Adjust various bignum functions to use BN_CTX for variables instead of
[openssl.git] / crypto / bn / bn_exp.c
index 8117323766e8093a561ccd46c483a231ee571778..d6bb2b4397cc91a1cad003bba0595f904849ab21 100644 (file)
  */
 
 
-#include <stdio.h>
 #include "cryptlib.h"
 #include "bn_lcl.h"
-#ifdef ATALLA
-# include <alloca.h>
-# include <atasi.h>
-# include <assert.h>
-# include <dlfcn.h>
-#endif
-
 
 #define TABLE_SIZE     32
 
-/* slow but works */
-int BN_mod_mul(BIGNUM *ret, BIGNUM *a, BIGNUM *b, const BIGNUM *m, BN_CTX *ctx)
-       {
-       BIGNUM *t;
-       int r=0;
-
-       bn_check_top(a);
-       bn_check_top(b);
-       bn_check_top(m);
-
-       BN_CTX_start(ctx);
-       if ((t = BN_CTX_get(ctx)) == NULL) goto err;
-       if (a == b)
-               { if (!BN_sqr(t,a,ctx)) goto err; }
-       else
-               { if (!BN_mul(t,a,b,ctx)) goto err; }
-       if (!BN_mod(ret,t,m,ctx)) goto err;
-       r=1;
-err:
-       BN_CTX_end(ctx);
-       return(r);
-       }
-
-
 /* this one works - simple but works */
-int BN_exp(BIGNUM *r, BIGNUM *a, BIGNUM *p, BN_CTX *ctx)
+int BN_exp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, BN_CTX *ctx)
        {
        int i,bits,ret=0;
        BIGNUM *v,*rr;
@@ -179,179 +147,12 @@ int BN_exp(BIGNUM *r, BIGNUM *a, BIGNUM *p, BN_CTX *ctx)
 err:
        if (r != rr) BN_copy(r,rr);
        BN_CTX_end(ctx);
+       bn_check_top(r);
        return(ret);
        }
 
