tree 7765664417360e5d3769dc9ccd06c978e4923c62
parent 7b6d8c27c8fcff9baf39792ff2c44faa87660b97
author Jaeden Amero <jaeden.amero@arm.com> 1571410927 +0100
committer Jaeden Amero <jaeden.amero@arm.com> 1573655378 +0000

x509write_csr: Reduce stack usage of mbedtls_x509write_csr_pem()

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).