Glossary | Private Key
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Private Key

A private key is a random 256 bit number.

It is used as the source of a public key.

Generate a Private Key

All you need to generate a private key is a reliable source of randomness.

An easy source of randomness on a Linux computer is /dev/urandom, which provides random bits of data from your computer. All you need to do is read from it:

# generate 256 bits of random data
urandom = File.open("/dev/urandom")    # urandom is a "file"
bytes = urandom.read(32)               # read 32 bytes from it (256 bits) 
privatekey = bytes.unpack("H*")[0]     # the data is binary, so unpack it to hexadecimal

# print the private key
puts privatekey

A private key can be almost any 256-bit number.

When you create a public key, your private key is put through a special mathematical function, and this function can only handle numbers up to just below 256 bits. The maximum value is:

max = 2**256 - 2**32 - 2**9 - 2**8 - 2**7 - 2**6 - 2**4 - 1
max = 115792089237316195423570985008687907853269984665640564039457584007908834671663

So when you generate a 256 bit number, you will want to check that it's below this maximum value.

Code (Ruby)

# print a warning if private key is too big
if privatekey.to_i(16) > 115792089237316195423570985008687907853269984665640564039457584007908834671663
    puts "Private key is too big. Try again."
end

Formats

A hexadecimal private key is 64 characters in length:

ef235aacf90d9f4aadd8c92e4b2562e1d9eb97f0df9ba3b508258739cb013db2 

If you're generating private keys for your own personal use, this is all you really need.

Wallet Import Format

However, you can convert your private key to a WIF Private Key, which basically makes it easier to copy.