openssl API Usage

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

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

int decryptFile(const char *inputFile, const char *outputFile, const char *passphrase)
{
    OpenSSL_add_all_algorithms();
    ERR_load_crypto_strings();

    FILE *inFile = fopen(inputFile, "rb");
    if (!inFile) {
        perror("Error opening input file");
        return 1;
    }

    FILE *outFile = fopen(outputFile, "wb");
    if (!outFile) {
        perror("Error opening output file");
        fclose(inFile);
        return 1;
    }

    const EVP_CIPHER *cipher = EVP_aes_256_cbc();
    const EVP_MD *digest = EVP_sha256();

    unsigned char key[EVP_MAX_KEY_LENGTH], iv[EVP_MAX_IV_LENGTH];
    unsigned char salt[8] = {0};
    fread(salt, 1, 8, inFile); // skip Salted__
    fread(salt, 1, 8, inFile); // store next 8 bytes
    if (!EVP_BytesToKey(cipher, digest, salt, (const unsigned char *)passphrase,
                                    strlen(passphrase), 1, key, iv)) {
#if 0
    if (!EVP_BytesToKey(cipher, digest, NULL, (const unsigned char *)passphrase, strlen(passphrase), 1, key, iv)) {
#endif
        perror("Error deriving key and IV");
        fclose(inFile);
        fclose(outFile);
        return 1;
    }

    EVP_CIPHER_CTX *ctx = EVP_CIPHER_CTX_new();
    if (!ctx) {
        perror("Error creating context");
        fclose(inFile);
        fclose(outFile);
        return 1;
    }

    if (1 != EVP_DecryptInit_ex(ctx, cipher, NULL, key, iv)) {
        perror("Error initializing decryption");
        EVP_CIPHER_CTX_free(ctx);
        fclose(inFile);
        fclose(outFile);
        return 1;
    }
    EVP_CIPHER_CTX_set_padding(ctx, 0);

    unsigned char inBuf[1024], outBuf[1024];
    int bytesRead, decryptedLen;

    while ((bytesRead = fread(inBuf, 1, sizeof(inBuf), inFile)) > 0) {
        if (1 != EVP_DecryptUpdate(ctx, outBuf, &decryptedLen, inBuf, bytesRead)) {
            perror("Error updating decryption");
            EVP_CIPHER_CTX_free(ctx);
            fclose(inFile);
            fclose(outFile);
            return 1;
        }

        fwrite(outBuf, 1, decryptedLen, outFile);
    }

    if (1 != EVP_DecryptFinal_ex(ctx, outBuf, &decryptedLen)) {
        ERR_print_errors_fp(stderr);
        perror("Error finalizing decryption");
        EVP_CIPHER_CTX_free(ctx);
        fclose(inFile);
        fclose(outFile);
        return 1;
    }

    fwrite(outBuf, 1, decryptedLen, outFile);

    EVP_CIPHER_CTX_free(ctx);
    fclose(inFile);
    fclose(outFile);

    return 0;
}

int main()
{
    const char *inputFile = "encrypted_file.enc";
    const char *outputFile = "decrypted_file.txt";
    const char *passphrase = "111111";

    if (decryptFile(inputFile, outputFile, passphrase) == 0) {
        printf("File decrypted successfully.\n");
    } else {
        fprintf(stderr, "Error decrypting file.\n");
    }

    ERR_free_strings();
    return 0;
}