int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
                              const BIGNUM *m, BN_CTX *ctx,
                              BN_MONT_CTX *in_mont)
{
    int i, bits, ret = 0, window, wvalue;
    int top;
    BN_MONT_CTX *mont = NULL;

    int numPowers;
    unsigned char *powerbufFree = NULL;
    int powerbufLen = 0;
    unsigned char *powerbuf = NULL;
    BIGNUM tmp, am;
#if defined(SPARC_T4_MONT)
    unsigned int t4 = 0;
#endif

    bn_check_top(a);
    bn_check_top(p);
    bn_check_top(m);

    if (!BN_is_odd(m)) {
        BNerr(BN_F_BN_MOD_EXP_MONT_CONSTTIME, BN_R_CALLED_WITH_EVEN_MODULUS);
        return (0);
    }

    top = m->top;

    bits = BN_num_bits(p);
    if (bits == 0) {
        /* x**0 mod 1 is still zero. */
        if (BN_is_one(m)) {
            ret = 1;
            BN_zero(rr);
        } else {
            ret = BN_one(rr);
        }
        return ret;
    }

    BN_CTX_start(ctx);

    /*
     * Allocate a montgomery context if it was not supplied by the caller. If
     * this is not done, things will break in the montgomery part.
     */
    if (in_mont != NULL)
        mont = in_mont;
    else {
        if ((mont = BN_MONT_CTX_new()) == NULL)
            goto err;
        if (!BN_MONT_CTX_set(mont, m, ctx))
            goto err;
    }

#ifdef RSAZ_ENABLED
    /*
     * If the size of the operands allow it, perform the optimized
     * RSAZ exponentiation. For further information see
     * crypto/bn/rsaz_exp.c and accompanying assembly modules.
     */
    if ((16 == a->top) && (16 == p->top) && (BN_num_bits(m) == 1024)
        && rsaz_avx2_eligible()) {
        if (NULL == bn_wexpand(rr, 16))
            goto err;
        RSAZ_1024_mod_exp_avx2(rr->d, a->d, p->d, m->d, mont->RR.d,
                               mont->n0[0]);
        rr->top = 16;
        rr->neg = 0;
        bn_correct_top(rr);
        ret = 1;
        goto err;
    } else if ((8 == a->top) && (8 == p->top) && (BN_num_bits(m) == 512)) {
        if (NULL == bn_wexpand(rr, 8))
            goto err;
        RSAZ_512_mod_exp(rr->d, a->d, p->d, m->d, mont->n0[0], mont->RR.d);
        rr->top = 8;
        rr->neg = 0;
        bn_correct_top(rr);
        ret = 1;
        goto err;
    }
#endif

    /* Get the window size to use with size of p. */
    window = BN_window_bits_for_ctime_exponent_size(bits);
#if defined(SPARC_T4_MONT)
    if (window >= 5 && (top & 15) == 0 && top <= 64 &&
        (OPENSSL_sparcv9cap_P[1] & (CFR_MONTMUL | CFR_MONTSQR)) ==
        (CFR_MONTMUL | CFR_MONTSQR) && (t4 = OPENSSL_sparcv9cap_P[0]))
        window = 5;
    else
#endif
#if defined(OPENSSL_BN_ASM_MONT5)
    if (window >= 5) {
        window = 5;             /* ~5% improvement for RSA2048 sign, and even
                                 * for RSA4096 */
        if ((top & 7) == 0)
            powerbufLen += 2 * top * sizeof(m->d[0]);
    }
#endif
    (void)0;

    /*
     * Allocate a buffer large enough to hold all of the pre-computed powers
     * of am, am itself and tmp.
     */
    numPowers = 1 << window;
    powerbufLen += sizeof(m->d[0]) * (top * numPowers +
                                      ((2 * top) >
                                       numPowers ? (2 * top) : numPowers));
#ifdef alloca
    if (powerbufLen < 3072)
        powerbufFree =
            alloca(powerbufLen + MOD_EXP_CTIME_MIN_CACHE_LINE_WIDTH);
    else
#endif
        if ((powerbufFree =
             OPENSSL_malloc(powerbufLen + MOD_EXP_CTIME_MIN_CACHE_LINE_WIDTH))
            == NULL)
        goto err;

    powerbuf = MOD_EXP_CTIME_ALIGN(powerbufFree);
    memset(powerbuf, 0, powerbufLen);

#ifdef alloca
    if (powerbufLen < 3072)
        powerbufFree = NULL;
#endif

    /* lay down tmp and am right after powers table */
    tmp.d = (BN_ULONG *)(powerbuf + sizeof(m->d[0]) * top * numPowers);
    am.d = tmp.d + top;
    tmp.top = am.top = 0;
    tmp.dmax = am.dmax = top;
    tmp.neg = am.neg = 0;
    tmp.flags = am.flags = BN_FLG_STATIC_DATA;

