HMAC is a secure way of generating a hash that combines your message with a secret key (e.g., "my_secret_123"). It is commonly used in API authentication. To use it, enter a secret string above and choose one or more hash algorithms (e.g., SHA-256). The result will be calculated as HMAC-[Algorithm]. If no key is entered, a normal hash will be used. When BLAKE2b is selected, the key is applied as keyed BLAKE2b rather than HMAC, and the key length must be 16 to 64 bytes.