From 4e0d7ca2331a07c263f9d2dded2e8bc2af69c918 Mon Sep 17 00:00:00 2001
From: Paul Bakker <p.j.bakker@polarssl.org>
Date: Thu, 29 Jan 2009 22:24:33 +0000
Subject: [PATCH]  - Fixed a bug in mpi_gcd() that prevented correct funtioning
 when both input numbers are even.

---
 ChangeLog        |  8 ++++++++
 library/bignum.c | 52 ++++++++++++++++++++++++++++++++++++++++++------
 2 files changed, 54 insertions(+), 6 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index bb3988ef2..74a40f8de 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,13 @@
 PolarSSL ChangeLog
 
+= Version 0.10.2 released on 2009-XXXX
+   * Fixed a bug in mpi_gcd() so that it also works when both
+     input numbers are even and added testcases to check
+     (found by Pierre Habouzit)
+
+= Version 0.10.1 released on 2009-01-25
+   * Added BSD License
+
 = Version 0.10.0 released on 2009-01-12
    * Migrated XySSL to PolarSSL
    * Added XTEA symmetric cipher
diff --git a/library/bignum.c b/library/bignum.c
index 97d4d120b..1685d308a 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1478,21 +1478,29 @@ cleanup:
  */
 int mpi_gcd( mpi *G, mpi *A, mpi *B )
 {
-    int ret;
+    int ret, lz, lzt;
     mpi TG, TA, TB;
 
     mpi_init( &TG, &TA, &TB, NULL );
 
-    MPI_CHK( mpi_lset( &TG, 1 ) );
     MPI_CHK( mpi_copy( &TA, A ) );
     MPI_CHK( mpi_copy( &TB, B ) );
 
+    lz = mpi_lsb( &TA );
+    lzt = mpi_lsb( &TB );
+
+    if ( lzt < lz )
+        lz = lzt;
+
+    MPI_CHK( mpi_shift_r( &TA, lz ) );
+    MPI_CHK( mpi_shift_r( &TB, lz ) );
+
     TA.s = TB.s = 1;
 
     while( mpi_cmp_int( &TA, 0 ) != 0 )
     {
-        while( ( TA.p[0] & 1 ) == 0 ) MPI_CHK( mpi_shift_r( &TA, 1 ) );
-        while( ( TB.p[0] & 1 ) == 0 ) MPI_CHK( mpi_shift_r( &TB, 1 ) );
+        MPI_CHK( mpi_shift_r( &TA, mpi_lsb( &TA ) ) );
+        MPI_CHK( mpi_shift_r( &TB, mpi_lsb( &TB ) ) );
 
         if( mpi_cmp_mpi( &TA, &TB ) >= 0 )
         {
@@ -1506,7 +1514,8 @@ int mpi_gcd( mpi *G, mpi *A, mpi *B )
         }
     }
 
-    MPI_CHK( mpi_mul_mpi( G, &TG, &TB ) );
+    MPI_CHK( mpi_shift_l( &TB, lz ) );
+    MPI_CHK( mpi_copy( G, &TB ) );
 
 cleanup:
 
@@ -1820,12 +1829,21 @@ cleanup:
 
 #if defined(POLARSSL_SELF_TEST)
 
+#define GCD_PAIR_COUNT	3
+
+static const int gcd_pairs[GCD_PAIR_COUNT][3] =
+{
+    { 693, 609, 21 },
+    { 1764, 868, 28 },
+    { 768454923, 542167814, 1 }
+};
+
 /*
  * Checkup routine
  */
 int mpi_self_test( int verbose )
 {
-    int ret;
+    int ret, i;
     mpi A, E, N, X, Y, U, V;
 
     mpi_init( &A, &E, &N, &X, &Y, &U, &V, NULL );
@@ -1939,6 +1957,28 @@ int mpi_self_test( int verbose )
     if( verbose != 0 )
         printf( "passed\n" );
 
+    if( verbose != 0 )
+        printf( "  MPI test #5 (simple gcd): " );
+
+    for ( i = 0; i < GCD_PAIR_COUNT; i++)
+    {
+        MPI_CHK( mpi_lset( &X, gcd_pairs[i][0] ) );
+	MPI_CHK( mpi_lset( &Y, gcd_pairs[i][1] ) );
+
+	MPI_CHK( mpi_gcd( &A, &X, &Y ) );
+
+	if( mpi_cmp_int( &A, gcd_pairs[i][2] ) != 0 )
+	{
+		if( verbose != 0 )
+			printf( "failed at %d\n", i );
+
+		return( 1 );
+	}
+    }
+
+    if( verbose != 0 )
+        printf( "passed\n" );
+
 cleanup:
 
     if( ret != 0 && verbose != 0 )