Commit d6482a82 authored by Andy Polyakov's avatar Andy Polyakov Committed by Matt Caswell
Browse files

bn/bn_exp.c: constant-time MOD_EXP_CTIME_COPY_FROM_PREBUF. 



Performance penalty varies from platform to platform, and even
key length. For rsa2048 sign it was observed to reach almost 10%.

CVE-2016-0702

Reviewed-by: default avatarRichard Levitte <levitte@openssl.org>
Reviewed-by: default avatarRich Salz <rsalz@openssl.org>
parent 2e0956ba

crypto/bn/bn_exp.c

+57 −18
Original line number Diff line number Diff line
@@ -109,6 +109,7 @@ 
 */



#include "internal/cryptlib.h"

#include "internal/constant_time_locl.h"

#include "bn_lcl.h"



#include <stdlib.h>
@@ -605,15 +606,17 @@ static BN_ULONG bn_get_bits(const BIGNUM *a, int bitpos) 


static int MOD_EXP_CTIME_COPY_TO_PREBUF(const BIGNUM *b, int top,

                                        unsigned char *buf, int idx,

                                        int width)

                                        int window)

{

    size_t i, j;

    int i, j;

    int width = 1 << window;

    BN_ULONG *table = (BN_ULONG *)buf;



    if (top > b->top)

        top = b->top;           /* this works because 'buf' is explicitly

                                 * zeroed */

    for (i = 0, j = idx; i < top * sizeof b->d[0]; i++, j += width) {

        buf[j] = ((unsigned char *)b->d)[i];

    for (i = 0, j = idx; i < top; i++, j += width) {

        table[j] = b->d[i];

    }



    return 1;
@@ -621,15 +624,51 @@ static int MOD_EXP_CTIME_COPY_TO_PREBUF(const BIGNUM *b, int top, 


static int MOD_EXP_CTIME_COPY_FROM_PREBUF(BIGNUM *b, int top,

                                          unsigned char *buf, int idx,

                                          int width)

                                          int window)

{

    size_t i, j;

    int i, j;

    int width = 1 << window;

    volatile BN_ULONG *table = (volatile BN_ULONG *)buf;



    if (bn_wexpand(b, top) == NULL)

        return 0;



    for (i = 0, j = idx; i < top * sizeof b->d[0]; i++, j += width) {

        ((unsigned char *)b->d)[i] = buf[j];

    if (window <= 3) {

        for (i = 0; i < top; i++, table += width) {

            BN_ULONG acc = 0;



            for (j = 0; j < width; j++) {

                acc |= table[j] &

                       ((BN_ULONG)0 - (constant_time_eq_int(j,idx)&1));

            }



            b->d[i] = acc;

        }

    } else {

        int xstride = 1 << (window - 2);

        BN_ULONG y0, y1, y2, y3;



        i = idx >> (window - 2);        /* equivalent of idx / xstride */

        idx &= xstride - 1;             /* equivalent of idx % xstride */



        y0 = (BN_ULONG)0 - (constant_time_eq_int(i,0)&1);

        y1 = (BN_ULONG)0 - (constant_time_eq_int(i,1)&1);

        y2 = (BN_ULONG)0 - (constant_time_eq_int(i,2)&1);

        y3 = (BN_ULONG)0 - (constant_time_eq_int(i,3)&1);



        for (i = 0; i < top; i++, table += width) {

            BN_ULONG acc = 0;



            for (j = 0; j < xstride; j++) {

                acc |= ( (table[j + 0 * xstride] & y0) |

                         (table[j + 1 * xstride] & y1) |

                         (table[j + 2 * xstride] & y2) |

                         (table[j + 3 * xstride] & y3) )

                       & ((BN_ULONG)0 - (constant_time_eq_int(j,idx)&1));

            }



            b->d[i] = acc;

        }

    }



    b->top = top;
@@ -1063,9 +1102,9 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p, 
    } else

#endif

    {

        if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&tmp, top, powerbuf, 0, numPowers))

        if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&tmp, top, powerbuf, 0, window))

            goto err;

        if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&am, top, powerbuf, 1, numPowers))

        if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&am, top, powerbuf, 1, window))

            goto err;



        /*
@@ -1077,15 +1116,15 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p, 
        if (window > 1) {

            if (!BN_mod_mul_montgomery(&tmp, &am, &am, mont, ctx))

                goto err;

            if (!MOD_EXP_CTIME_COPY_TO_PREBUF

                (&tmp, top, powerbuf, 2, numPowers))

            if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&tmp, top, powerbuf, 2,

                                              window))

                goto err;

            for (i = 3; i < numPowers; i++) {

                /* Calculate a^i = a^(i-1) * a */

                if (!BN_mod_mul_montgomery(&tmp, &am, &tmp, mont, ctx))

                    goto err;

                if (!MOD_EXP_CTIME_COPY_TO_PREBUF

                    (&tmp, top, powerbuf, i, numPowers))

                if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&tmp, top, powerbuf, i,

                                                  window))

                    goto err;

            }

        }
@@ -1093,8 +1132,8 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p, 
        bits--;

        for (wvalue = 0, i = bits % window; i >= 0; i--, bits--)

            wvalue = (wvalue << 1) + BN_is_bit_set(p, bits);

        if (!MOD_EXP_CTIME_COPY_FROM_PREBUF

            (&tmp, top, powerbuf, wvalue, numPowers))

        if (!MOD_EXP_CTIME_COPY_FROM_PREBUF(&tmp, top, powerbuf, wvalue,

                                            window))

            goto err;



        /*
@@ -1114,8 +1153,8 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p, 
            /*

             * Fetch the appropriate pre-computed value from the pre-buf

             */

            if (!MOD_EXP_CTIME_COPY_FROM_PREBUF

                (&am, top, powerbuf, wvalue, numPowers))

            if (!MOD_EXP_CTIME_COPY_FROM_PREBUF(&am, top, powerbuf, wvalue,

                                                window))

                goto err;



            /* Multiply the result into the intermediate result */