# Weaknesses arising from using same key in both channel directions

I came across the following question:

Which of the following risk may arise, when same key is used to encrypt both directions of a communication channel, that are not present if using different keys in both direction?

The answer to the question was reflection attack. The other options provided were denial of service, eavesdropping and none of the above.

I presume it could be eavesdropping. Eg: When a Diffie-Hellman key exchange is performed, a man-in-the-middle attack (type of eavesdropping attack) could occur.

So is the answer reflection attack or eavesdropping or something else entirely and why ? I tried looking up resources to justify and figure out the right answer but this is what I could conclude with.

Suppose that the keys used in both directions are the same. You listen to something sent from A to B:

kjhsdfiugdsa8ltr634o87qy5843jlgrwjklfjgsfds


You then listen to something sent from B to A:

jh35i798fdgkjh53jkh87gfhkjsfhtjk35h1o8798ug


Does it matter that the key is the same? Can you understand any of the messages?

If the answer to these questions is negative, then eavesdropping can be ruled out.

In contrast, suppose that you sent the following message from B to A:

kjhsdfiugdsa8ltr634o87qy5843jlgrwjklfjgsfds


You don't necessarily have any idea what this means, but it is identical to a message sent from A to B.

Because the key is the same, whatever meaning the original message had, so does this one.

This kind of attack (which is easy to thwart even if the keys are the same) is known as replay.

• Is there a way a reflection attack can be used in this scenario ? – mahesh Rao Sep 3 at 18:06
• It's the same thing as a replay attack. – Yuval Filmus Sep 3 at 18:20