BitCoin Project. 1 Description. 1.1 Version exchange. 1.2 Ping-Pong. CMSC 417 Computer Networks Fall 2013

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1 CMSC 417 Computer Networks Fall 2013 BitCoin Project Assigned: Nov 20th Due: around Dec 16th 1 Description In this project you will develop a rudimentary BitCoin client. As your initial codebase you will be using CBitcoin, an open-source C library for working with blockchain data written by Matthew L. Mitchell. For the purpose of this project we have removed the networking code from the version of the CBitCoin provided. You may work in a group of two. Your code must be written in C, and use either select or poll to multiplex network connections. 1.1 Version exchange Connect to a BitCoin client. Your goal is to send a version message within the first 60 seconds of the connection. Failure to send the message will cause a disconnect from the other end. Upon successful of transmission of the version message the other end will reply back with its own version message along with a version acknowledgment message: verack. Within the sample code you will find examples that implement a request-reply version exchange. The filename is pingpong.c and is located under the examples folder. Before you do anything else for this project you must make sure you understand the code template we provided. Note: To create a version message you should use the CBNewVersion. Something that we don t cover in the example is that for every CBNew* function you have to call the appropriate CBRelease* function. If you don t call the release functions your implementation will leak memory. The header of every message that you have to construct is always 24 bytes. The library does not provide an predefined way to construct the headers; you have to construct them manually. The experience you have from the distance vector assignment will help you construct the headers. You need to construct a checksum for every message you send; the examples demonstrate this for the version message. Make sure to take advantage of all the provided functionality from the library. Functions like CBNewByteArrayFromString, CBInt32ToArray will be useful. Therefore, study the provided code very carefully. 1.2 Ping-Pong A standard BitCoin (e.g, satoshi) client send you ping messages. Upon the reception of a ping message your client should respond with a pong message. You client should be sending pings to the satoshi client as well. You should ping each peer every 60 seconds. 1

2 1.3 Interactive client interface You should have a way to interactively send each command you implement. Consider using the GNU readline library to implement your command line. Note: You have to use select or poll functions for all the I/O operations. Furthermore, you need to make sure that you properly handle concurrent actions. 1.4 Multiple Connections and connection bookkeeping You should implement the correct data structures (feel free to use the ones provided) to keep track of the peer addresses your client learns. Your goal should be to connect to as many of them as you can. When a node sends you an Addr message containing peer addresses it knows about, add only the new ones to your address data structure. A good strategy to quickly learn as many nodes as possible is to send a GetAddr messages to the initial set of nodes you will retrieve from the initial node you connect. Do not add private addresses to the address data structure. We have created a fork of the BitCoin network called umdnet, and we are running a public node you can connect to at kale.cs.umd.edu: You should also try connecting to two other forks: the Bitcoin mainnet, and the testnet. The different forks are distinguished in three ways: a) different ports (28333 for umdnet, for testnet, and 8333 for mainnet); b) a 4-byte netmagic code that is sent at the very beginning of each message: 0xf9beb4d0 for umdnet, 0x B for testnet, 0xf9beb4d9 for mainnet; and c) a unique genesis block (look in CBBlock.c to see how the mainnet and umdnet blocks are initialized). Advertise your client s address by sending Addr messages to the nodes you learn about. When nodes connect to you, accept the incoming connections and always wait for a version message that will initiate the version-verack exchange. After the initial version exchange you can treat the node the same as the peers that you are already connected to. 1.5 Download and serve blocks In order to validate transactions, you will need to maintain a representation of the current state of the ledger also known as the set of Unspent TX Outputs as well as forks for validation. You can use the blockchain database and validation library provided with cbitcoin. This library keeps track of the current main chain (the valid chain with the most cumulative work) as well as orphan blocks and side branches, and will automatically reorganize if the main chain needs to switch. The subset of the BitCoin messages you need to use: Version, Verack: initial handshake Ping, Pong: checks if a node is still connected GetAddr, Addr: announce or ask for node ip addresses GetBlocks: ask for block hashes Inv, GetData: announce or ask for blocks and transactions Block, Tx: block and transaction data In order to download blocks, you will need to construct a GetBlocks message. The GetBlocks message includes a block locator (i.e., CBChainDescriptor), which is built by enumerating backwards from the most recent block in the main branch. Follow the procedure described in the bitcoin wiki: 2