 
-#ifdef ATALLA
-
-/*
- * This routine will dynamically check for the existance of an Atalla AXL-200
- * SSL accelerator module.  If one is found, the variable
- * asi_accelerator_present is set to 1 and the function pointers
- * ptr_ASI_xxxxxx above will be initialized to corresponding ASI API calls.
- */
-typedef int tfnASI_GetPerformanceStatistics(int reset_flag,
-                                           unsigned int *ret_buf);
-typedef int tfnASI_GetHardwareConfig(long card_num, unsigned int *ret_buf);
-typedef int tfnASI_RSAPrivateKeyOpFn(RSAPrivateKey * rsaKey,
-                                    unsigned char *output,
-                                    unsigned char *input,
-                                    unsigned int modulus_len);
-
-static tfnASI_GetHardwareConfig *ptr_ASI_GetHardwareConfig;
-static tfnASI_RSAPrivateKeyOpFn *ptr_ASI_RSAPrivateKeyOpFn;
-static tfnASI_GetPerformanceStatistics *ptr_ASI_GetPerformanceStatistics;
-static int asi_accelerator_present;
-static int tried_atalla;
-
-void atalla_initialize_accelerator_handle(void)
-       {
-       void *dl_handle;
-       int status;
-       unsigned int config_buf[1024]; 
-       static int tested;
-
-       if(tested)
-               return;
-
-       tested=1;
-
-       bzero((void *)config_buf, 1024);
-
-       /*
-        * Check to see if the library is present on the system
-        */
-       dl_handle = dlopen("atasi.so", RTLD_NOW);
-       if (dl_handle == (void *) NULL)
-               {
-/*             printf("atasi.so library is not present on the system\n");
-               printf("No HW acceleration available\n");*/
-               return;
-               }
-
-       /*
-        * The library is present.  Now we'll check to insure that the
-        * LDM is up and running. First we'll get the address of the
-        * function in the atasi library that we need to see if the
-        * LDM is operating.
-        */
-
-       ptr_ASI_GetHardwareConfig =
-         (tfnASI_GetHardwareConfig *)dlsym(dl_handle,"ASI_GetHardwareConfig");
-
-       if (ptr_ASI_GetHardwareConfig)
-               {
-               /*
-                * We found the call, now we'll get our config
-                * status.  If we get a non 0 result, the LDM is not
-                * running and we cannot use the Atalla ASI *
-                * library.
-                */
-               status = (*ptr_ASI_GetHardwareConfig)(0L, config_buf);
-               if (status != 0)
-                       {
-                       printf("atasi.so library is present but not initialized\n");
-                       printf("No HW acceleration available\n");
-                       return;
-                       }    
-               }
-       else
-               {
-/*             printf("We found the library, but not the function. Very Strange!\n");*/
-               return ;
-               }
-
-       /* 
-        * It looks like we have acceleration capabilities.  Load up the
-        * pointers to our ASI API calls.
-        */
-       ptr_ASI_RSAPrivateKeyOpFn=
-         (tfnASI_RSAPrivateKeyOpFn *)dlsym(dl_handle, "ASI_RSAPrivateKeyOpFn");
-       if (ptr_ASI_RSAPrivateKeyOpFn == NULL)
-               {
-/*             printf("We found the library, but no RSA function. Very Strange!\n");*/
-               return;
-               }
-
-       ptr_ASI_GetPerformanceStatistics =
-         (tfnASI_GetPerformanceStatistics *)dlsym(dl_handle, "ASI_GetPerformanceStatistics");
-       if (ptr_ASI_GetPerformanceStatistics == NULL)
-               {
-/*             printf("We found the library, but no stat function. Very Strange!\n");*/
-               return;
-             }
-
-       /*
-        * Indicate that acceleration is available
-        */
-       asi_accelerator_present = 1;
-
-/*     printf("This system has acceleration!\n");*/
-
-       return;
-       }
-
-/* make sure this only gets called once when bn_mod_exp calls bn_mod_exp_mont */
-int BN_mod_exp_atalla(BIGNUM *r, BIGNUM *a, const BIGNUM *p, const BIGNUM *m)
-       {
-       unsigned char *abin;
-       unsigned char *pbin;
-       unsigned char *mbin;
-       unsigned char *rbin;
-       int an,pn,mn,ret;
-       RSAPrivateKey keydata;
-
-       atalla_initialize_accelerator_handle();
-       if(!asi_accelerator_present)
-               return 0;
-
-
-/* We should be able to run without size testing */
-# define ASIZE 128
-       an=BN_num_bytes(a);
-       pn=BN_num_bytes(p);
-       mn=BN_num_bytes(m);
-
-       if(an <= ASIZE && pn <= ASIZE && mn <= ASIZE)
-           {
-           int size=mn;
-
-           assert(an <= mn);
-           abin=alloca(size);
-           memset(abin,'\0',mn);
-           BN_bn2bin(a,abin+size-an);
-
-           pbin=alloca(pn);
-           BN_bn2bin(p,pbin);
-
-           mbin=alloca(size);
-           memset(mbin,'\0',mn);
-           BN_bn2bin(m,mbin+size-mn);
-
-           rbin=alloca(size);
-
-           memset(&keydata,'\0',sizeof keydata);
-           keydata.privateExponent.data=pbin;
-           keydata.privateExponent.len=pn;
-           keydata.modulus.data=mbin;
-           keydata.modulus.len=size;
-
-           ret=(*ptr_ASI_RSAPrivateKeyOpFn)(&keydata,rbin,abin,keydata.modulus.len);
-/*fprintf(stderr,"!%s\n",BN_bn2hex(a));*/
-           if(!ret)
-               {
-               BN_bin2bn(rbin,keydata.modulus.len,r);
-/*fprintf(stderr,"?%s\n",BN_bn2hex(r));*/
-               return 1;
-               }
-           }
-       return 0;
-        }
-#endif /* def ATALLA */
-
-
-int BN_mod_exp(BIGNUM *r, BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
+int BN_mod_exp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
               BN_CTX *ctx)
        {
        int ret;
@@ -360,12 +161,39 @@ int BN_mod_exp(BIGNUM *r, BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
        bn_check_top(p);
        bn_check_top(m);
 
