Those managing 'blogs are frequently told that the administrative account should not have a username of
admin nor of
administrator. Indeed, 'bots attacking this 'blog try the username
admin multiple times every day. None-the-less, I think that concern about easily guessed usernames is quite misplaced.
Ordinary access to an account requires two pieces of identification, the username and a passcode. We can conceptualize these jointly as a single string, the first part of which is practically fixed, the second part of which is changeable. For example, if one had the username
admin and the passcode
h3Ll0p0p3y3, then the string would be
adminh3Ll0p0p3y3 Some might imagine that two strings represent two hoops and therefore more security; but, actually, each character is a hoop. If usernames and passcodes were equally secure, then the username-passcode pairs
kelsey5 dO0DL3bug and
kelsey 5dO0DL3bug would be perfectly equivalent as far as security were concerned. So we can imagine the two strings concatenated, so long as we remember that one set of its characters are unchangeable, while the others may be changed. In general, the form of the string can be conceptualized as u1u2…ump1p2…pn where each
ui represents an unchangeable username character and each
pj represents a changeable passcode character. Now, if we simply know that the administrative account username is
adminp1p2…pn unauthorized access is a matter of guessing the characters of the passcode, without knowing how many they might be. (How passcodes are stored may limit or effectively limit the length of passcodes, but this will typically not have much effect unless those limits are very tight.) On the other hand, if the administrative username is completely unknown, then the string is the apparently more mysterious u1u2…ump1p2…pn That might seem significantly more secure. However, the number of characters in the passcode is unknown to the opponent, and u1u2…um-kp1p2…pn+k is more secure for all 0 < k ≤ m, because usernames are unchangeable. (Were usernames as changeable as are passcode, then the two would be equally secure.) And
adminp1p2…pn+m is more secure than u1u2…ump1p2…pn
So real security here is to be found in long and strong passcodes, for which secret usernames are poor substitutes, and one can easily compensate for a readily guessed username by having a stronger passcode.
[0 (2016:03/30, 04/09)] I've fleshed-out this entry a bit, in an attempt to make in more easily understood.
 See, for example, the entry for 23 March at the Wordfence 'blog.
 The case k = m represents a zero-length username, which really is to say no username at all. It would be quite possible to create a system with just passcodes and no distinct usernames — or, equivalently, a system with very changeable usernames and no passcodes — though this would present some practical difficulties.