1/13 In honor of Bitcoin White Paper day, here’s a thread on things that work differently in Bitcoin today than they were outlined 14 years ago 🧵👇🏽
2/13 “Nodes always consider the longest chain to be the correct one and will keep working on extending it.” 
3/13 Instead of the longest chain, nodes follow the chain with the most work. This is due to the risk of timewarp attacks, in which malicious actors can generate a longer chain with less work by manipulating MedianTimePast, reducing difficulty. bitcoin.stackexchange.com/questions/7583…
4/13 “We define an electronic coin as a chain of digital signatures.”
5/13 A Bitcoin is not defined as a chain of digital signatures, as it is possible to spend and receive coins without any signatures at all. Rather, a Bitcoin is defined as an Unspent Transaction Output, or UTXO.
en.bitcoin.it/wiki/Script#Tr…
en.bitcoin.it/wiki/Script#Tr…
6/13 “Reclaiming Disk Space: Once the latest transaction in a coin is buried under enough blocks, the spent transactions before it can be discarded to save disk space.”
7/13 This concept of pruning was never implemented. Instead of discarding txs and keeping blocks, we keep the txs (UTXO set) and discard old blocks. lists.linuxfoundation.org/pipermail/bitc…
8/13 “The simplified method can be fooled by an attacker's fabricated transactions for as long as the attacker can continue to overpower the network. One strategy to protect against this would be to accept alerts from network nodes when they detect an invalid block.”
9/13 Alerts were never implemented, as you cannot verify a block by itself. To verify a block, all the blocks before it are needed. The only way to implement alerts for simplified payment verification are ZKPs.
10/13 Sidenote: SPV is rarely used. With SPV, users trust miners to tell the truth. SPV is implemented in e.g. Schildbach Wallet, the 1st btc mobile wallet. Instead of using SPV, users today rather use someone else’s node, trusting the node operator.
wallet.schildbach.de
wallet.schildbach.de
11/13 “Normally there will be either a single input from a larger previous transaction or multiple inputs combining smaller amounts, and at most two outputs: one for the payment, and one returning the change, if any, back to the sender.”
12/13 Today we have txs with much more than two outputs, such as with conjoins. For example, just yesterday a tx was created with 407 outputs 🤯 mempool.space/tx/d465033214f…
13/13 Oh how far you've come. Happy #Bitcoin White Paper Day! 🧡🎉
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