Detecting covert data channels?

It is reasonably trivial to encode data within packet headers, and
even encrypt said data as most are probably aware.  There are past
examples where control information has been sent within ICMP and other
packets using header fields.

My question surrounds detection; given that IDS tends to be payload
focused, if a covert channel exists that has encrypted data in a
packet header, how do we go about detecting it?

My initial thought leans toward the fact that encrypted data blocks
are statistically flat over time.  Given say 'snort', how can we use
this idea?   I am not a snort expert by any means, so please no
flames!

I would be happy to summarize opinions.

-Joff Thyer

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