Compressed data vulnerable to known-plaintext?

[anonymous-remailer at shell.portal.com]

Someone who claimed to be Mark M. said on Sat, 1 Jun 1996:

    (I said:)
> > Why not simply use two session keys, and encrypt the headers with one 
> > while encrypting the actual data with the other? That seems to solve both 
> > problems, except that more CPU cycles are required.
> 
> An easier solution would be to just strip of the headers.  If the header is
> always the same, then it is redundant.  If it varies, then it cannot be used
> as known-plaintext.

But then you still have the problem of identifying the contents.  If there
were no headers, one could not tell if the message was compressed using
ZIP, LHA, StuffIt, tar*, compress, gzip, Alice's Magical Supercompressor,
or even if it was left alone.  One could also not tell if the decryption
happened successfully.

( * Yes, I know tar is not compression. )