How To Encrypt String C

C++ Programming

Today, I want to share with you my knowledge and experience on how to encrypt a string in the C programming language. Encryption is an essential technique in computer security that ensures sensitive information remains confidential. By encrypting a string, we transform it into an unreadable form, making it significantly more challenging for unauthorized individuals to access the original data. In this article, I will guide you through the process of encrypting a string in C, providing detailed explanations and personal insights along the way.

Understanding Encryption

Before we dive into the code, let’s take a moment to understand the basics of encryption. Encryption involves using an algorithm to convert plain text (our original string) into cipher text (the encrypted form). To encrypt a string, we need a key – a special sequence of characters that controls the encryption process. Without the correct key, it is nearly impossible to decipher the encrypted string and obtain the original data.

The C Programming Language

C is a powerful and widely-used programming language known for its efficiency and low-level control. It provides various libraries and functions that make it suitable for working with encryption algorithms. In this article, we will focus on using the OpenSSL library, a popular and trusted library for encryption in C.

Installing and Including OpenSSL

Before we begin, we need to install the OpenSSL library if it is not already available on our system. Detailed instructions can be found on the OpenSSL website ( Once installed, we need to include the appropriate header files in our C program. The most commonly used OpenSSL headers for encryption are:

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

These headers provide the necessary functions and data structures to work with encryption algorithms in C.

Encrypting a String

Now that we have OpenSSL installed and the necessary headers included, we can proceed to encrypting a string. Here is a step-by-step guide:

  1. Define your string and key: In your C program, define the string you want to encrypt and the key you want to use for encryption. Be sure to choose a strong key and keep it secure.
  2. char* plaintext = "Hello, World!";
    char* key = "ThisIsMySecretKey";

  3. Create a cipher context: In OpenSSL, we need to create a cipher context, which will hold the encryption algorithm and other relevant information.
  4. EVP_CIPHER_CTX* ctx = EVP_CIPHER_CTX_new();

  5. Initialize the cipher context: We need to initialize the cipher context with the appropriate encryption algorithm, mode, and key. For example, if we want to use AES encryption in CBC mode, we can use the following code:
  6. EVP_EncryptInit_ex(ctx, EVP_aes_256_cbc(), NULL, key, iv);

    Note: The ‘iv’ parameter represents the initialization vector, which adds an additional layer of security to the encryption process. It should be a random value and should be kept secret along with the key.

  7. Perform the encryption: Now that the cipher context is initialized, we can use the EVP_EncryptUpdate() function to encrypt our string. This function takes the cipher context, the output buffer, the input buffer, and the length of the input buffer as parameters. Here is an example:
  8. unsigned char ciphertext[128];
    int ciphertext_len;
    EVP_EncryptUpdate(ctx, ciphertext, &ciphertext_len, plaintext, strlen(plaintext));

  9. Finalize the encryption: After encrypting the input string, we need to finalize the encryption process by calling the EVP_EncryptFinal_ex() function. This function ensures that any remaining data is processed and adds padding if necessary.
  10. EVP_EncryptFinal_ex(ctx, ciphertext + ciphertext_len, &len);

  11. Cleanup: Finally, we need to clean up the cipher context and free any resources it used.
  12. EVP_CIPHER_CTX_free(ctx);


Encrypting a string in C can be a complex but essential task when it comes to protecting sensitive data. In this article, we explored the basics of encryption, the use of the C programming language, and the OpenSSL library to encrypt a string. By following the steps outlined in this article, you can ensure the confidentiality of your data and make it significantly more challenging for unauthorized individuals to access the original information.

Remember, encryption is just one piece of the puzzle when it comes to securing your data. It is crucial to consider other security measures, such as secure key management and secure communication channels, to create a robust and reliable security system.