1 #include "os.h"
2 #include <mp.h>
3 #include <libsec.h>
4
5 // decrypt rsa using garner's algorithm for the chinese remainder theorem
6 // seminumerical algorithms, knuth, pp 253-254
7 // applied cryptography, menezes et al, pg 612
8 mpint*
rsadecrypt(RSApriv * rsa,mpint * in,mpint * out)9 rsadecrypt(RSApriv *rsa, mpint *in, mpint *out)
10 {
11 mpint *v1, *v2;
12
13 if(out == nil)
14 out = mpnew(0);
15
16 // convert in to modular representation
17 v1 = mpnew(0);
18 mpmod(in, rsa->p, v1);
19 v2 = mpnew(0);
20 mpmod(in, rsa->q, v2);
21
22 // exponentiate the modular rep
23 mpexp(v1, rsa->kp, rsa->p, v1);
24 mpexp(v2, rsa->kq, rsa->q, v2);
25
26 // out = v1 + p*((v2-v1)*c2 mod q)
27 mpsub(v2, v1, v2);
28 mpmul(v2, rsa->c2, v2);
29 mpmod(v2, rsa->q, v2);
30 mpmul(v2, rsa->p, out);
31 mpadd(v1, out, out);
32
33 mpfree(v1);
34 mpfree(v2);
35
36 return out;
37 }
38