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With respect to collisions, hashing twice can not increase security, because if $x$ and $x'\ne x$ collide for $H$, that is $H(x)=H(x')$, then $H(H(x))=H(H(x'))$. Otherwise said, any collision for $H$ is a collision for the double hash $H\circ H$. It is therefore trivial to exhibit collisions for $\operatorname{MD5}\circ\operatorname{MD5}$. Hence the answer ...


2

SipHash doesn't claim to be a secure hash function. Only a secure MAC. So if you try to use it as a hash function, with a constant, public key, you are on your own. SHA-512/64 should be a "secure" 64-bit hash, which is of course not enough for a truly secure hash, since it only has 32-bit collision resistance. However, since you only desire preimage ...


1

A quick resarch showed that there are no (good) attacks on Siphash. For SHA-512 there are defintely no known attacks. The first 64 bits of SHA-512 should have the same security guarantees as full SHA-512 has. So breaking any of the two comes down to how fast they are. SHA-512 is slower, in particular it achieves 192.5 cycles / byte in a 64-bit C ...



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