How do you check if a crypto address is valid?

You check a crypto address is valid mainly by letting wallet software test its format and checksum, then confirming with the recipient that it is the right address on the right network. The first step catches typos. The second catches mistakes and scams that a format check cannot see. The basics of formats are in what a wallet address is.

What is an address checksum?

A checksum is a short piece of an address that is calculated from the rest of it. If any character is changed, the calculation no longer matches, and the wallet refuses the address.

• Bitcoin addresses starting with 1 or 3 use an older format called Base58. The Bitcoin Wiki says a mistyped address of this kind passes the check only about once in 4.29 billion attempts. These addresses are case sensitive.
• Bitcoin addresses starting with bc1 use a format called Bech32, defined in BIP-173. It is guaranteed to detect any error affecting up to four characters and leaves out characters that look alike, such as "1", "b", "i" and "o" in the data part.
• Ethereum addresses start with 0x and use the letters a to f plus digits. EIP-55, proposed in January 2016, hides a checksum in the pattern of upper and lower case letters. A random typo slips through about 0.0247% of the time.

The Bitcoin Wiki warns that checking length, allowed characters or the prefix by eye is not enough. Software does the real check, which is one reason to copy and paste rather than type an address.

How do you verify a wallet address belongs to the right person?

A checksum cannot tell you who owns an address. A well-formed address belonging to a stranger passes every test. Common ways to reduce the risk:

• Compare the whole address, not just the first and last few characters. Scammers create look-alike addresses for address poisoning.
• Confirm the address with the recipient through a second channel, or scan their QR code directly.
• Check the network matches. Many networks share Ethereum's address format, so an address can pass the checksum on a chain the recipient does not use.
• Send a small test amount and have the recipient confirm it arrived.

Can you look up an address before sending?

Yes. Pasting it into a blockchain explorer shows whether it has any history, what it holds and, on smart contract chains, whether it is a contract rather than a personal wallet.

How do you check if a crypto address is flagged?

The US Treasury's Office of Foreign Assets Control (OFAC) adds some crypto addresses to its sanctions list, the SDN List, under entries labelled "Digital Currency Address". Its free Sanctions List Search tool finds them when the full address is entered in the ID field, and it returns exact matches only. OFAC says its address listings are not likely to be exhaustive, so a clean result is not a guarantee. Exchanges and analytics firms run their own screening on top of this, and scam reports also appear as labels on some explorers.