mbedTLS: Update to new LTS v3.6.0

Keep module compatibility with mbedtls 2.x (old LTS branch).

A patch has been added to allow compiling after removing all the `psa_*`
files from the library folder (will look into upstreaming it).

Note: mbedTLS 3.6 finally enabled TLSv1.3 by default, but it requires
some module changes, and to enable PSA crypto (new "standard" API
specification), so it might be best done in a separate commit/PR.

1 files changed, 895 insertions, 0 deletions

diff --git a/thirdparty/mbedtls/library/bignum_core.c b/thirdparty/mbedtls/library/bignum_core.c
new file mode 100644
index 0000000000..1a3e0b9b6f
--- /dev/null
+++ b/thirdparty/mbedtls/library/bignum_core.c
@@ -0,0 +1,895 @@
+/*
+ *  Core bignum functions
+ *
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
+ */
+
+#include "common.h"
+
+#if defined(MBEDTLS_BIGNUM_C)
+
+#include <string.h>
+
+#include "mbedtls/error.h"
+#include "mbedtls/platform_util.h"
+#include "constant_time_internal.h"
+
+#include "mbedtls/platform.h"
+
+#include "bignum_core.h"
+#include "bn_mul.h"
+#include "constant_time_internal.h"
+
+size_t mbedtls_mpi_core_clz(mbedtls_mpi_uint a)
+{
+#if defined(__has_builtin)
+#if (MBEDTLS_MPI_UINT_MAX == UINT_MAX) && __has_builtin(__builtin_clz)
+    #define core_clz __builtin_clz
+#elif (MBEDTLS_MPI_UINT_MAX == ULONG_MAX) && __has_builtin(__builtin_clzl)
+    #define core_clz __builtin_clzl
+#elif (MBEDTLS_MPI_UINT_MAX == ULLONG_MAX) && __has_builtin(__builtin_clzll)
+    #define core_clz __builtin_clzll
+#endif
+#endif
+#if defined(core_clz)
+    return (size_t) core_clz(a);
+#else
+    size_t j;
+    mbedtls_mpi_uint mask = (mbedtls_mpi_uint) 1 << (biL - 1);
+
+    for (j = 0; j < biL; j++) {
+        if (a & mask) {
+            break;
+        }
+
+        mask >>= 1;
+    }
+
+    return j;
+#endif
+}
+
+size_t mbedtls_mpi_core_bitlen(const mbedtls_mpi_uint *A, size_t A_limbs)
+{
+    int i;
+    size_t j;
+
+    for (i = ((int) A_limbs) - 1; i >= 0; i--) {
+        if (A[i] != 0) {
+            j = biL - mbedtls_mpi_core_clz(A[i]);
+            return (i * biL) + j;
+        }
+    }
+
+    return 0;
+}
+
+static mbedtls_mpi_uint mpi_bigendian_to_host(mbedtls_mpi_uint a)
+{
+    if (MBEDTLS_IS_BIG_ENDIAN) {
+        /* Nothing to do on bigendian systems. */
+        return a;
+    } else {
+#if defined(MBEDTLS_HAVE_INT32)
+        return (mbedtls_mpi_uint) MBEDTLS_BSWAP32(a);
+#elif defined(MBEDTLS_HAVE_INT64)
+        return (mbedtls_mpi_uint) MBEDTLS_BSWAP64(a);
+#endif
+    }
+}
+
+void mbedtls_mpi_core_bigendian_to_host(mbedtls_mpi_uint *A,
+                                        size_t A_limbs)
+{
+    mbedtls_mpi_uint *cur_limb_left;
+    mbedtls_mpi_uint *cur_limb_right;
+    if (A_limbs == 0) {
+        return;
+    }
+
+    /*
+     * Traverse limbs and
+     * - adapt byte-order in each limb
+     * - swap the limbs themselves.
+     * For that, simultaneously traverse the limbs from left to right
+     * and from right to left, as long as the left index is not bigger
+     * than the right index (it's not a problem if limbs is odd and the
+     * indices coincide in the last iteration).
+     */
+    for (cur_limb_left = A, cur_limb_right = A + (A_limbs - 1);
+         cur_limb_left <= cur_limb_right;
+         cur_limb_left++, cur_limb_right--) {
+        mbedtls_mpi_uint tmp;
+        /* Note that if cur_limb_left == cur_limb_right,
+         * this code effectively swaps the bytes only once. */
+        tmp             = mpi_bigendian_to_host(*cur_limb_left);
+        *cur_limb_left  = mpi_bigendian_to_host(*cur_limb_right);
+        *cur_limb_right = tmp;
+    }
+}
+
+/* Whether min <= A, in constant time.
