Tuesday, June 21, 2011

OpenSSL: decrypt with RSA private key in C++

The following C++ code demonstrates how to decrypt with RSA private key with OpenSSL library. For practical use, you will need to add error handling.

For how to use the OpenSSL commands to encrypt and decrypt files, see this post.

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

char *private_key_file_name = "private_key.pem";
char *encrypted_file_name = "encrypted_file";
char *decrypted_file_name = "decrypted_file";

int main()
    FILE *fp = fopen(private_key_file_name, "r");

    RSA *rsa = PEM_read_RSAPrivateKey(fp, NULL, NULL, NULL);


    std::ifstream enc_file;

    enc_file.open(encrypted_file_name, std::ifstream::in);

    int keysize = RSA_size(rsa);

    std::auto_ptr<unsigned char> rsa_in(new unsigned char[keysize * 2]);
    std::auto_ptr<unsigned char> rsa_out(new unsigned char[keysize]);

    memset(rsa_in.get(), 0, keysize * 2);
    memset(rsa_out.get(), 0, keysize);

    enc_file.read(reinterpret_cast<char *>(rsa_in.get()), keysize * 2);

    int rsa_inlen = enc_file.gcount();

    int rsa_outlen  = RSA_private_decrypt(
            rsa_inlen, rsa_in.get(), rsa_out.get(),
            rsa, RSA_PKCS1_PADDING);

    std::ofstream dec_file;

    dec_file.open(decrypted_file_name, std::ifstream::out);

    dec_file.write(reinterpret_cast<char *>(rsa_out.get()), rsa_outlen);



    return 0;

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