How can I decrypt data using OpenSSL in C++11?

How can I decrypt data using OpenSSL in C++11?

To decrypt data using OpenSSL in C++11, you can use the EVP (Envelope) API. First, ensure you have initialized the OpenSSL library.

What are the steps to initialize OpenSSL?

You need to call OPENSSL_init_crypto() to initialize the library. Then, set up the cipher context with EVP_CIPHER_CTX_new().

Can you provide a code example for decryption?

Sure! Here's a simple example:

#include <openssl/evp.h>
#include <openssl/err.h>

void decrypt(const unsigned char *ciphertext, int ciphertext_len, const unsigned char *key, const unsigned char *iv, unsigned char *plaintext) {
    EVP_CIPHER_CTX *ctx;
    int len;
    int plaintext_len;

    ctx = EVP_CIPHER_CTX_new();
    EVP_DecryptInit_ex(ctx, EVP_aes_256_cbc(), NULL, key, iv);

    EVP_DecryptUpdate(ctx, plaintext, &len, ciphertext, ciphertext_len);
    plaintext_len = len;

    EVP_DecryptFinal_ex(ctx, plaintext + len, &len);
    plaintext_len += len;

    EVP_CIPHER_CTX_free(ctx);
}

This function takes the ciphertext, key, and IV to produce the plaintext.

What should I do if I encounter errors during decryption?

You can use the ERR_get_error() function to retrieve error codes and then use ERR_error_string() to get a human-readable string for debugging.