    /* prepare a^0 in Montgomery domain */
#if 1                           /* by Shay Gueron's suggestion */
    if (m->d[top - 1] & (((BN_ULONG)1) << (BN_BITS2 - 1))) {
        /* 2^(top*BN_BITS2) - m */
        tmp.d[0] = (0 - m->d[0]) & BN_MASK2;
        for (i = 1; i < top; i++)
            tmp.d[i] = (~m->d[i]) & BN_MASK2;
        tmp.top = top;
    } else
#endif
    if (!BN_to_montgomery(&tmp, BN_value_one(), mont, ctx))
        goto err;

    /* prepare a^1 in Montgomery domain */
    if (a->neg || BN_ucmp(a, m) >= 0) {
        if (!BN_mod(&am, a, m, ctx))
            goto err;
        if (!BN_to_montgomery(&am, &am, mont, ctx))
            goto err;
    } else if (!BN_to_montgomery(&am, a, mont, ctx))
        goto err;

#if defined(SPARC_T4_MONT)
    if (t4) {
        typedef int (*bn_pwr5_mont_f) (BN_ULONG *tp, const BN_ULONG *np,
                                       const BN_ULONG *n0, const void *table,
                                       int power, int bits);
        int bn_pwr5_mont_t4_8(BN_ULONG *tp, const BN_ULONG *np,
                              const BN_ULONG *n0, const void *table,
                              int power, int bits);
        int bn_pwr5_mont_t4_16(BN_ULONG *tp, const BN_ULONG *np,
                               const BN_ULONG *n0, const void *table,
                               int power, int bits);
        int bn_pwr5_mont_t4_24(BN_ULONG *tp, const BN_ULONG *np,
                               const BN_ULONG *n0, const void *table,
                               int power, int bits);
        int bn_pwr5_mont_t4_32(BN_ULONG *tp, const BN_ULONG *np,
                               const BN_ULONG *n0, const void *table,
                               int power, int bits);
        static const bn_pwr5_mont_f pwr5_funcs[4] = {
            bn_pwr5_mont_t4_8, bn_pwr5_mont_t4_16,
            bn_pwr5_mont_t4_24, bn_pwr5_mont_t4_32
        };
        bn_pwr5_mont_f pwr5_worker = pwr5_funcs[top / 16 - 1];

        typedef int (*bn_mul_mont_f) (BN_ULONG *rp, const BN_ULONG *ap,
                                      const void *bp, const BN_ULONG *np,
                                      const BN_ULONG *n0);
        int bn_mul_mont_t4_8(BN_ULONG *rp, const BN_ULONG *ap, const void *bp,
                             const BN_ULONG *np, const BN_ULONG *n0);
        int bn_mul_mont_t4_16(BN_ULONG *rp, const BN_ULONG *ap,
                              const void *bp, const BN_ULONG *np,
                              const BN_ULONG *n0);
        int bn_mul_mont_t4_24(BN_ULONG *rp, const BN_ULONG *ap,
                              const void *bp, const BN_ULONG *np,
                              const BN_ULONG *n0);
        int bn_mul_mont_t4_32(BN_ULONG *rp, const BN_ULONG *ap,
                              const void *bp, const BN_ULONG *np,
                              const BN_ULONG *n0);
        static const bn_mul_mont_f mul_funcs[4] = {
            bn_mul_mont_t4_8, bn_mul_mont_t4_16,
            bn_mul_mont_t4_24, bn_mul_mont_t4_32
        };
        bn_mul_mont_f mul_worker = mul_funcs[top / 16 - 1];

        void bn_mul_mont_vis3(BN_ULONG *rp, const BN_ULONG *ap,
                              const void *bp, const BN_ULONG *np,
                              const BN_ULONG *n0, int num);
        void bn_mul_mont_t4(BN_ULONG *rp, const BN_ULONG *ap,
                            const void *bp, const BN_ULONG *np,
                            const BN_ULONG *n0, int num);
        void bn_mul_mont_gather5_t4(BN_ULONG *rp, const BN_ULONG *ap,
                                    const void *table, const BN_ULONG *np,
                                    const BN_ULONG *n0, int num, int power);
        void bn_flip_n_scatter5_t4(const BN_ULONG *inp, size_t num,
                                   void *table, size_t power);
        void bn_gather5_t4(BN_ULONG *out, size_t num,
                           void *table, size_t power);
        void bn_flip_t4(BN_ULONG *dst, BN_ULONG *src, size_t num);

        BN_ULONG *np = mont->N.d, *n0 = mont->n0;
        int stride = 5 * (6 - (top / 16 - 1)); /* multiple of 5, but less
                                                * than 32 */

