Add constant for shared secret buffer size

We can remove the magic number currently used for length of the
`SharedSecret` array by defining a const `BUF_SIZE`.

Author: tcharding

src/ecdh.rs

@@ -23,6 +23,9 @@ use key::{SecretKey, PublicKey};
 use ffi::{self, CPtr};
 use secp256k1_sys::types::{c_int, c_uchar, c_void};
 
+/// The buffer size (in bytes) used by `SharedSecret`.
+const BUF_SIZE: usize = 256;
+
 /// Enables two parties to create a shared secret without revealing their own secrets.
 ///
 /// # Examples
@@ -42,20 +45,20 @@ use secp256k1_sys::types::{c_int, c_uchar, c_void};
 // ```
 #[derive(Copy, Clone)]
 pub struct SharedSecret {
-    data: [u8; 256],
+    data: [u8; BUF_SIZE],
     len: usize,
 }
 impl_raw_debug!(SharedSecret);
 
 // This implementes `From<N>` for all `[u8; N]` arrays from 128bits(16 byte) to 2048bits allowing known hash lengths.
 // Lower than 128 bits isn't resistant to collisions any more.
-impl_from_array_len!(SharedSecret, 256, (16 20 28 32 48 64 96 128 256));
+impl_from_array_len!(SharedSecret, BUF_SIZE, (16 20 28 32 48 64 96 128 256)); // BUF_SIZE == 256
 
 impl SharedSecret {
     /// Create an empty SharedSecret
     pub(crate) fn empty() ->  SharedSecret {
         SharedSecret {
-            data: [0u8; 256],
+            data: [0u8; BUF_SIZE],
             len: 0,
         }
     }