Hi Shariful,

First, please note that the library called PolarSSL with functions like rsa_private() and memory_buffer_alloc_init() has not been supported for several years. You should upgrade to Mbed TLS, with functions like mbedtls_rsa_private() and mbedtls_memory_buffer_alloc_init(). That being said, the memory_buffer_alloc module works in the same way.

Normally, applications call mbedtls_memory_buffer_alloc_init() in the startup code of the initial thread, before creating other threads. The alloc/free functions are thread-safe, but the initialization and deinitialization functions aren't. If you must call mbedtls_memory_buffer_alloc_init() after creating other threads, make sure that no thread calls mbedtls_calloc until mbedtls_memory_buffer_alloc_init() has returned.

The same principle applies to other parts of Mbed TLS that are thread-safe. For example, only the RSA operations (encryption, decryption, signature, verification, and also the low-level functions mbedtls_rsa_public() and mbedtls_rsa_private()) are protected. So you must finish setting up the RSA key inside one thread before you pass a pointer to other threads. Similarly, only mbedtls_xxx_drbg_random() is thread-safe, and the RNG setup (including mbedtls_xxx_drgb_seed()) should be done as part of the initial application startup.

Finally, note that mbedtls_rsa_private() alone cannot decrypt a message: all it does it to apply the private key operation. To decrypt a simple message encrypted with RSA-OAEP, call mbedtls_rsa_rsaes_oaep_decrypt() or mbedtls_rsa_pkcs1_decrypt() with a key set up for MBEDTLS_RSA_PKCS_V21 encoding. To use the legacy PKCS#1v1.5 mechanism, call mbedtls_rsa_rsaes_pkcs1_v15_decrypt() or mbedtls_rsa_pkcs1_decrypt() with a key set up for .MBEDTLS_RSA_PKCS_V15. To decrypt a message using a RSA FDH hybrid scheme, you do need to call mbedtls_rsa_private() since Mbed TLS doesn't support it natively, but what this gives you is the intermediate secret from which you then need to derive a symmetric key, not the message itself.

Best regards,