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myarbrough@oreilly.com 2014-11-18 08:26:34 -08:00
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@ -165,13 +165,13 @@ Simplified payment verification verifies transactions by reference to their _dep
For example, when examining a transaction in block 300,000, a full node links all 300,000 blocks down to the genesis block and builds a full database of UTXO, establishing the validity of the transaction by confirming that the UTXO remains unspent. An SPV node cannot validate whether the UTXO is unspent. Instead, the SPV node will establish a link between the transaction and the block that contains it, using a((("merkle trees","SPV and"))) _merkle path_ (see <<merkle_trees>>). Then, the SPV node waits until it sees the six blocks 300,001 through 300,006 piled on top of the block containing the transaction and verifies it by establishing its depth under blocks 300,006 to 300,001. The fact that other nodes on the network accepted block 300,000 and then did the necessary work to produce six more blocks on top of it is proof, by proxy, that the transaction was not a double-spend.
An SPV node cannot be persuaded that a transaction exists in a block when the transaction does not in fact exist. The SPV node establishes the existence of a transaction in a block by requesting a merkle path proof and by validating the proof of work in the chain of blocks. However, a transaction's existence can be "hidden" from an SPV node. An SPV node can definitely prove that a transaction exists but cannot verify that a transaction, such as a double-spend of the same UTXO, doesn't exist because it doesn't have a record of all transactions. This vulnerability can be used in a denial-of-service attack or for a double-spending attack against SPV nodes. To defend against this, an SPV node needs to connect randomly to several nodes, to increase the probability that it is in contact with at least one honest node. This need to randomly connect means that SPV nodes are vulnerable to network partitioning attacks or Sybil attacks, where they are connected to fake nodes or fake networks and do not have access to honest nodes or the real bitcoin network.
An SPV node cannot be persuaded that a transaction exists in a block when the transaction does not in fact exist. The SPV node establishes the existence of a transaction in a block by requesting a merkle path proof and by validating the proof of work in the chain of blocks. However, a transaction's existence can be "hidden" from an SPV node. An SPV node can definitely prove that a transaction exists but cannot verify that a transaction, such as a double-spend of the same UTXO, doesn't exist because it doesn't have a record of all transactions. This vulnerability can be used in a denial-of-service attack or for a double-spending attack against SPV nodes. To defend against this, an SPV node needs to connect randomly to several nodes, to increase the probability that it is in contact with at least one honest node. This need to randomly connect means that SPV nodes also are vulnerable to network partitioning attacks or Sybil attacks, where they are connected to fake nodes or fake networks and do not have access to honest nodes or the real bitcoin network.
For most practical purposes, well-connected SPV nodes are secure enough, striking the right balance between resource needs, practicality, and security. For the truly security conscious, however, nothing beats running a full-blockchain node.
For most practical purposes, well-connected SPV nodes are secure enough, striking the right balance between resource needs, practicality, and security. For infallible security, however, nothing beats running a full block chain node.
[TIP]
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((("Simplified Payment Verification (SPV) nodes","verification")))A full-blockchain node verifies a transaction by checking the chain of thousands of blocks below it and checks that the UTXO is not spent, whereas an SPV node checks how deep the block is buried by a handful of blocks above it.
((("simplified payment verification (SPV) nodes","verification")))A full block chain node verifies a transaction by checking the entire chain of thousands of blocks below it in order to guarantee that the UTXO is not spent, whereas an SPV node checks how deep the block is buried by a handful of blocks above it.
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((("block headers","getting on SPV nodes")))To get the block headers, SPV nodes use a((("getheaders message"))) +getheaders+ message instead of +getblocks+. The responding peer will send up to 2,000 block headers using a single +headers+ message. The process is otherwise the same as that used by a full node to retrieve full blocks. SPV nodes also set a filter on the connection to peers, to filter the stream of future blocks and transactions sent by the peers. Any transactions of interest are retrieved using a +getdata+ request. The peer generates a((("tx messages"))) +tx+ message containing the transactions, in response. <<spv_synchronization>> shows the synchronization of block headers.