-#ifdef ATALLA
-       if(BN_mod_exp_atalla(r,a,p,m))
-           return 1;
-/* If it fails, try the other methods (but don't try atalla again) */
-       tried_atalla=1;
-#endif
+       /* For even modulus  m = 2^k*m_odd,  it might make sense to compute
+        * a^p mod m_odd  and  a^p mod 2^k  separately (with Montgomery
+        * exponentiation for the odd part), using appropriate exponent
+        * reductions, and combine the results using the CRT.
+        *
+        * For now, we use Montgomery only if the modulus is odd; otherwise,
+        * exponentiation using the reciprocal-based quick remaindering
+        * algorithm is used.
+        *
+        * (Timing obtained with expspeed.c [computations  a^p mod m
+        * where  a, p, m  are of the same length: 256, 512, 1024, 2048,
+        * 4096, 8192 bits], compared to the running time of the
+        * standard algorithm:
+        *
+        *   BN_mod_exp_mont   33 .. 40 %  [AMD K6-2, Linux, debug configuration]
+         *                     55 .. 77 %  [UltraSparc processor, but
+        *                                  debug-solaris-sparcv8-gcc conf.]
+        * 
+        *   BN_mod_exp_recp   50 .. 70 %  [AMD K6-2, Linux, debug configuration]
+        *                     62 .. 118 % [UltraSparc, debug-solaris-sparcv8-gcc]
+        *
+        * On the Sparc, BN_mod_exp_recp was faster than BN_mod_exp_mont
+        * at 2048 and more bits, but at 512 and 1024 bits, it was
+        * slower even than the standard algorithm!
+        *
+        * "Real" timings [linux-elf, solaris-sparcv9-gcc configurations]
+        * should be obtained when the new Montgomery reduction code
+        * has been integrated into OpenSSL.)
+        */
+
+#define MONT_MUL_MOD
+#define MONT_EXP_WORD
+#define RECP_MUL_MOD
 
 #ifdef MONT_MUL_MOD
        /* I have finally been able to take out this pre-condition of
@@ -376,12 +204,14 @@ int BN_mod_exp(BIGNUM *r, BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
 
        if (BN_is_odd(m))
                {
-               if (a->top == 1)
+#  ifdef MONT_EXP_WORD
+               if (a->top == 1 && !a->neg)
                        {
                        BN_ULONG A = a->d[0];
                        ret=BN_mod_exp_mont_word(r,A,p,m,ctx,NULL);
                        }
                else
+#  endif
                        ret=BN_mod_exp_mont(r,a,p,m,ctx,NULL);
                }
        else
@@ -392,10 +222,7 @@ int BN_mod_exp(BIGNUM *r, BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
                { ret=BN_mod_exp_simple(r,a,p,m,ctx); }
 #endif
 
-#ifdef ATALLA
-       tried_atalla=0;
-#endif
-
+       bn_check_top(r);
        return(ret);
        }
 
@@ -404,43 +231,59 @@ int BN_mod_exp_recp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
                    const BIGNUM *m, BN_CTX *ctx)
        {
        int i,j,bits,ret=0,wstart,wend,window,wvalue;
-       int start=1,ts=0;
+       int start=1;
        BIGNUM *aa;
-       BIGNUM val[TABLE_SIZE];
+       /* Table of variables obtained from 'ctx' */
+       BIGNUM *val[TABLE_SIZE];
        BN_RECP_CTX recp;
 
        bits=BN_num_bits(p);
 
        if (bits == 0)
                {
-               BN_one(r);
-               return(1);
+               ret = BN_one(r);
+               return ret;
                }
 
        BN_CTX_start(ctx);
-       if ((aa = BN_CTX_get(ctx)) == NULL) goto err;
+       aa = BN_CTX_get(ctx);
+       val[0] = BN_CTX_get(ctx);
+       if(!aa || !val[0]) goto err;
 
        BN_RECP_CTX_init(&recp);
-       if (BN_RECP_CTX_set(&recp,m,ctx) <= 0) goto err;
-
-       BN_init(&(val[0]));
-       ts=1;
+       if (m->neg)
+               {
+               /* ignore sign of 'm' */
+               if (!BN_copy(aa, m)) goto err;
+               aa->neg = 0;
+               if (BN_RECP_CTX_set(&recp,aa,ctx) <= 0) goto err;
+               }
+       else
+               {
+               if (BN_RECP_CTX_set(&recp,m,ctx) <= 0) goto err;
+               }
 
