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It is stated in Winternitz One time signature security that MD5 is not safe for Winternitz due to collision attack. Given that WOTS generates multiple, say 32 private keys then hashing them a number of times to obtain 32 public keys. How does a collision attack from MD5 break the Winternitz OTS? Does this mean I have to find 32 collisions of MD5 ?

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I believe that, in the question you cited, the answer included:

BTW: this appears to be more about the proof technique used to prove W-OTS, rather than the actual security; we can create MD5 collisions (invalidating the proof), however we don't know how to use those collisions to actually attack W-OTS-MD5; hence it would appear to be secure (but we can't prove it).

That is, we don't know how a collision attack would break Winternitz; we just can't prove that it can't.

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  • $\begingroup$ "That is, we don't know how a collision attack would break Winternitz; we just can't prove that it can't." -- That's an amazing scientific remark $\endgroup$ – hola Jul 21 at 16:55

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