tests/suites/test_suite_bignum_mod.function

/* BEGIN_HEADER */
#include "mbedtls/bignum.h"
#include "mbedtls/entropy.h"
#include "bignum_mod.h"
#include "bignum_mod_raw.h"
#include "constant_time_internal.h"
#include "test/constant_flow.h"

#define TEST_COMPARE_MPI_RESIDUES(a, b) \
    ASSERT_COMPARE((a).p, (a).limbs * sizeof(mbedtls_mpi_uint), \
                   (b).p, (b).limbs * sizeof(mbedtls_mpi_uint))

static int test_read_residue(mbedtls_mpi_mod_residue *r,
                             const mbedtls_mpi_mod_modulus *m,
                             char *input,
                             int skip_limbs_and_value_checks)
{
    mbedtls_mpi_uint *p = NULL;
    size_t limbs;

    int ret = mbedtls_test_read_mpi_core(&p, &limbs, input);
    if (ret != 0) {
        return ret;
    }

    if (skip_limbs_and_value_checks) {
        r->p = p;
        r->limbs = limbs;
        return 0;
    }

    /* mbedtls_mpi_mod_residue_setup() checks limbs, and that value < m */
    return mbedtls_mpi_mod_residue_setup(r, m, p, limbs);
}
/* END_HEADER */

/* BEGIN_DEPENDENCIES
 * depends_on:MBEDTLS_BIGNUM_C
 * END_DEPENDENCIES
 */

/* BEGIN_CASE */
void mpi_mod_setup(int int_rep, int iret)
{
    #define MLIMBS 8
    mbedtls_mpi_uint mp[MLIMBS];
    mbedtls_mpi_mod_modulus m;
    int ret;

    memset(mp, 0xFF, sizeof(mp));

    mbedtls_mpi_mod_modulus_init(&m);

    switch (int_rep) {
        case MBEDTLS_MPI_MOD_REP_MONTGOMERY:
            ret = mbedtls_mpi_mod_modulus_setup(&m, mp, MLIMBS);
            break;
        case MBEDTLS_MPI_MOD_REP_OPT_RED:
            ret = mbedtls_mpi_mod_optred_modulus_setup(&m, mp, MLIMBS, NULL);
            break;
        default:
            ret = MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
            break;
    }

    TEST_EQUAL(ret, iret);

    /* Only test if the constants have been set-up  */
    if (ret == 0 && int_rep == MBEDTLS_MPI_MOD_REP_MONTGOMERY) {
        /* Test that the consts have been calculated */
        TEST_ASSERT(m.rep.mont.rr != NULL);
        TEST_ASSERT(m.rep.mont.mm != 0);

    }

    /* Address sanitiser should catch if we try to free mp */
    mbedtls_mpi_mod_modulus_free(&m);

    /* Make sure that the modulus doesn't have reference to mp anymore */
    TEST_ASSERT(m.p != mp);

    /* Only test if the constants have been set-up  */
    if (ret == 0 && int_rep == MBEDTLS_MPI_MOD_REP_MONTGOMERY) {
        /* Verify the data and pointers allocated have been properly wiped */
        TEST_ASSERT(m.rep.mont.rr == NULL);
        TEST_ASSERT(m.rep.mont.mm == 0);
    }
exit:
    /* It should be safe to call an mbedtls free several times */
    mbedtls_mpi_mod_modulus_free(&m);

    #undef MLIMBS
}
/* END_CASE */

/* BEGIN_CASE */
void mpi_mod_mul(char *input_A,
                 char *input_B,
                 char *input_N,
                 char *result)
{
    mbedtls_mpi_uint *X = NULL;

    mbedtls_mpi_mod_residue rA = { NULL, 0 };
    mbedtls_mpi_mod_residue rB = { NULL, 0 };
    mbedtls_mpi_mod_residue rR = { NULL, 0 };
    mbedtls_mpi_mod_residue rX = { NULL, 0 };

    mbedtls_mpi_mod_modulus m;
    mbedtls_mpi_mod_modulus_init(&m);

