rephrase Features

Author: tomato42

README.md

@@ -15,12 +15,13 @@ into other protocols.
 ## Features
 
 This library provides key generation, signing, and verifying, for five
-popular NIST "Suite B" GF(p) curves, with key lengths of 192, 224, 256, 384,
-and 521 bits. The "short names" for these curves, as known by the OpenSSL
-tool (`openssl ecparam -list_curves`), are: prime192v1, secp224r1,
-prime256v1, secp384r1, and secp521r1. It also includes the 256-bit curve used
-by Bitcoin, whose short name is secp256k1. No other curves are included, but
-it would not be too hard to add more.
+popular NIST "Suite B" GF(p) (_prime field_) curves, with key lengths of 192,
+224, 256, 384, and 521 bits. The "short names" for these curves, as known by
+the OpenSSL tool (`openssl ecparam -list_curves`), are: `prime192v1`,
+`secp224r1`, `prime256v1`, `secp384r1`, and `secp521r1`. It also includes the
+256-bit curve `secp256k1` used by Bitcoin. No other curves
+are included, but it is not too hard to add support for more curves
+over prime fields.
 
 ## Dependencies