Ownership of bitcoin is established through _digital keys_, _bitcoin addresses_ and _digital signatures_. The digital keys are not actually stored in the network, but are instead created and stored by end-users in a file, or simple database, called a _wallet_. The digital keys in a user's wallet are completely independent of the bitcoin protocol and can be generated and managed by the user's wallet software without reference to the blockchain or access to the network. Keys enable many of the interesting properties of bitcoin, including de-centralized trust and control, ownership attestation and the cryptographic-proof security model.
The digital keys within each user's wallet allow the user to sign transactions, thereby providing cryptographic proof of the ownership of the bitcoins sourced by the transaction. Keys come in pairs consisting of a private (secret) and public key. Think of the public key as similar to a bank account number and the private key as similar to the secret PIN number, or signature on a cheque, that provides control over the account. These digital keys are very rarely seen by the users of bitcoin. For the most part, they are stored inside the wallet file and managed by the bitcoin wallet software.
In transactions, the public key is represented by its digital fingeprint called a _bitcoin address_ which is used as the beneficiary name on a cheque (ie. "Pay to the order of"). In most cases a bitcoin address is a generated from and corresponds to a public key. However, like a beneficiary name on a cheque, some bitcoin addresses do not represent a public key and instead represent other beneficiaries such as scripts, as we will see later in this chapter. This way, bitcoin addresses abstract the recipient of funds, making transaction destinations flexible, similar to paper cheques: a single payment instrument that can be used to pay into people's accounts, company accounts, pay for bills or pay to cash. The bitcoin address is the only part of the wallet that users will routinely see.
In this chapter we will introduce wallets, which contain cryptographic keys. We will look at how keys are generated, stored and managed. We will review the various encoding formats used to represent private and public keys, addresses and script addresses. Finally we will look at special uses of keys: to sign messages, to prove ownership and to create vanity addresses and paper wallets.
Public key cryptography was invented in the 1970s and is mathematics applied to computer security. Since the invention of public key cryptography, several suitable mathematical functions, such as prime number exponentiation and elliptic curve multiplication, have been discovered. These mathematical functions are practically irreversible, meaning that they are easy to calculate in one direction and infeasible to calculate in the opposite direction. Based on these mathematical functions, cryptography enables the creation of digital secrets and unforgeable digital signatures. Bitcoin uses elliptic curve multiplication as the basis for its public key cryptography.
In bitcoin, we use public key cryptography to create a key pair that controls access to bitcoins. The key pair consists of a private key and derived from it, a unique public key. The public key is used to receive bitcoins and the private key is used to sign transactions to spend those bitcoins. There is a special relationship between the public key and private key that allows the private key to be used to generate a signature. This signature can be validated against the public key without revealing the private key. When spending bitcoins, the current bitcoin owner presents their public key and a signature in a transaction to spend those bitcoins. Through the presentation of the public key and signature everyone in the bitcoin network can verify and accept that transaction as valid, meaning the person transfering the bitcoin owned them at the time of the transfer.
In most implementations, the private and public keys are stored together as a _key pair_ for convenience. However, it is trivial to reproduce the public key if one has the private key, so storing only the private key is also possible.
Elliptic Curve Cryptography is a type of asymmetric or public-key cryptography based on the discrete logarithm problem as expressed by addition and multiplication on the points of an elliptic curve.
[[ecc_addition]]
.Elliptic Curve Cryptography: Visualizing the addition operator on the points of an elliptic curve
image::images/ecc-addition.png["Addition operator on points of an elliptic curve"]
Bitcoin specifically uses the +secp256k1+ elliptic curve:
((("secp256k1")))
[latexmath]
++++
\begin{equation}
{y^2 = (x^3 + 7)} \text{over} \mathbb{F}_p
\end{equation}
++++
or
[latexmath]
++++
\begin{equation}
{y^2 \mod p = (x^3 + 7) \mod p}
\end{equation}
++++
where +latexmath:[\(p = 2^256 - 2^32 - 2^9 - 2^8 - 2^7 - 2^6 - 2^4 - 1\)]+, a very large prime number.
