RSA

rsa.c

/* 
   An answer to Task3 by [email protected]
   rsa.c
 
   $gcc -o rsa.exe rsa.c -lcrypto
 
   *http://www.cis.syr.edu/~wedu/seed/Labs_16.04/Crypto/Crypto_RSA/Crypto_RSA.pdf
   *https://goo.gl/HuwEPn
*/

#include <stdio.h>
#include <string.h>
#include <openssl/bn.h>


void hex_encode(char *hex, char *text){
   int i, len;
   len = strlen(text);
   for(i = 0; i<len; i++)
      sprintf(hex+i*2, "%02X", text[i]);
}

void hex_decode(char *text, char *hex){
   unsigned int ch ;
   for( ; sscanf( (const char*)hex, "%02X", &ch ) == 1 ; hex += 2 )
      *text++ = ch ;
   *text = '\0';
}

BIGNUM *encrypt(BN_CTX *ctx, BIGNUM *M, BIGNUM *e, BIGNUM *n){
   BIGNUM *C = BN_new();
   BN_mod_exp(C,M,e,n,ctx);
   return C;
}

BIGNUM *decrypt(BN_CTX *ctx, BIGNUM *C, BIGNUM *d, BIGNUM *n){
   BIGNUM *M = BN_new();
   BN_mod_exp(M,C,d,n,ctx);
   return M;
}

void printBN(char *msg, BIGNUM * a)
{
   /* Use BN_bn2hex(a) for hex string
   * Use BN_bn2dec(a) for decimal string */
   char * number_str = BN_bn2hex(a);
   printf("%s %s\n", msg, number_str);
   OPENSSL_free(number_str);
}

int main ()
{
   char text[256]="Password is dees";
   char hex[1024]="";
   char tmp[1024]="";

   BN_CTX *ctx = BN_CTX_new();
   BIGNUM *n = BN_new();
   BIGNUM *e = BN_new();
   BIGNUM *d = BN_new();
   BIGNUM *C = BN_new();
   BIGNUM *M = BN_new();

   BN_hex2bn(&n, "DCBFFE3E51F62E09CE7032E2677A78946A849DC4CDDE3A4D0CB81629242FB1A5");
   BN_hex2bn(&e, "010001");
   BN_hex2bn(&d, "74D806F9F3A62BAE331FFE3F0A68AFE35B3D2E4794148AACBC26AA381CD7D30D");

   //BN_hex2bn(&C, "8C0F971DF2F3672B28811407E2DABBE1DA0FEBBBDFC7DCB67396567EA1E2493F");  
 
   printf("Converting message to hex and bignum..\n");
   hex_encode(hex,text);
   printf("Hex: %s\n", hex);
   BN_hex2bn(&M,hex);

   printf("Encrypting message..\n");
   //printBN("M=", decrypt(ctx,C,d,n));
   printBN("C=",C=encrypt(ctx,M,e,n));
   
   printf("Decrypting message..\n");
   printBN("M=", M=decrypt(ctx,C,d,n));
   
   hex_decode(tmp,BN_bn2hex(M));
   printf("Text: %s\n", tmp);
   
   return 0;

}

rsadec.c

/* An answer to Task3 by [email protected]
   rsadec.c
 
   $gcc -o rsadec.exe  rsadec.c -lcrypto
 
   *http://www.cis.syr.edu/~wedu/seed/Labs_16.04/Crypto/Crypto_RSA/Crypto_RSA.pdf
   *https://goo.gl/HuwEPn
*/

#include <stdio.h>
#include <openssl/bn.h>

void printBN(char *msg, BIGNUM * a)
{
   /* Use BN_bn2hex(a) for hex string
   * Use BN_bn2dec(a) for decimal string */
   char * number_str = BN_bn2hex(a);
   printf("%s %s\n", msg, number_str);
   OPENSSL_free(number_str);
}

int main ()
{
   BN_CTX *ctx = BN_CTX_new();
   BIGNUM *n = BN_new();
   BIGNUM *e = BN_new();
   BIGNUM *d = BN_new();
   BIGNUM *C = BN_new();
   BIGNUM *M = BN_new();

   BN_hex2bn(&n, "DCBFFE3E51F62E09CE7032E2677A78946A849DC4CDDE3A4D0CB81629242FB1A5");
   BN_hex2bn(&e, "010001");
   BN_hex2bn(&d, "74D806F9F3A62BAE331FFE3F0A68AFE35B3D2E4794148AACBC26AA381CD7D30D");
   BN_hex2bn(&C, "8C0F971DF2F3672B28811407E2DABBE1DA0FEBBBDFC7DCB67396567EA1E2493F");  
 
   BN_mod_exp(M,C,d,n,ctx);

   printBN("M=", M);
   
   return 0;

}