256' }
const payload = { exp: Date.now() + duration, aud: tokenServerUrl, iss: authServerUrl }

const body = [
  Buffer.from(JSON.stringify(header)).toString('base64url'),
  Buffer.from(JSON.stringify(payload)).toString('base64url')
].join('.')

const signature = crypto.sign('SHA256', body, {
  key: kp.privateKey,
  dsaEncoding: 'ieee-p1363'
}).toString('base64url')

const signedObject = [body, signature].join('.')

Believe it or not, this is all it takes to make a valid JWT. Not so bad, isn’t it?

Making the backed identity assertion

Since we have a valid cert and signedObject at that point, that part stays the same. All we need to do is to bundle them together with the ~ character (tilde).

const backedAssertion = [cert, signedObject].join('~')

This is the value we can pass in the Authorization header such as:

Authorization: BrowserID <backedAssertion>

Wrapping up

And that’s it! We now have a fully working BrowserID implementation, with both RSA and DSA support, that only depend on the native crypto module!

There were some incompatibilities with DSA that we needed to work around, especially the fact that it is not supported by JWK, forcing us to fallback to the OpenSSL CLI to extract the key parameters, and also that common JWT libraries don’t support it either, leading us to write our own (dead simple) JWT implementation.

This makes the code a bit simpler if we only use RSA, where it’s barely longer than the initial browserid-crypto version, while the DSA version is a bit hairier with the code to invoke OpenSSL and parse its output, as well as the custom JWT signature.

Because of that, unlike in the previous post, I won’t include the full code here. You should easily be able to put together the pieces that you need from this article.

But this is probably not an exercise you should be interested in anyways, because as I later found out, it’s not just browserid-crypto that’s unmaintained, but the BrowserID protocol altogether that’s deprecated! In the next stop of this journey, we’ll look at the OAuth version, which turned out to be much easier to support than it first looked like.

Check out the other posts in this series!

  1. A journey to scripting Firefox Sync / Lockwise: existing clients
  2. A journey to scripting Firefox Sync / Lockwise: figuring the protocol
  3. A journey to scripting Firefox Sync / Lockwise: understanding BrowserID
  4. A journey to scripting Firefox Sync / Lockwise: hybrid OAuth
  5. A journey to scripting Firefox Sync / Lockwise: complete OAuth

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