    TEST_EQUAL(mbedtls_test_read_mpi_modulus(&m, input_N,
               MBEDTLS_MPI_MOD_REP_MONTGOMERY), 0);

    TEST_EQUAL(test_read_residue(&rA, &m, input_A, 0), 0);
    TEST_EQUAL(test_read_residue(&rB, &m, input_B, 0), 0);
    TEST_EQUAL(test_read_residue(&rR, &m, result,  0), 0);

    const size_t limbs = m.limbs;
    const size_t bytes = limbs * sizeof(mbedtls_mpi_uint);

    TEST_EQUAL(rA.limbs, limbs);
    TEST_EQUAL(rB.limbs, limbs);
    TEST_EQUAL(rR.limbs, limbs);

    ASSERT_ALLOC(X, limbs);

    TEST_EQUAL(mbedtls_mpi_mod_residue_setup(&rX, &m, X, limbs), 0);

    TEST_EQUAL(mbedtls_mpi_mod_mul(&rX, &rA, &rB, &m), 0);
    ASSERT_COMPARE(rX.p, bytes, rR.p, bytes);

    /* alias X to A */
    memcpy(rX.p, rA.p, bytes);
    TEST_EQUAL(mbedtls_mpi_mod_mul(&rX, &rX, &rB, &m), 0);
    ASSERT_COMPARE(rX.p, bytes, rR.p, bytes);

    /* alias X to B */
    memcpy(rX.p, rB.p, bytes);
    TEST_EQUAL(mbedtls_mpi_mod_mul(&rX, &rA, &rX, &m), 0);
    ASSERT_COMPARE(rX.p, bytes, rR.p, bytes);

    /* A == B: alias A and B */
    if (memcmp(rA.p, rB.p, bytes) == 0) {
        TEST_EQUAL(mbedtls_mpi_mod_mul(&rX, &rA, &rA, &m), 0);
        ASSERT_COMPARE(rX.p, bytes, rR.p, bytes);

        /* X, A, B all aliased together */
        memcpy(rX.p, rA.p, bytes);
        TEST_EQUAL(mbedtls_mpi_mod_mul(&rX, &rX, &rX, &m), 0);
        ASSERT_COMPARE(rX.p, bytes, rR.p, bytes);
    }
    /* A != B: test B * A */
    else {
        TEST_EQUAL(mbedtls_mpi_mod_mul(&rX, &rB, &rA, &m), 0);
        ASSERT_COMPARE(rX.p, bytes, rR.p, bytes);

        /* B * A: alias X to A */
        memcpy(rX.p, rA.p, bytes);
        TEST_EQUAL(mbedtls_mpi_mod_mul(&rX, &rB, &rX, &m), 0);
        ASSERT_COMPARE(rX.p, bytes, rR.p, bytes);

        /* B + A: alias X to B */
        memcpy(rX.p, rB.p, bytes);
        TEST_EQUAL(mbedtls_mpi_mod_mul(&rX, &rX, &rA, &m), 0);
        ASSERT_COMPARE(rX.p, bytes, rR.p, bytes);
    }

exit:
    mbedtls_free(rA.p);
    mbedtls_free(rB.p);
    mbedtls_free(rR.p);
    mbedtls_free(X);
    mbedtls_free((mbedtls_mpi_uint *) m.p);

    mbedtls_mpi_mod_modulus_free(&m);
}
/* END_CASE */

/* BEGIN_CASE */
void mpi_mod_mul_neg(char *input_A,
                     char *input_B,
                     char *input_N,
                     char *result,
                     int exp_ret)
{
    mbedtls_mpi_uint *X = NULL;

    mbedtls_mpi_mod_residue rA = { NULL, 0 };
    mbedtls_mpi_mod_residue rB = { NULL, 0 };
    mbedtls_mpi_mod_residue rR = { NULL, 0 };
    mbedtls_mpi_mod_residue rX = { NULL, 0 };

    mbedtls_mpi_mod_modulus m;
    mbedtls_mpi_mod_modulus_init(&m);

    mbedtls_mpi_mod_modulus fake_m;
    mbedtls_mpi_mod_modulus_init(&fake_m);

