Make definitions easier to understand
1. removed if !defined to make things clearer
2. changed comments respectively
#define MBEDTLS_CTR_DRBG_BLOCKSIZE 16 /**< The block size used by the cipher. */
-#if !defined(MBEDTLS_CTR_DRBG_USE_128_BIT_KEY) /* Use default 256-bit key for CTR_DRBG. */
-#define MBEDTLS_CTR_DRBG_KEYSIZE 32 /**< The key size used by the cipher. */
+#if defined(MBEDTLS_CTR_DRBG_USE_128_BIT_KEY)
+#define MBEDTLS_CTR_DRBG_KEYSIZE 16 /**< The key size used by the cipher (compile-time choice: 128 bits). */
#else
-#define MBEDTLS_CTR_DRBG_KEYSIZE 16 /**< The key size used by the cipher. */
+#define MBEDTLS_CTR_DRBG_KEYSIZE 32 /**< The key size used by the cipher (compile-time choice: 256 bits). */
#endif
+
#define MBEDTLS_CTR_DRBG_KEYBITS ( MBEDTLS_CTR_DRBG_KEYSIZE * 8 ) /**< The key size for the DRBG operation, in bits. */
#define MBEDTLS_CTR_DRBG_SEEDLEN ( MBEDTLS_CTR_DRBG_KEYSIZE + MBEDTLS_CTR_DRBG_BLOCKSIZE ) /**< The seed length, calculated as (counter + AES key). */