-       if (!BN_mod(&(val[0]),a,m,ctx)) goto err;               /* 1 */
+       if (!BN_nnmod(val[0],a,m,ctx)) goto err;                /* 1 */
+       if (BN_is_zero(val[0]))
+               {
+               BN_zero(r);
+               ret = 1;
+               goto err;
+               }
 
        window = BN_window_bits_for_exponent_size(bits);
        if (window > 1)
                {
-               if (!BN_mod_mul_reciprocal(aa,&(val[0]),&(val[0]),&recp,ctx))
+               if (!BN_mod_mul_reciprocal(aa,val[0],val[0],&recp,ctx))
                        goto err;                               /* 2 */
                j=1<<(window-1);
                for (i=1; i<j; i++)
                        {
-                       BN_init(&val[i]);
-                       if (!BN_mod_mul_reciprocal(&(val[i]),&(val[i-1]),aa,&recp,ctx))
+                       if(((val[i] = BN_CTX_get(ctx)) == NULL) ||
+                                       !BN_mod_mul_reciprocal(val[i],val[i-1],
+                                               aa,&recp,ctx))
                                goto err;
                        }
-               ts=i;
                }
                
        start=1;        /* This is used to avoid multiplication etc
@@ -492,7 +335,7 @@ int BN_mod_exp_recp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
                                }
                
                /* wvalue will be an odd number < 2^window */
-               if (!BN_mod_mul_reciprocal(r,r,&(val[wvalue>>1]),&recp,ctx))
+               if (!BN_mod_mul_reciprocal(r,r,val[wvalue>>1],&recp,ctx))
                        goto err;
 
                /* move the 'window' down further */
@@ -504,34 +347,28 @@ int BN_mod_exp_recp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
        ret=1;
 err:
        BN_CTX_end(ctx);
-       for (i=0; i<ts; i++)
-               BN_clear_free(&(val[i]));
        BN_RECP_CTX_free(&recp);
+       bn_check_top(r);
        return(ret);
        }
 
 
-int BN_mod_exp_mont(BIGNUM *rr, BIGNUM *a, const BIGNUM *p,
+int BN_mod_exp_mont(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
                    const BIGNUM *m, BN_CTX *ctx, BN_MONT_CTX *in_mont)
        {
        int i,j,bits,ret=0,wstart,wend,window,wvalue;
-       int start=1,ts=0;
+       int start=1;
        BIGNUM *d,*r;
-       BIGNUM *aa;
-       BIGNUM val[TABLE_SIZE];
+       const BIGNUM *aa;
+       /* Table of variables obtained from 'ctx' */
+       BIGNUM *val[TABLE_SIZE];
        BN_MONT_CTX *mont=NULL;
 
        bn_check_top(a);
        bn_check_top(p);
        bn_check_top(m);
 
-#ifdef ATALLA
-       if(!tried_atalla && BN_mod_exp_atalla(rr,a,p,m))
-           return 1;
-/* If it fails, try the other methods */
-#endif
-
-       if (!(m->d[0] & 1))
+       if (!BN_is_odd(m))
                {
                BNerr(BN_F_BN_MOD_EXP_MONT,BN_R_CALLED_WITH_EVEN_MODULUS);
                return(0);
@@ -539,13 +376,15 @@ int BN_mod_exp_mont(BIGNUM *rr, BIGNUM *a, const BIGNUM *p,
        bits=BN_num_bits(p);
        if (bits == 0)
                {
-               BN_one(rr);
-               return(1);
+               ret = BN_one(rr);
+               return ret;
                }
+
        BN_CTX_start(ctx);
        d = BN_CTX_get(ctx);
        r = BN_CTX_get(ctx);
-       if (d == NULL || r == NULL) goto err;
+       val[0] = BN_CTX_get(ctx);
+       if (!d || !r || !val[0]) goto err;
 
        /* If this is not done, things will break in the montgomery
         * part */
@@ -558,30 +397,34 @@ int BN_mod_exp_mont(BIGNUM *rr, BIGNUM *a, const BIGNUM *p,
                if (!BN_MONT_CTX_set(mont,m,ctx)) goto err;
                }
 
