Four ways to compute the base64 md5 checksum used by ActiveStorage

base64-md5-test.rb

require 'digest'

# From activestorage
def compute_checksum_in_chunks(io)
  Digest::MD5.new.tap do |checksum|
    while chunk = io.read(5242880)
      checksum << chunk
    end
    io.rewind
  end.base64digest
end

path = ARGV[0]
puts "Path:                 #{path}"
puts "Chunks:               " + compute_checksum_in_chunks(File.open(path))
puts "MD5.file:             " + Digest::MD5.file(path).base64digest
puts "openssl md5 | base64: " + `openssl md5 -binary #{path} | base64`
puts "python:               " + `python -c 'import hashlib; import base64; print(base64.b64encode(hashlib.md5(open("#{path}").read()).digest()))'`

# https://tools.ietf.org/html/rfc2045#section-6.8