How are different addresses generated from the same private key?

Question

I was reading in the "Technical background of version 1 Bitcoin addresses" that addresses are a function of the private key. More or less:

Private Key -> Public Key -> Address

If the public key is the only input needed to generate an address, how come a wallet generates a different address every time? What is changing?

Answer 1

Private and public key correspond to a point on the secp256k1 curve. They have a one-to-one relationship.

The address is derived from the public key by performing a ripemd160 hash after a sha256 hash on the public key. Multiple public keys hash to the same address, as the address space is only 160 bit, while the public key space is 256 bit.

Since both derivations (private key > public key > address) are deterministic, you cannot derive more than one address from one private key.^*

What you can do, is derive additional private keys from a "master private key" with a given derivation rule. These additional private keys obviously correspond to new addresses. This type of address management is called "hierarchical deterministic wallet".

^{*Strictly speaking, you can create multiple addresses by differentiating between compressed and uncompressed keys. See Sven's and skaht's answers for details.}

Answer 2

With Bitcoin, a single private key will have associated compressed and uncompressed private/public key pairs. Uncompressed public key addresses are larger in size than newer compressed public addresses. (Contrast 1b and 2b below.) Uncompressed and compressed public keys shall have different associated Bitcoin addresses. Private keys encoded in wallet input format (WIF) will implicitly communicate to a hot wallet if uncompressed or compressed keys are to be used. A Blockchain records funds sent to either address uncompressed or compressed independently.

Here are illustrative examples using libbitcoin's bitcoin-explorer (bx) command line interface using one of the absolute worst hexadecimal encoded private keys in the world 0000111122223333444455556666777788889999aaaabbbbccccddddeeeeffff that is obviously 256 bits in length.

1a) Uncompressed Private Key WIF-encoded:

% echo "0000111122223333444455556666777788889999aaaabbbbccccddddeeeeffff" | bx base58check-encode -v 128

5HpHb4pzVWwsDAHNwwUS3VViCkwzcutaSJ57T4GNFw5UBNLSrRV

1b) Uncompressed Public Key:

% echo "0000111122223333444455556666777788889999aaaabbbbccccddddeeeeffff" | bx ec-to-public -u

040d47568a5e517067a2836c3823fbc58169a7662bfae934a4d41da3e23c98d816e7202dd702ffe038147f78aee4973a581972960a1460312ffb6f3f0f13d4a52c

1c) Uncompressed Public Address:

% echo "0000111122223333444455556666777788889999aaaabbbbccccddddeeeeffff" | bx ec-to-public -u | bx ec-to-address -v 0

1NGoV1EGZrwM7yvUYqRC7TMBMj7ftpjR2B

2a) Compressed Private Key WIF-encoded:

% echo "0000111122223333444455556666777788889999aaaabbbbccccddddeeeeffff01" | bx base58check-encode -v 128

KwDiDMtpksBAcfyHsVS5XzmirtyjKWSeaeM9U1QppugixMUeKMqp

2b) Compressed Public Key:

% echo "0000111122223333444455556666777788889999aaaabbbbccccddddeeeeffff" | bx ec-to-public

020d47568a5e517067a2836c3823fbc58169a7662bfae934a4d41da3e23c98d816

2c) Compressed Public Address:

% echo "0000111122223333444455556666777788889999aaaabbbbccccddddeeeeffff" | bx ec-to-public | bx ec-to-address -v 0

1PbStXjfDNBU6FZA2iSeisVWwCFN9GK1eQ <- No bot was fast enough here to intercept my experimentation

Answer 3

Whenever you are attempting to explain Bitcoin to anyone, it makes a lot of sense to simply state that the derivation private key > public key > address is deterministic. However, if you are interested in writing your own code or studying the existing code base from various libraries, you may wish to know that a private key can actually lead to two public keys (one in compressed form and another in uncompressed form), both being encodings of the same point on the secp256k1 elliptic curve but leading to two different hash values. Hence, we have two possible hash values and each hash value can in turn lead to two different (pay-to-public-key-hash) addresses (one for the main bitcoin network and one for the testing network). So all in all (from a developer perspective rather than a mere user of Bitcoin), a private key can lead to 4 possible addresses. I attach a java snippet:

import java.math.BigInteger;
import org.bitcoinj.core.ECKey;
import org.bitcoinj.core.NetworkParameters;
import org.bitcoinj.core.Address;
import org.bitcoinj.params.MainNetParams;
import org.bitcoinj.params.TestNet3Params;

public class Test {

  public static void main(String[] args){

  // An example of private key from the book 'Mastering Bitcoin'
  String k = "1E99423A4ED27608A15A2616A2B0E9E52CED330AC530EDCC32C8FFC6A526AEDD";

  // Converting our string encoding as an actual number
  BigInteger priv = new BigInteger(k,16);

  // Creating a key object from our private key, with compressed public key
  ECKey k1 = ECKey.fromPrivate(priv, true);

  // Creating a key object from our private key, with uncompressed public key
  ECKey k2 = ECKey.fromPrivate(priv, false);


  // 03f028892bad7ed57d2fb57bf33081d5cfcf6f9ed3d3d7f159c2e2fff579dc341a
  System.out.println(k1.getPublicKeyAsHex()); // compressed

  // 04f028892bad7ed57d2fb57bf33081d5cfcf6f9ed3d3d7f159c2e2fff579dc341a...
  //...07cf33da18bd734c600b96a72bbc4749d5141c90ec8ac328ae52ddfe2e505bdb
  System.out.println(k2.getPublicKeyAsHex()); // uncompressed

  NetworkParameters main = MainNetParams.get();   // main bitcoin network
  NetworkParameters test = TestNet3Params.get();  // test bitcoin network

  Address addr1 = k1.toAddress(main); // main network, compressed
  Address addr2 = k1.toAddress(test); // test network, compressed
  Address addr3 = k2.toAddress(main); // main network, uncompressed
  Address addr4 = k2.toAddress(test); // test network, uncompressed

  System.out.println(addr1.toString()); // 1J7mdg5rbQyUHENYdx39WVWK7fsLpEoXZy
  System.out.println(addr2.toString()); // mxdivjAqQSQj4LrAMX1XLQidyfU3pCWeS7
  System.out.println(addr3.toString()); // 1424C2F4bC9JidNjjTUZCbUxv6Sa1Mt62x
  System.out.println(addr4.toString()); // miY1V5L3QDaZVjrMT2Sw2WhHn63GzsNFQB

  }
}

Answer 4

You can generate unlimited number of addresses from the same private key. Private key is simply a number. from which you can generate compressed and uncompressed public keys. from the public keys you can generate addresses for each network (test, main, reg) for different types of altcoins by changing network byte.