Private Key

A large randomly generated number.

A private key is a random number. It is a 256 bit number.

It is used as the source of a public key.

Try it! - Generate Private Key

Hexadecimal (Base 16)
Decimal (Base 10)
Binary (Base 2)

Never use a private key generated by a website or by someone else. Always generate your own private keys secretly on your own computer.

Generating 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 = 115792089237316195423570985008687907852837564279074904382605163141518161494336

This number is n-1, where n is the number of points on the elliptic curve used in Bitcoin. So when you generate a 256 bit number, you will want to check that it’s not above this maximum value.

Formats

A hexadecimal private key is 32 bytes (64 characters):

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 and import in to wallets.