+ * A_limbs must be at least 1. */
+mbedtls_ct_condition_t mbedtls_mpi_core_uint_le_mpi(mbedtls_mpi_uint min,
+                                                    const mbedtls_mpi_uint *A,
+                                                    size_t A_limbs)
+{
+    /* min <= least significant limb? */
+    mbedtls_ct_condition_t min_le_lsl = mbedtls_ct_uint_ge(A[0], min);
+
+    /* limbs other than the least significant one are all zero? */
+    mbedtls_ct_condition_t msll_mask = MBEDTLS_CT_FALSE;
+    for (size_t i = 1; i < A_limbs; i++) {
+        msll_mask = mbedtls_ct_bool_or(msll_mask, mbedtls_ct_bool(A[i]));
+    }
+
+    /* min <= A iff the lowest limb of A is >= min or the other limbs
+     * are not all zero. */
+    return mbedtls_ct_bool_or(msll_mask, min_le_lsl);
+}
+
+mbedtls_ct_condition_t mbedtls_mpi_core_lt_ct(const mbedtls_mpi_uint *A,
+                                              const mbedtls_mpi_uint *B,
+                                              size_t limbs)
+{
+    mbedtls_ct_condition_t ret = MBEDTLS_CT_FALSE, cond = MBEDTLS_CT_FALSE, done = MBEDTLS_CT_FALSE;
+
+    for (size_t i = limbs; i > 0; i--) {
+        /*
+         * If B[i - 1] < A[i - 1] then A < B is false and the result must
+         * remain 0.
+         *
+         * Again even if we can make a decision, we just mark the result and
+         * the fact that we are done and continue looping.
+         */
+        cond = mbedtls_ct_uint_lt(B[i - 1], A[i - 1]);
+        done = mbedtls_ct_bool_or(done, cond);
+
+        /*
+         * If A[i - 1] < B[i - 1] then A < B is true.
+         *
+         * Again even if we can make a decision, we just mark the result and
+         * the fact that we are done and continue looping.
+         */
+        cond = mbedtls_ct_uint_lt(A[i - 1], B[i - 1]);
+        ret  = mbedtls_ct_bool_or(ret, mbedtls_ct_bool_and(cond, mbedtls_ct_bool_not(done)));
+        done = mbedtls_ct_bool_or(done, cond);
+    }
+
+    /*
+     * If all the limbs were equal, then the numbers are equal, A < B is false
+     * and leaving the result 0 is correct.
+     */
+
+    return ret;
+}
+
+void mbedtls_mpi_core_cond_assign(mbedtls_mpi_uint *X,
+                                  const mbedtls_mpi_uint *A,
+                                  size_t limbs,
+                                  mbedtls_ct_condition_t assign)
+{
+    if (X == A) {
+        return;
+    }
+
+    /* This function is very performance-sensitive for RSA. For this reason
+     * we have the loop below, instead of calling mbedtls_ct_memcpy_if
+     * (this is more optimal since here we don't have to handle the case where
+     * we copy awkwardly sized data).
+     */
+    for (size_t i = 0; i < limbs; i++) {
+        X[i] = mbedtls_ct_mpi_uint_if(assign, A[i], X[i]);
+    }
+}
+
+void mbedtls_mpi_core_cond_swap(mbedtls_mpi_uint *X,
+                                mbedtls_mpi_uint *Y,
+                                size_t limbs,
+                                mbedtls_ct_condition_t swap)
+{
+    if (X == Y) {
+        return;
+    }
+
+    for (size_t i = 0; i < limbs; i++) {
+        mbedtls_mpi_uint tmp = X[i];
+        X[i] = mbedtls_ct_mpi_uint_if(swap, Y[i], X[i]);
+        Y[i] = mbedtls_ct_mpi_uint_if(swap, tmp, Y[i]);
+    }
+}
+
+int mbedtls_mpi_core_read_le(mbedtls_mpi_uint *X,
+                             size_t X_limbs,
+                             const unsigned char *input,
+                             size_t input_length)
+{
+    const size_t limbs = CHARS_TO_LIMBS(input_length);
+
+    if (X_limbs < limbs) {
+        return MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL;
+    }
+
+    if (X != NULL) {
+        memset(X, 0, X_limbs * ciL);
+
+        for (size_t i = 0; i < input_length; i++) {
+            size_t offset = ((i % ciL) << 3);
+            X[i / ciL] |= ((mbedtls_mpi_uint) input[i]) << offset;
+        }
+    }
+
+    return 0;
+}
+
+int mbedtls_mpi_core_read_be(mbedtls_mpi_uint *X,
+                             size_t X_limbs,
+                             const unsigned char *input,
+                             size_t input_length)
+{
+    const size_t limbs = CHARS_TO_LIMBS(input_length);
+
+    if (X_limbs < limbs) {
+        return MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL;
+    }
+
+    /* If X_limbs is 0, input_length must also be 0 (from previous test).
