+`y` (byte array) \[const uint8_t \*, size\_t\]
+
+: Convert the byte array whose length is given to
+ its base64url string representation.
+
+`Y` (byte array) \[const uint8_t \*, size\_t\]
+
+: Like 'y' but output is base64.
+
+`y?`, `Y?` (byte array or null) \[const uint8_t \*, size\_t\]
+
+: Like 'y' or 'Y' but allows to output a JSON null value
+ either when the buffer is *NULL* or when the size is *0*.
+
+`y*`, `y*` (optional byte array) \[const uint8_t \*, size\_t\]
+
+: Like 'y' or 'Y' but do not put JSON value
+ either when the buffer is *NULL* or when the size is *0*.
+ This format can only be used inside an object or an array. If used
+ inside an object, the corresponding key is additionally suppressed
+ when the value is omitted. See below for an example.
+