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Support negative zero as MPI test input

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The bignum module does not officially support "negative zero" (an
mbedtls_mpi object with s=-1 and all limbs zero). However, we have a
history of bugs where a function that should produce an official
zero (with s=1), produces a negative zero in some circumstances. So it's
good to check that the bignum functions are robust when passed a negative
zero as input. And for that, we need a way to construct a negative zero
from test case arguments.

There are checks that functions don't produce negative zeros as output in
the test suite. Skip those checks if there's a negative zero input: we
don't want functions to _create_ negative zeros, but we don't mind if
they _propagate_ negative zeros.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine
2022-11-09 21:08:44 +01:00
parent 806c9588ef
commit ca6e8aac58
4 changed files with 84 additions and 11 deletions
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26
tests/include/test/helpers.h
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@ -295,13 +295,19 @@ int mbedtls_test_read_mpi_core( mbedtls_mpi_uint **pX, size_t *plimbs,
/** Read an MPI from a hexadecimal string.
*
* Like mbedtls_mpi_read_string(), but size the resulting bignum based
* on the number of digits in the string. In particular, construct a
* bignum with 0 limbs for an empty string, and a bignum with leading 0
* limbs if the string has sufficiently many leading 0 digits.
* Like mbedtls_mpi_read_string(), but with tighter guarantees around
* edge cases.
*
* This is important so that the "0 (null)" and "0 (1 limb)" and
* "leading zeros" test cases do what they claim.
* - This function guarantees that if \p s begins with '-' then the sign
* bit of the result will be negative, even if the value is 0.
* When this function encounters such a "negative 0", it
* increments #mbedtls_test_read_mpi.
* - The size of the result is exactly the minimum number of limbs needed
* to fit the digits in the input. In particular, this function constructs
* a bignum with 0 limbs for an empty string, and a bignum with leading 0
* limbs if the string has sufficiently many leading 0 digits.
* This is important so that the "0 (null)" and "0 (1 limb)" and
* "leading zeros" test cases do what they claim.
*
* \param[out] X The MPI object to populate. It must be initialized.
* \param[in] s The null-terminated hexadecimal string to read from.
@ -309,6 +315,14 @@ int mbedtls_test_read_mpi_core( mbedtls_mpi_uint **pX, size_t *plimbs,
* \return \c 0 on success, an \c MBEDTLS_ERR_MPI_xxx error code otherwise.
*/
int mbedtls_test_read_mpi( mbedtls_mpi *X, const char *s );
/** Nonzero if the current test case had an input parsed with
* mbedtls_test_read_mpi() that is a negative 0 (`"-"`, `"-0"`, `"-00"`, etc.,
* constructing a result with the sign bit set to -1 and the value being
* all-limbs-0, which is not a valid representation in #mbedtls_mpi but is
* tested for robustness).
*/
extern unsigned mbedtls_test_case_uses_negative_0;
#endif /* MBEDTLS_BIGNUM_C */
#endif /* TEST_HELPERS_H */