+     * Nothing to do. */
+    if (X_limbs == 0) {
+        return 0;
+    }
+
+    memset(X, 0, X_limbs * ciL);
+
+    /* memcpy() with (NULL, 0) is undefined behaviour */
+    if (input_length != 0) {
+        size_t overhead = (X_limbs * ciL) - input_length;
+        unsigned char *Xp = (unsigned char *) X;
+        memcpy(Xp + overhead, input, input_length);
+    }
+
+    mbedtls_mpi_core_bigendian_to_host(X, X_limbs);
+
+    return 0;
+}
+
+int mbedtls_mpi_core_write_le(const mbedtls_mpi_uint *A,
+                              size_t A_limbs,
+                              unsigned char *output,
+                              size_t output_length)
+{
+    size_t stored_bytes = A_limbs * ciL;
+    size_t bytes_to_copy;
+
+    if (stored_bytes < output_length) {
+        bytes_to_copy = stored_bytes;
+    } else {
+        bytes_to_copy = output_length;
+
+        /* The output buffer is smaller than the allocated size of A.
+         * However A may fit if its leading bytes are zero. */
+        for (size_t i = bytes_to_copy; i < stored_bytes; i++) {
+            if (GET_BYTE(A, i) != 0) {
+                return MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL;
+            }
+        }
+    }
+
+    for (size_t i = 0; i < bytes_to_copy; i++) {
+        output[i] = GET_BYTE(A, i);
+    }
+
+    if (stored_bytes < output_length) {
+        /* Write trailing 0 bytes */
+        memset(output + stored_bytes, 0, output_length - stored_bytes);
+    }
+
+    return 0;
+}
+
+int mbedtls_mpi_core_write_be(const mbedtls_mpi_uint *X,
+                              size_t X_limbs,
+                              unsigned char *output,
+                              size_t output_length)
+{
+    size_t stored_bytes;
+    size_t bytes_to_copy;
+    unsigned char *p;
+
+    stored_bytes = X_limbs * ciL;
+
+    if (stored_bytes < output_length) {
+        /* There is enough space in the output buffer. Write initial
+         * null bytes and record the position at which to start
+         * writing the significant bytes. In this case, the execution
+         * trace of this function does not depend on the value of the
+         * number. */
+        bytes_to_copy = stored_bytes;
+        p = output + output_length - stored_bytes;
+        memset(output, 0, output_length - stored_bytes);
+    } else {
+        /* The output buffer is smaller than the allocated size of X.
