tree 26d0e4b0b7fab916df31443e9cbd502a5d2d4b18
parent 0bea4d14e04c3c4c4cb5c0d52ec683877c89a065
author Gilles Peskine <Gilles.Peskine@arm.com> 1623169954 +0200
committer Gilles Peskine <Gilles.Peskine@arm.com> 1624358357 +0200

mbedtls_mpi_read_string: make an empty bignum for an empty string

In mbedtls_mpi_read_string, if the string is empty, return an empty bignum
rather than a bignum with one limb with the value 0.

Both representations are correct, so this is not, in principle, a
user-visible change. The change does leak however through
mbedtls_mpi_write_string in base 16 (but not in other bases), as it writes a
bignum with 0 limbs as "" but a bignum with the value 0 and at least one
limb as "00".

This change makes it possible to construct an empty bignum through
mbedtls_mpi_read_string, which is especially useful to construct test
cases (a common use of mbedtls_mpi_read_string, as most formats use in
production encode numbers in binary, to be read with mbedtls_mpi_read_binary
or mbedtls_mpi_read_binary_le).

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>