Crypto glossary
Address checksum (EIP-55)
An address checksum, defined for Ethereum in EIP-55, mixes upper and lower case letters in an address so wallets can catch most typing errors.
Ethereum addresses are 40 hexadecimal characters, and hex is case-insensitive, so EIP-55 uses the case as a hidden check. The address is hashed with Keccak-256, and each letter is written in capitals if the matching character of the hash is high, lowercase otherwise. A wallet that receives a mixed-case address can recompute the pattern and warn if a single character was mistyped.
Most wallets, explorers and exchanges display and expect checksummed addresses, and libraries reject a mixed-case address whose capitals do not match. Bitcoin formats build a checksum into the address itself through Base58Check or Bech32, and some EVM networks use a variant that includes the chain ID, which can confuse tools that only know EIP-55.
A checksum detects accidents, not fraud. Address poisoning works precisely because the attacker generates a perfectly valid address that shares the first and last characters with yours. An all-lowercase address is also valid but carries no check at all, so copying it by hand removes the protection. Comparing the full address remains the only real safeguard.
Example
A user pastes an address with one character changed; because the capitalisation no longer matches the EIP-55 pattern, the wallet refuses to send and flags an invalid checksum.