+         * However X may fit if its leading bytes are zero. */
+        bytes_to_copy = output_length;
+        p = output;
+        for (size_t i = bytes_to_copy; i < stored_bytes; i++) {
+            if (GET_BYTE(X, i) != 0) {
+                return MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL;
+            }
+        }
+    }
+
+    for (size_t i = 0; i < bytes_to_copy; i++) {
+        p[bytes_to_copy - i - 1] = GET_BYTE(X, i);
+    }
+
+    return 0;
+}
+
+void mbedtls_mpi_core_shift_r(mbedtls_mpi_uint *X, size_t limbs,
+                              size_t count)
+{
+    size_t i, v0, v1;
+    mbedtls_mpi_uint r0 = 0, r1;
+
+    v0 = count /  biL;
+    v1 = count & (biL - 1);
+
+    if (v0 > limbs || (v0 == limbs && v1 > 0)) {
+        memset(X, 0, limbs * ciL);
+        return;
+    }
+
+    /*
+     * shift by count / limb_size
+     */
+    if (v0 > 0) {
+        for (i = 0; i < limbs - v0; i++) {
+            X[i] = X[i + v0];
+        }
+
+        for (; i < limbs; i++) {
+            X[i] = 0;
+        }
+    }
+
+    /*
+     * shift by count % limb_size
+     */
+    if (v1 > 0) {
+        for (i = limbs; i > 0; i--) {
+            r1 = X[i - 1] << (biL - v1);
+            X[i - 1] >>= v1;
+            X[i - 1] |= r0;
+            r0 = r1;
+        }
+    }
+}
+
+void mbedtls_mpi_core_shift_l(mbedtls_mpi_uint *X, size_t limbs,
+                              size_t count)
+{
+    size_t i, v0, v1;
+    mbedtls_mpi_uint r0 = 0, r1;
+
+    v0 = count / (biL);
+    v1 = count & (biL - 1);
+
+    /*
+     * shift by count / limb_size
+     */
+    if (v0 > 0) {
+        for (i = limbs; i > v0; i--) {
+            X[i - 1] = X[i - v0 - 1];
+        }
+
+        for (; i > 0; i--) {
+            X[i - 1] = 0;
+        }
+    }
+
+    /*
+     * shift by count % limb_size
+     */
+    if (v1 > 0) {
+        for (i = v0; i < limbs; i++) {
+            r1 = X[i] >> (biL - v1);
+            X[i] <<= v1;
+            X[i] |= r0;
+            r0 = r1;
+        }
+    }
+}
+
+mbedtls_mpi_uint mbedtls_mpi_core_add(mbedtls_mpi_uint *X,
+                                      const mbedtls_mpi_uint *A,
+                                      const mbedtls_mpi_uint *B,
+                                      size_t limbs)
+{
+    mbedtls_mpi_uint c = 0;
+
+    for (size_t i = 0; i < limbs; i++) {
+        mbedtls_mpi_uint t = c + A[i];
+        c = (t < A[i]);
+        t += B[i];
+        c += (t < B[i]);
+        X[i] = t;
+    }
+
+    return c;
+}
+
+mbedtls_mpi_uint mbedtls_mpi_core_add_if(mbedtls_mpi_uint *X,
+                                         const mbedtls_mpi_uint *A,
+                                         size_t limbs,
+                                         unsigned cond)
+{
+    mbedtls_mpi_uint c = 0;
+
+    mbedtls_ct_condition_t do_add = mbedtls_ct_bool(cond);
+
+    for (size_t i = 0; i < limbs; i++) {
+        mbedtls_mpi_uint add = mbedtls_ct_mpi_uint_if_else_0(do_add, A[i]);
+        mbedtls_mpi_uint t = c + X[i];
+        c = (t < X[i]);
+        t += add;
+        c += (t < add);
+        X[i] = t;
+    }
+
+    return c;
+}
+
+mbedtls_mpi_uint mbedtls_mpi_core_sub(mbedtls_mpi_uint *X,
+                                      const mbedtls_mpi_uint *A,
+                                      const mbedtls_mpi_uint *B,
+                                      size_t limbs)
+{
+    mbedtls_mpi_uint c = 0;
+
+    for (size_t i = 0; i < limbs; i++) {
+        mbedtls_mpi_uint z = (A[i] < c);
+        mbedtls_mpi_uint t = A[i] - c;
+        c = (t < B[i]) + z;
+        X[i] = t - B[i];
+    }
+
+    return c;
+}
+
+mbedtls_mpi_uint mbedtls_mpi_core_mla(mbedtls_mpi_uint *d, size_t d_len,
+                                      const mbedtls_mpi_uint *s, size_t s_len,
+                                      mbedtls_mpi_uint b)
+{
+    mbedtls_mpi_uint c = 0; /* carry */
+    /*
+     * It is a documented precondition of this function that d_len >= s_len.
+     * If that's not the case, we swap these round: this turns what would be
+     * a buffer overflow into an incorrect result.
