this post was submitted on 26 Sep 2024
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I remember hearing to not layer encryptions or hashes on top of themselves. It didn't make any sense to me at the time. It was presented as if that weakened the encryption somehow, though wasn't elaborated on (it was a security focused class, not encryption focused, so didn't go heavy into the math).
Like my thought was, if doing more encryption weakened the encryption that was already there, couldn't an attacker just do more encryption themselves to reduce entropy?
The class was overall good, but this was still a university level CS course and I really wish I had pressed on that bit of "advice" more. Best guess at this point is that I misunderstood what was really being said because it just never made any sense at all to me.
It's because layering doesn't really gain you anything so it only has downsides. It's important to differentiate encryption and hashing from here on since the dangers are different.
With hashing, layering different hashing algorithms can lead to increased collision chance and if done wrong a reduced entropy (for instance hashing a 256 bit hash with a 16 bit hashing algorithm). Done correctly it's probably fine and in fact rehashing a hash with the same algorithm is standard practice, but care should be taken.
With encryption things get much worse. When layering encryption algorithms a flaw in one can severely compromise them all. Presumably you're using the same secret across them all. If the attacker has a known piece of input or can potentially control the input a variety of potential attack vectors open up. If there's a flaw in one of the algorithms used that can make the process of extracting the encryption key much easier. Often times the key is more valuable than any single piece of input because keys are often shared across many encrypted files or data streams.
With the hash one, it doesn't look like that could be exploited by an attacker doing the bad hashing themselves, since any collisions they do find will only be relevant to the extra hashing they do on their end.
But that encryption one still sounds like it could be exploited by an attacker applying more encryption themselves. Though I'm assuming there's a public key the attacker has access to and if more layers of encryption make it easier to determine the associated private key, then just do that?
Though when you say they share the same secret, my assumption is that a public key for one algorithm doesn't map to the same private key as another algorithm, so wouldn't cracking one layer still be uncorrelated with cracking the other layers? Assuming it's not reusing a one time pad or something like that, so I guess context matters here.