Right!
Copied from DES.php, I should admit.
I thought it was used by the Crypt_Base class.

By the way: I've seen you included some comments for developers in Crypt/Base.php. Would it be possible to complete this doc? List the fields and the functions that have to be defined by sublcasses, with their semantics. Give some info about the inline code: the $in parameter, the disallowed variable names (those you use in the code inlining the snippets), etc. This would be great even for people familiar with this code.... just a suggestion :-)