RSA optimizaation.

diff --git a/lib/silccrypt/rsa.c b/lib/silccrypt/rsa.c
index df7932985705698b18c9720af40ddfbca56f9c3e..c65283e7422c28a9fd9bf32745b64f9ac4e4dbc4 100644 (file)
--- a/lib/silccrypt/rsa.c
+++ b/lib/silccrypt/rsa.c
@@ -4,7 +4,7 @@
  *
  * Author: Pekka Riikonen <priikone@poseidon.pspt.fi>
  *
- * Copyright (C) 1997 - 2000 Pekka Riikonen
+ * Copyright (C) 1997 - 2001 Pekka Riikonen
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -28,7 +28,7 @@
  *     Public key exponent:
  *     e   relatively prime to (p-1) * (q-1)
  *     Private key exponent:
- *     d = e ^ -1 mod ((p-1) * (q-1))
+ *     d = e ^ -1 mod lcm(((p-1) * (q-1)))
  *
  *     Encryption:
  *     c = m ^ e mod n
@@ -45,6 +45,17 @@
  */
 /* $Id$ */
 
+/*
+   ChangeLog
+
+   o Mon Feb 12 11:20:32 EET 2001  Pekka
+
+     Changed RSA private exponent generation to what PKCS #1 suggests.  We
+     try to find the smallest possible d by doing modinv(e, lcm(phi)) instead
+     of modinv(e, phi).  Note: this is not security fix but optimization.
+
+*/
+
 #include "silcincludes.h"
 #include "rsa.h"
 
@@ -460,7 +471,7 @@ void rsa_generate_keys(RsaKey *key, unsigned int bits,
                       SilcInt *p, SilcInt *q)
 {
   SilcInt phi, hlp;
-  SilcInt dq;
+  SilcInt div, lcm;
   SilcInt pm1, qm1;
   
   /* Initialize variables */
@@ -471,7 +482,8 @@ void rsa_generate_keys(RsaKey *key, unsigned int bits,
   silc_mp_init(&key->d);
   silc_mp_init(&phi);
   silc_mp_init(&hlp);
-  silc_mp_init(&dq);
+  silc_mp_init(&div);
+  silc_mp_init(&lcm);
   silc_mp_init(&pm1);
   silc_mp_init(&qm1);
 
@@ -500,14 +512,15 @@ void rsa_generate_keys(RsaKey *key, unsigned int bits,
     goto retry_e;
   }
   
-  /* Find d, the private exponent. First we do phi / 2, to get it a 
-     bit smaller */
-  silc_mp_div_ui(&dq, &phi, 2);
-  silc_mp_modinv(&key->d, &key->e, &dq);
+  /* Find d, the private exponent. */
+  silc_mp_gcd(&div, &pm1, &qm1);
+  silc_mp_fdiv_q(&lcm, &phi, &div);
+  silc_mp_modinv(&key->d, &key->e, &lcm);
   
   silc_mp_clear(&phi);
   silc_mp_clear(&hlp);
-  silc_mp_clear(&dq);
+  silc_mp_clear(&div);
+  silc_mp_clear(&lcm);
   silc_mp_clear(&pm1);
   silc_mp_clear(&qm1);
 }