rsa encryption decryption using c

Question

I'm trying to write C code of RSA encryption and decryption using Open SSL. But I am not able to do so. I googled it but whatever code I got from internet it was out of my head. main function goes here which I got from stack overflow. I tried using it … but its not working. May be my bad.

encrypt(FILE *rsa_Pkey_fole,FILE *in_file,FILE *out_file){

}

int main(int argc, char *argv[])
{
    FILE *rsa_pkey_file, *infile;
    int rv;

    if (argc < 2) {
            fprintf(stderr, "Usage: %s <PEM RSA Public Key File>\n", argv[0]);
            exit(1);
    }

    rsa_pkey_file = fopen(argv[1], "rb");
    infile = fopen(argv[2], "w");
    if (!rsa_pkey_file) {
        perror(argv[1]);
        fprintf(stderr, "Error loading PEM RSA Public Key File.\n");
        exit(2);
    }

    rv = encrypt(rsa_pkey_file, infile.txt, stdout);
    fclose(rsa_pkey_file);

    return rv;
}

And similar way decryption.

How can I do RSA encryption and decryption of a file using Open SSL library in C in simple way?

"it's not working" isn't a particularly useful problem description. catb.org/esr/faqs/smart-questions.html — Randy Howard
– Randy Howard, Commented Mar 27, 2013 at 6:13
Please post the function encrypt, this main() won't tell us much... — Deepu
– Deepu, Commented Mar 27, 2013 at 6:20
sure.. i ll post the encryption part.. its little big.. and i m not able to understand.. — geek
– geek, Commented Mar 27, 2013 at 7:20
@sakshi Could you please at least try to use proper english? This is not 4chan. — fuz
– fuz, Commented Mar 27, 2013 at 8:58
@ FUZxxl hope my question is in human readable format.. so if possible try to answer :) — geek
– geek, Commented Mar 27, 2013 at 9:49

zmo · Accepted Answer · 2014-02-26 15:51:56Z

4

Steps for RSA encryption are as follows:

Read the public key into RSA * structure. It depends on your key format. If key is in PEM format, use PEM_read_RSA_PUBKEY functions. If it is in DER form, use d2i_RSA.
Encrypt your data using RSA public key. Use RSA_public_encrypt function.
Write the data to file or whatever you want to do.

Steps for RSA decryption are:

Read the private key into RSA * structure. It is similar to step 1 in RSA encryption with some minor difference.
Decrypt the data using RSA_private_decrypt. Use RSA_private_decrypt.

You can look OpenSSL documentation which is quite useful and its names are intuitive. I give you just broad level idea. If you need more help, I can post the code example.

edited Feb 26, 2014 at 15:51

zmo

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answered Mar 27, 2013 at 10:29

doptimusprime

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1 Comment

Michael Over a year ago

the man page for RSA_public_encrypt says you need to seed the RNG before calling, implying you need to get some random data from somewhere to pass to RAND_seed...

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