How to Compute MD5 or SHA Hash of a File in C Using OpenSSL

C is a widely used programming language in various applications, including cryptographic functions. Computing the MD5 or SHA hash of a file is a common task in security and data integrity verification. In this article, we will explore how to achieve this using the OpenSSL library, a popular choice for cryptographic functions. We will provide detailed examples in C and cover the necessary steps to install, include, and use the library effectively.

Overview of OpenSSL

OpenSSL is a robust, semi-free, and widely used software library for the transport layer security (TLS) and secure socket layer (SSL) protocols. It is used to create secure network connections and other cryptographic operations.

Prerequisites

To get started, it's important to ensure that OpenSSL is installed on your system. The following steps will guide you through the installation process.

Installing OpenSSL

Debian/Ubuntu: sudo apt-get install libssl-dev macOS: brew install openssl

Example Code: Computing MD5 Hash

Below is an example of how to compute the MD5 hash of a file using C and OpenSSL.

#include iostream#include fstream#include openssl/md5.hvoid computeMD5(const std::string filename) {    unsigned char c[MD5_DIGEST_LENGTH];    MD5_CTX md5Context;    MD5_Init(md5Context);    std::ifstream file(filename, std::ifstream::binary);    if (!file) {        std::cerr  "Error: could not open file
";        return;    }    char buffer[1024];    while ((buffer, sizeof(buffer))) {        MD5_Update(md5Context, buffer, file.gcount());    }    MD5_Final(c, md5Context);    std::cout  "MD5: ";    for (int i  0; i  MD5_DIGEST_LENGTH; i  ) {        printf(x, c[i]);    }    std::cout  std::endl;}

Example Code: Computing SHA-256 Hash

Here is an example of how to compute the SHA-256 hash of a file using C and OpenSSL.

#include iostream#include fstream#include openssl/sha.hvoid computeSHA256(const std::string filename) {    unsigned char hash[SHA256_DIGEST_LENGTH];    SHA256_CTX sha256Context;    SHA256_Init(sha256Context);    std::ifstream file(filename, std::ifstream::binary);    if (!file) {        std::cerr  "Error: could not open file
";        return;    }    char buffer[1024];    while ((buffer, sizeof(buffer))) {        SHA256_Update(sha256Context, buffer, file.gcount());    }    SHA256_Final(hash, sha256Context);    std::cout  "SHA-256: ";    for (int i  0; i  SHA256_DIGEST_LENGTH; i  ) {        printf(x, hash[i]);    }    std::cout  std::endl;}

Putting It All Together

You can combine the above functions in your main program:

int main() {    std::string filename  "example.txt";    computeMD5(filename);    computeSHA256(filename);    return 0;}

Compiling the Code

To compile the code, link against the OpenSSL library:

g   -o hash_example hash_example.cpp -lssl -lcrypto

After compilation, you can run the program:

./hash_example

The program will output the MD5 and SHA-256 hashes for the specified file.

Conclusion

This guide provides you with a straightforward approach to computing MD5 and SHA-256 hashes of files in C, leveraging the power of OpenSSL. With this knowledge, you can enhance the security and integrity verification of your applications.