CBC - Recovering the key when the IV and the key are the same

August 17, 2021

This post is based on the “Recover the key from CBC with IV=Key” exercise from Cryptopals. I highly recommend attempting the previous CBC exercises yourself as they do a great job ramping up your knowledge on the subject.


In order for this attack to be successful two things must be in place:

  1. The encryption process must use the KEY as the IV
  2. The server throws an error when decryption fails and reflects the decoded message to the attacker


Given the conditions above we can exploit this in the following way:

  1. Make a plaintext with a length of at least 3 blocks
  2. Encrypt the plaintext and get the resulting ciphertext
  3. Modify the second block of the ciphertext to contain only zeros
  4. Modify the third block of the ciphertext to be the same as the first block
  5. Decrypt the ciphertext and get the invalid plaintext result
  6. XOR the first and third blocks of the invalid plaintext
  7. That’s our key!


This seems magical at first, but let’s review the algorithm that is performed by CBC during decryption:

CBC Decryption by WhiteTimberwolf

Deconstructing what needs to happen in order to decrypt our first block:

  1. result = AES_Decrypt(first_block_ciphertext, KEY)
  2. result XOR KEY (remember that we are using the KEY as the IV)

Deconstructing what needs to happen in order to decrypt our third block:

  1. result = AES_Decrypt(third_block_ciphertext, KEY)
  2. result XOR second_block_ciphertext

Let’s XOR these operations together:

AES_Decrypt(first_block_ciphertext, KEY) XOR KEY XOR AES_Decrypt(third_block_ciphertext, KEY) XOR second_block_ciphertext

Given that our first ciphertext block is the same as our third ciphertext block (step 4 of attack) we know that the following operations will produce the same result:

  • AES_Decrypt(first_block_ciphertext, KEY)
  • AES_Decrypt(third_block_ciphertext, KEY)

When we XOR these two operations together the result will be zero. This leaves us with:

=> 0 XOR KEY XOR second_block_ciphertext => KEY XOR second_block_ciphertext

Remember that we made our second ciphertext block contain only zeroes (step 3 of attack), so this becomes:

=> KEY XOR 0 => KEY

And that’s the reason we can extract the KEY using this algorithm.

Implementation based on Cryptopal’s requirements

class InvalidFormat < StandardError; end KEY = 16.times.map { rand(0..255) } def encode_cookie(input) prefix = 'comment1=cooking%20MCs;userdata=' suffix = ';comment2=%20like%20a%20pound%20of%20bacon' plaintext = prefix + input.tr(';=', '') + suffix raise InvalidFormat.new(plaintext) unless plaintext.ascii_only? aes_cbc_encrypt(pkcs7_pad(plaintext.bytes, 16), KEY, KEY) end def decode_cookie(ciphertext) plaintext = pkcs7_unpad( aes_cbc_decrypt(ciphertext, KEY, KEY) ).pack('C*') raise InvalidFormat.new(plaintext) unless plaintext.ascii_only? config = plaintext.split(';').map { |kv| kv.split('=') }.to_h puts "Decoded data: #{config}" puts "Admin detected: #{config['admin'] == 'true'}" end def exploit_server(input) cookie = encode_cookie(input) # Step 3 of the attack 16.times { |i| cookie[16 + i] = 0 } # Step 4 of the attack 16.times { |i| cookie[32 + i] = cookie[i] } begin decode_cookie(cookie) rescue InvalidFormat => e puts "Invalid message!" e.message end end input = 'A' * (16 * 3) # Step 5 of the attack result = exploit_server(input) blocks = result.bytes.each_slice(16).to_a puts 'Original key:' puts KEY.inspect puts 'Leaked Key:' # Step 6 of the attack puts xor_bytes(blocks[0], blocks[2]).inspect

In order to keep the code short and to the point I’ve opted to hide methods that we have seem in previous posts, they are:

  • pkcs7_pad
  • pkcs7_unpad
  • aes_cbc_encrypt
  • aes_cbc_decrypt

See CBC Padding Oracle for more information about each method.

And we have reached the end of our exercise! This should be our last post on CBC, on future posts we will start investigating a new block mode called CTR. Please reach out to me on Twitter or by email if you have any questions or suggestions on how to improve this or future posts.

Bernardo de Araujo

Application Security Engineer @Shopify.

© Bernardo de Araujo 2021