CS120: Bitcoin for Developers I

Coinbase transactions do not have an unlocking script (aka, scriptSig) field. Instead, this field is replaced by coinbase data, which must be between 2 and 100 bytes. Except for the first few bytes, the rest of the coinbase data can be used by miners in any way they want; it is arbitrary data.

In the genesis block, for example, Satoshi Nakamoto added the text "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks" in the coinbase data, using it as a proof of the date and to convey a message. Currently, miners use the coinbase data to include extra nonce values and strings identifying the mining pool.

The first few bytes of the coinbase used to be arbitrary, but that is no longer the case. As per BIP-34, version-2 blocks (blocks with the version field set to 2) must contain the block height index as a script "push" operation in the beginning of the coinbase field.

In block 277,316 we see that the coinbase, which is in the unlocking script or scriptSig field of the transaction input, contains the hexadecimal value 03443b0403858402062f503253482f. Let's decode this value.

The first byte, 03, instructs the script execution engine to push the next three bytes onto the script stack. The next three bytes, 0x443b04, are the block height encoded in little-endian format (backward, least-significant byte first). Reverse the order of the bytes and the result is 0x043b44, which is 277,316 in decimal.

The next few hexadecimal digits (0385840206) are used to encode an extra nonce, or random value, used to find a suitable Proof-of-Work solution.

The final part of the coinbase data (2f503253482f) is the ASCII-encoded string /P2SH/, which indicates that the mining node that mined this block provides support for the P2SH improvement defined in BIP-16. The introduction of the P2SH capability required signaling by miners to endorse either BIP-16 or BIP-17. Those endorsing the BIP-16 implementation were to include the string /P2SH/ in their coinbase data. Those endorsing the BIP-17 implementation of P2SH were to include the string p2sh/CHV in their coinbase data. Finally, the BIP-16 was elected as the winner, and many miners continued including the string /P2SH/ in their coinbase to indicate that they provide support for this feature.

Extract the coinbase data from the genesis block uses the libbitcoin library introduced in [alt_libraries] to extract the coinbase data from the genesis block, displaying Satoshi's message. Note that the libbitcoin library contains a static copy of the genesis block, so the example code can retrieve the genesis block directly from the library.

Example 6. Extract the coinbase data from the genesis block

link:code/satoshi-words.cpp[]

We compile the code with the GNU C++ compiler and run the resulting executable, as shown in Compiling and running the satoshi-words example code.

Example 7. Compiling and running the satoshi-words example code

# Compile the code
$  g++ -o satoshi-words satoshi-words.cpp $(pkg-config --cflags --libs libbitcoin)
# Run the executable
$ ./satoshi-words
^D��<GS>^A^DEThe Times 03/Jan/2009 Chancellor on brink of second bailout for banks