Hello,
The entropy module uses a hash to mix the entropy. It uses SHA-512 if present and SHA-256 otherwise. Depending on the hash function, the entropy module can return either up to 64 bytes (SHA-512) or 32 bytes (SHA-256). The CTR_DRBG module knows about this and requests only 32 bytes at a time if the entropy module only delivers 32 bytes at a time.
It looks like something goes wrong when CTR_DRBG tries to request 64 bytes. This could be, for example, because a buffer is too small somewhere, or because of a limitation of the entropy source. You would need to debug the call to mbedtls_entropy_func to know more.
As a workaround, you can enable MBEDTLS_ENTROPY_FORCE_SHA256. Then the entropy module will use SHA-256 (even if MBEDTLS_SHA512_C is enabled) and only return 32 bytes at a time, and CTR_DRBG understands this and will only request 32 bytes.
mbed-tls@lists.trustedfirmware.org
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Gilles Peskine