+.SS x25519
+.PP
+\fBx25519
+.PP
+A premade \fIdh closure\fR
+which uses Daniel Bernstein's X25519 key-exchange function.
+This uses an elliptic curve called Curve25519,
+defined over a 255-bit field.
+The function is fast and very well-studied.
+.PP
+A \fIdh closure\fR defines a group to be used for key exchange.
+The
+.B x25519
+Diffie\(enHellman group always uses capability number 24.
+
+.SS x448
+.PP
+\fBx448
+.PP
+A premade \fIdh closure\fR
+which uses Mike Hamburg's X448 key-exchange function.
+This uses an elliptic curve called Ed448-Goldilocks,
+defined over a 448-bit field.
+The function is unusually quick and fairly well studied.
+.PP
+A \fIdh closure\fR defines a group to be used for key exchange.
+The
+.B x448
+Diffie\(enHellman group always uses capability number 25.
+