Bitcoin

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Revision as of 05:15, 24 September 2015 by Leo (talk | contribs) (→Address / Keys)
This page was last edited on 24 September 2015, at 05:15.

Bitcoin is a proof of work cryptocurrency.

This article covers some specific aspects of the Bitcoin protocol.

My Bitcoin Address is: 1HDp1gU3rgrRNKqrKorz8uWCpHUNdyGqXm

Overview

Bitcoin is a decentralized protocol that operates in an untrusted peer to peer environment while still being a reliable method to exchange credits (bitcoins) between people.

For detailed information, read the Bitcoin paper or one of the many sites out there.

Hashes & Encoding

You will see hashes as part of the Bitcoin protocol everywhere.

Types of Hashes

Here are a few common hashes you will encounter.

40 character long / 160-bit value

37f332f68db77bd9d7edd4969571ad671cf9dd3b

Generated using RIPEMD-160

64 character long / 256-bit value

600FFE422B4E00731A59557A5CCA46CC183944191006324A447BDB2D98D4B408

Generated using SHA256 of any number of rounds. This is typically seen as a transaction ID. The private key of an address also uses a 256-bit value.

130 character long

0450863AD64A87AE8A2FE83C1AF1A8403CB53F53E486D8511DAD8A04887E5B23522CD470243453A299FA9E77237716103ABC11A1DF38855ED6F2EE187E9C582BA6

The private key of an address

Hash160 (Transactions)

Transaction numbers are represented using Hash160 of the public key. That is: 

TransactionID = Hash160 = RIPEMD160(SHA256(PubKey))

It is outputted as a 160-bit value in hex (64 characters long). For example: 

49123c475ef78814414f1318eff0a349bcbafd182d8b3e110684152ee0e44081

Hash

Hashes in Bitcoin are done using two rounds of SHA256. 

Hash = SHA256(SHA256(X))

Base58Check

Base58Check is used to generate Bitcoin addresses.

Address / Keys

A Bitcoin address is a 160-bit hash of the ECDSA public/private keypair.

Generating An Address

  1. Take a random 160-bit value as the private key.
  2. Generate a public key with it
  3. Hash = SHA256(PublicKey)
  4. Hash160 = RIPEMD160(Hash)
  5. Address = Base58Check(Hash160)

Where Base58Check takes a Hash160 hash value:

  1. BaseAddress = 00 + RIPEMD160(Hash160)
  2. Check = SHA256(SHA256(BaseAddress))
  3. FinalAddress = BaseAddress + (first 4 bytes of Check)
  4. Address = Base58CheckEncode(FinalAddress)

See Also: https://en.bitcoin.it/wiki/Technical_background_of_version_1_Bitcoin_addresses

Brain Wallets

A brain wallet allows for the generation of the public/private key pair using a passphrase. It is accomplished by:

  1. Converting a pass phrase into a private key
    • PrivateKey = SHA256(PassPhrase)
  2. Generate a public key using the private key
    • PublicKey = privateToPublic(PrivateKey)
  3. Generate the bitcoin address
    • Hash = SHA256(PublicKey)
    • Hash160 = RIPEMD160(Hash) (used by transactions)
    • Address = Base58Check(Hash160)

The publickey can either be uncompressed or compressed. A compressed public key is simply a truncated key where the missing parts can be recomputed.

Attack

Ryan Castellucci did a talk on Defcon 23 on cracking brain wallets using his program called Brainflayer, available at https://github.com/ryancdotorg/brainflayer.

The basic idea is to extract all unique addresses in the bitcoin system. Dump the addresses into a bloom filter for almost constant time search, then use Brainflayer to bruteforce check on a word list that matches a specific address hash.

That is:

  1. get all Bitcoin Addresses as Hash160 > hashes.hex
  2. hex2blf hashes.hex hashes.blf
  3. brainflayer -b hashes.blf -i phraselist.txt
  4. or cat password.txt | brainflayer -b hashes.blf
  5. or john --incremental --stdout | brainflayer -b hashes.blf