Generate a Ethereum key pair / Get public key from a given private key (if a parameter passed)

eth-key.rb

#!/usr/bin/env ruby
# gem install digest-sha3

require 'openssl'
require 'digest/sha3'
require 'open-uri'
require 'json'

def decode(s, base)
  syms = '0123456789abcdef'.freeze
  s = s.downcase if base == 16

  result = 0
  while s.size > 0
    result *= base
    result += syms.index(s[0])
    s = s[1..-1]
  end

  result
end

def encode(v, base)
  syms = 256.times.map {|i| i.chr }.join.freeze

  result = ''
  while v > 0
    result = syms[v % base] + result
    v /= base
  end

  result
end

def public_to_address(str)
  str = str[2, 128]
  Digest::SHA3.hexdigest(encode(decode(str, 16), 256), 256)[-40..-1]
end

private_key = ARGV[0]
if private_key
  group = OpenSSL::PKey::EC::Group.new('secp256k1')
  public_key = group.generator.mul(private_key.hex).to_bn.to_s(16)
else
  curve = OpenSSL::PKey::EC.new('secp256k1')
  curve.generate_key
  private_key = curve.private_key.to_s(16)
  public_key = curve.public_key.to_bn.to_s(16)
end

public_key = public_to_address(public_key)

puts "Private Key: #{private_key}"
puts "Address: 0x#{public_key}"

# Just in case 😉
url = "https://api.etherscan.io/api?module=account&action=balance&address=0x#{public_key}&tag=latest"
eth_balance = JSON.load(open(url))['result'].to_f / 10**18
puts "Balance: #{eth_balance} ETH"

Usage:

~$ ethKey
Private Key: E0D46700050E218B448C945B5E19C1E89A536068298E4938909537A35A38A939
Address: 0x5b268746ececf276e5502ec586172ae949ee96bb
Balance: 0.0 ETH

~$ ethKey E0D46700050E218B448C945B5E19C1E89A536068298E4938909537A35A38A939
Private Key: E0D46700050E218B448C945B5E19C1E89A536068298E4938909537A35A38A939
Address: 0x5b268746ececf276e5502ec586172ae949ee96bb
Balance: 0.0 ETH