Is it not possible to decrypt partial AES messages?

[Christina Penn]

Hello,



Here is some example code of me trying to decrypt a partial AES message. It
doesn't work.. is there a way I can do something like this? It only works if
I call DecryptMessage() with the entire encrypted string. Why?



Thanks!!



#include <openssl/evp.h>
#include <iostream>
#include <string>
using namespace std;
#pragma comment (lib, "libeay32MDd.lib")

EVP_CIPHER_CTX enCTX, deCTX;
int blockSize=128;

string DecryptMessage(const string message)
{
    try
    {
        int p_len = message.length(), f_len = 0;
        unsigned char* plaintext = (unsigned char*)malloc(p_len);
        if(!EVP_DecryptInit_ex(&deCTX, NULL, NULL, NULL, NULL))
            cerr << "ERROR in EVP_DecryptInit_ex" << endl;
        if(!EVP_DecryptUpdate(&deCTX, plaintext, &p_len, (unsigned
char*)message.data(),
message.length()))
            cerr << "ERROR in EVP_DecryptUpdate" << endl;
        if(!EVP_DecryptFinal_ex(&deCTX, plaintext+p_len, &f_len))
            cerr << "ERROR in EVP_DecryptFinal_ex" << endl;

        return string((char*)plaintext, p_len + f_len-1);
    }
    catch(...)
    {
        return message;
    }
}

string EncryptMessage(const string message)
{
    try
    {
        // max ciphertext len for a n bytes of plaintext is n +
AES_BLOCK_SIZE bytes
        int c_len = message.length() + blockSize;
        unsigned char *ciphertext = (unsigned char*)malloc(c_len);
        if(!EVP_EncryptInit_ex(&enCTX, NULL, NULL, NULL, NULL))
            cerr << "ERROR in EVP_EncryptInit_ex" << endl;
        // update ciphertext, c_len is filled with the length of ciphertext
generated
        if(!EVP_EncryptUpdate(&enCTX, ciphertext, &c_len, (unsigned
char*)message.data(),
message.length()+1))
            cerr << "ERROR in EVP_EncryptUpdate" << endl;
        // update ciphertext with the final remaining bytes
        int f_len = 0;
        if(!EVP_EncryptFinal_ex(&enCTX, ciphertext+c_len, &f_len))
            cerr << "ERROR in EVP_EncryptFinal_ex" << endl;

        return string((char*)ciphertext, c_len + f_len);
    }
    catch(...)
    {
        return message;
    }
}

int main()
{
    // ssl init
    const EVP_CIPHER* c = EVP_aes_128_cbc();
    unsigned char key[32], iv[32];
    string passphrase="test";
    string salt="12345678";
    int rounds=5;
    EVP_BytesToKey(c, EVP_sha1(), (unsigned char*)salt.data(), (unsigned
char*)passphrase.data(), passphrase.length(), rounds, key, iv);
    EVP_CIPHER_CTX_init(&deCTX);
    EVP_DecryptInit_ex(&deCTX, c, NULL, key, iv);
    EVP_CIPHER_CTX_init(&enCTX);
    EVP_EncryptInit_ex(&enCTX, c, NULL, key, iv);

    // trying to encrypt and decrypt
    string plaintext = "0000015this is a test!";
    cout << "plaintext: " << plaintext << endl;
    string ciphertext = EncryptMessage(plaintext);
    cout << "ciphertext: " << ciphertext << endl;
    string header = ciphertext.substr(0, 7);
    cout << "header: " << header << endl;
    string decrypted_header = DecryptMessage(header);
    cout << "decrypted header: " << decrypted_header << endl;
}