test.rb

require 'crxmake'
require 'digest/sha2'

# generate id from pem

def generate_id pkey
  key = CrxMake::KEY + pkey.public_key.to_der
  id = Digest::SHA256.hexdigest(key)
  id[0..32].split('').map{|ch| ('a'.ord + ch.hex).chr }.join('')
end

File.open('path/to/pem.pem', 'rb') do |file|
  puts generate_id OpenSSL::PKey::RSA.new(file)
end

# OR extract id from crx package

def extract_id file
  content = file.read.force_encoding('ascii-8bit')
  size = content[8...12].unpack('V')[0]
  key = content[16, size]
  id = Digest::SHA256.hexdigest(key)
  id[0..32].split('').map{|ch| ('a'.ord + ch.hex).chr }.join('')
end

File.open('path/to/extension.crx', 'rb') do |file|
  puts extract_id file
end