Public Key

A public key is like an account number that you use to receive bitcoins.

It is created from your private key, which is like a password for that account number.

You should never enter your private key in to a website, or use a private key generated by a website. Websites can save these private keys and use them to steal any bitcoins you send to its address.

How do you generate a public key?

You use your private key (which is just a big random number) to generate a corresponding public key.

You perform elliptic curve multiplication using your private key, which will give you a final resting point on the elliptic curve. The x and y coordinate of this point is your public key.

Code

Here’s some basic code for creating a public key from a private key.

I haven’t explained how the elliptic curve mathematics works, but I’ve included this code anyway to show how you can get started with calculating a public key for yourself.

# example private key
private_key = "ef235aacf90d9f4aadd8c92e4b2562e1d9eb97f0df9ba3b508258739cb013db2"

# --------------------------
# Secp256k1 Curve Parameters
# --------------------------
# y^2 = x^3 + ax + b
$a = 0
$b = 7 # using global variables for convenience

# prime modulus
$p = 2 ** 256 - 2 ** 32 - 2 ** 9 - 2 ** 8 - 2 ** 7 - 2 ** 6 - 2 ** 4 - 1

# number of points on the curve
$n = 115792089237316195423570985008687907852837564279074904382605163141518161494337

# generator point (the starting point on the curve used for all calculations)
$g = {
  x: 55066263022277343669578718895168534326250603453777594175500187360389116729240,
  y: 32670510020758816978083085130507043184471273380659243275938904335757337482424,
}

# --------------------------
# Elliptic Curve Mathematics
# --------------------------
# Modular Inverse - Ruby doesn't have a built-in function for finding modular inverses, so here's one using the extended Euclidean algorithm.
def modinv(a, m = $p)
  a = a % m if a < 0 # make sure a is positive
  prevy, y = 0, 1
  while a > 1
    q = m / a
    y, prevy = prevy - q * y, y
    a, m = m % a, a
  end
  return y
end

# Double - Add a point on the curve to itself.
def double(point)
  # slope = (3x^2 + a) / 2y
  slope = ((3 * point[:x] ** 2) * modinv((2 * point[:y]))) % $p # using modular inverse to perform "division"

  # new x = slope^2 - 2x
  x = (slope ** 2 - (2 * point[:x])) % $p

  # new y = slope * (x - new x) * y
  y = (slope * (point[:x] - x) - point[:y]) % $p

  # return x, y coordinates of point
  return { x: x, y: y }
end

# Add - Add two points together.
def add(point1, point2)
  # double if both points are the same
  return double(point1) if point1 == point2

  # slope = (y1 - y2) / (x1 - x2)
  slope = ((point1[:y] - point2[:y]) * modinv(point1[:x] - point2[:x])) % $p

  # new x = slope^2 - x1 - x2
  x = (slope ** 2 - point1[:x] - point2[:x]) % $p

  # new y = slope * (x1 - new x) - y1
  y = ((slope * (point1[:x] - x)) - point1[:y]) % $p

  # return x, y coordinates of point
  return { x: x, y: y }
end

# Multiply - Use the double and add operations to quickly multiply a point by an integer (e.g. a private key).
def multiply(k, point = $g) # multiply the generator point by default
  # create a copy the initial starting point (for use in addition later on)
  current = point

  # convert integer to binary representation (for use in the double and add algorithm)
  binary = k.to_s(2)

  # double and add algorithm for fast multiplication
  binary.split("").drop(1).each do |char| # ignore first binary character
    # 0 = double
    current = double(current)

    # 1 = double and add
    if char == "1"
      current = add(current, point)
    end
  end

  # return the final point
  return current
end

# -------------------------
# Private Key To Public Key
# -------------------------
# convert private key to an integer
k = private_key.to_i(16)

# multiply generator point by this private key
point = multiply(k, $g) # this point is the public key

# convert x and y values of this point to hexadecimal
x = point[:x].to_s(16).rjust(64, "0")
y = point[:y].to_s(16).rjust(64, "0")

# uncompressed public key format (not used much these days, just showing how it looks)
public_key_uncompressed = "04" + x + y

# compressed public key format (every x value has a y that could be one of two possible points)
if (point[:y] % 2 == 0)
  prefix = "02" # if y is even
else
  prefix = "03" # if y is odd
end

public_key_compressed = prefix + x # only uses the full x coordinate

# -------
# Results
# -------
puts private_key           #=> ef235aacf90d9f4aadd8c92e4b2562e1d9eb97f0df9ba3b508258739cb013db2
puts public_key_compressed #=> 02b4632d08485ff1df2db55b9dafd23347d1c47a457072a1e87be26896549a8737

Why is the elliptic curve used?

Public Key Format

A public key is just the x and y co-ordinate of a point on the elliptic curve. It’s usually stored in hexadecimal format.

There are two formats for public keys:

1. Uncompressed

This is the old format. It has generally stopped being used in favor of the shorter compressed format.

Bitcoin originally used both the x and y coordinate to store the public key.

In this uncompressed format, you just place the x and y coordinate next to each other, then prefix the whole thing with an 04 to show that it is an uncompressed public key:

Here’s an example of what an uncompressed public key looks like:

public key (uncompressed) = 04b4632d08485ff1df2db55b9dafd23347d1c47a457072a1e87be26896549a87378ec38ff91d43e8c2092ebda601780485263da089465619e0358a5c1be7ac91f4

2. Compressed

However, because the elliptic curve is symmetrical along its x-axis, each x coordinate will only ever have one of two possible y coordinates.