    TEST_EQUAL(mbedtls_test_read_mpi_modulus(&m, input_N,
               MBEDTLS_MPI_MOD_REP_MONTGOMERY), 0);

    TEST_EQUAL(test_read_residue(&rA, &m, input_A, 1), 0);
    TEST_EQUAL(test_read_residue(&rB, &m, input_B, 1), 0);
    TEST_EQUAL(test_read_residue(&rR, &m, result,  1), 0);

    const size_t limbs = m.limbs;

    ASSERT_ALLOC(X, limbs);

    TEST_EQUAL(mbedtls_mpi_mod_residue_setup(&rX, &m, X, limbs), 0);
    rX.limbs = rR.limbs;

    TEST_EQUAL(mbedtls_mpi_mod_mul(&rX, &rA, &rB, &m), exp_ret);

    /* Check when m is not initialized */
    TEST_EQUAL(mbedtls_mpi_mod_mul(&rX, &rA, &rB, &fake_m),
               MBEDTLS_ERR_MPI_BAD_INPUT_DATA);

exit:
    mbedtls_free(rA.p);
    mbedtls_free(rB.p);
    mbedtls_free(rR.p);
    mbedtls_free(X);
    mbedtls_free((mbedtls_mpi_uint *) m.p);

    mbedtls_mpi_mod_modulus_free(&m);
    mbedtls_mpi_mod_modulus_free(&fake_m);
}
/* END_CASE */

/* BEGIN_CASE */
void mpi_mod_sub(char *input_N,
                 char *input_A, char *input_B,
                 char *input_D, int expected_ret)
{
    mbedtls_mpi_mod_residue a = { NULL, 0 };
    mbedtls_mpi_mod_residue b = { NULL, 0 };
    mbedtls_mpi_mod_residue d = { NULL, 0 };
    mbedtls_mpi_mod_residue x = { NULL, 0 };
    mbedtls_mpi_uint *X_raw = NULL;

    mbedtls_mpi_mod_modulus m;
    mbedtls_mpi_mod_modulus_init(&m);

    TEST_EQUAL(0,
               mbedtls_test_read_mpi_modulus(&m, input_N,
               MBEDTLS_MPI_MOD_REP_MONTGOMERY));

    /* test_read_residue() normally checks that inputs have the same number of
     * limbs as the modulus. For negative testing we can ask it to skip this
     * with a non-zero final parameter. */
    TEST_EQUAL(0, test_read_residue(&a, &m, input_A, expected_ret != 0));
    TEST_EQUAL(0, test_read_residue(&b, &m, input_B, expected_ret != 0));
    TEST_EQUAL(0, test_read_residue(&d, &m, input_D, expected_ret != 0));

    size_t limbs = m.limbs;
    size_t bytes = limbs * sizeof(*X_raw);

    if (expected_ret == 0) {
        /* Negative test with too many limbs in output */
        ASSERT_ALLOC(X_raw, limbs + 1);

        x.p = X_raw;
        x.limbs = limbs + 1;
        TEST_EQUAL(MBEDTLS_ERR_MPI_BAD_INPUT_DATA,
                   mbedtls_mpi_mod_sub(&x, &a, &b, &m));

        mbedtls_free(X_raw);
        X_raw = NULL;

        /* Negative test with too few limbs in output */
        if (limbs > 1) {
            ASSERT_ALLOC(X_raw, limbs - 1);

            x.p = X_raw;
            x.limbs = limbs - 1;
            TEST_EQUAL(MBEDTLS_ERR_MPI_BAD_INPUT_DATA,
                       mbedtls_mpi_mod_sub(&x, &a, &b, &m));

            mbedtls_free(X_raw);
            X_raw = NULL;
        }

        /* Negative testing with too many/too few limbs in a and b is covered by
         * manually-written test cases with expected_ret != 0. */
    }

    ASSERT_ALLOC(X_raw, limbs);

