Initialize an AES context with just mbedtls_aes_setkey_enc() for both encrypting or decrypting; Put a random value in nonce_counter (This is your nonce + counter within the nonce) Use something like CTR-DRBG. Have a local variable nc_off with a value of 0. the value is updated by the function so it knows where in a 'AES block' it left off.