What does murmurhash do in BTC? | Bitcointalksearch.org

nuggetbram

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Quote from: gmaxwell on June 28, 2014, 12:37:43 AM

Quote from: nuggetbram on June 27, 2014, 10:02:53 PM

What/why is there murmurhash in the middle of bitcoin?
What does it do, excatly, and why have I never heard of this anywhere? I'm not very proficient with cryptography, so a detailed explanation would be great Wink

It's not used for a cryptographic purpose.

See BIP0037 (or grep the code base for where it is used!).

Aah, that makes much more sense. Thanks!

gmaxwell

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Quote from: nuggetbram on June 27, 2014, 10:02:53 PM

What/why is there murmurhash in the middle of bitcoin?
What does it do, excatly, and why have I never heard of this anywhere? I'm not very proficient with cryptography, so a detailed explanation would be great Wink

It's not used for a cryptographic purpose.

See BIP0037 (or grep the code base for where it is used!).

nuggetbram

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Activity: 326

Merit: 100

After inspecting bitcoin's hashing section, I discovered this in hash.cpp

Code:

unsigned int MurmurHash3(unsigned int nHashSeed, const std::vector& vDataToHash)
{
    // The following is MurmurHash3 (x86_32), see http://code.google.com/p/smhasher/source/browse/trunk/MurmurHash3.cpp
    uint32_t h1 = nHashSeed;
    const uint32_t c1 = 0xcc9e2d51;
    const uint32_t c2 = 0x1b873593;

    const int nblocks = vDataToHash.size() / 4;

    //----------
    // body
    const uint32_t * blocks = (const uint32_t *)(&vDataToHash[0] + nblocks*4);

    for(int i = -nblocks; i; i++)
    {
        uint32_t k1 = blocks[i];

        k1 *= c1;
        k1 = ROTL32(k1,15);
        k1 *= c2;

        h1 ^= k1;
        h1 = ROTL32(h1,13);
        h1 = h1*5+0xe6546b64;
    }

    //----------
    // tail
    const uint8_t * tail = (const uint8_t*)(&vDataToHash[0] + nblocks*4);

    uint32_t k1 = 0;

    switch(vDataToHash.size() & 3)
    {
    case 3: k1 ^= tail[2] << 16;
    case 2: k1 ^= tail[1] << 8;
    case 1: k1 ^= tail[0];
            k1 *= c1; k1 = ROTL32(k1,15); k1 *= c2; h1 ^= k1;
    };

    //----------
    // finalization
    h1 ^= vDataToHash.size();
    h1 ^= h1 >> 16;
    h1 *= 0x85ebca6b;
    h1 ^= h1 >> 13;
    h1 *= 0xc2b2ae35;
    h1 ^= h1 >> 16;

    return h1;
}

What/why is there murmurhash in the middle of bitcoin?

What does it do, excatly, and why have I never heard of this anywhere? I'm not very proficient with cryptography, so a detailed explanation would be great Wink

Thanks,

Topic: What does murmurhash do in BTC? (Read 1122 times)