    TEST_EQUAL(0, mbedtls_mpi_mod_residue_setup(&x, &m, X_raw, limbs));

    /* a - b => Correct result, or expected error */
    TEST_EQUAL(expected_ret, mbedtls_mpi_mod_sub(&x, &a, &b, &m));
    if (expected_ret != 0) {
        goto exit;
    }

    TEST_COMPARE_MPI_RESIDUES(x, d);

    /* a - b: alias x to a => Correct result */
    memcpy(x.p, a.p, bytes);
    TEST_EQUAL(0, mbedtls_mpi_mod_sub(&x, &x, &b, &m));
    TEST_COMPARE_MPI_RESIDUES(x, d);

    /* a - b: alias x to b => Correct result */
    memcpy(x.p, b.p, bytes);
    TEST_EQUAL(0, mbedtls_mpi_mod_sub(&x, &a, &x, &m));
    TEST_COMPARE_MPI_RESIDUES(x, d);

    if (memcmp(a.p, b.p, bytes) == 0) {
        /* a == b: alias a and b */

        /* a - a => Correct result */
        TEST_EQUAL(0, mbedtls_mpi_mod_sub(&x, &a, &a, &m));
        TEST_COMPARE_MPI_RESIDUES(x, d);

        /* a - a: x, a, b all aliased together => Correct result */
        memcpy(x.p, a.p, bytes);
        TEST_EQUAL(0, mbedtls_mpi_mod_sub(&x, &x, &x, &m));
        TEST_COMPARE_MPI_RESIDUES(x, d);
    }

exit:
    mbedtls_free((void *) m.p);  /* mbedtls_mpi_mod_modulus_free() sets m.p = NULL */
    mbedtls_mpi_mod_modulus_free(&m);

    mbedtls_free(a.p);
    mbedtls_free(b.p);
    mbedtls_free(d.p);
    mbedtls_free(X_raw);
}
/* END_CASE */

/* BEGIN_CASE */
void mpi_mod_inv_mont(char *input_N,
                      char *input_A, char *input_I,
                      int expected_ret)
{
    mbedtls_mpi_mod_residue a = { NULL, 0 };    /* argument */
    mbedtls_mpi_mod_residue i = { NULL, 0 };    /* expected inverse wrt N */
    mbedtls_mpi_mod_residue x = { NULL, 0 };    /* output */
    mbedtls_mpi_uint *X_raw = NULL;

    mbedtls_mpi_mod_modulus N;
    mbedtls_mpi_mod_modulus_init(&N);

    TEST_EQUAL(0,
               mbedtls_test_read_mpi_modulus(&N, input_N,
               MBEDTLS_MPI_MOD_REP_MONTGOMERY));

    /* test_read_residue() normally checks that inputs have the same number of
     * limbs as the modulus. For negative testing we can ask it to skip this
     * with a non-zero final parameter. */
    TEST_EQUAL(0, test_read_residue(&a, &N, input_A, expected_ret != 0));
    TEST_EQUAL(0, test_read_residue(&i, &N, input_I, expected_ret != 0));

    size_t limbs = N.limbs;
    size_t bytes = limbs * sizeof(*X_raw);

    ASSERT_ALLOC(X_raw, limbs);

    TEST_EQUAL(0, mbedtls_mpi_mod_residue_setup(&x, &N, X_raw, limbs));

    TEST_EQUAL(expected_ret, mbedtls_mpi_mod_inv(&x, &a, &N));
    if (expected_ret == 0) {
        TEST_COMPARE_MPI_RESIDUES(x, i);

        /* a^-1: alias x to a => Correct result */
        memcpy(x.p, a.p, bytes);
        TEST_EQUAL(0, mbedtls_mpi_mod_inv(&x, &x, &N));
        TEST_COMPARE_MPI_RESIDUES(x, i);
    }

exit:
    mbedtls_free((void *) N.p);  /* mbedtls_mpi_mod_modulus_free() sets N.p = NULL */
    mbedtls_mpi_mod_modulus_free(&N);

    mbedtls_free(a.p);
    mbedtls_free(i.p);
    mbedtls_free(X_raw);
}
/* END_CASE */

