accurate description of rng

README.md

@@ -86,9 +86,10 @@ U2F layer:
 
 ## Random number generation
 
-U2F Zero has a [true random number generator](https://en.wikipedia.org/wiki/Hardware_random_number_generator) 
-(TRNG).  It's used internally
-for key generation and signing but it's also exposed to the user for other use cases because
+U2F Zero has a [cryptographically secure 
+random number generator](https://en.wikipedia.org/wiki/Cryptographically_secure_pseudorandom_number_generator)
+(CSPRNG) that implements [CTR_DRBG](https://en.wikipedia.org/wiki/NIST_SP_800-90A).
+It's used internally for key generation and signing but it's also exposed to the user because
 a good source of entropy can be useful.
 
 Generate random numbers from the device: