Noob attempt to translate Web Crypto API calls into Elixir

Questions / Help
1

Hi,

I’ve been trying to implement the following javascript code provided by Cloudflare to generate tokens to secure video streams (Secure your Stream · Cloudflare Stream docs)

// Main function
async function streamSignedUrl () {
  const encoder = new TextEncoder()
  const expiresIn = Math.floor(Date.now() / 1000) + expiresTimeInS
  const headers = {
    "alg": "RS256",
    "kid": keyID
  }
  const data = {
    "sub": videoUID,
    "kid": keyID,
    "exp": expiresIn,
    "accessRules": [
      {
        "type": "ip.geoip.country",
        "action": "allow",
        "country": [
          "GB"
        ]
      },
      {
        "type": "any",
        "action": "block"
      }
    ]
  }

  const token = `${objectToBase64url(headers)}.${objectToBase64url(data)}`

  const jwk = JSON.parse(atob(jwkKey))

  const key = await crypto.subtle.importKey(
    "jwk", jwk,
    {
      name: 'RSASSA-PKCS1-v1_5',
      hash: 'SHA-256',
    },
    false, [ "sign" ]
  )

  const signature = await crypto.subtle.sign(
    { name: 'RSASSA-PKCS1-v1_5' }, key,
    encoder.encode(token)
  )

  const signedToken = `${token}.${arrayBufferToBase64Url(signature)}`

  return signedToken
}

// Utilities functions
function arrayBufferToBase64Url(buffer) {
  return btoa(String.fromCharCode(...new Uint8Array(buffer)))
    .replace(/=/g, '')
    .replace(/\+/g, '-')
    .replace(/\//g, '_')
}

function objectToBase64url(payload) {
  return arrayBufferToBase64Url(
    new TextEncoder().encode(JSON.stringify(payload)),
  )
}

I was able to produce the token variable successfully by comparing the outputs of the javascript code with the output I generate on my elixir code, and then produce the same object assigned to jwk variable in the shape of a elixir map, but then I got got stuck.

I don’t quite understand what would be the equivalent (if there is one) in Elixir for what is produced by crypto.subtle.importKey and crypto.subtle.sign.

crypto.subtle.importKey returns a CryptoKey object that can be used as argument in crypto.subtle.sign but I couldn’t find elixir/erlang counterpart for this.

Could anyone give me some guidance on how to proceed or where to get more information on this?
I don’t even know what to lookup.

Thanks!

2

Take a look at Joken and the underlying JOSE library, they may do what you need.

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