/* BEGIN_CASE */
void mpi_mod_inv_non_mont(char *input_N,
                          char *input_A, char *input_I,
                          int expected_ret)
{
    mbedtls_mpi_mod_residue a = { NULL, 0 };    /* argument */
    mbedtls_mpi_mod_residue i = { NULL, 0 };    /* expected inverse wrt N */
    mbedtls_mpi_mod_residue x = { NULL, 0 };    /* output */
    mbedtls_mpi_uint *X_raw = NULL;

    mbedtls_mpi_mod_modulus N;
    mbedtls_mpi_mod_modulus_init(&N);

    TEST_EQUAL(0,
               mbedtls_test_read_mpi_modulus(&N, input_N,
               MBEDTLS_MPI_MOD_REP_OPT_RED));

    /* test_read_residue() normally checks that inputs have the same number of
     * limbs as the modulus. For negative testing we can ask it to skip this
     * with a non-zero final parameter. */
    TEST_EQUAL(0, test_read_residue(&a, &N, input_A, expected_ret != 0));
    TEST_EQUAL(0, test_read_residue(&i, &N, input_I, expected_ret != 0));

    size_t limbs = N.limbs;
    size_t bytes = limbs * sizeof(*X_raw);

    ASSERT_ALLOC(X_raw, limbs);

    TEST_EQUAL(0, mbedtls_mpi_mod_residue_setup(&x, &N, X_raw, limbs));

    TEST_EQUAL(expected_ret, mbedtls_mpi_mod_inv(&x, &a, &N));
    if (expected_ret == 0) {
        TEST_COMPARE_MPI_RESIDUES(x, i);

        /* a^-1: alias x to a => Correct result */
        memcpy(x.p, a.p, bytes);
        TEST_EQUAL(0, mbedtls_mpi_mod_inv(&x, &x, &N));
        TEST_COMPARE_MPI_RESIDUES(x, i);
    }

exit:
    mbedtls_free((void *) N.p);  /* mbedtls_mpi_mod_modulus_free() sets N.p = NULL */
    mbedtls_mpi_mod_modulus_free(&N);

    mbedtls_free(a.p);
    mbedtls_free(i.p);
    mbedtls_free(X_raw);
}
/* END_CASE */

/* BEGIN_CASE */
void mpi_mod_add(char *input_N,
                 char *input_A, char *input_B,
                 char *input_S, int expected_ret)
{
    mbedtls_mpi_mod_residue a = { NULL, 0 };
    mbedtls_mpi_mod_residue b = { NULL, 0 };
    mbedtls_mpi_mod_residue s = { NULL, 0 };
    mbedtls_mpi_mod_residue x = { NULL, 0 };
    mbedtls_mpi_uint *X_raw = NULL;

    mbedtls_mpi_mod_modulus m;
    mbedtls_mpi_mod_modulus_init(&m);

    TEST_EQUAL(0,
               mbedtls_test_read_mpi_modulus(&m, input_N,
               MBEDTLS_MPI_MOD_REP_MONTGOMERY));

    /* test_read_residue() normally checks that inputs have the same number of
     * limbs as the modulus. For negative testing we can ask it to skip this
     * with a non-zero final parameter. */
    TEST_EQUAL(0, test_read_residue(&a, &m, input_A, expected_ret != 0));
    TEST_EQUAL(0, test_read_residue(&b, &m, input_B, expected_ret != 0));
    TEST_EQUAL(0, test_read_residue(&s, &m, input_S, expected_ret != 0));

    size_t limbs = m.limbs;
    size_t bytes = limbs * sizeof(*X_raw);

    if (expected_ret == 0) {
        /* Negative test with too many limbs in output */
        ASSERT_ALLOC(X_raw, limbs + 1);

        x.p = X_raw;
        x.limbs = limbs + 1;
        TEST_EQUAL(MBEDTLS_ERR_MPI_BAD_INPUT_DATA,
                   mbedtls_mpi_mod_add(&x, &a, &b, &m));

        mbedtls_free(X_raw);
        X_raw = NULL;

