How is the encoding of extraNonce done in the genesis block?

You can see here the coinbase of the genesis block

txNew.vin[0].scriptSig = CScript() << 486604799 << CBigNum(4) << vector<unsigned char>((const unsigned char*)pszTimestamp, (const unsigned char*)pszTimestamp + strlen(pszTimestamp));

Broken down into hex it looks like this

Size of nBits as varint

0x04

nBits themselves

0xffff001d

Satoshi's message in HEX

0x5468652054696d65732030332f4a616e2f32303039204368616e63656c6c6f72206f6e206272696e6b206f66207365636f6e64206261696c6f757420666f722062616e6b73

And between the nBits and Satoshi's message we have these three bytes which should be the extraNonce

0x010445

Which correspond to the code CBigNum(4), since 4 is an integer, it's 4 bytes, if this was encoded as a varint, it's value is < 253 as such it would've taken only 1 byte, but as you see there are three bytes. Upon asking the bitcoin developers on IRC, they told me it's because the BigNum library encodes bits differently.

I started poking around in the Bitcoin source code, specifically the Bignum wrapper, I found many overloaded constructors and operators, but ultimately failed to find how these three bytes are calculated and returned because of my little experience with C++.

However, I believe it's this function

void setulong(unsigned long n)
{
    if (!BN_set_word(this, n))
        throw bignum_error("CBigNum conversion from unsigned long : BN_set_word failed");
}

However it's still not returning these bytes. So my question is, how is the integer 4 which should still be 0x04 in hex encoded to be this sequence of bytes 0x010445(big-endian) so that I can do it in C?

coinbase-transaction

farmdve

Posted 2013-04-18T01:20:13.197

Reputation: 173

Answers

Looks like you're misinterpreting the code.

CBigNum is just a class that stores numbers. CBigNum(4) simply stores the number 4, and CBigNum(SHA256(block)) would store the hash of a block as a number (after all, SHA256 gives us numbers). CBigNum does what it says: it stores a big number. As it has a function that exports the number as big endian, it's useful here, and the Bitcoin source will often use it to concatenate numbers in a data stream.

Let's take a quick look at the genesis block's script, without looking at the code. Please open the relevant wiki page: Script

04FFFF001D0104455468652054696D65732030332F4A616E2F32303039204368616E63656C6C6F72206F6E206272696E6B206F66207365636F6E64206261696C6F757420666F722062616E6B73

Part of this can be interpreted as :

04 -> PUSH 4  bytes (FFFF001D)
01 -> PUSH 1  byte  (04)
45 -> PUSH 69 bytes (The Times 03/Jan/2009 Chancellor on brink of second bailout for banks)

Tom van der Woerdt

Posted 2013-04-18T01:20:13.197

Reputation: 2 397

Thanks. You have to understand though, the code is hardly documented inside. The protocol is documented mostly well, but the code not so much. CBigNum is said to be a wrapper to be a C++ wrapper to BigNum and nothing more. I could not have known what it does. – farmdve – 2013-04-18T09:52:51.863

I've never even looked at that code, for this exact reason. The answer I gave you is based on information I found on the wiki only. – Tom van der Woerdt – 2013-04-18T09:53:53.660