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Author: Admin | 2025-04-28
A very large random number A private key is a very large random number.It's used as the source for creating a public key.GeneratingHow do you create a private key?To create a private key you just need to generate a random 256-bit number*.The critical part to generating a private key is to use a reliable source of randomness. If you're using Linux, a reliable source of randomness is /dev/urandom:# generate 256 bits of random dataurandom = 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 keyputs privatekey Private key range A valid private key is any number between (and including) the following range of numbers: min: 1max: 115792089237316195423570985008687907852837564279074904382605163141518161494336 This maximum value is n-1, where n is the number of points on the elliptic curve used in Bitcoin (secp256k1). This is slightly less than the maximum value for a 256-bit number. So if you're generating a random 256-bit (32-byte) number, you want to check it's not above the maximum value before using it. Cryptographically secure random numbers The default random number functions in programming languages are typically not secure enough for generating private keys. The standard "rand()" functions in most languages are just quick and easy ways for generating a "random" looking numbers, but they're not random enough to use for cryptographic purposes, such as generating private keys. For example: # simple random number (do not use for generating private keys)puts rand(1..115792089237316195423570985008687907852837564279074904382605163141518161494336)# cryptographically secure random number (can use for generating private keys)require 'securerandom'puts SecureRandom.random_number(1..115792089237316195423570985008687907852837564279074904382605163141518161494336)# NOTE: These random number functions include the given minimum and maximum values as part of the range of possible results. So for whatever programming language you're using, make sure you search for how to generate "cryptographically secure random numbers" to find out which function you should be using (instead of the default functions you may be familiar with). For example, the libbitcoin library (specifically the bx seed command line tool) caused the loss of over $900,000 worth of bitcoin in 2023 due to not using cryptographically secure random numbers for generating seed phrases. For a full explanation of what happened and why, see Milk Sad. It's sometimes easier to generate random bytes instead of random numbers. So you can always just generate 32 random bytes and use that as your private key, as that's equivalent to generating a 256-bit number. If you're on Linux, the secure random numbers are usually sourced from /dev/urandom anyway, so it's more straightforward to get your bytes from that source directly.FormatsWhat does a private key look like?DecimalA private key is ultimately just a random number, so it's perfectly fine to store
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