        /* Negative test with too few limbs in output */
        if (limbs > 1) {
            ASSERT_ALLOC(X_raw, limbs - 1);

            x.p = X_raw;
            x.limbs = limbs - 1;
            TEST_EQUAL(MBEDTLS_ERR_MPI_BAD_INPUT_DATA,
                       mbedtls_mpi_mod_add(&x, &a, &b, &m));

            mbedtls_free(X_raw);
            X_raw = NULL;
        }

        /* Negative testing with too many/too few limbs in a and b is covered by
         * manually-written test cases with oret != 0. */
    }

    /* Allocate correct number of limbs for X_raw */
    ASSERT_ALLOC(X_raw, limbs);

    TEST_EQUAL(0, mbedtls_mpi_mod_residue_setup(&x, &m, X_raw, limbs));

    /* A + B => Correct result or expected error */
    TEST_EQUAL(expected_ret, mbedtls_mpi_mod_add(&x, &a, &b, &m));
    if (expected_ret != 0) {
        goto exit;
    }

    TEST_COMPARE_MPI_RESIDUES(x, s);

    /* a + b: alias x to a => Correct result */
    memcpy(x.p, a.p, bytes);
    TEST_EQUAL(0, mbedtls_mpi_mod_add(&x, &x, &b, &m));
    TEST_COMPARE_MPI_RESIDUES(x, s);

    /* a + b: alias x to b => Correct result */
    memcpy(x.p, b.p, bytes);
    TEST_EQUAL(0, mbedtls_mpi_mod_add(&x, &a, &x, &m));
    TEST_COMPARE_MPI_RESIDUES(x, s);

    if (memcmp(a.p, b.p, bytes) == 0) {
        /* a == b: alias a and b */

        /* a + a => Correct result */
        TEST_EQUAL(0, mbedtls_mpi_mod_add(&x, &a, &a, &m));
        TEST_COMPARE_MPI_RESIDUES(x, s);

        /* a + a: x, a, b all aliased together => Correct result */
        memcpy(x.p, a.p, bytes);
        TEST_EQUAL(0, mbedtls_mpi_mod_add(&x, &x, &x, &m));
        TEST_COMPARE_MPI_RESIDUES(x, s);
    }

exit:
    mbedtls_free((void *) m.p);  /* mbedtls_mpi_mod_modulus_free() sets m.p = NULL */
    mbedtls_mpi_mod_modulus_free(&m);

    mbedtls_free(a.p);
    mbedtls_free(b.p);
    mbedtls_free(s.p);
    mbedtls_free(X_raw);
}
/* END_CASE */

/* BEGIN_CASE */
void mpi_residue_setup(char *input_N, char *input_R, int ret)
{
    mbedtls_mpi_uint *N = NULL;
    mbedtls_mpi_uint *R = NULL;
    size_t n_limbs, r_limbs;
    mbedtls_mpi_mod_modulus m;
    mbedtls_mpi_mod_residue r;

    mbedtls_mpi_mod_modulus_init(&m);

    /* Allocate the memory for intermediate data structures */
    TEST_EQUAL(0, mbedtls_test_read_mpi_core(&N, &n_limbs, input_N));
    TEST_EQUAL(0, mbedtls_test_read_mpi_core(&R, &r_limbs, input_R));

    TEST_EQUAL(0, mbedtls_mpi_mod_modulus_setup(&m, N, n_limbs));

    TEST_EQUAL(ret, mbedtls_mpi_mod_residue_setup(&r, &m, R, r_limbs));

    if (ret == 0) {
        TEST_EQUAL(r.limbs, r_limbs);
        TEST_ASSERT(r.p == R);
    }

exit:
    mbedtls_mpi_mod_modulus_free(&m);
    mbedtls_free(N);
    mbedtls_free(R);
}
/* END_CASE */

