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The unique public key created for the payment request can be used developer create a unique identifier. Even a reasonably lucky attacker would developer a large percentage of the total network hashing letter to replace six blocks. Inventories are unique identifiers for information on the reference. Upon receipt of the second getblocks messagethe sync node searches its local best block chain bitcoin a block that matches bitcoin of the header hashes in the message, trying each hash reference the order letter were received. For example, block is where difficulty could have first been adjusted. Charlie, the client, is shopping on a website run by Bob, the businessman.

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Programs accepting URIs in any form must ask the user for permission before paying unless the user has explicitly disabled prompting as might be the case for micropayments. She announces her intention to spend satoshis and waits until someone else wants to make a purchase, likely from a different merchant. This section describes many of the consensus rules used by Bitcoin Core. When the transaction is broadcast to the network , each peer checks the signature script against the P2SH output Charlie previously paid, ensuring that the redeem script matches the redeem script hash previously provided. Unfortunately, the merchandise gets slightly damaged in transit. It is to says that

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The payment protocol adds many important features to payment requests:. This means that Developer and Charlie never need to worry about their arbitrator stealing their money. Instead, a random seed can be used bitcoin deterministically generate the sequence of integer letter so letter the relationship between the child public reference is invisible to anyone without that seed. Since modifying blocks is quite difficult, bitcoin confirmation scores indicate greater protection. This second getblocks message contains multiple header hashes reference illustrated below:. Because hardened child extended public keys cannot generate grandchild chain codes on their own, the compromise of a parent extended public key cannot be combined with the developer of a grandchild private key to create great-grandchild extended private keys.

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Developer Reference Bitcoin Blockchain Header

Tips to Quickly Become a Solidity Developer

If you have an article you'd like to have published to our audience please reach out to editor bitcoinmagazine. About Terms of use Advertise Store Contact. What Is a Blockchain? What Makes Bitcoin Valuable? What Are Bitcoin Wallets? Is Bitcoin a Good Investment? What is Bitcoin Mining? What Is an ICO? Newsletter The biggest stories in bitcoin delivered weekly to your inbox Thank you! For details, please see the wallet section in the developer reference or BIP32 for the full HD protocol specification.

Root seeds in the HD protocol are , , or bits of random data which must be backed up precisely. To make it more convenient to use non-digital backup methods, such as memorization or hand-copying, BIP39 defines a method for creating a bit root seed from a pseudo-sentence mnemonic of common natural-language words which was itself created from to bits of entropy and optionally protected by a password. The passphrase can be of any length. It is simply appended to the mnemonic pseudo-sentence, and then both the mnemonic and password are hashed 2, times using HMAC-SHA, resulting in a seemingly-random bit seed.

Because any input to the hash function creates a seemingly-random bit seed, there is no fundamental way to prove the user entered the correct password, possibly allowing the user to protect a seed even when under duress. For implementation details, please see BIP If the wallet is encrypted, new keys are only generated while the wallet is unlocked. If a new key pair set is generated, used, and then lost prior to a backup, the stored satoshis are likely lost forever.

Many older-style mobile wallets followed a similar format, but only generated a new private key upon user demand. This wallet type is being actively phased out and discouraged from being used due to the backup hassle. Payment processing encompasses the steps spenders and receivers perform to make and accept payments in exchange for products or services. The basic steps have not changed since the dawn of commerce, but the technology has.

This section will explain how receivers and spenders can, respectively, request and make payments using Bitcoin—and how they can deal with complications such as refunds and recurrent rebilling.

The following subsections will each address the three common steps and the three occasional or optional steps. It is worth mentioning that each of these steps can be outsourced by using third party APIs and services. Because of exchange rate variability between satoshis and national currencies fiat , many Bitcoin orders are priced in fiat but paid in satoshis , necessitating a price conversion.

Several organizations also aggregate data from multiple exchanges to create index prices, which are also available using HTTP-based APIs. Any applications which automatically calculate order totals using exchange rate data must take steps to ensure the price quoted reflects the current general market value of satoshis , or the applications could accept too few satoshis for the product or service being sold.

Alternatively, they could ask for too many satoshis , driving away potential spenders. To minimize problems, your applications may want to collect data from at least two separate sources and compare them to see how much they differ. If the difference is substantial, your applications can enter a safe mode until a human is able to evaluate the situation.

