RE: RSA encryption and Decryption code in C language

[enrico d'urso]

Hi,

I'm implementing a software very similar to yours.

This is a small function that I used to generate private and public key:
#include <openssl/pem.h>

int main()
{
    char * file_pem = "key_priv";
    char * file_pem_pub = "key_pub";
    FILE * fp;
    int bits = 1024;
    unsigned long exp = RSA_F4;
    RSA * rsa;
    rsa = RSA_generate_key(bits,exp,NULL,NULL);
    
    fp = fopen(file_pem,"w");
    unsigned char* kstr ="XXXX"; //Password, change it ,it's just an example
    
PEM_write_RSAPrivateKey(fp,rsa,EVP_des_ede3_cbc(),kstr,strlen(kstr),NULL,NULL);
    
    close(fp);
    
    fp = fopen(file_pem_pub,"w");
    
    PEM_write_RSAPublicKey(fp,rsa);
    close(fp);
    RSA_free(rsa);


}

This function is called by client, it is self explicative:
RSA * create_and_set_context()
{
    RSA * rsa = RSA_new();
    FILE * fp = fopen("key_pub","r");
    
    if( fp == NULL)    
        return NULL;
    
    RSA * rs = PEM_read_RSAPublicKey(fp, &rsa, NULL,NULL);

    return rs;
}

This is function called by client to encrypt symmetric key to send to Server.

/* Key is the simmetryc key, to is a buffer */
int encrypt_simmetric_key(unsigned char *key, unsigned char *to, int size, RSA 
* rsa)
{
    return RSA_public_encrypt(size, key, to, rsa, RSA_PKCS1_PADDING );
 
}
***********************

Now, server side:
RSA * create_and_set_context()
{
    OpenSSL_add_all_algorithms();
    RSA * rsa = RSA_new();
    FILE * fp = fopen("key_priv","r");
    unsigned char* kstr ="XXXXXX";
    if( fp == NULL)    
        return NULL;
    
    RSA * rs = PEM_read_RSAPrivateKey(fp, &rsa, NULL,kstr);

    return rs;
}


Then, supposed that buf is the buffer where is stored the symmetric key just 
received with a socket by Server:

unsigned char* getSimKey(char * buf, RSA* rsa)
{
    unsigned char* to = malloc(RSA_size(rsa)); // RSA_size(rsa) is the modulus
    if( to == NULL)
        return NULL;
    int k = RSA_private_decrypt(RSA_size(rsa), (unsigned char*)buf, to, rsa, 
RSA_PKCS1_PADDING);
    if( k == -1)
        return NULL;
    printf("K: %d\n",k);
    int i = 0;
    for(; i< k; i++)
        printbyte(to[i]);
    return to;
    
}

That's all.

Sorry for my bad english, I hope my code will help.

Bye

Enrico