主要摘自 Mastering Bitcoin - chapter07

区块链是一串使用密码学方法相关联产生的数据块。每一个区块都有一个唯一的哈希值，区块头还有一个指向前一个区块。每个区块只能有一个父区块，但可以暂时拥有多个子区块。

当父区块有任何改动时，父区块的哈希值也随之变化，因此子区块的「父区块哈希值」也改变，从而又将导致子区块的哈希值发生改变。以此类推，一旦一个区块有很多代以后，这种瀑布效应将保证该区块不会被改变，除非强制重新计算该区块所有后续的区块。而重新计算需要耗费巨大的计算量，所以一个长区块链的存在可以让区块链的历史不可改变。

Structure of a Block

Size	Field (low)	Desciption (high)
4 bytes	Block Size	The size of the block, in bytes, following this field
80 bytes	Block Header	Several fields form the block header
1-9 bytes (VarInt)	Transaction Counter	How many transactions follow
Variable	Transactions	The transactions recorded in this block

Block Header

Size	Field (low)	Desciption (high)
4 bytes	Version	A version number to track software/protocol upgrades
32 bytes	Previous Block Hash	A reference to the hash of the previous (parent) block in the chain
32 bytes	Merkle Root	A hash of the root of the merkle tree of this block’s transactions
4 bytes	Timestamp	The approximate creation time of this block (seconds from Unix Epoch)
4 bytes	Difficulty Target	The proof-of-work algorithm difficulty target for this block
4 bytes	Nonce	A counter used for the proof-of-work algorithm

Linking Blocks in the Blockchain

{
	"size" : 43560,
	"version" : 2,
	"previousblockhash" :
    	"00000000000000027e7ba6fe7bad39faf3b5a83daed765f05f7d1b71a1632249",
	"merkleroot" :
    	"5e049f4030e0ab2debb92378f53c0a6e09548aea083f3ab25e1d94ea1155e29d",
	"time" : 1388185038,
	"difficulty" : 1180923195.25802612,
	"nonce" : 4215469401,
	"tx" : [
    	"257e7497fb8bc68421eb2c7b699dbab234831600e7352f0d9e6522c7cf3f6c77",
    	... many more transactions omitted ...
    	"05cfd38f6ae6aa83674cc99e4d75a1458c165b7ab84725eda41d018a09176634"
    ]
} 

Merkle tree

区块链中的每个区块都包含了产生于该区块的所有交易，且以 Merkle 树表示。

Merkle Tree 就是不断两两哈希，最后得到一整棵树的最后哈希值，也就是根节点, Merkle Root.

相比 Hash List, Merkle Tree 的一个好处是可以单独拿一个分支出来对部分数据进行校验，也能很容易就找到哪一小块数据是不对的。