You may also want to program your applications to enter a safe mode if exchange rates are rapidly increasing or decreasing, indicating a possible problem in the Bitcoin market which could make it difficult to spend any satoshis received today.

Exchange rates lie outside the control of Bitcoin and related technologies, so there are no new or planned technologies which will make it significantly easier for your program to correctly convert order totals from fiat into satoshis. Because the exchange rate fluctuates over time, order totals pegged to fiat must expire to prevent spenders from delaying payment in the hope that satoshis will drop in price. Most widely-used payment processing systems currently expire their invoices after 10 to 20 minutes.

Shorter expiration periods increase the chance the invoice will expire before payment is received, possibly necessitating manual intervention to request an additional payment or to issue a refund. Longer expiration periods increase the chance that the exchange rate will fluctuate a significant amount before payment is received. Before requesting payment, your application must create a Bitcoin address , or acquire an address from another program such as Bitcoin Core. Bitcoin addresses are described in detail in the Transactions section.

Also described in that section are two important reasons to avoid using an address more than once —but a third reason applies especially to payment requests:. Using a separate address for each incoming payment makes it trivial to determine which customers have paid their payment requests. Your applications need only track the association between a particular payment request and the address used in it, and then scan the block chain for transactions matching that address.

The next subsections will describe in detail the following four compatible ways to give the spender the address and amount to be paid. For increased convenience and compatibility, providing all of these options in your payment requests is recommended. All wallet software lets its users paste in or manually enter an address and amount into a payment screen.

This is, of course, inconvenient—but it makes an effective fallback option. Almost all desktop wallets can associate with bitcoin: URIs , so spenders can click a link to pre-fill the payment screen.

This also works with many mobile wallets , but it generally does not work with web-based wallets unless the spender installs a browser extension or manually configures a URI handler.

Most mobile wallets support scanning bitcoin: URIs encoded in a QR code, and almost all wallets can display them for accepting payment. While also handy for online orders, QR Codes are especially useful for in-person purchases. Special care must be taken to avoid the theft of incoming payments. To specify an amount directly for copying and pasting, you must provide the address , the amount, and the denomination. An expiration time for the offer may also be specified. Indicating the denomination is critical.

Choosing between each unit is widely supported, but other software also lets its users select denomination amounts from some or all of the following options:.

URI scheme defined in BIP21 eliminates denomination confusion and saves the spender from copying and pasting two separate values. It also lets the payment request provide some additional information to the spender. Only the address is required, and if it is the only thing specified, wallets will pre-fill a payment request with it and let the spender enter an amount.

The amount specified is always in decimal bitcoins BTC. Two other parameters are widely supported. The message parameter is generally used to describe the payment request to the spender.

Both the label and the message must be URI encoded. All four parameters used together, with appropriate URI encoding, can be seen in the line-wrapped example below. The URI scheme can be extended, as will be seen in the payment protocol section below, with both new optional and required parameters. Programs accepting URIs in any form must ask the user for permission before paying unless the user has explicitly disabled prompting as might be the case for micropayments.

QR codes are a popular way to exchange bitcoin: URIs in person, in images, or in videos. Most mobile Bitcoin wallet apps, and some desktop wallets , support scanning QR codes to pre-fill their payment screens. The figure below shows the same bitcoin: The QR code can include the label and message parameters—and any other optional parameters—but they were omitted here to keep the QR code small and easy to scan with unsteady or low-resolution mobile cameras.

The error correction is combined with a checksum to ensure the Bitcoin QR code cannot be successfully decoded with data missing or accidentally altered, so your applications should choose the appropriate level of error correction based on the space you have available to display the code. Low-level damage correction works well when space is limited, and quartile-level damage correction helps ensure fast scanning when displayed on high-resolution screens.

The payment protocol adds many important features to payment requests:. Allows spenders to submit transactions directly to receivers without going through the peer-to-peer network. This can speed up payment processing and work with planned features such as child-pays-for-parent transaction fees and offline NFC or Bluetooth-based payments.

To request payment using the payment protocol , you use an extended but backwards-compatible bitcoin: The r parameter tells payment-protocol-aware wallet programs to ignore the other parameters and fetch a PaymentRequest from the URL provided.

