add some comments to common-hash for new APIs

www/common/common-hash.js

@@ -85,16 +85,27 @@ define([
         return id;
     };
 
+    /*  Given a base64-encoded public key, deterministically derive a channel id
+        Used for support mailboxes
+    */
     Hash.getChannelIdFromKey = function (publicKey) {
         if (!publicKey) { return; }
         return uint8ArrayToHex(Hash.decodeBase64(publicKey).subarray(0,16));
     };
+
+    /*  Given a base64-encoded asymmetric private key
+        derive the corresponding public key
+    */
     Hash.getBoxPublicFromSecret = function (priv) {
         if (!priv) { return; }
         var u8_priv = Hash.decodeBase64(priv);
         var pair = Nacl.box.keyPair.fromSecretKey(u8_priv);
         return Hash.encodeBase64(pair.publicKey);
     };
+
+    /*  Given a base64-encoded private key and public key
+        check that the keys are part of a valid keypair
+    */
     Hash.checkBoxKeyPair = function (priv, pub) {
         if (!pub || !priv) { return false; }
         var u8_priv = Hash.decodeBase64(priv);