OpenSSL

OpenSSL was originally developed in 1998 and has become the de facto standard library for implementing secure communications in C/C++ applications. It is widely used in web servers, email servers, VPNs, and many security-sensitive software systems for encryption and certificate management.

OpenSSL is a robust, full-featured open-source toolkit implementing the Secure Sockets Layer (SSL) and Transport Layer Security (TLS) protocols. It provides cryptographic functions such as encryption, decryption, hashing, key generation, and certificate management.

Installation

Usage

OpenSSL provides APIs for symmetric and asymmetric encryption, digital signatures, hashing, SSL/TLS communication, and certificate handling. It supports a wide range of cryptographic algorithms including AES, RSA, ECC, SHA, and HMAC.

#include <openssl/rsa.h>
#include <openssl/pem.h>

int main() {
    RSA *rsa = RSA_generate_key(2048, RSA_F4, NULL, NULL);
    FILE *fp = fopen("private.pem", "wb");
    PEM_write_RSAPrivateKey(fp, rsa, NULL, NULL, 0, NULL, NULL);
    fclose(fp);
    RSA_free(rsa);
    return 0;
}

#include <openssl/sha.h>
#include <stdio.h>
#include <string.h>

int main() {
    unsigned char digest[SHA256_DIGEST_LENGTH];
    char str[] = "Hello, OpenSSL!";
    SHA256((unsigned char*)str, strlen(str), digest);
    for(int i = 0; i < SHA256_DIGEST_LENGTH; i++)
        printf("%02x", digest[i]);
    printf("\n");
    return 0;
}

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

int main() {
    unsigned char key[32] = "01234567890123456789012345678901";
    unsigned char iv[16] = "0123456789012345";
    unsigned char plaintext[] = "Hello, OpenSSL AES!";
    unsigned char ciphertext[128];
    int len, ciphertext_len;

    EVP_CIPHER_CTX *ctx = EVP_CIPHER_CTX_new();
    EVP_EncryptInit_ex(ctx, EVP_aes_256_cbc(), NULL, key, iv);
    EVP_EncryptUpdate(ctx, ciphertext, &len, plaintext, strlen((char*)plaintext));
    ciphertext_len = len;
    EVP_EncryptFinal_ex(ctx, ciphertext + len, &len);
    ciphertext_len += len;
    EVP_CIPHER_CTX_free(ctx);

    printf("Encrypted text length: %d\n", ciphertext_len);
    return 0;
}

#include <openssl/rsa.h>
#include <openssl/pem.h>
#include <openssl/err.h>
#include <openssl/sha.h>

int main() {
    // Load private key
    FILE *fp = fopen("private.pem", "r");
    RSA *rsa = PEM_read_RSAPrivateKey(fp, NULL, NULL, NULL);
    fclose(fp);

    unsigned char msg[] = "Hello";
    unsigned char sig[256];
    unsigned int sig_len;

    SHA256(msg, strlen((char*)msg), sig);
    RSA_sign(NID_sha256, sig, SHA256_DIGEST_LENGTH, sig, &sig_len, rsa);
    RSA_free(rsa);

    printf("Signature length: %u\n", sig_len);
    return 0;
}

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

int main() {
    SSL_library_init();
    SSL_CTX *ctx = SSL_CTX_new(TLS_client_method());
    SSL *ssl = SSL_new(ctx);
    // connect socket code omitted for brevity
    SSL_connect(ssl);
    SSL_shutdown(ssl);
    SSL_free(ssl);
    SSL_CTX_free(ctx);
    return 0;
}

Error Handling

Best Practices

Always check return values for errors to ensure cryptographic operations succeed.

Use updated and recommended algorithms (e.g., AES, SHA-256, RSA-2048+).

Securely store private keys and sensitive data in memory or protected files.

Use random number generators from OpenSSL for key generation.

Keep OpenSSL library updated to patch security vulnerabilities.