[kat/rsa] Prevent stack smashing when copying RSA plaintext

An incorrect array size was used for the message buffer which resulted
in memory writes that went beyond the allocated array corrupting the
test data in the process.

Signed-off-by: Andrea Caforio <[email protected]>

Author: andrea-caforio

sw/device/tests/crypto/cryptotest/firmware/rsa.c

@@ -143,7 +143,7 @@ status_t handle_rsa_encrypt(ujson_t *uj) {
   TRY(otcrypto_rsa_public_key_construct(rsa_size, modulus, &public_key));
 
   // Create input message.
-  uint8_t msg_buf[rsa_num_words];
+  uint8_t msg_buf[uj_input.plaintext_len];
   memset(msg_buf, 0, sizeof(msg_buf));
   memcpy(msg_buf, uj_input.plaintext, uj_input.plaintext_len);
   otcrypto_const_byte_buf_t input_message = {