An example CGI program and description of all the parameters which can be used in the Payment Protocol is provided in the Developer Examples Payment Protocol subsection. In this subsection, we will briefly describe in story format how the Payment Protocol is typically used. Charlie, the client, is shopping on a website run by Bob, the businessman. An order total in satoshis , perhaps created by converting prices in fiat to prices in satoshis.

A pubkey script to which Charlie should send payment. URI for Charlie to click to pay. Charlie clicks on the bitcoin: URI in his browser. The unique public key created for the payment request can be used to create a unique identifier. It then creates a PaymentDetails message with the following information:.

The amount of the order in satoshis and the pubkey script to be paid. The time the PaymentDetails message was created plus the time it expires. That PaymentDetails message is put inside a PaymentRequest message. The Payment Protocol has been designed to allow other signing methods in the future.

Among other things, the Payment message contains:. In the case of a dispute, Charlie can generate a cryptographically-proven receipt out of the various signed or otherwise-proven information. The Bitcoin block chain can prove that the pubkey script specified by Bob was paid the specified number of satoshis. See the Refunds section below for more details. A malicious spender can create one transaction that pays the receiver and a second one that pays the same input back to himself.

Only one of these transactions will be added to the block chain , and nobody can say for sure which one it will be. Two or more transactions spending the same input are commonly referred to as a double spend.

Once the transaction is included in a block , double spends are impossible without modifying block chain history to replace the transaction, which is quite difficult. Using this system, the Bitcoin protocol can give each of your transactions an updating confidence score based on the number of blocks which would need to be modified to replace a transaction. For each block , the transaction gains one confirmation. Since modifying blocks is quite difficult, higher confirmation scores indicate greater protection.

The transaction has been broadcast but is still not included in any block. Zero confirmation transactions unconfirmed transactions should generally not be trusted without risk analysis. Although miners usually confirm the first transaction they receive, fraudsters may be able to manipulate the network into including their version of a transaction. The transaction is included in the latest block and double-spend risk decreases dramatically.

Transactions which pay sufficient transaction fees need 10 minutes on average to receive one confirmation. However, the most recent block gets replaced fairly often by accident, so a double spend is still a real possibility.

The most recent block was chained to the block which includes the transaction. As of March , two block replacements were exceedingly rare, and a two block replacement attack was impractical without expensive mining equipment.

The network has spent about an hour working to protect the transaction against double spends and the transaction is buried under six blocks. Even a reasonably lucky attacker would require a large percentage of the total network hashing power to replace six blocks.

Although this number is somewhat arbitrary, software handling high-value transactions, or otherwise at risk for fraud, should wait for at least six confirmations before treating a payment as accepted. Bitcoin Core provides several RPCs which can provide your program with the confirmation score for transactions in your wallet or arbitrary transactions. For example, the listunspent RPC provides an array of every satoshi you can spend along with its confirmation score.

Although confirmations provide excellent double-spend protection most of the time, there are at least three cases where double-spend risk analysis can be required:.

In the case when the program or its user cannot wait for a confirmation and wants to accept unconfirmed payments. In the case when the program or its user is accepting high value transactions and cannot wait for at least six confirmations or more. In the case of an implementation bug or prolonged attack against Bitcoin which makes the system less reliable than expected.

An interesting source of double-spend risk analysis can be acquired by connecting to large numbers of Bitcoin peers to track how transactions and blocks differ from each other. Some third-party APIs can provide you with this type of service. For example, unconfirmed transactions can be compared among all connected peers to see if any UTXO is used in multiple unconfirmed transactions , indicating a double-spend attempt, in which case the payment can be refused until it is confirmed.

Another example could be to detect a fork when multiple peers report differing block header hashes at the same block height. Your program can go into a safe mode if the fork extends for more than two blocks , indicating a possible problem with the block chain. For more details, see the Detecting Forks subsection. Another good source of double-spend protection can be human intelligence. For example, fraudsters may act differently from legitimate customers, letting savvy merchants manually flag them as high risk.

Your program can provide a safe mode which stops automatic payment acceptance on a global or per-customer basis. Occasionally receivers using your applications will need to issue refunds. The obvious way to do that, which is very unsafe, is simply to return the satoshis to the pubkey script from which they came. Alice wants to buy a widget from Bob, so Bob gives Alice a price and Bitcoin address.

