Efficiently throttling user authentication

At a client computer system including at least one processor and a memory, in a computer networking environment including a plurality of computing systems, a computer-implemented method for providing login error messages while preventing enumeration attacks, the method comprising:

• an act of the processor sending user login credentials from a user to an authentication server at a specified time, the user login credentials including a user identifier and a password, the authentication server making at least one of the following determinations:
• determining that the user identifier does not match any existing user account;
• determining that the user identifier matches at least one existing user account, but the user's account is in a locked state;
• and
• determining that the user identifier matches at least one existing user account, but the user's password does not match the user identifier;
• and
• an act of receiving a response message from the authentication server indicating that the user's credentials are invalid, the authentication server having generated a variable delay based on the time the login credentials were received at the authentication server, the generated delay accounting for the amount of time taken by the authentication server to make the at least one determination, the generated delay ensuring that each response message is returned to the user at the same elapsed time since the user's login credentials were received, regardless of which determination was made, wherein the length of the delay is variable and is dynamically generated for each login attempt to ensure that each response is sent after the same amount of time has elapsed since the user's login credentials were received;
• and
• wherein the response message prevents the user from determining which of the credentials was invalid as the response message is the same for each determination and is received by the user after a measured response time that is the same for each determination.

A computer system comprising the following:

• one or more processors;
• system memory;
• one or more computer-readable storage media having stored thereon computer-executable instructions that, when executed by the one or more processors, causes the computing system to perform a method for efficiently authenticating users while preventing enumeration attacks, the method comprising the following:
• an act of receiving user login credentials from a user at a specified time, the user login credentials including a user identifier and a password;
• an act of making at least one of the following determinations:
• determining that the user identifier does not match any existing user account;
• determining that the user identifier matches at least one existing user account, but the user's account is in a locked state;
• and
• determining that the user identifier matches at least one existing user account, but the user's password does not match the user identifier;
• an act of generating a variable delay based on the time the login credentials were received at the computer system, the generated delay accounting for the amount of time taken to make the at least one determination, the generated delay ensuring that each response message is returned to the user at substantially the same elapsed time since the user's login credentials were received, regardless of which determination was made, wherein the length of the delay is variable and is dynamically generated for each login attempt to ensure that each response is sent after the same amount of time has elapsed since the user's login credentials were received;
• and
• upon application of the generated delay, an act of returning to the user the same response message regardless of which determination is made, the response indicating that the user's login credentials are invalid, wherein the response message prevents the user from determining which of the credentials was invalid as the response message is the same for each determination and is sent to the user after a measured response time that is the same for each determination.