Private Keys

A private key is simply a number, picked at random. Ownership and control of the private key is the root of user control over all funds associated with the corresponding Ethereum address, as well as access to contracts that authorize that address. The private key is used to create signatures required to spend ether by proving ownership of funds used in a transaction. The private key must remain secret at all times, because revealing it to third parties is equivalent to giving them control over the ether and contracts secured by that private key. The private key must also be backed up and protected from accidental loss. If it’s lost, it cannot be recovered and the funds secured by it are lost forever too.

Generating a Private Key from a Random Number

The first and most important step in generating keys is to find a secure source of entropy, or randomness. Creating an Ethereum private key essentially involves picking a number between 1 and 2256. The exact method you use to pick that number does not matter as long as it is not predictable or deterministic. Ethereum software uses the underlying operating system’s random number generator to produce 256 random bits. 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, or press random keys on your keyboard. An alternative could be cosmic radiation noise on the computer’s microphone channel.

More precisely, a private key can be any nonzero number up to a very large number slightly less than 2256—a huge 78-digit number, roughly 1.158 * 1077. The exact number shares the first 38 digits with 2256 and is defined as the order of the elliptic curve used in Ethereum (see Elliptic Curve Cryptography Explained). To create a private key, we randomly pick a 256-bit number and check that it is within the valid range. In programming terms, this is usually achieved by feeding an even larger string of random bits (collected from a cryptographically secure source of randomness) into a 256-bit hash algorithm such as Keccak-256 or SHA-256, both of which will conveniently produce a 256-bit number. If the result is within the valid range, we have a suitable private key. Otherwise, we simply try again with another random number.

Note that the private key generation process is an offline one; it does not require any communication with the Ethereum network, or indeed any communication with anyone at all. As such, in order to pick a number that no one else will ever pick, it needs to be truly random. If you choose the number yourself, the chance that someone else will try it (and then run off with your ether) is too high. Using a bad random number generator (like the pseudorandom rand function in most programming languages) is even worse, because it is even more obvious and even easier to replicate. Just like with passwords for online accounts, the private key needs to be unguessable. Fortunately, you never need to remember your private key, so you can take the best possible approach for picking it: namely, true randomness.

The following is a randomly generated private key shown in hexadecimal format (256 bits shown as 64 hexadecimal digits, each 4 bits):

f8f8a2f43c8376ccb0871305060d7b27b0554d2cc72bccf41b2705608452f315