-       BN_init(&val[0]);
-       ts=1;
-       if (BN_ucmp(a,m) >= 0)
+       if (a->neg || BN_ucmp(a,m) >= 0)
                {
-               if (!BN_mod(&(val[0]),a,m,ctx))
+               if (!BN_nnmod(val[0],a,m,ctx))
                        goto err;
-               aa= &(val[0]);
+               aa= val[0];
                }
        else
                aa=a;
-       if (!BN_to_montgomery(&(val[0]),aa,mont,ctx)) goto err; /* 1 */
+       if (BN_is_zero(aa))
+               {
+               BN_zero(rr);
+               ret = 1;
+               goto err;
+               }
+       if (!BN_to_montgomery(val[0],aa,mont,ctx)) goto err; /* 1 */
 
        window = BN_window_bits_for_exponent_size(bits);
        if (window > 1)
                {
-               if (!BN_mod_mul_montgomery(d,&(val[0]),&(val[0]),mont,ctx)) goto err; /* 2 */
+               if (!BN_mod_mul_montgomery(d,val[0],val[0],mont,ctx)) goto err; /* 2 */
                j=1<<(window-1);
                for (i=1; i<j; i++)
                        {
-                       BN_init(&(val[i]));
-                       if (!BN_mod_mul_montgomery(&(val[i]),&(val[i-1]),d,mont,ctx))
+                       if(((val[i] = BN_CTX_get(ctx)) == NULL) ||
+                                       !BN_mod_mul_montgomery(val[i],val[i-1],
+                                               d,mont,ctx))
                                goto err;
                        }
-               ts=i;
                }
 
        start=1;        /* This is used to avoid multiplication etc
@@ -634,7 +477,7 @@ int BN_mod_exp_mont(BIGNUM *rr, BIGNUM *a, const BIGNUM *p,
                                }
                
                /* wvalue will be an odd number < 2^window */
-               if (!BN_mod_mul_montgomery(r,r,&(val[wvalue>>1]),mont,ctx))
+               if (!BN_mod_mul_montgomery(r,r,val[wvalue>>1],mont,ctx))
                        goto err;
 
                /* move the 'window' down further */
@@ -648,8 +491,7 @@ int BN_mod_exp_mont(BIGNUM *rr, BIGNUM *a, const BIGNUM *p,
 err:
        if ((in_mont == NULL) && (mont != NULL)) BN_MONT_CTX_free(mont);
        BN_CTX_end(ctx);
-       for (i=0; i<ts; i++)
-               BN_clear_free(&(val[i]));
+       bn_check_top(rr);
        return(ret);
        }
 
@@ -667,45 +509,46 @@ int BN_mod_exp_mont_word(BIGNUM *rr, BN_ULONG a, const BIGNUM *p,
                (/* BN_ucmp(r, (m)) < 0 ? 1 :*/  \
                        (BN_mod(t, r, m, ctx) && (swap_tmp = r, r = t, t = swap_tmp, 1))))
                /* BN_MOD_MUL_WORD is only used with 'w' large,
-                 * so the BN_ucmp test is probably more overhead
-                 * than always using BN_mod (which uses BN_copy if
-                 * a similar test returns true). */
+                * so the BN_ucmp test is probably more overhead
+                * than always using BN_mod (which uses BN_copy if
+                * a similar test returns true). */
+               /* We can use BN_mod and do not need BN_nnmod because our
+                * accumulator is never negative (the result of BN_mod does
+                * not depend on the sign of the modulus).
+                */
 #define BN_TO_MONTGOMERY_WORD(r, w, mont) \
                (BN_set_word(r, (w)) && BN_to_montgomery(r, r, (mont), ctx))
 
        bn_check_top(p);
        bn_check_top(m);
 
-       if (!(m->d[0] & 1))
+       if (!BN_is_odd(m))
                {
                BNerr(BN_F_BN_MOD_EXP_MONT_WORD,BN_R_CALLED_WITH_EVEN_MODULUS);
                return(0);
                }
+       if (m->top == 1)
+               a %= m->d[0]; /* make sure that 'a' is reduced */
+
        bits = BN_num_bits(p);
        if (bits == 0)
                {
-               BN_one(rr);
-               return(1);
+               ret = BN_one(rr);
+               return ret;
+               }
+       if (a == 0)
+               {
+               BN_zero(rr);
+               ret = 1;
+               return ret;
                }
+
        BN_CTX_start(ctx);
        d = BN_CTX_get(ctx);
        r = BN_CTX_get(ctx);
        t = BN_CTX_get(ctx);
        if (d == NULL || r == NULL || t == NULL) goto err;
 