/* BEGIN_CASE */
void mpi_mod_io_neg(char *input_N, data_t *buf, int ret)
{
    mbedtls_mpi_uint *N = NULL;
    mbedtls_mpi_uint *R = NULL;

    mbedtls_mpi_mod_modulus m;
    mbedtls_mpi_mod_residue r = { NULL, 0 };
    mbedtls_mpi_mod_ext_rep endian = MBEDTLS_MPI_MOD_EXT_REP_LE;

    mbedtls_mpi_mod_modulus_init(&m);

    size_t n_limbs;
    TEST_EQUAL(0, mbedtls_test_read_mpi_core(&N, &n_limbs, input_N));
    size_t r_limbs = n_limbs;
    ASSERT_ALLOC(R, r_limbs);

    /* modulus->p == NULL || residue->p == NULL ( m has not been set-up ) */
    TEST_EQUAL(MBEDTLS_ERR_MPI_BAD_INPUT_DATA,
               mbedtls_mpi_mod_read(&r, &m, buf->x, buf->len, endian));

    TEST_EQUAL(MBEDTLS_ERR_MPI_BAD_INPUT_DATA,
               mbedtls_mpi_mod_write(&r, &m, buf->x, buf->len, endian));

    /* Set up modulus and test with residue->p == NULL */
    TEST_EQUAL(0, mbedtls_mpi_mod_modulus_setup(&m, N, n_limbs));

    TEST_EQUAL(MBEDTLS_ERR_MPI_BAD_INPUT_DATA,
               mbedtls_mpi_mod_read(&r, &m, buf->x, buf->len, endian));
    TEST_EQUAL(MBEDTLS_ERR_MPI_BAD_INPUT_DATA,
               mbedtls_mpi_mod_write(&r, &m, buf->x, buf->len, endian));

    /* Do the rest of the tests with a residue set up with the input data */
    TEST_EQUAL(0, mbedtls_mpi_mod_residue_setup(&r, &m, R, r_limbs));

    /* Fail for r_limbs < m->limbs */
    r.limbs--;
    TEST_ASSERT(r.limbs < m.limbs);
    TEST_EQUAL(MBEDTLS_ERR_MPI_BAD_INPUT_DATA,
               mbedtls_mpi_mod_read(&r, &m, buf->x, buf->len, endian));
    TEST_EQUAL(MBEDTLS_ERR_MPI_BAD_INPUT_DATA,
               mbedtls_mpi_mod_write(&r, &m, buf->x, buf->len, endian));
    r.limbs++;

    /* Fail for r_limbs > m->limbs */
    m.limbs--;
    TEST_ASSERT(r.limbs > m.limbs);
    TEST_EQUAL(MBEDTLS_ERR_MPI_BAD_INPUT_DATA,
               mbedtls_mpi_mod_read(&r, &m, buf->x, buf->len, endian));
    TEST_EQUAL(MBEDTLS_ERR_MPI_BAD_INPUT_DATA,
               mbedtls_mpi_mod_write(&r, &m, buf->x, buf->len, endian));
    m.limbs++;

    /* Test the read */
    TEST_EQUAL(ret, mbedtls_mpi_mod_read(&r, &m, buf->x, buf->len, endian));

    /* Test write overflow only when the representation is large and read is successful  */
    if (r.limbs > 1 && ret == 0) {
        TEST_EQUAL(MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL,
                   mbedtls_mpi_mod_write(&r, &m, buf->x, 1, endian));
    }

exit:
    mbedtls_mpi_mod_residue_release(&r);
    mbedtls_mpi_mod_modulus_free(&m);
    mbedtls_free(N);
    mbedtls_free(R);
}
/* END_CASE */

/* BEGIN_CASE */
void mpi_mod_io(char *input_N, data_t *input_A, int endian)
{
    mbedtls_mpi_uint *N = NULL;
    mbedtls_mpi_uint *R = NULL;
    mbedtls_mpi_uint *R_COPY = NULL;
    unsigned char *obuf = NULL;
    unsigned char *ref_buf = NULL;
    mbedtls_mpi_mod_modulus m;
    mbedtls_mpi_mod_residue r;
    mbedtls_mpi_mod_residue r_copy;
    size_t n_limbs, n_bytes, a_bytes;

    mbedtls_mpi_mod_modulus_init(&m);

