Fixing minor nits: conversion to u16 and removing keypair constructor

(cherry picked from commit c8c57785af)

src/key_exchange/tripledh.rs

@@ -24,9 +24,8 @@ use generic_bytes::SizedBytes;
 use hkdf::Hkdf;
 use hmac::{Hmac, Mac, NewMac};
 use rand::{CryptoRng, RngCore};
-use zeroize::Zeroize;
-
 use std::convert::TryFrom;
+use zeroize::Zeroize;
 
 const KEY_LEN: usize = 32;
 pub(crate) type NonceLen = U32;
@@ -540,7 +539,9 @@ fn hkdf_expand_label_extracted<D: Hash>(
     let mut okm = vec![0u8; length];
 
     let mut hkdf_label: Vec<u8> = Vec::new();
-    hkdf_label.extend_from_slice(&length.to_be_bytes()[std::mem::size_of::<usize>() - 2..]);
+
+    let length_u16: u16 = u16::try_from(length).map_err(|_| PakeError::SerializationError)?;
+    hkdf_label.extend_from_slice(&length_u16.to_be_bytes());
 
     let mut opaque_label: Vec<u8> = Vec::new();
     opaque_label.extend_from_slice(&STR_OPAQUE);