The +mod p+ indicates that this curve is over a finite field of prime order +p+, also written as latexmath:[\(\mathbb{F}_p\)]. The curve looks like a pattern of dots scattered in two dimensions, which makes it difficult to visualize. However, the math is identical as that of an elliptic curve over the real numbers shown above.
[[ecc-over-F37-math]]
.Elliptic Curve Cryptography: Visualizing the addition operator on the points of an elliptic curve over F(p)
image::images/ecc-over-F37-math.png["Addition operator on points of an elliptic curve over F(p)"]
Once a private key has been generated, the public key equivalent can be derived from it using the elliptic curve multiplication function. Many software implementations of bitcoin use the OpenSSL library, specifically the https://www.openssl.org/docs/crypto/ec.html[Elliptic Curve library].
[TIP]
====
The size of bitcoin's private key, 2^256^ is a truly unfathomable number. It is equal to approximately 10^77^ in decimal. The visible universe contains approximately 10^80^ atoms.
In the most simple form, the +private key+ is a number. The private key can be used to create a corresponding +public key+. The public key can then be converted into a +bitcoin address+, which is shared with anyone who we want to send us bitcoin. Ownership and control over the private key is the root of user control over all funds associated with the corresponding bitcoin address.
A private key is a number between +1+ and +n - 1+ where latexmath:[\(n = 1.158 * 10^\(77\) \)] is the order of the elliptic curve used in bitcoin (See <<secp256k1>>). To create such a key, we just pick a 256-bit random number and check that it is less than +n - 1+. The constant +n+ is defined in any elliptic curve cryptography library. In programming terms, this is usually achieved by feeding a larger string of random bits, collected from a cryptographically-secure source of randomness, into the SHA-256 hash algorithm which will conveniently produce a 256-bit number.
Do not try and design your own pseudo random number generator (PRNG). Use a cryptographically-secure pseudo-random number generator (CSPRNG) with a seed from a source of sufficient entropy, the choice of which depends on the operating-system. Correct implementation of the CSPRNG is critical to the security of the keys. DIY is highly discouraged unless you are a professional cryptographer.
To generate a new key with bitcoind, use the +getnewaddress+ command. For security reasons it displays the public key only, not the private key. To ask bitcoind to expose the private key, use the +dumpprivkey+ command. Here's an example of both commands:
The +dumpprivkey+ command is opening the wallet and extracting the private key that was generated by the +getnewaddress+ command. It is not otherwise possible for bitcoind to know the private key from the public key, unless they are both stored in the wallet. In the example above, we see that the private key has a "K" prefix, indicating it is encoded as a WIF-compressed format. This means that the key-pair is stored in the wallet with both keys compressed, saving 31 bytes of space. If the prefix had been "5", indicating the WIF format, we would know that the key-pair is uncompressed.
You can also use +sx tools+ to generate keys and convert them between formats:
The public key is calculated from the private key using elliptic curve multiplication, which is irreversible: latexmath:[\(K = k * G\)]+ where +k+ is the private key, +G+ is a constant point called the _Generator Point_ and +K+ is the resulting public key. The reverse (division), or calculating +k+ if you know +K+ is as difficult as trying all possible values of +k+, i.e. a brute-force search.
Starting with a private key in the form of a randomly generated number +k+, we multiply it with a predetermined point on the curve called the _generator point_ +G+ to produce another point somewhere else on the curve, which is the corresponding public key +K+.
[latexmath]
++++
\begin{equation}
{K = k * G}
\end{equation}
++++
[[key_derivation]]
where +k+ is the private key, +G+ is a fixed point on the curve called the _generator point_, ((("generator point"))) and +K+ is the resulting public key, another point on the curve.
