The ModHex Calculator is a simple tool to convert values between different data encoding formats.
The Modified Hexadecimal encoding scheme was invented to cope with potential keyboard mapping ambiguities, namely the inconstant locations of keys between different keyboard layouts. The YubiKey uses this special data encoding format known as “modhex” rather than normal hex encoding or base64 encoding. Modhex is similar to hex encoding but with a different encoding alphabet. The reason is to achieve a keyboard layout independent encoding.
Learn more about the YubiKey and ModHex with the YubiKey Manual.
YubiKey Technical Manual (pdf)