From bda577bb0bd003206c7234ced60950faaa8a7ef7 Mon Sep 17 00:00:00 2001 From: Paul Elliott Date: Tue, 6 Feb 2024 17:49:20 +0000 Subject: [PATCH] Fix confusing comment in ctr drbg thread test Make it clearer where the magic number chosen for entropy_len actually comes from, and why we chose this value. Signed-off-by: Paul Elliott --- tests/suites/test_suite_ctr_drbg.function | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-) diff --git a/tests/suites/test_suite_ctr_drbg.function b/tests/suites/test_suite_ctr_drbg.function index 425c43ef1..63524f25a 100644 --- a/tests/suites/test_suite_ctr_drbg.function +++ b/tests/suites/test_suite_ctr_drbg.function @@ -354,7 +354,12 @@ void ctr_drbg_threads(data_t *expected_result, int reseed, int arg_thread_count) const size_t n_random_calls = thread_count * thread_random_reps + 1; - /* Based on the size of MBEDTLS_CTR_DRBG_ENTROPY_LEN for SHA512. */ + /* This is a known-answer test, and although tests use a mock entropy + * function the input entropy length will still affect the output. + * We therefore need to pick a fixed entropy length, rather than using the + * default entropy length (MBEDTLS_CTR_DRBG_ENTROPY_LEN). We've chosen to + * use the default value of MBEDTLS_CTR_DRBG_ENTROPY_LEN for SHA-512, + * as this was the value used when the expected answers were calculated. */ const size_t entropy_len = 48; AES_PSA_INIT(); @@ -367,8 +372,8 @@ void ctr_drbg_threads(data_t *expected_result, int reseed, int arg_thread_count) test_offset_idx = 0; - /* Need to do this, otherwise if we are forced into using SHA256 for - * whaever reason, output will differ. */ + /* Need to set a non-default fixed entropy len, to ensure same output across + * all configs - see above for details. */ mbedtls_ctr_drbg_set_entropy_len(&ctx, entropy_len); if (reseed == 0) {