    /* Read inputs */
    TEST_EQUAL(0, mbedtls_test_read_mpi_core(&N, &n_limbs, input_N));
    n_bytes = n_limbs * sizeof(mbedtls_mpi_uint);
    a_bytes = input_A->len;

    /* Allocate the memory for intermediate data structures */
    ASSERT_ALLOC(R, n_bytes);
    ASSERT_ALLOC(R_COPY, n_bytes);

    /* Test that input's size is not greater to modulo's */
    TEST_LE_U(a_bytes, n_bytes);

    /* Init Structures */
    TEST_EQUAL(0, mbedtls_mpi_mod_modulus_setup(&m, N, n_limbs));

    /* Enforcing p_limbs >= m->limbs */
    TEST_EQUAL(0, mbedtls_mpi_mod_residue_setup(&r, &m, R, n_limbs));

    TEST_EQUAL(0, mbedtls_mpi_mod_read(&r, &m, input_A->x, input_A->len,
                                       endian));

    /* Read a copy for checking that writing didn't change the value of r */
    TEST_EQUAL(0, mbedtls_mpi_mod_residue_setup(&r_copy, &m,
                                                R_COPY, n_limbs));
    TEST_EQUAL(0, mbedtls_mpi_mod_read(&r_copy, &m, input_A->x, input_A->len,
                                       endian));

    /* Get number of bytes without leading zeroes */
    size_t a_bytes_trimmed = a_bytes;
    while (a_bytes_trimmed > 0) {
        unsigned char *r_byte_array = (unsigned char *) r.p;
        if (r_byte_array[--a_bytes_trimmed] != 0) {
            break;
        }
    }
    a_bytes_trimmed++;

    /* Test write with three output buffer sizes: tight, same as input and
     * longer than the input */
    size_t obuf_sizes[3];
    const size_t obuf_sizes_len = sizeof(obuf_sizes) / sizeof(obuf_sizes[0]);
    obuf_sizes[0] = a_bytes_trimmed;
    obuf_sizes[1] = a_bytes;
    obuf_sizes[2] = a_bytes + 8;

    for (size_t i = 0; i < obuf_sizes_len; i++) {
        ASSERT_ALLOC(obuf, obuf_sizes[i]);
        TEST_EQUAL(0, mbedtls_mpi_mod_write(&r, &m, obuf, obuf_sizes[i], endian));

        /* Make sure that writing didn't corrupt the value of r */
        ASSERT_COMPARE(r.p, r.limbs, r_copy.p, r_copy.limbs);

        /* Set up reference output for checking the result */
        ASSERT_ALLOC(ref_buf, obuf_sizes[i]);
        switch (endian) {
            case MBEDTLS_MPI_MOD_EXT_REP_LE:
                memcpy(ref_buf, input_A->x, a_bytes_trimmed);
                break;
            case MBEDTLS_MPI_MOD_EXT_REP_BE:
            {
                size_t a_offset = input_A->len - a_bytes_trimmed;
                size_t ref_offset = obuf_sizes[i] - a_bytes_trimmed;
                memcpy(ref_buf + ref_offset, input_A->x + a_offset,
                       a_bytes_trimmed);
            }
            break;
            default:
                TEST_ASSERT(0);
        }

        /* Check the result */
        ASSERT_COMPARE(obuf, obuf_sizes[i], ref_buf, obuf_sizes[i]);

        mbedtls_free(ref_buf);
        ref_buf = NULL;
        mbedtls_free(obuf);
        obuf = NULL;
    }

exit:
    mbedtls_mpi_mod_modulus_free(&m);
    mbedtls_free(N);
    mbedtls_free(R);
    mbedtls_free(R_COPY);
    mbedtls_free(obuf);
}
/* END_CASE */