+     */
+    if (d_len < s_len) {
+        s_len = d_len;
+    }
+    size_t excess_len = d_len - s_len;
+    size_t steps_x8 = s_len / 8;
+    size_t steps_x1 = s_len & 7;
+
+    while (steps_x8--) {
+        MULADDC_X8_INIT
+        MULADDC_X8_CORE
+            MULADDC_X8_STOP
+    }
+
+    while (steps_x1--) {
+        MULADDC_X1_INIT
+        MULADDC_X1_CORE
+            MULADDC_X1_STOP
+    }
+
+    while (excess_len--) {
+        *d += c;
+        c = (*d < c);
+        d++;
+    }
+
+    return c;
+}
+
+void mbedtls_mpi_core_mul(mbedtls_mpi_uint *X,
+                          const mbedtls_mpi_uint *A, size_t A_limbs,
+                          const mbedtls_mpi_uint *B, size_t B_limbs)
+{
+    memset(X, 0, (A_limbs + B_limbs) * ciL);
+
+    for (size_t i = 0; i < B_limbs; i++) {
+        (void) mbedtls_mpi_core_mla(X + i, A_limbs + 1, A, A_limbs, B[i]);
+    }
+}
+
+/*
+ * Fast Montgomery initialization (thanks to Tom St Denis).
+ */
+mbedtls_mpi_uint mbedtls_mpi_core_montmul_init(const mbedtls_mpi_uint *N)
+{
+    mbedtls_mpi_uint x = N[0];
+
+    x += ((N[0] + 2) & 4) << 1;
+
+    for (unsigned int i = biL; i >= 8; i /= 2) {
+        x *= (2 - (N[0] * x));
+    }
+
+    return ~x + 1;
+}
+
+void mbedtls_mpi_core_montmul(mbedtls_mpi_uint *X,
+                              const mbedtls_mpi_uint *A,
+                              const mbedtls_mpi_uint *B,
+                              size_t B_limbs,
+                              const mbedtls_mpi_uint *N,
+                              size_t AN_limbs,
+                              mbedtls_mpi_uint mm,
+                              mbedtls_mpi_uint *T)
+{
+    memset(T, 0, (2 * AN_limbs + 1) * ciL);
+
+    for (size_t i = 0; i < AN_limbs; i++) {
+        /* T = (T + u0*B + u1*N) / 2^biL */
+        mbedtls_mpi_uint u0 = A[i];
+        mbedtls_mpi_uint u1 = (T[0] + u0 * B[0]) * mm;
+
+        (void) mbedtls_mpi_core_mla(T, AN_limbs + 2, B, B_limbs, u0);
+        (void) mbedtls_mpi_core_mla(T, AN_limbs + 2, N, AN_limbs, u1);
+
+        T++;
+    }
+
+    /*
+     * The result we want is (T >= N) ? T - N : T.
+     *
+     * For better constant-time properties in this function, we always do the
+     * subtraction, with the result in X.
+     *
+     * We also look to see if there was any carry in the final additions in the
+     * loop above.
+     */
+
+    mbedtls_mpi_uint carry  = T[AN_limbs];
+    mbedtls_mpi_uint borrow = mbedtls_mpi_core_sub(X, T, N, AN_limbs);
+
+    /*
+     * Using R as the Montgomery radix (auxiliary modulus) i.e. 2^(biL*AN_limbs):
+     *
+     * T can be in one of 3 ranges:
+     *
+     * 1) T < N      : (carry, borrow) = (0, 1): we want T
+     * 2) N <= T < R : (carry, borrow) = (0, 0): we want X
+     * 3) T >= R     : (carry, borrow) = (1, 1): we want X
+     *
+     * and (carry, borrow) = (1, 0) can't happen.
+     *
+     * So the correct return value is already in X if (carry ^ borrow) = 0,
+     * but is in (the lower AN_limbs limbs of) T if (carry ^ borrow) = 1.
+     */
+    mbedtls_ct_memcpy_if(mbedtls_ct_bool(carry ^ borrow),
+                         (unsigned char *) X,
+                         (unsigned char *) T,
+                         NULL,
+                         AN_limbs * sizeof(mbedtls_mpi_uint));
+}
+
+int mbedtls_mpi_core_get_mont_r2_unsafe(mbedtls_mpi *X,
+                                        const mbedtls_mpi *N)
+{
+    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
+
+    MBEDTLS_MPI_CHK(mbedtls_mpi_lset(X, 1));
+    MBEDTLS_MPI_CHK(mbedtls_mpi_shift_l(X, N->n * 2 * biL));
+    MBEDTLS_MPI_CHK(mbedtls_mpi_mod_mpi(X, X, N));
+    MBEDTLS_MPI_CHK(mbedtls_mpi_shrink(X, N->n));
+
+cleanup:
+    return ret;
+}
+
+MBEDTLS_STATIC_TESTABLE
+void mbedtls_mpi_core_ct_uint_table_lookup(mbedtls_mpi_uint *dest,
+                                           const mbedtls_mpi_uint *table,
+                                           size_t limbs,
+                                           size_t count,
+                                           size_t index)
+{
+    for (size_t i = 0; i < count; i++, table += limbs) {
+        mbedtls_ct_condition_t assign = mbedtls_ct_uint_eq(i, index);
+        mbedtls_mpi_core_cond_assign(dest, table, limbs, assign);
+    }
+}
+
+/* Fill X with n_bytes random bytes.
