void aes_decrypt_core(aes_cipher_state_t* state, const aes_genctx_t* ks, uint8_t rounds){
uint8_t i;
aes_dec_firstround(state, &(ks->key[i=rounds]));
for(;rounds>1;--rounds){
--i;
aes_dec_round(state, &(ks->key[i]));
}
for(i=0; i<16; ++i){
state->s[i] ^= ks->key[0].ks[i];
}
}
// when decrypting the algorithm has the opposite order compared to the encryption
// the PDF gives us the encryption direction
// the comments use the naming conventions from the PDF
void aes_decrypt_core(aes_cipher_state_t* state, const aes_genctx_t* ks, uint8_t rounds){
uint8_t i;
// buffer == state
// aes_cipher_state_t represents the 4x4 byte cipher-matrix
// "last round"
aes_dec_firstround(state, &(ks->key[i=rounds])); // start with rounds == 10
// 9 "normal rounds"
// including "first round" and "second last round"
for(;rounds>1;--rounds){
// i is the counter for the roundkey which is applied backwards
--i;
aes_dec_round(state, &(ks->key[i]));
}
// "Initial Round"
// adding roundkey
for(i=0; i<16; ++i){
state->s[i] ^= ks->key[0].ks[i];
}
}
/*
* aes_decrypt_core
*
* This function performs the steps to undo the encryption
*
*/
void aes_decrypt_core( aes_cipher_state_t* state,
const aes_genctx_t* ks,
uint8_t rounds)
{
uint8_t i;
// Firstly, undo the final encryption step
// using the round keys from backwards
aes_dec_firstround(state, &(ks->key[i=rounds]));
// Secondly, undo all the remaining rounds
// using the round keys from backwards
for(; rounds>1; --rounds)
{
--i;
aes_dec_round(state, &(ks->key[i]));
}
// Finally add the key to get the original message decrypted from the input
for(i=0; i<16; ++i)
{
state->s[i] ^= ks->key[0].ks[i];
}
}
