AES - Higher level functions


Making our toy AES implementation slightly useable.

Our implementation now has a useable way to encrypt and decrypt stuff according to the AES standard, although it’s not very straightforward:

my $plain = shift // 'AES is a fun toy'; # 16 octets (block size)
my $key   = shift // 'YELLOW SUBMARINE'; # 16 octets => AES 128

my $cipher_schedule = key_expansion($key);
my $cipher = cipher($plain, $cipher_schedule);

my $decipher_schedule = modify_key_schedule_copy($cipher_schedule);
my $decrypted = eq_inv_cipher($cipher, $decipher_schedule);

Let’s remember that cipher/eq_inv_cipher operate on one single block of 16 octets, always and independently of the key length.

We can do better from a useability point of view, e.g.:

sub block_decrypt ($ct, $key) { block_decrypter($key)->($ct) }

sub block_decrypter ($key) {
   $key = modify_key_schedule_inplace(key_expansion($key));
   return sub ($ct) { return equivalent_inv_cipher($ct, $key) };

sub block_encrypt ($pt, $key) { block_encrypter($key)->($pt) }

sub block_encrypter ($key) {
   $key = key_expansion($key);
   return sub ($plaintext) { return cipher($plaintext, $key) };

The function names all start with block_ to remind us that they’re useful for working on a single block of data, which means 16 octets in AES. Anything that has to work on more (or less!) data will have to be dealt with separately (with some mode of operation, like ECB, CBC, …).

The two block_*crypter functions take a key and give back a function to apply the right AES algorithm to a single block, like this:

my $encrypter = block_encrypter($key);
my $decrypter = block_decrypter($key);
die if $decrypter->($encrypter->($plain)) ne $plain;

They are useful if we have to perform multiple block encryption/decryption with the same key. On the other hand, we also have one-off block-level operations that take a block of plain/cipher data and a key, and do the right thing:

die if block_decrypt(block_encrypt($plain, $key), $key) ne $plain;

So well, this is it for our little AES toy implementation. If you want to experiment with it, it’s been encapsulated within a small Perl module AesBasic, which you can download here (I don’t see any reason to publish it to CPAN).

This also gives us the tool to continue our journey through Cryptopals challenges. Stay tuned and safe!

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