bn_exp.c: fix corner case in new constant-time code.

Submitted by: Emilia Kasper

diff --git a/crypto/bn/bn_exp.c b/crypto/bn/bn_exp.c
index 5c492365f3172b1faa94a1473e0781f3dbc61667..8454d42f84609d64025687697f23cf2e149f61a7 100644 (file)
--- a/crypto/bn/bn_exp.c
+++ b/crypto/bn/bn_exp.c
@@ -693,6 +693,11 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
 
        BN_ULONG *np=mont->N.d, *n0=mont->n0;
 
+       /* BN_to_montgomery can contaminate words above .top
+        * [in BN_DEBUG[_DEBUG] build]... */
+       for (i=am.top; i<top; i++)      am.d[i]=0;
+       for (i=tmp.top; i<top; i++)     tmp.d[i]=0;
+
        bn_scatter5(tmp.d,top,powerbuf,0);
        bn_scatter5(am.d,am.top,powerbuf,1);
        bn_mul_mont(tmp.d,am.d,am.d,np,n0,top);