Transcript Lecture 10
Bitcoin What is Bitcoin? • A P2P network for electronic payments • Benefits: – – – – – Low fees No middlemen No central authority Can be anonymous Each payment goes through, no censorship How Does it Work? • Does not rely on real currency but one can buy bitcoins for money – Valuable because they are useful and limited in supply • New coins are ”mined” by performing cryptographic operations that take lots of CPU • A user can package the new coin together with a block containing new transactions of others – “Verifying a block” brings the user 25 BTC reward – Reward is halved every 4 years How Does it Work? • Each transaction is signed by the sender’s private key – So no one can spend others’ money • Transaction = bitcoins, public key of recipient • Each transaction is broadcast in the P2P network • Recipients collect and verify transactions in a block – On the average 6 blocks per hour are generated How Does it Work? • Blocks are chained together so that latter blocks depend on previous ones • If multiple chains appear only the longest and fastest-growing chain is accepted as true – This prevents double-spending • Anonymity can be achieved by using a new public/private key pair for each receiving transaction Transaction Format Field Description Version In-counter List of inputs Out-counter 1 Int>0 Txin Int>0 List of outputs Lock_time Txout When is the transaction final, only valid if sequence_no < 0xFFFFFFFF Txin Format Field Description Previous Transaction hash Previous Txout-index Txin-script length Txin-script/scriptSig Which output of prev Tx is used Int >= 0 Sequence_no normally 0xFFFFFFFF Txout Format Field Description Value number of BTC*108 to be transferred Txout-script length Int >= 0 Txout-xcript/scriptPubKey Sample Transaction • Input 50 bitcoins from transaction with hash f5d… then send them to a Bitcoin address 4043 .. • If the input was greater than the amount you want to spend create another output to yourself with the rest • Any part of input not redeemed is the transaction fee and belongs to whoever generates a block Transaction Verification • Use a scripting system – describes how the transaction amount can be redeemed • Input’s scriptSig is evaluated first – this leaves some state on the stack • Output’s scriptPubKey is evaluated second • Through scripting system one can create different sorts of payments with different conditions Types of Transactions • Transfer to IP address • Transfer to Bitcoin address • Generation Blocks • A block is a record of some or all transactions that have not been recorded so far • Each block contains: – A reference to the block immediately before it – chaining (roughly one block is added each 10 min) – A solution to a cryptographic puzzle – A generation transaction so fee can be claimed for generating the block • Transactions are broadcast and peers compete in trying to generate new blocks • More about block rules https://en.bitcoin.it/wiki/Protocol_rules#Blocks Puzzles • A puzzle is finding an input to a hash algorithm that, when combined with the block’s hash results in output with many zeros • The difficulty of the puzzle is automatically adjusted so that roughly 6 puzzles per hour can be solved • Every two weeks all clients compare how much off target they were and adjust puzzle difficulty • Clients accept the chain with the highest combined difficulty of puzzles as valid Double-Spending • Could one spend the same coin twice? – Bitcoin peers verify all transactions they add to the block to ensure that the same coin wasn’t spent elsewhere – There are some race conditions that may lead to double-spending before transactions are verified – As the confirmations increase no double-spending is possible Some Statistics Some Statistics Some Statistics Some Statistics Some Statistics Some Statistics Some Statistics Drive-By Downloads What are Drive-By Downloads Are These Just Exploits? • If my computer is well-protected should I worry? • Drive-by downloads occur in two ways: – There’s a pop-up that a user clicks on or sometimes even closes. This is interpreted as permission to download and install malware onto your machine – There’s a script on the page a user visited that initiates file download. Some browsers may run such files automatically, infecting the machine • Some browsers will force downloads and warn on downloads of executable software What If I Visit Well-Known Sites? • Even such sites can have vulnerable servers that get infected by attackers • Or they may serve ads that distribute malware • Exploit kits for Web servers proliferate on black markets – They are also very versatile and customizable Are There Any Defenses? • Lots of research in the area from top security experts • BLADEhttp://www.youtube.com/watch?v=9emHejh8 hW “all executable files delivered through browser downloads must result from explicit user consent”