Address Checksum

A checksum is redundant information built into an address so that software can tell whether the string it was given is internally consistent. Change one character and the check fails, which is why wallets reject a mistyped address instead of sending funds into nowhere. The mechanism varies by chain and address format: some encodings carry a dedicated checksum section, and some use letter casing to encode the check, which is why altering the capitalisation of certain addresses can make them invalid. What a checksum does not do is verify ownership or intent. An address substituted wholesale by malware, or supplied by an attacker, has a perfectly valid checksum and will be accepted without complaint. It catches accidents, not adversaries — which is why it does not replace comparing an address against a source you trust before sending.