        /*
         * 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_flip_n_scatter5_t4(tmp.d, top, powerbuf, 0);
        bn_flip_n_scatter5_t4(am.d, top, powerbuf, 1);
        if (!(*mul_worker) (tmp.d, am.d, am.d, np, n0) &&
            !(*mul_worker) (tmp.d, am.d, am.d, np, n0))
            bn_mul_mont_vis3(tmp.d, am.d, am.d, np, n0, top);
        bn_flip_n_scatter5_t4(tmp.d, top, powerbuf, 2);

        for (i = 3; i < 32; i++) {
            /* Calculate a^i = a^(i-1) * a */
            if (!(*mul_worker) (tmp.d, tmp.d, am.d, np, n0) &&
                !(*mul_worker) (tmp.d, tmp.d, am.d, np, n0))
                bn_mul_mont_vis3(tmp.d, tmp.d, am.d, np, n0, top);
            bn_flip_n_scatter5_t4(tmp.d, top, powerbuf, i);
        }

        /* switch to 64-bit domain */
        np = alloca(top * sizeof(BN_ULONG));
        top /= 2;
        bn_flip_t4(np, mont->N.d, top);

        bits--;
        for (wvalue = 0, i = bits % 5; i >= 0; i--, bits--)
            wvalue = (wvalue << 1) + BN_is_bit_set(p, bits);
        bn_gather5_t4(tmp.d, top, powerbuf, wvalue);

        /*
         * Scan the exponent one window at a time starting from the most
         * significant bits.
         */
        while (bits >= 0) {
            if (bits < stride)
                stride = bits + 1;
            bits -= stride;
            wvalue = bn_get_bits(p, bits + 1);

            if ((*pwr5_worker) (tmp.d, np, n0, powerbuf, wvalue, stride))
                continue;
            /* retry once and fall back */
            if ((*pwr5_worker) (tmp.d, np, n0, powerbuf, wvalue, stride))
                continue;

            bits += stride - 5;
            wvalue >>= stride - 5;
            wvalue &= 31;
            bn_mul_mont_t4(tmp.d, tmp.d, tmp.d, np, n0, top);
            bn_mul_mont_t4(tmp.d, tmp.d, tmp.d, np, n0, top);
            bn_mul_mont_t4(tmp.d, tmp.d, tmp.d, np, n0, top);
            bn_mul_mont_t4(tmp.d, tmp.d, tmp.d, np, n0, top);
            bn_mul_mont_t4(tmp.d, tmp.d, tmp.d, np, n0, top);
            bn_mul_mont_gather5_t4(tmp.d, tmp.d, powerbuf, np, n0, top,
                                   wvalue);
        }

        bn_flip_t4(tmp.d, tmp.d, top);
        top *= 2;
        /* back to 32-bit domain */
        tmp.top = top;
        bn_correct_top(&tmp);
        OPENSSL_cleanse(np, top * sizeof(BN_ULONG));
    } else
#endif
#if defined(OPENSSL_BN_ASM_MONT5)
    if (window == 5 && top > 1) {
        /*
         * This optimization uses ideas from http://eprint.iacr.org/2011/239,
         * specifically optimization of cache-timing attack countermeasures
         * and pre-computation optimization.
         */

        /*
         * Dedicated window==4 case improves 512-bit RSA sign by ~15%, but as
         * 512-bit RSA is hardly relevant, we omit it to spare size...
         */
        void bn_mul_mont_gather5(BN_ULONG *rp, const BN_ULONG *ap,
                                 const void *table, const BN_ULONG *np,
                                 const BN_ULONG *n0, int num, int power);
        void bn_scatter5(const BN_ULONG *inp, size_t num,
                         void *table, size_t power);
        void bn_gather5(BN_ULONG *out, size_t num, void *table, size_t power);
        void bn_power5(BN_ULONG *rp, const BN_ULONG *ap,
                       const void *table, const BN_ULONG *np,
                       const BN_ULONG *n0, int num, int power);
        int bn_get_bits5(const BN_ULONG *ap, int off);
        int bn_from_montgomery(BN_ULONG *rp, const BN_ULONG *ap,
                               const BN_ULONG *not_used, const BN_ULONG *np,
                               const BN_ULONG *n0, int num);

        BN_ULONG *np = mont->N.d, *n0 = mont->n0, *np2;

        /*
         * 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;

        if (top & 7)
            np2 = np;
        else
            for (np2 = am.d + top, i = 0; i < top; i++)
                np2[2 * i] = np[i];

        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);
        bn_scatter5(tmp.d, top, powerbuf, 2);