[TIP]
====
The private key is just a number. A public key can be generated from any private key. Therefore, a public key can be generated from any number, up to 256 bits long. You can pick your keys randomly using a method as simple as tossing a coin, pencil and paper. Toss a coin 256 times and you have the binary digits of a random private key you can use in a bitcoin wallet. Keys really are just a pair of numbers, one calculated from the other.
====
The public key is a point on the elliptic curve, and consists of a pair of coordinates +(x,y)+, normally represented by a 512-bit number with the added prefix +04+.
Here's the public key generated by the private key we created above, shown as the coordinates +(x,y)+
.Public Key K defined as a point +K = (x,y)+
----
x = 32 5D 52 E3 B7 ... E5 D3 78
y = 7A 3D 41 E6 70 ... CD 90 C2
----
Here's the same public key shown as a 512-bit number (130 hex digits) with the prefix +04+ followed by +x+ and then +y+
.Uncompressed Public Key K shown in hex (130 hex digits) as +04 x y+
----
K = 04 32 5D 52 E3 B7 ... CD 90 C2
----
[TIP]
====
A private key can be converted into a public key, but a public key cannot be converted back into a private key because the math only works one way.
====
==== Addresses
An address is a string of digits and characters that can be shared with anyone who wants to send you money. In bitcoin, addresses begin with the digit "1". This is an address made by hashing the public key twice through two different hashing algorithms.
All of the above representations are different ways of showing the same number, the same private key. They look different, but any one format can easily be converted to any other format.
===== Decoded from the Base58Check encoding to Hex
The +y+ coordinate can be deduced from the +x+ coordinate, since they both lie on the same curved line defined by the elliptic curve equation. This makes it possible to store the public key _compressed_, with the +y+ omitted. A +compressed public key+ has the prefix +02+ if the +y+ is above the x-axis, and +03+ if it is below the x-axis, allowing the software to calculate it from +x+.
Here's the same public key above, shown as a +compressed public key+ stored in 264-bits (66 hex digits) with the prefix +02+ indicating the +y+ coordinate has a positive sign:
There are many ways to generate keys for use in bitcoin. The simplest is to pick a large random number and turn it into a key pair (See <<key_derivation>>). A random key can be generated with very simple hardware or even manually with pen, paper and dice. The disadvantage of random keys is that if you generate many of them you must keep copies of all of them. Another method for making keys is _deterministic key generation_. Here you generate each new key as a function of the previous key, linking them in a sequence. As long as you can re-create that sequence, you only need the first key to generate them all. In this section we will examine the different methods for key generation.
Wallets contain keys, not coins. The coins are stored on the blockchain in the form of transaction-outputs (often noted as vout or txout). Each user has a wallet containing keys. Wallets are really keychains containing pairs of private/public keys (See <<public key>>). Users sign transactions with the keys, thereby proving they own the transaction outputs (their coins).
The first and most important step in generating keys is to find a secure source of entropy, or randomness. The private key is a 256-bit number, which must be selected at random. Creating a bitcoin key is essentially the same as "Pick a number between 1 and 2^256^". The exact method you use to pick that number does not matter as long as it is not predictable or repeatable. Bitcoin software will use the underlying operating system's random number generators to produce 256 bits of entropy. Usually, the OS random number generator is initialized by a human source of randomness, which is why you may be asked to wiggle your mouse around for a few seconds. For the truly paranoid, nothing beats dice, pencil and paper.
The bitcoin private key is just a number. A public key can be generated from any private key. Therefore, a public key can be generated from any number, up to 256 bits long. You can pick your keys randomly using a method as simple as dice, pencil and paper.
The size of bitcoin's private key, 2^256^ is a truly unfathomable number. It is equal to approximately 10^77^ in decimal. The visible universe contains approximately 10^80^ atoms.
This most basic form of key generation generates what are known as _Type-0_ or _Non-Deterministic_ (i.e. random) keys. When a sequence of keys is generated for a single user's wallet, each key is randomly generated when needed.
