Usage: echo -n text_to_append | ruby shattered_length_extension.rb

shattered_length_extension.rb

#!/usr/bin/env ruby

# output format:
#   => collided sha1
#   => message 1 base64
#   => message 2 base64

require 'base64'
require 'digest/sha1'

p1 = Base64.decode64(<<EOF)
JVBERi0xLjMKJeLjz9MKCgoxIDAgb2JqCjw8L1dpZHRoIDIgMCBSL0hlaWdodCAzIDAgUi9UeXBl
IDQgMCBSL1N1YnR5cGUgNSAwIFIvRmlsdGVyIDYgMCBSL0NvbG9yU3BhY2UgNyAwIFIvTGVuZ3Ro
IDggMCBSL0JpdHNQZXJDb21wb25lbnQgOD4+CnN0cmVhbQr/2P/+ACRTSEEtMSBpcyBkZWFkISEh
ISGFL+wJIzl1nDmxocY8TJfh//4Bc0bckWa2fhGPApq2IbJWD/nKZ8yox/hbqEx5AwwrPeIY+G2z
qQkB1d9FwU8m/t+z3DjpasIv571yjw5FvOBG0jxXD+sUE5i7VS71oKgr4zH+pIA3uLXXHw4zLt+T
rDUA603cDezBqGR5DHgsdiFWYN0wl5HQa9CvP5jNpLxGKbE=
EOF

p2 = Base64.decode64(<<EOF)
JVBERi0xLjMKJeLjz9MKCgoxIDAgb2JqCjw8L1dpZHRoIDIgMCBSL0hlaWdodCAzIDAgUi9UeXBl
IDQgMCBSL1N1YnR5cGUgNSAwIFIvRmlsdGVyIDYgMCBSL0NvbG9yU3BhY2UgNyAwIFIvTGVuZ3Ro
IDggMCBSL0JpdHNQZXJDb21wb25lbnQgOD4+CnN0cmVhbQr/2P/+ACRTSEEtMSBpcyBkZWFkISEh
ISGFL+wJIzl1nDmxocY8TJfh//4Bf0bck6a2fgE7ApqqHbJWC0XKZ9aIx/hLjEx5H+ArPfYU+G2x
aQkBxWtFwVMK/t+3YDjpcnIv561yjw5JBOBGwjBXD+nUE5ir4S71vJQr4zVCpIAtmLXXDyozLsN/
rDUU503cDyzBqHTNDHgwWiFWZGEwl4lga9C/P5jNqARGKaE=
EOF

extension = STDIN.read
m1 = p1 + extension
m2 = p2 + extension

m1_sha = Digest::SHA1.hexdigest(m1)
m2_sha = Digest::SHA1.hexdigest(m2)

if m1_sha == m2_sha and m1 != m2
  STDOUT.puts m1_sha
  STDOUT.puts Base64.strict_encode64(m1)
  STDOUT.puts Base64.strict_encode64(m2)
else
  raise "failed attempt at length extension"
end