3 If you receive an Inv message containing blocks, you should check each one to see if you already have it. Collect all the ones you do not have and ask for them with a GetData message. If you receive a Block message, pass it to the ProcessBlock function in the library so that it will be checked and stored. The GetData message contains the same data as an Inv message: a serialized CBInventoryBroadcast. The Block and Transaction messages just consist of a serialized CBBlock and CBTransaction respectively. If you receive a GetBlocks message, then you should return an Inv message containing the list of blocks starting right after the last known hash in the block locator object, up to hash stop or 500 blocks, whichever comes first. Note that it is allowed to send in fewer known hashes down to a minimum of just one hash. If the peer detects that you are off the main chain, it will send in block hashes which are earlier than your last known block. So if you just send in your last known hash and it is off the main chain, the peer starts over at block #1. If a node asks for block data with GetData message, send a Block message for every one of them you have. 1.6 Transactions: Creation and broadcast Use the tool provided under the examples folder to create a new address for yourself. Scan each block in the blockchain database to find coins you can spend with that address. Construct a transaction that spends your coins and broadcasts it to the network. We will send you some umdnet coins you can spend. The broadcast works in two stages. First, you send everyone an Inv message containing the hash of the transaction. Add the transaction to a collection of transactions you know about. 1.7 Relay: Transactions and blocks When your client receives a new block or information about a new transaction, send an Inv message to every node you re connected to. Respond to GetData requests for both transactions and blocks. You must relay only valid blocks and transactions. You will have to write additional logic to validate transactions that are not yet in blocks. 2 Mine for new blocks Construct a new block out of any transactions that you know about. Create a coinbase transaction that pays the reward (block bonus + any fees from the transactions) to an address you own. Try different values for the nnonce field until the hash of the block falls below the difficulty threshold. Broadcast an Inv message as soon as you find a winning one. 3 Build a Distributed Hash Table (DHT) Rather than each node storing the whole blockchain, we would like to split the task across multiple nodes. Add your own communication layer between two or more of your own nodes such that when a GetData message is received, if you dont have it on disk but you know who might, then request it from that node and relay the response. More details about DHTs in forthcoming class. 1. Chord: 2. Pastry: 3. Kadmelia: 3

4 4 Remarks If you work on linuxlab in order to compile the provided code you should follow these steps: 1. Download the latest open-ssl from the following link: 2. To download use the command : wget 3. Create a folder in your account lex_openssl. (You can use any name you want. Make sure you use the same name in the following steps). 4. Execute the following command to decompress the openssl source code you downloaded in step 1. The command is: tar xvzf openssl-1.01e.tar.gz 5. Step inside the openssl directory and run the following sequence of commands:./configure --prefix=~/cs4170xx/lex_openssl; make; make install. Make sure to change the cs4170xx to the number of your linuxlab account. At the end of this step the newest openssl version will be installed in your linuxlab account at the folder lex_openssl. 6. Enter the cbitcoin directory and change the file Makefile.in to include the local directory that you installed openssl in the previous step. In this step you should replace the directives /usr/local/ssl/include and /usr/local/ssl/lib with the ones in your lex_openssl folder. 7. Run the following sequence:./configure; make Access your code by from the following repository: Understand the BitCoin protocol. Read the specification thoroughly: Read Satoshi s original paper for more information: Understand the provided code. This is part of the work you have to do for this project. Treat the example code as a template. The examples use libev. Your code must use select/poll. However, in order to run the examples follow these steps: 1. Download libev: 2. To download use the command : wget 3. Create a folder in your account lex_libev. (You can use any name you want. Make sure you use the same name in the following steps). 4. Execute the following command to decompress the libev source code you downloaded in step 1. The command is: tar xvzf libev-4.15.tar.gz 5. Step inside the openssl directory and run the following sequence of commands:./configure --prefix=~/cs4170xx/lex_libev; make; make install. Make sure to change the cs4170xx to the number of your linuxlab account. At the 4

5 end of this step the newest libev version will be installed in your linuxlab account at the folder lex_libev. 6. Enter the cbitcoin directory and change the file Makefile.in to include the local directory that you installed openssl in the previous step. In this step you should add the directives -I/afs/csic.umd.edu/class/cmsc417/0101/cs417XXX/lex_libev/include and -I/afs/csic.umd.edu/class/cmsc417/0101/cs417XXX/lex_libev/lib in the CFLAGS and LFLAGS respectively. 7. Run the following sequence:./configure; make examples-build 5