+ * X must already have room for those bytes.
+ * The ordering of the bytes returned from the RNG is suitable for
+ * deterministic ECDSA (see RFC 6979 §3.3 and the specification of
+ * mbedtls_mpi_core_random()).
+ */
+int mbedtls_mpi_core_fill_random(
+    mbedtls_mpi_uint *X, size_t X_limbs,
+    size_t n_bytes,
+    int (*f_rng)(void *, unsigned char *, size_t), void *p_rng)
+{
+    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
+    const size_t limbs = CHARS_TO_LIMBS(n_bytes);
+    const size_t overhead = (limbs * ciL) - n_bytes;
+
+    if (X_limbs < limbs) {
+        return MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
+    }
+
+    memset(X, 0, overhead);
+    memset((unsigned char *) X + limbs * ciL, 0, (X_limbs - limbs) * ciL);
+    MBEDTLS_MPI_CHK(f_rng(p_rng, (unsigned char *) X + overhead, n_bytes));
+    mbedtls_mpi_core_bigendian_to_host(X, limbs);
+
+cleanup:
+    return ret;
+}
+
+int mbedtls_mpi_core_random(mbedtls_mpi_uint *X,
+                            mbedtls_mpi_uint min,
+                            const mbedtls_mpi_uint *N,
+                            size_t limbs,
+                            int (*f_rng)(void *, unsigned char *, size_t),
+                            void *p_rng)
+{
+    mbedtls_ct_condition_t ge_lower = MBEDTLS_CT_TRUE, lt_upper = MBEDTLS_CT_FALSE;
+    size_t n_bits = mbedtls_mpi_core_bitlen(N, limbs);
+    size_t n_bytes = (n_bits + 7) / 8;
+    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
+
+    /*
+     * When min == 0, each try has at worst a probability 1/2 of failing
+     * (the msb has a probability 1/2 of being 0, and then the result will
+     * be < N), so after 30 tries failure probability is a most 2**(-30).
+     *
+     * When N is just below a power of 2, as is the case when generating
+     * a random scalar on most elliptic curves, 1 try is enough with
+     * overwhelming probability. When N is just above a power of 2,
+     * as when generating a random scalar on secp224k1, each try has
+     * a probability of failing that is almost 1/2.
+     *
+     * The probabilities are almost the same if min is nonzero but negligible
+     * compared to N. This is always the case when N is crypto-sized, but
+     * it's convenient to support small N for testing purposes. When N
+     * is small, use a higher repeat count, otherwise the probability of
+     * failure is macroscopic.
+     */
+    int count = (n_bytes > 4 ? 30 : 250);
+
+    /*
+     * Match the procedure given in RFC 6979 §3.3 (deterministic ECDSA)
+     * when f_rng is a suitably parametrized instance of HMAC_DRBG:
+     * - use the same byte ordering;
+     * - keep the leftmost n_bits bits of the generated octet string;
+     * - try until result is in the desired range.
+     * This also avoids any bias, which is especially important for ECDSA.
