Getting an extension's ID

I’ve gone in-depth on the structure of CRX files, but left out a crucial portion for anyone looking to host their own CRX from a server.

In order to get the advantages of auto-updating your extension, you’ll need to host an autoupdate manifest file. From the documentation of this feature, the file should be similar to the following:

<?xml version='1.0' encoding='UTF-8'?>
<gupdate xmlns='http://www.google.com/update2/response' protocol='2.0'>
  <app appid='aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'>
    <updatecheck codebase='http://example.com/extension.crx' version='2.0' />
  </app>
</gupdate>

Note that the appid attribute requires the 32 character long identifier for the extension (the same one printed in each extension’s listing on the chrome://extensions page).

While it’s easy enough to obtain this value by installing an extension and copying and pasting from the extensions information page, this approach is not ideal if you’re generating your own signing keys, which the previous article covered in depth.

I thought it might be good to cover that as an addendum here. Luckily, Erik Kay explained the format of this number in this post on StackOverflow:

To be precise, it's the first 128 bits of the SHA256 of an RSA public key
encoded in base 16.

Another random bit of trivia is that the encoding uses a-p instead of 0-9a-f.
The reason is that leading numeric characters in the host field of an origin
can wind up being treated as potential IP addresses by Chrome. We refer to it
internally as "mpdecimal" after the guy who came up with it.

Generating the number is easy enough, especially considering that I’ve already covered the process to obtain an RSA public key.

Here’s the code from last time, which generates the key and DER encodes it:

from pyasn1.codec.der import encoder
from pyasn1.type import univ
from Crypto.PublicKey import RSA
import os

...

# Generate an RSA key
rsakey = RSA.generate(1024, os.urandom)

# Get a RSAPublicKey structure
pkinfo = univ.Sequence()
pkinfo.setComponentByPosition(0, univ.Integer(rsakey.n))
pkinfo.setComponentByPosition(1, univ.Integer(rsakey.e))

#Convert the key into a bit string
def to_bitstring(self, num):
  buf = ''
  while num > 1:
    buf = str(num & 1) + buf
    num = num >> 1
  buf = str(num) + buf
  return buf
pklong = long(encoder.encode(pkinfo).encode('hex'), 16)
pkbitstring = univ.BitString("'00%s'B" % to_bitstring(pklong))

# Get the rsaEncryption identifier:
idrsaencryption = univ.ObjectIdentifier('1.2.840.113549.1.1.1')

# Get the AlgorithmIdentifier for rsaEncryption
idinfo = univ.Sequence()
idinfo.setComponentByPosition(0, idrsaencryption)
idinfo.setComponentByPosition(1, univ.Null(''))

# Get the SubjectPublicKeyInfo structure
publickeyinfo = univ.Sequence()
publickeyinfo.setComponentByPosition(0, idinfo)
publickeyinfo.setComponentByPosition(1, pkbitstring)

# Encode the public key structure
publickey = encoder.encode(publickeyinfo)

The result is that publickey contains the raw bytes of a DER encoded RSA public key.

According to Erik’s instructions, we need to generate the sha256 hash of this key, then take the first 128 bits and encode it to hex. 128 bits worth of hex is 32 characters, so we’ll use the Python hashlib module to sha256 the public key, and pull the first 32 characters from the hex version of the hash:

import hashlib

...

digest = hashlib.sha256(publickey).hexdigest()[:32]

Finally, encode the “mpdecimal” version of the hex-encoded digest variable. This just involves shifting each character in the hex string over by 10 characters:

extension_id = ''.join(["abcdefghijklmnop"[int(i,16)] for i in digest])

VoilĂ ! extension_id now contains a 32 character string that represents the extension’s ID number. This is suitable for including in the extension’s autoupdate manifest file, as described above.

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