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2

There are no known attacks on SHA3 series that are faster than the generic attacks. Your problem is the 2nd pre-image attack: given a message $m_1$ finding another message $m_2$ such that $m_1 \neq m_2$ and $Hash(m_1)=Hash(m_2)$. SHA3-256 has $2^{256}$ 2nd preimage resistance. Now, you only allow the attacker first 20-byte which is 160-bit of your data. ...


3

Lets get one thing out of the way: forcing one bit to 0 or 1 does not change the output size of the hash. A hash output is not a number, so the output size would not be affected. Reducing hash output is common practice. Although maybe not a direct requirement, generally the output of a hash is considered indistinguishable from random - if the input is ...


18

With all well-regarded hash functions, the bits of the hash all have equal worth: as far as anyone knows (unless they aren't telling), the bits are not correlated. If you take $k$ bits of an $n$-bit hash, you get a $k$-bit hash function. Truncating SHA-256 to 255 bits gives you a hash that's almost as good as SHA-256: it has $2^{255}$ strength against ...


6

Apart from the slightly reduced resistances, there is no problem: Resistances for SHA3-512; Pre-image resistance decreased to $2^{511}$ or $2^{504}$, if 1 bit or 1 byte trimmed, respectively. Secondary preimage resistance decreased to o $2^{511}$ or $2^{504}$, if 1 bit or 1 byte trimmed, respectively. Collision resistance decreased to o $\sqrt{2^{511}} = \...


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