+     */
+    do {
+        MBEDTLS_MPI_CHK(mbedtls_mpi_core_fill_random(X, limbs,
+                                                     n_bytes,
+                                                     f_rng, p_rng));
+        mbedtls_mpi_core_shift_r(X, limbs, 8 * n_bytes - n_bits);
+
+        if (--count == 0) {
+            ret = MBEDTLS_ERR_MPI_NOT_ACCEPTABLE;
+            goto cleanup;
+        }
+
+        ge_lower = mbedtls_mpi_core_uint_le_mpi(min, X, limbs);
+        lt_upper = mbedtls_mpi_core_lt_ct(X, N, limbs);
+    } while (mbedtls_ct_bool_and(ge_lower, lt_upper) == MBEDTLS_CT_FALSE);
+
+cleanup:
+    return ret;
+}
+
+static size_t exp_mod_get_window_size(size_t Ebits)
+{
+#if MBEDTLS_MPI_WINDOW_SIZE >= 6
+    return (Ebits > 671) ? 6 : (Ebits > 239) ? 5 : (Ebits >  79) ? 4 : 1;
+#elif MBEDTLS_MPI_WINDOW_SIZE == 5
+    return (Ebits > 239) ? 5 : (Ebits >  79) ? 4 : 1;
+#elif MBEDTLS_MPI_WINDOW_SIZE > 1
+    return (Ebits >  79) ? MBEDTLS_MPI_WINDOW_SIZE : 1;
+#else
+    (void) Ebits;
+    return 1;
+#endif
+}
+
+size_t mbedtls_mpi_core_exp_mod_working_limbs(size_t AN_limbs, size_t E_limbs)
+{
+    const size_t wsize = exp_mod_get_window_size(E_limbs * biL);
+    const size_t welem = ((size_t) 1) << wsize;
+
+    /* How big does each part of the working memory pool need to be? */
+    const size_t table_limbs   = welem * AN_limbs;
+    const size_t select_limbs  = AN_limbs;
+    const size_t temp_limbs    = 2 * AN_limbs + 1;
+
+    return table_limbs + select_limbs + temp_limbs;
+}
+
+static void exp_mod_precompute_window(const mbedtls_mpi_uint *A,
+                                      const mbedtls_mpi_uint *N,
+                                      size_t AN_limbs,
+                                      mbedtls_mpi_uint mm,
+                                      const mbedtls_mpi_uint *RR,
+                                      size_t welem,
+                                      mbedtls_mpi_uint *Wtable,
+                                      mbedtls_mpi_uint *temp)
+{
+    /* W[0] = 1 (in Montgomery presentation) */
+    memset(Wtable, 0, AN_limbs * ciL);
+    Wtable[0] = 1;
+    mbedtls_mpi_core_montmul(Wtable, Wtable, RR, AN_limbs, N, AN_limbs, mm, temp);
+
+    /* W[1] = A (already in Montgomery presentation) */
+    mbedtls_mpi_uint *W1 = Wtable + AN_limbs;
+    memcpy(W1, A, AN_limbs * ciL);
+
+    /* W[i+1] = W[i] * W[1], i >= 2 */
+    mbedtls_mpi_uint *Wprev = W1;
+    for (size_t i = 2; i < welem; i++) {
+        mbedtls_mpi_uint *Wcur = Wprev + AN_limbs;
+        mbedtls_mpi_core_montmul(Wcur, Wprev, W1, AN_limbs, N, AN_limbs, mm, temp);
+        Wprev = Wcur;
+    }
+}
+
+/* Exponentiation: X := A^E mod N.
+ *
+ * A must already be in Montgomery form.
+ *
+ * As in other bignum functions, assume that AN_limbs and E_limbs are nonzero.
+ *
+ * RR must contain 2^{2*biL} mod N.
+ *
+ * The algorithm is a variant of Left-to-right k-ary exponentiation: HAC 14.82
+ * (The difference is that the body in our loop processes a single bit instead
+ * of a full window.)
+ */
+void mbedtls_mpi_core_exp_mod(mbedtls_mpi_uint *X,
+                              const mbedtls_mpi_uint *A,
+                              const mbedtls_mpi_uint *N,
+                              size_t AN_limbs,
+                              const mbedtls_mpi_uint *E,
+                              size_t E_limbs,
+                              const mbedtls_mpi_uint *RR,
+                              mbedtls_mpi_uint *T)
+{
+    const size_t wsize = exp_mod_get_window_size(E_limbs * biL);
+    const size_t welem = ((size_t) 1) << wsize;
+
+    /* This is how we will use the temporary storage T, which must have space
+     * for table_limbs, select_limbs and (2 * AN_limbs + 1) for montmul. */
+    const size_t table_limbs  = welem * AN_limbs;
+    const size_t select_limbs = AN_limbs;
+
+    /* Pointers to specific parts of the temporary working memory pool */
+    mbedtls_mpi_uint *const Wtable  = T;
+    mbedtls_mpi_uint *const Wselect = Wtable  +  table_limbs;
+    mbedtls_mpi_uint *const temp    = Wselect + select_limbs;
+
+    /*
+     * Window precomputation
+     */
+
+    const mbedtls_mpi_uint mm = mbedtls_mpi_core_montmul_init(N);
+
+    /* Set Wtable[i] = A^(2^i) (in Montgomery representation) */
+    exp_mod_precompute_window(A, N, AN_limbs,
+                              mm, RR,
+                              welem, Wtable, temp);
+
+    /*
+     * Fixed window exponentiation
+     */
+
+    /* X = 1 (in Montgomery presentation) initially */
+    memcpy(X, Wtable, AN_limbs * ciL);
+
+    /* We'll process the bits of E from most significant
+     * (limb_index=E_limbs-1, E_bit_index=biL-1) to least significant
+     * (limb_index=0, E_bit_index=0). */
+    size_t E_limb_index = E_limbs;
+    size_t E_bit_index = 0;
+    /* At any given time, window contains window_bits bits from E.
