Using 4096 bytes of stack for the temporary buffer used for holding a
throw-away DER-formatted CSR limits the portability of generating
certificate signing requests to only devices with lots of stack space.
To increase portability, use the mbedtls_pem_write_buffer() in-place
capability instead, using the same buffer for input and output. This
works since the DER encoding for some given data is always smaller than
that same data PEM-encoded.
PEM format is desirable to use even on stack-constrained devices as the
format is easy to work with (for example, copy-pasting from a tiny
device's serial console output, for CSRs generated on tiny devices
without the private key leaving said tiny device).
Add a new function mbedtls_asn1_write_named_bitstring() that removes
trailing 0s at the end of DER encoded bitstrings. The function is
implemented according to Hanno Becker's suggestions.
This commit also changes the functions x509write_crt_set_ns_cert_type
and crt_set_key_usage to call the new function as the use named
bitstrings instead of the regular bitstrings.
This commit removes all the static occurrencies of the function
mbedtls_zeroize() in each of the individual .c modules. Instead the
function has been moved to utils.h that is included in each of the
modules.