And here’s the trick…

• If y is even, it corresponds to one of the points.
• If y is odd, it corresponds to the other.

So in the compressed public key format, we just store the full x coordinate, along with a prefix that indicates whether the y is even or odd.

Here’s an example of what a compressed public key looks like compared to an uncompressed public key:

public key (uncompressed) = 04b4632d08485ff1df2db55b9dafd23347d1c47a457072a1e87be26896549a87378ec38ff91d43e8c2092ebda601780485263da089465619e0358a5c1be7ac91f4
public key (compressed)   = 02b4632d08485ff1df2db55b9dafd23347d1c47a457072a1e87be26896549a8737

This compressed format ultimately allows us to work out the full x and y coordinates, but saves a lot of space inside the blockchain (i.e. when we create transactions that lock outputs to specific public keys).

How to decompress a public key

# Compressed public key
compressed = "02b4632d08485ff1df2db55b9dafd23347d1c47a457072a1e87be26896549a8737"

# Split compressed key in to prefix and x-coordinate
prefix = compressed[0..1]
x      = compressed[2..-1].to_i(16)

# Secp256k1 curve parameters
p = 0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2f

# Work out y values using the curve equation y^2 = x^3 + 7
y_sq = (x**3 + 7) % p # everything is modulo p

# Secp256k1 is chosen in a special way so that the square root of y is y^((p+1)/4)
y = y_sq.pow((p+1)/4, p) # use modular exponentation

# Use prefix to select the correct value for y
# * 02 prefix = y is even
# * 03 prefix = y is odd 
if (prefix == "02" && y % 2 != 0) # if prefix is 02 and y isn't even, use other y value
    y = (p - y) % p
end
if (prefix == "03" && y % 2 == 0) # if prefix is 03 and y is even, use other y value
    y = (p - y) % p
end

# Construct the uncompressed public key
x = x.to_s(16).rjust(64, "0") # convert to hex and make sure it's 32 bytes (64 characters)
y = y.to_s(16).rjust(64, "0")
uncompressed = "04" + x + y

# Result
puts uncompressed #=> 04b4632d08485ff1df2db55b9dafd23347d1c47a457072a1e87be26896549a87378ec38ff91d43e8c2092ebda601780485263da089465619e0358a5c1be7ac91f4

How are public keys used in Bitcoin?

You can give your public key away to people so that they can include it in the the locking script of an output when they create a transaction.

However, in Bitcoin we now more commonly hash160 our public key before giving it away. The public key is then used only when we come to unlocking the output. (The initial lock will then want to check that the public key hashes correctly first before going on to check it against the signature.)

Where can you find public keys inside the blockchain?

If your looking through raw blockchain data, public keys can typically be found inside transaction data.

In a standard P2PKH transaction, for example:

1. The public key hash sits within the locking code (scriptPubKey) of an output.

01000000017dc9d3eaa91ef9886e48929285243d945a20be621a7483d5442872c2f4bbf432000000004a493046022100ea5812a1cbcf9c8c49fdfb4ed7ef89c05d9d11a5df941fd76546e9031fefbef5022100f5458675ebd56db5517510527afa5ff7c98e08d7ed83a8b180f7ac841531a3ac01ffffffff0100f2052a010000001976a914fc50c5907d86fed474ba5ce8b12a66e0a4c139d888ac00000000

Transaction: 24c1f77de7c580eccd46c41d4c708da4ace04880559ee6bce97a67217466d7f7

Then in the next transaction that spends the bitcoins…

2. The original public key can then be found inside the unlocking code (scriptSig) of the input.

0100000000100000001f7d7667421677ae9bce69e558048e0aca48d704c1dc446cdec80c5e77df7c124000000008b483045022100b92b0d78a1a72b25179260e96a15efe95f98962622fb232f92d6c6ef20e15e9b022061c946c3f976339e370eabd256d91aa4711bb9985330f7d18ee77987b0ca24300141046c04c02f1138f440e8c5e9099db938bfba93d0389528bb7f6bf423ae203a2edcfba133f0409023d7ea13ac01c5aeedaf0bbfbeb8b82e9b48410d93a296da5b0cffffffff0100f2052a010000001976a914e6a874331cddf113e6f424f547aa93c10755d5e688ac00000000

Transaction: 33ab606c34dce6e43673d20c1a72c7b0bce314d9d21e227c04092bbdaf8aaed5

As you can see, the 04 at the start of the public key indicates that it’s an uncompressed public key. This makes it almost twice as long as the compressed public keys typically used today.

Libraries

In most languages today you can use an existing elliptic curve library to help you create public keys (instead of having to code the mathematics yourself). For example:

require 'ecdsa' # sudo gem install ecdsa

# This private key is just an example
privatekey = "ef235aacf90d9f4aadd8c92e4b2562e1d9eb97f0df9ba3b508258739cb013db2"

# Elliptic curve multiplication
group = ECDSA::Group::Secp256k1 # Select the curve used in Bitcoin
point = group.generator.multiply_by_scalar(privatekey.to_i(16)) # Multiply by integer (not hex)

# Convert public key point to a compressed hexadecimal public key
publickey = ECDSA::Format::PointOctetString.encode(point, compression: true).unpack("H*").join

puts publickey

• Ruby: github.com/DavidEGrayson/ruby_ecdsa
• PHP: github.com/BitcoinPHP/BitcoinECDSA.php

Resources

• Bitcoin 101 - Generating a Public Key in Python
• Graphical Address Generator