Title: Password Security: A Case History 
Authors: Robert Morris Ken Thompson   
File name:pubdocpasswordsmorris.password_security_case_study.ps.Z 
Filesize: 19228 bytes 

Abstract: This paper describes the history of
the design of the password security scheme on a remotely accessed
time-sharing system. The present design was the result of
countering observed attempts to penetrate the system.  The result
is a compromise between extreme security and ease of use.