// Compile this code via gcc -o openssl_encrypt openssl_encrypt.c -lssl -lcrypto

#include <openssl/evp.h>
#include <openssl/aes.h>
#include <openssl/err.h>
#include <string.h>
#include <stdio.h>

void handleErrors(void) {
    ERR_print_errors_fp(stderr);
    abort();
}

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

    if(!(ctx = EVP_CIPHER_CTX_new())) handleErrors();

    if(1 != EVP_EncryptInit_ex(ctx, EVP_aes_256_cbc(), NULL, key, iv))
        handleErrors();

    if(1 != EVP_EncryptUpdate(ctx, ciphertext, &len, plaintext, plaintext_len))
        handleErrors();
    ciphertext_len = len;

    if(1 != EVP_EncryptFinal_ex(ctx, ciphertext + len, &len)) handleErrors();
    ciphertext_len += len;

    EVP_CIPHER_CTX_free(ctx);

    return ciphertext_len;
}

int main(void) {
    unsigned char *key = (unsigned char *)"01234567890123456789012345678901";
    unsigned char *iv = (unsigned char *)"0123456789012345";
    unsigned char *plaintext = (unsigned char *)"This is a secret message";
    unsigned char ciphertext[128];

    int ciphertext_len = encrypt(plaintext, strlen((char *)plaintext), key, iv, ciphertext);

    printf("PLaintext: %s\n", plaintext);
    printf("Ciphertext:\n");
    BIO_dump_fp(stdout, (const char *)ciphertext, ciphertext_len);

    return 0;
}