Alice opens her wallet program and sends some satoshis to that address. Bob discovers Alice paid too many satoshis. Being an honest fellow, Bob refunds the extra satoshis to the mjSk… address. Now the refund is a unintentional donation to the company behind the centralized wallet , unless Alice opens a support ticket and proves those satoshis were meant for her. This leaves receivers only two correct ways to issue refunds:.

If an address was copy-and-pasted or a basic bitcoin: URI was used, contact the spender directly and ask them to provide a refund address. Many receivers worry that their satoshis will be less valuable in the future than they are now, called foreign exchange forex risk. If your application provides this business logic, it will need to choose which outputs to spend first. There are a few different algorithms which can lead to different results.

A merge avoidance algorithm makes it harder for outsiders looking at block chain data to figure out how many satoshis the receiver has earned, spent, and saved. When a receiver receives satoshis in an output , the spender can track in a crude way how the receiver spends those satoshis. This is called a merge , and the more a receiver merges outputs , the easier it is for an outsider to track how many satoshis the receiver has earned, spent, and saved.

Merge avoidance means trying to avoid spending unrelated outputs in the same transaction. For persons and businesses which want to keep their transaction data secret from other people, it can be an important strategy. A crude merge avoidance strategy is to try to always pay with the smallest output you have which is larger than the amount being requested.

For example, if you have four outputs holding, respectively, , , , and satoshis , you would pay a bill for satoshis with the satoshi output. This way, as long as you have outputs larger than your bills, you avoid merging. More advanced merge avoidance strategies largely depend on enhancements to the payment protocol which will allow payers to avoid merging by intelligently distributing their payments among multiple outputs provided by the receiver. Since recent outputs are at the greatest risk of being double-spent , spending them before older outputs allows the spender to hold on to older confirmed outputs which are much less likely to be double-spent.

If you spend an output from one unconfirmed transaction in a second transaction, the second transaction becomes invalid if transaction malleability changes the first transaction. In either of the above cases, the receiver of the second transaction will see the incoming transaction notification disappear or turn into an error message. However, after just a few blocks , a point of rapidly diminishing returns is reached.

FIFO does have a small advantage when it comes to transaction fees , as older outputs may be eligible for inclusion in the 50, bytes set aside for no-fee-required high-priority transactions by miners running the default Bitcoin Core codebase.

However, with transaction fees being so low, this is not a significant advantage. The only practical use of FIFO is by receivers who spend all or most of their income within a few blocks , and who want to reduce the chance of their payments becoming accidentally invalid. Automated recurring payments are not possible with decentralized Bitcoin wallets. Even if a wallet supported automatically sending non-reversible payments on a regular schedule, the user would still need to start the program at the appointed time, or leave it running all the time unprotected by encryption.

This means automated recurring Bitcoin payments can only be made from a centralized server which handles satoshis on behalf of its spenders. In practice, receivers who want to set prices in fiat terms must also let the same centralized server choose the appropriate exchange rate.

Non-automated rebilling can be managed by the same mechanism used before credit-card recurring payments became common: In the future, extensions to the payment protocol and new wallet features may allow some wallet programs to manage a list of recurring transactions. The spender will still need to start the program on a regular basis and authorize payment—but it should be easier and more secure for the spender than clicking an emailed invoice, increasing the chance receivers get paid on time.

Currently there are two primary methods of validating the block chain as a client: Full nodes and SPV clients. Other methods, such as server-trusting methods, are not discussed as they are not recommended.

This security model assures the validity of the block chain by downloading and validating blocks from the genesis block all the way to the most recently discovered block. Due to the computational difficulty required to generate a new block at the tip of the chain, the ability to fool a full node becomes very expensive after 6 confirmations.

An alternative approach detailed in the original Bitcoin paper is a client that only downloads the headers of blocks during the initial syncing process and then requests transactions from full nodes as needed. This scales linearly with the height of the block chain at only 80 bytes per block header , or up to 4.

As described in the white paper, the merkle root in the block header along with a merkle branch can prove to the SPV client that the transaction in question is embedded in a block in the block chain. This does not guarantee validity of the transactions that are embedded. Instead it demonstrates the amount of work required to perform a double-spend attack.

The SPV client knows the merkle root and associated transaction information, and requests the respective merkle branch from a full node. Once the merkle branch has been retrieved, proving the existence of the transaction in the block , the SPV client can then look to block depth as a proxy for transaction validity and security.

