j"1LAnZuoQdcKCkpDBKQMCgziGMoPC4VQUckM½<div class="post">Hi,<br/><br/>The Hash() function in util.h forms the backbone of most of bitcoin's crypto:<br/><br/><div class="codeheader">Code:</div><div class="code">template<typename T1><br/>inline uint256 Hash(const T1 pbegin, const T1 pend)<br/>{<br/> uint256 hash1;<br/> SHA256((unsigned char*)&pbegin[0], (pend - pbegin) * sizeof(pbegin[0]), (unsigned char*)&hash1);<br/> uint256 hash2;<br/> SHA256((unsigned char*)&hash1, sizeof(hash1), (unsigned char*)&hash2);<br/> return hash2;<br/>}<br/></div><br/>As you can see, this tries to be more secure by hashing twice. However, this actually reduces security. To break pure SHA256, an attacker needs to find a d' such that SHA256(d') == SHA256(d), for a known d. This is also sufficient to break Hash(). However the attacker can also attack the outer layer of the hash, finding a d' such that SHA256(SHA256(d')) == SHA256(SHA256(d)), even though SHA256(d') != SHA256(d). As you can see, the double hashing here makes it _easier_ to break the hash!<br/><br/>A better solution would be something like:<br/><br/><div class="codeheader">Code:</div><div class="code">template<typename T1><br/>inline vector<unsigned char> HashV(const T1 pbegin, const T1 pend)<br/>{<br/> uint256 sharesult;<br/> uint160 riperesult;<br/> SHA256((unsigned char*)&pbegin[0], (pend - pbegin) * sizeof(pbegin[0]), (unsigned char *)&sharesult);<br/> RIPEMD160((unsigned char*)&pbegin[0], (pend - pbegin) * sizeof(pbegin[0]), (unsigned char *)&riperesult);<br/><br/> vector<unsigned char> ret;<br/> ret.insert(ret.end(), (unsigned char *)(&sharesult), (unsigned char *)(&sharesult + 1));<br/> ret.insert(ret.end(), (unsigned char *)(&riperesult), (unsigned char *)(&riperesult + 1));<br/> return ret;<br/>}<br/></div><br/>The key is to concatenate the hashes, not merge them. This means the attacker has to break both hashes - even in the worst case, this cannot be less secure than a single hash.<br/><br/>Unfortunately, changing hashes would break the chain...</div> text/html
https://whatsonchain.com/tx/b6ecbb74dfbbafa519597e2ff849de0a4e0eda268b1d17f12c5decce33b3950d