cryptanalysis question: Does knowing some of the content of the message make the full message vulnerable to decryption?

[Donald Morgan Jr.]

If you know a user has a signature that they use to always end a message
with, does that data aid in the decryption of the file? Would this exploit
be applicable to symmetric encryption methods as well?
-------------- next part --------------
An HTML attachment was scrubbed...
URL: </pipermail/attachments/20140130/2964936e/attachment.html>