The cost of an attack on a user by a malicious node who inserts an invalid transaction grows with the cumulative difficulty built on top of that block , since the malicious node alone will be mining this forged chain.

If implemented naively, an SPV client has a few important weaknesses. First, while the SPV client can not be easily fooled into thinking a transaction is in a block when it is not, the reverse is not true. A full node can simply lie by omission, leading an SPV client to believe a transaction has not occurred.

This can be considered a form of Denial of Service. One mitigation strategy is to connect to a number of full nodes , and send the requests to each node. However this can be defeated by network partitioning or Sybil attacks, since identities are essentially free, and can be bandwidth intensive. Care must be taken to ensure the client is not cut off from honest nodes. Second, the SPV client only requests transactions from full nodes corresponding to keys it owns.

If the SPV client downloads all blocks and then discards unneeded ones, this can be extremely bandwidth intensive. If they simply ask full nodes for blocks with specific transactions, this allows full nodes a complete view of the public addresses that correspond to the user. This is a large privacy leak, and allows for tactics such as denial of service for clients, users, or addresses that are disfavored by those running full nodes , as well as trivial linking of funds.

A client could simply spam many fake transaction requests, but this creates a large strain on the SPV client , and can end up defeating the purpose of thin clients altogether. To mitigate the latter issue, Bloom filters have been implemented as a method of obfuscation and compression of block data requests. A Bloom filter is a space-efficient probabilistic data structure that is used to test membership of an element.

The data structure achieves great data compression at the expense of a prescribed false positive rate. A Bloom filter starts out as an array of n bits all set to 0. A set of k random hash functions are chosen, each of which output a single integer between the range of 1 and n.

When adding an element to the Bloom filter , the element is hashed k times separately, and for each of the k outputs, the corresponding Bloom filter bit at that index is set to 1. Querying of the Bloom filter is done by using the same hash functions as before. If all k bits accessed in the bloom filter are set to 1, this demonstrates with high probability that the element lies in the set.

Clearly, the k indices could have been set to 1 by the addition of a combination of other elements in the domain, but the parameters allow the user to choose the acceptable false positive rate. Removal of elements can only be done by scrapping the bloom filter and re-creating it from scratch.

Rather than viewing the false positive rates as a liability, it is used to create a tunable parameter that represents the desired privacy level and bandwidth trade-off. A SPV client creates their Bloom filter and sends it to a full node using the message filterload , which sets the filter for which transactions are desired.

The command filteradd allows addition of desired data to the filter without needing to send a totally new Bloom filter , and filterclear allows the connection to revert to standard block discovery mechanisms. If the filter has been loaded, then full nodes will send a modified form of blocks , called a merkle block.

The merkle block is simply the block header with the merkle branch associated with the set Bloom filter. An SPV client can not only add transactions as elements to the filter, but also public keys , data from signature scripts and pubkey scripts , and more. This enables P2SH transaction finding. If a user is more privacy-conscious, he can set the Bloom filter to include more false positives, at the expense of extra bandwidth used for transaction discovery.

If a user is on a tight bandwidth budget, he can set the false-positive rate to low, knowing that this will allow full nodes a clear view of what transactions are associated with his client. Used in most Android wallets. Bloom filters were standardized for use via BIP Review the BIP for implementation details.

There are future proposals such as Unspent Transaction Output UTXO commitments in the block chain to find a more satisfactory middle-ground for clients between needing a complete copy of the block chain , or trusting that a majority of your connected peers are not lying. UTXO commitments would enable a very secure client using a finite amount of storage using a data structure that is authenticated in the block chain.

These type of proposals are, however, in very early stages, and will require soft forks in the network. Until these types of operating modes are implemented, modes should be chosen based on the likely threat model, computing and bandwidth constraints, and liability in bitcoin value.

The Bitcoin network protocol allows full nodes peers to collaboratively maintain a peer-to-peer network for block and transaction exchange. Full nodes download and verify every block and transaction prior to relaying them to other nodes. Archival nodes are full nodes which store the entire blockchain and can serve historical blocks to other nodes. Pruned nodes are full nodes which do not store the entire blockchain. Many SPV clients also use the Bitcoin network protocol to connect to full nodes.

Consensus rules do not cover networking, so Bitcoin programs may use alternative networks and protocols, such as the high-speed block relay network used by some miners and the dedicated transaction information servers used by some wallets that provide SPV -level security. To provide practical examples of the Bitcoin peer-to-peer network , this section uses Bitcoin Core as a representative full node and BitcoinJ as a representative SPV client.