+     * window_bits can go up to wsize. */
+    size_t window_bits = 0;
+    mbedtls_mpi_uint window = 0;
+
+    do {
+        /* Square */
+        mbedtls_mpi_core_montmul(X, X, X, AN_limbs, N, AN_limbs, mm, temp);
+
+        /* Move to the next bit of the exponent */
+        if (E_bit_index == 0) {
+            --E_limb_index;
+            E_bit_index = biL - 1;
+        } else {
+            --E_bit_index;
+        }
+        /* Insert next exponent bit into window */
+        ++window_bits;
+        window <<= 1;
+        window |= (E[E_limb_index] >> E_bit_index) & 1;
+
+        /* Clear window if it's full. Also clear the window at the end,
+         * when we've finished processing the exponent. */
+        if (window_bits == wsize ||
+            (E_bit_index == 0 && E_limb_index == 0)) {
+            /* Select Wtable[window] without leaking window through
+             * memory access patterns. */
+            mbedtls_mpi_core_ct_uint_table_lookup(Wselect, Wtable,
+                                                  AN_limbs, welem, window);
+            /* Multiply X by the selected element. */
+            mbedtls_mpi_core_montmul(X, X, Wselect, AN_limbs, N, AN_limbs, mm,
+                                     temp);
+            window = 0;
+            window_bits = 0;
+        }
+    } while (!(E_bit_index == 0 && E_limb_index == 0));
+}
+
+mbedtls_mpi_uint mbedtls_mpi_core_sub_int(mbedtls_mpi_uint *X,
+                                          const mbedtls_mpi_uint *A,
+                                          mbedtls_mpi_uint c,  /* doubles as carry */
+                                          size_t limbs)
+{
+    for (size_t i = 0; i < limbs; i++) {
+        mbedtls_mpi_uint s = A[i];
+        mbedtls_mpi_uint t = s - c;
+        c = (t > s);
+        X[i] = t;
+    }
+
+    return c;
+}
+
+mbedtls_ct_condition_t mbedtls_mpi_core_check_zero_ct(const mbedtls_mpi_uint *A,
+                                                      size_t limbs)
+{
+    volatile const mbedtls_mpi_uint *force_read_A = A;
+    mbedtls_mpi_uint bits = 0;
+
+    for (size_t i = 0; i < limbs; i++) {
+        bits |= force_read_A[i];
+    }
+
+    return mbedtls_ct_bool(bits);
+}
+
+void mbedtls_mpi_core_to_mont_rep(mbedtls_mpi_uint *X,
+                                  const mbedtls_mpi_uint *A,
+                                  const mbedtls_mpi_uint *N,
+                                  size_t AN_limbs,
+                                  mbedtls_mpi_uint mm,
+                                  const mbedtls_mpi_uint *rr,
+                                  mbedtls_mpi_uint *T)
+{
+    mbedtls_mpi_core_montmul(X, A, rr, AN_limbs, N, AN_limbs, mm, T);
+}
+
+void mbedtls_mpi_core_from_mont_rep(mbedtls_mpi_uint *X,
+                                    const mbedtls_mpi_uint *A,
+                                    const mbedtls_mpi_uint *N,
+                                    size_t AN_limbs,
+                                    mbedtls_mpi_uint mm,
+                                    mbedtls_mpi_uint *T)
+{
+    const mbedtls_mpi_uint Rinv = 1;    /* 1/R in Mont. rep => 1 */
+
+    mbedtls_mpi_core_montmul(X, A, &Rinv, 1, N, AN_limbs, mm, T);
+}
+
+#endif /* MBEDTLS_BIGNUM_C */