Since trim also removes several other characters that might end up in an encrypted string (potentially at the end), you shouldn't have been using it in the first place. An encrypted string is meant to be used EXACTLY as created, with no changes to it afterwards.

Char(26) (Ctrl-Z) is frequently the EOF marker for text files, and should be removed with trim as it's not a standard displayable text character (some font families don't have it defined at all).

trim( string) should only trim off spaces.

trim(string, true) should trim off all types of whitespace - although char(26) is not listed as one of the characters that would be removed.

you should submit this as a bug

I agree with Brad, you shouldn't be using Trim on an encrypted string. Anything you do to modify the encrypted string will prevent it from being decrypted later.