Validate and generate mixed-case checksums for Ethereum addresses per EIP-55. Optional EIP-1191 mode adds chain-specific checksum variance (Rootstock, etc.). 100% client-side.
An Ethereum address is 20 bytes (40 hex chars) and is case-insensitive for the network. EIP-55 adds a checksum by capitalising hex letters (a-f) based on the Keccak-256 hash of the lowercase address: if the corresponding hash nibble is ≥ 8, the letter is uppercase. Wallets reject addresses with an invalid mixed-case checksum, catching typos before funds are lost.
EIP-1191 prefixes the chain ID to the address before hashing, producing different checksums per chain so an address valid on Ethereum is marked invalid on Rootstock.