@ -149,15 +149,11 @@ Merkle trees are used in bitcoin to summarize all the transactions in a block, p
The merkle tree is constructed bottom-up. In the example below, we start with four transactions A, B, C and D, which form the _leaves_ of the Merkle Tree, shown in the diagram at the bottom. The transactions are not stored in the merkle tree, rather their data is hashed and the resulting hash is stored in each leaf node as H~A~, H~B~, H~C~ and H~D~:
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H~A~ = SHA256(SHA256(Transaction A))
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+H~A~ = SHA256(SHA256(Transaction A))+
Consecutive pairs of leaf nodes are then summarized in a parent node, by concatenating the two hashes and hashing them together. For example, to construct the parent node H~AB~, the two 32-byte hashes of the children are concatenated to create a 64-byte string. That string is then double-hashed to produce the parent node's hash:
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H~AB~ = SHA256(SHA256(H~A~ + H~B~))
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+H~AB~ = SHA256(SHA256(H~A~ + H~B~))+
The process continues until there is only one node at the top, the node known as the Merkle Root. That 32-byte hash is stored in the block header and summarizes all the data in all four transactions.
Andreas M. Antonopoulos
10 years ago