Alphanumeric

Alphanumeric describes any string built only from the 26 letters of the Latin alphabet (upper or lower case) and the digits 0 through 9, with no spaces, punctuation, or special symbols. It is a general computing term, but in cryptocurrency it shows up constantly because so many of the identifiers people copy and paste, such as wallet addresses, private keys, and transaction hashes, are alphanumeric strings generated by an underlying cryptographic algorithm rather than chosen by a person.

Different blockchains encode these strings in slightly different alphanumeric alphabets. Bitcoin's older legacy and script addresses use Base58, a 58-character set that deliberately drops the digit 0, the capital letter O, the capital letter I, and the lowercase letter l because they look too similar in many fonts. Newer SegWit addresses starting with "bc1" use Bech32, a 32-character lowercase-only alphabet with a built-in checksum that flags typos before a transaction is broadcast. A raw private key or an Ethereum address, by contrast, is usually shown in hexadecimal, using only 0-9 and a-f, a narrower alphanumeric subset.

Not every crypto secret is alphanumeric. A BIP-39 recovery seed phrase is deliberately made of 12 or 24 plain English words instead, because whole words are easier for humans to read aloud, write down, and spot errors in than a long alphanumeric string.

Because alphanumeric addresses and keys are long and easy to mistype, users are advised to copy them directly, scan a QR code, or double-check the first and last few characters rather than retype them manually, since malware that silently swaps a copied address for a similar-looking alphanumeric one is a known theft technique.