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#include "lib2.h"
#include "lib.h"
/**
* appends PKCS#7 padding to string. devide string in blocks of size blocksize
* and append to last block so that it is also of blocksize length
*/
char *pkcs7_padding(char *string, int length_string, int blocksize)
{
char *result = NULL;
int i;
int value = blocksize - (length_string % blocksize);
result = malloc(length_string+value+1);
memcpy(result, string, length_string);
for(i=0;i<value;i++) {
result[length_string+i] = (char) value;
}
result[length_string+i] = '\0';
return result;
}
/**
* decrypts content which is encrypted in AES CBC mode
* @param in input content
* @param length_in length of parametere in
* @param out place where the decrypted content should be written to
* @param string_key key with which the content in in has been decrypted
* @param iv initalization vector
*/
int aes_cbc(char *in, int length_in, char *out, unsigned char *string_key, char *init_vector)
{
char iv[16];
AES_KEY key;
int number_blocks = length_in / 16;
int i, j;
unsigned char ciphertext[128+1];
unsigned char tmp_after_aes[128+1];
unsigned char cleartext[128+1];
// set the key and bits
AES_set_decrypt_key(string_key, 128, &key);
memcpy(init_vector, iv, 16);
// implement cbc mode
for(i=0;i<number_blocks;i++) {
//do aes decryption
AES_decrypt(&in[i*16], tmp_after_aes, &key);
// xor
xor_string(iv, tmp_after_aes, &out[i*16], 16, 16);
// this ciphertext block is the next iv
for(j=0;j<16;j++) {
iv[j] = in[i*16+j];
}
}
return 0;
}
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