zephyr: correct the RNG seeding description in the README - #23
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The README claimed that wolfPSA's boot init registers a Zephyr-CSPRNG seed callback, so that the shared wolfCrypt Hash-DRBG is seeded from the platform entropy source via sys_csrand_get(). That describes a design that no longer exists: no seed callback is registered by either module, and seeding through sys_csrand_get() would recurse back into the DRBG that implements it. Describe what actually happens instead. The shared Hash-DRBG is seeded on demand by wolfCrypt's own wc_GenerateSeed(), which on Zephyr reads the hardware entropy driver directly via entropy_get_entropy() on the zephyr,entropy chosen node, and falls back to sys_rand_get() on boards that have no such node. A dead entropy source makes wc_InitRng() fail with RNG_FAILURE_E rather than yield weak output. Also note that the Hash-DRBG requirement is enforced by a build-time #error on HAVE_HASHDRBG. Fix a second stale claim in the same file while here: wolfPSA's Kconfig no longer selects WOLFSSL_HASH_DRBG or WOLFSSL_RNG_SEED_CB. Neither symbol exists in the wolfSSL module Kconfig, and wolfPSA selects no wolfCrypt feature knobs at all; the structural profile comes from the wolfSSL module's own defaults. The same paragraph named WOLFSSL_CRYPTO_ONLY where it meant the WOLFCRYPT_ONLY macro. Documentation only; no functional change.
dgarske
approved these changes
Aug 7, 2026
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The README claimed that wolfPSA's boot init registers a Zephyr-CSPRNG seed callback, so that the shared wolfCrypt Hash-DRBG is seeded from the platform entropy source via sys_csrand_get(). That describes a design that no longer exists: no seed callback is registered by either module, and seeding through sys_csrand_get() would recurse back into the DRBG that implements it.
Describe what actually happens instead. The shared Hash-DRBG is seeded on demand by wolfCrypt's own wc_GenerateSeed(), which on Zephyr reads the hardware entropy driver directly via entropy_get_entropy() on the zephyr,entropy chosen node, and falls back to sys_rand_get() on boards that have no such node. A dead entropy source makes wc_InitRng() fail with RNG_FAILURE_E rather than yield weak output. Also note that the Hash-DRBG requirement is enforced by a build-time #error on HAVE_HASHDRBG.
Fix a second stale claim in the same file while here: wolfPSA's Kconfig no longer selects WOLFSSL_HASH_DRBG or WOLFSSL_RNG_SEED_CB. Neither symbol exists in the wolfSSL module Kconfig, and wolfPSA selects no wolfCrypt feature knobs at all; the structural profile comes from the wolfSSL module's own defaults. The same paragraph named WOLFSSL_CRYPTO_ONLY where it meant the WOLFCRYPT_ONLY macro.
Documentation only; no functional change.