Good point! Since Harry's ruse seems to have succeeded, this looks like an oversight on Eliezer's part.
Next time maybe let's not be so lazy with doing a few Caesar shifts, eh?
Which reminds me: While this cypher may be the kid-brother type, it used to be the major-governments type. So for unEnlightened Wizards, maybe it still is enough to fool major governments!
In Order of the Phoenix, Sirius Black seems to know that codes exist, at least. Although he dismisses them as "breakable" and decides to communicate over a floo network connection instead (which makes me wonder if wizards know about one-time pads, and if there are any spells which can break them).
Update: Discussion has moved on to a new thread.
After 61 chapters of Harry Potter and the Methods of Rationality and 5 discussion threads with over 500 comments each, HPMOR discussion has graduated from the main page and moved into the Less Wrong discussion section (which seems like a more appropriate location). You can post all of your insights, speculation, and, well, discussion about Eliezer Yudkowsky's Harry Potter fanfic here.
Previous threads are available under the harry_potter tag on the main page (or: one, two, three, four, five); this and future threads will be found under the discussion section tag (since there is a separate tag system for the discussion section). See also the author page for (almost) all things HPMOR, and AdeleneDawner's Author's Notes archive for one thing that the author page is missing.
As a reminder, it's useful to indicate at the start of your comment which chapter you are commenting on. Time passes but your comment stays the same.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: