CODEWITHSUNDEEP
pythoncencryptioncryptographyopenssl
Hunter
Hunter

Reputation: 39

How to decrypt ciphertext using openssl in C?

How do I able to decrypt a cipher text which is encrypted using AES in python.

Encrypt.py

Using this I made cipher text using AES and concatenated that with the IV and wrote that in to a file file.txt.

from Crypto.Cipher import AES
import hashlib, os
Plain = 'This is a string'             #String to Encrypt
key = 'mysecretpassword'      
#key = hashlib.md5(key).digest()       #Key
IV = 'InitializationVector'
IV = hashlib.md5(IV).digest()          #Initialization Vector
print len(IV)
Obj1 = AES.new(key, AES.MODE_CBC, IV)
Cipher = Obj1.encrypt(Plain)           #Cipher text
File = open('file.txt','w')
File.write(Cipher+IV)      #Concatenated the string and IV and
                           #wrote that to a file file.txt 
File.close()

Decrypt.c

Using this now I got the Cipher text and IV from file.txt. Now how do I able to decrypt the Cipher using openssl or any other libraries? 

#include <stdio.h>
#include <string.h>
int main ()
{
  char filename[] = "file.txt";
  FILE *file = fopen ( filename, "r" );
  char key[] = "mysecretpassword";
  if (file != NULL) {
    char line [1000];
    char *p = line;
    char *array = line;
    while(fgets(line,sizeof line,file)!= NULL) {
      fprintf(stdout,"%s\n",line);
      char otherString[strlen(line)-15];
      strncpy(otherString, p, strlen(line)-16);
      otherString[strlen(otherString)-1] = '\0';
      printf("%s\n", otherString);//Here I got the Encrypted string
      array=array+(strlen(array)-16);
      printf("%s\n",array);//Here I got the IV
      //Here how to decrypt the Cipher text using "IV" and "key"
    }
    fclose(file);
  }
  else {
    perror(filename);
  }
  return 0;
}

Really I am a newbie. Please forgive for the errors in my question and please feel free to help me, that would be your most kindness. Thanks a lot in advance.

Upvotes: 2

Views: 2207

Answers (1)

varren
varren

Reputation: 14731

#include "openssl/aes.h"

  char buffer[1000];
  AES_KEY dec_key;
  AES_set_decrypt_key(key, 128, &dec_key);
  AES_cbc_encrypt(otherString, buffer, strlen(line) - 16, 
                     &dec_key,  array, AES_DECRYPT);
//AES_KEY dec_key;
//AES_set_decrypt_key(key, keySize, &dec_key);
//AES_cbc_encrypt(ciphertext, result, textLen, &dec_key, iv, AES_DECRYPT);

This works for me. 128 is bits for 16-byte key.

But i believe, you also have bug somewhere in -1 -15 -16 string length. This part of while loop can be changed to fix the problem:

  int strLen = strlen(line) - 16;

  char otherString[strLen + 1];
  strncpy(otherString, p, strLen);
  otherString[strLen] = '\0';

  array = array + strLen;

Here is also nice AES CBC encryption/decryption example code and your working code.

Upvotes: 1

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