-#ifdef ATALLA
-       if (!tried_atalla)
-               {
-               BN_set_word(t, a);
-               if (BN_mod_exp_atalla(rr, t, p, m))
-                       {
-                       BN_CTX_end(ctx);
-                       return 1;
-                       }
-               }
-/* If it fails, try the other methods */
-#endif
-
        if (in_mont != NULL)
                mont=in_mont;
        else
@@ -790,47 +633,54 @@ int BN_mod_exp_mont_word(BIGNUM *rr, BN_ULONG a, const BIGNUM *p,
 err:
        if ((in_mont == NULL) && (mont != NULL)) BN_MONT_CTX_free(mont);
        BN_CTX_end(ctx);
+       bn_check_top(rr);
        return(ret);
        }
 
 
 /* The old fallback, simple version :-) */
-int BN_mod_exp_simple(BIGNUM *r, BIGNUM *a, BIGNUM *p, BIGNUM *m,
-            BN_CTX *ctx)
+int BN_mod_exp_simple(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
+               const BIGNUM *m, BN_CTX *ctx)
        {
-       int i,j,bits,ret=0,wstart,wend,window,wvalue,ts=0;
+       int i,j,bits,ret=0,wstart,wend,window,wvalue;
        int start=1;
        BIGNUM *d;
-       BIGNUM val[TABLE_SIZE];
+       /* Table of variables obtained from 'ctx' */
+       BIGNUM *val[TABLE_SIZE];
 
        bits=BN_num_bits(p);
 
        if (bits == 0)
                {
-               BN_one(r);
-               return(1);
+               ret = BN_one(r);
+               return ret;
                }
 
        BN_CTX_start(ctx);
-       if ((d = BN_CTX_get(ctx)) == NULL) goto err;
+       d = BN_CTX_get(ctx);
+       val[0] = BN_CTX_get(ctx);
+       if(!d || !val[0]) goto err;
 
-       BN_init(&(val[0]));
-       ts=1;
-       if (!BN_mod(&(val[0]),a,m,ctx)) goto err;               /* 1 */
+       if (!BN_nnmod(val[0],a,m,ctx)) goto err;                /* 1 */
+       if (BN_is_zero(val[0]))
+               {
+               BN_zero(r);
+               ret = 1;
+               goto err;
+               }
 
        window = BN_window_bits_for_exponent_size(bits);
        if (window > 1)
                {
-               if (!BN_mod_mul(d,&(val[0]),&(val[0]),m,ctx))
+               if (!BN_mod_mul(d,val[0],val[0],m,ctx))
                        goto err;                               /* 2 */
                j=1<<(window-1);
                for (i=1; i<j; i++)
                        {
-                       BN_init(&(val[i]));
-                       if (!BN_mod_mul(&(val[i]),&(val[i-1]),d,m,ctx))
+                       if(((val[i] = BN_CTX_get(ctx)) == NULL) ||
+                                       !BN_mod_mul(val[i],val[i-1],d,m,ctx))
                                goto err;
                        }
-               ts=i;
                }
 
        start=1;        /* This is used to avoid multiplication etc
@@ -882,7 +732,7 @@ int BN_mod_exp_simple(BIGNUM *r, BIGNUM *a, BIGNUM *p, BIGNUM *m,
                                }
                
                /* wvalue will be an odd number < 2^window */
-               if (!BN_mod_mul(r,r,&(val[wvalue>>1]),m,ctx))
+               if (!BN_mod_mul(r,r,val[wvalue>>1],m,ctx))
                        goto err;
 
                /* move the 'window' down further */
@@ -894,8 +744,7 @@ int BN_mod_exp_simple(BIGNUM *r, BIGNUM *a, BIGNUM *p, BIGNUM *m,
        ret=1;
 err:
        BN_CTX_end(ctx);
-       for (i=0; i<ts; i++)
-               BN_clear_free(&(val[i]));
+       bn_check_top(r);
        return(ret);
        }