# if 0
        for (i = 3; i < 32; i++) {
            /* Calculate a^i = a^(i-1) * a */
            bn_mul_mont_gather5(tmp.d, am.d, powerbuf, np2, n0, top, i - 1);
            bn_scatter5(tmp.d, top, powerbuf, i);
        }
# else
        /* same as above, but uses squaring for 1/2 of operations */
        for (i = 4; i < 32; i *= 2) {
            bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
            bn_scatter5(tmp.d, top, powerbuf, i);
        }
        for (i = 3; i < 8; i += 2) {
            int j;
            bn_mul_mont_gather5(tmp.d, am.d, powerbuf, np2, n0, top, i - 1);
            bn_scatter5(tmp.d, top, powerbuf, i);
            for (j = 2 * i; j < 32; j *= 2) {
                bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
                bn_scatter5(tmp.d, top, powerbuf, j);
            }
        }
        for (; i < 16; i += 2) {
            bn_mul_mont_gather5(tmp.d, am.d, powerbuf, np2, n0, top, i - 1);
            bn_scatter5(tmp.d, top, powerbuf, i);
            bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
            bn_scatter5(tmp.d, top, powerbuf, 2 * i);
        }
        for (; i < 32; i += 2) {
            bn_mul_mont_gather5(tmp.d, am.d, powerbuf, np2, n0, top, i - 1);
            bn_scatter5(tmp.d, top, powerbuf, i);
        }
# endif
        bits--;
        for (wvalue = 0, i = bits % 5; i >= 0; i--, bits--)
            wvalue = (wvalue << 1) + BN_is_bit_set(p, bits);
        bn_gather5(tmp.d, top, powerbuf, wvalue);

        /*
         * Scan the exponent one window at a time starting from the most
         * significant bits.
         */
        if (top & 7)
            while (bits >= 0) {
                for (wvalue = 0, i = 0; i < 5; i++, bits--)
                    wvalue = (wvalue << 1) + BN_is_bit_set(p, bits);

                bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
                bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
                bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
                bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
                bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
                bn_mul_mont_gather5(tmp.d, tmp.d, powerbuf, np, n0, top,
                                    wvalue);
        } else {
            while (bits >= 0) {
                wvalue = bn_get_bits5(p->d, bits - 4);
                bits -= 5;
                bn_power5(tmp.d, tmp.d, powerbuf, np2, n0, top, wvalue);
            }
        }

        ret = bn_from_montgomery(tmp.d, tmp.d, NULL, np2, n0, top);
        tmp.top = top;
        bn_correct_top(&tmp);
        if (ret) {
            if (!BN_copy(rr, &tmp))
                ret = 0;
            goto err;           /* non-zero ret means it's not error */
        }
    } else
#endif
    {
        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, window))
            goto err;

        /*
         * If the window size is greater than 1, then calculate
         * val[i=2..2^winsize-1]. Powers are computed as a*a^(i-1) (even
         * powers could instead be computed as (a^(i/2))^2 to use the slight
         * performance advantage of sqr over mul).
         */
        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,
                                              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,
                                                  window))
                    goto err;
            }
        }

        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,
                                            window))
            goto err;

        /*
         * Scan the exponent one window at a time starting from the most
         * significant bits.
         */
        while (bits >= 0) {
            wvalue = 0;         /* The 'value' of the window */

            /* Scan the window, squaring the result as we go */
            for (i = 0; i < window; i++, bits--) {
                if (!BN_mod_mul_montgomery(&tmp, &tmp, &tmp, mont, ctx))
                    goto err;
                wvalue = (wvalue << 1) + BN_is_bit_set(p, bits);
            }

            /*
             * Fetch the appropriate pre-computed value from the pre-buf
             */
            if (!MOD_EXP_CTIME_COPY_FROM_PREBUF(&am, top, powerbuf, wvalue,
                                                window))
                goto err;

            /* Multiply the result into the intermediate result */
            if (!BN_mod_mul_montgomery(&tmp, &tmp, &am, mont, ctx))
                goto err;
        }
    }

    /* Convert the final result from montgomery to standard format */
#if defined(SPARC_T4_MONT)
    if (OPENSSL_sparcv9cap_P[0] & (SPARCV9_VIS3 | SPARCV9_PREFER_FPU)) {
        am.d[0] = 1;            /* borrow am */
        for (i = 1; i < top; i++)
            am.d[i] = 0;
        if (!BN_mod_mul_montgomery(rr, &tmp, &am, mont, ctx))
            goto err;
    } else
#endif
    if (!BN_from_montgomery(rr, &tmp, mont, ctx))
        goto err;
    ret = 1;
 err:
    if (in_mont == NULL)
        BN_MONT_CTX_free(mont);
    if (powerbuf != NULL) {
        OPENSSL_cleanse(powerbuf, powerbufLen);
        OPENSSL_free(powerbufFree);
    }
    BN_CTX_end(ctx);
    return (ret);
}
The product exposes sensitive information to an actor that is not explicitly authorized to have access to that information.