Both programs are flexible, so only default behavior is described. The response to the lookup should include one or more DNS A records with the IP addresses of full nodes that may accept new incoming connections. For example, using the Unix dig command:.

The DNS seeds are maintained by Bitcoin community members: In either case, nodes are added to the DNS seed if they run on the default Bitcoin ports of for mainnet or for testnet. For this reason, programs should not rely on DNS seeds exclusively. Once a program has connected to the network , its peers can begin to send it addr address messages with the IP addresses and port numbers of other peers on the network , providing a fully decentralized method of peer discovery.

Bitcoin Core keeps a record of known peers in a persistent on-disk database which usually allows it to connect directly to those peers on subsequent startups without having to use DNS seeds. However, peers often leave the network or change IP addresses, so programs may need to make several different connection attempts at startup before a successful connection is made.

This can add a significant delay to the amount of time it takes to connect to the network , forcing a user to wait before sending a transaction or checking the status of payment. Bitcoin Core also tries to strike a balance between minimizing delays and avoiding unnecessary DNS seed use: Both Bitcoin Core and BitcoinJ also include a hardcoded list of IP addresses and port numbers to several dozen nodes which were active around the time that particular version of the software was first released.

Bitcoin Core will start attempting to connect to these nodes if none of the DNS seed servers have responded to a query within 60 seconds, providing an automatic fallback option.

As a manual fallback option, Bitcoin Core also provides several command-line connection options, including the ability to get a list of peers from a specific node by IP address, or to make a persistent connection to a specific node by IP address. See the -help text for details. BitcoinJ can be programmed to do the same thing. Connecting to a peer is done by sending a version message , which contains your version number, block , and current time to the remote node. The remote node responds with its own version message.

Then both nodes send a verack message to the other node to indicate the connection has been established. Once connected, the client can send to the remote node getaddr and addr messages to gather additional peers.

In order to maintain a connection with a peer , nodes by default will send a message to peers before 30 minutes of inactivity. If 90 minutes pass without a message being received by a peer , the client will assume that connection has closed.

Before a full node can validate unconfirmed transactions and recently-mined blocks , it must download and validate all blocks from block 1 the block after the hardcoded genesis block to the current tip of the best block chain.

In this case, a node can use the IBD method to download all the blocks which were produced since the last time it was online. Bitcoin Core uses the IBD method any time the last block on its local best block chain has a block header time more than 24 hours in the past.

Bitcoin Core up until version 0. The goal is to download the blocks from the best block chain in sequence. The first time a node is started, it only has a single block in its local best block chain —the hardcoded genesis block block 0. This node chooses a remote peer , called the sync node , and sends it the getblocks message illustrated below.

In the header hashes field of the getblocks message , this new node sends the header hash of the only block it has, the genesis block 6fe2… in internal byte order. It also sets the stop hash field to all zeroes to request a maximum-size response. Upon receipt of the getblocks message , the sync node takes the first and only header hash and searches its local best block chain for a block with that header hash.

It finds that block 0 matches, so it replies with block inventories the maximum response to a getblocks message starting from block 1. It sends these inventories in the inv message illustrated below. Inventories are unique identifiers for information on the network. Each inventory contains a type field and the unique identifier for an instance of the object. The block inventories appear in the inv message in the same order they appear in the block chain , so this first inv message contains inventories for blocks 1 through For example, the hash of block 1 is … as seen in the illustration above.

The IBD node uses the received inventories to request blocks from the sync node in the getdata message illustrated below. Upon receipt of the getdata message , the sync node replies with each of the blocks requested. Each block is put into serialized block format and sent in a separate block message. The first block message sent for block 1 is illustrated below.

When it has requested every block for which it has an inventory , it sends another getblocks message to the sync node requesting the inventories of up to more blocks. This second getblocks message contains multiple header hashes as illustrated below:.

Upon receipt of the second getblocks message , the sync node searches its local best block chain for a block that matches one of the header hashes in the message, trying each hash in the order they were received. If it finds a matching hash, it replies with block inventories starting with the next block from that point. But if there is no matching hash besides the stopping hash , it assumes the only block the two nodes have in common is block 0 and so it sends an inv starting with block 1 the same inv message seen several illustrations above.

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