How big the number is
Start with size. The Bitcoin developer guide calls a standard private key “simply a 256-bit number,” allowed anywhere from 1 up to a ceiling a little under 2 to the power of 256 that the secp256k1 curve sets. Written out in decimal, the biggest valid key has 78 digits.
Ethereum follows the same idea. According to ethereum.org’s accounts documentation, a private key is 64 hex characters, the public key is generated from it with the elliptic curve digital signature algorithm, and the address is the last 20 bytes of the Keccak-256 hash of that public key with “0x” in front. You cannot work backward from an address to the key.
Most people never see the raw number. Wallet maker Ledger says on its academy page that a device’s recovery phrase acts as a master key to every private key it manages, so the words stand in for many keys at once.
What happens to a key that is not random
For this entry, the number 1 was put through both derivations.
Anyone can repeat the sum and land on the same public addresses. Block explorer mempool.space, checked at 08:38 UTC on November 8, 2026, showed the older, uncompressed Bitcoin address for key 1 with 7.84927386 BTC received over 1,178 transaction outputs. Every one of those outputs had been spent.
The same key also has a compressed form, a second address that the developer guide explains. That one took in 0.25034358 BTC across 101 outputs, all spent too, and explorer Blockscout listed the matching Ethereum address at a zero ether balance after 6,120 transactions.
Why does money vanish there? A key is private only while nobody else can produce it. The SEC’s custody bulletin of December 12, 2025 calls a private key “a randomly generated alpha numeric passcode,” and ethereum.org notes that most software libraries generate a random one for you. A number a person picks can be picked by someone else.
Theft and what to do about it
Once created, a private key “cannot be changed or replaced,” the SEC bulletin says, so a leaked key stays leaked. Law enforcement has described one way keys leak. A public service announcement from the FBI dated September 3, 2024 described North Korean hackers luring crypto company staff with fake job offers, and it advised against storing seed phrases and private keys on internet-connected devices.
If a backup may have been exposed, wallet maker Trezor’s guide says to transfer the funds to a new wallet immediately. The old key can never be revoked. Moving the money is the only repair.