Converting Letters into Numbers for Creating an Authenticator
Normalize the input and encode it using UTF-8. That gets you octet strings. Then the particular block size depends on the system you're using. For Poly1305, treat 16-byte long octet strings as little-endian integers. RFC8439 provides the way to convert a message composed of bytes into appropriately sized blocks (for Poly1305, other such systems need different sizes):

o ...divide the message into 16-byte blocks.  The last one might be shorter:


o  Read the block as a little-endian number.


o  Add one bit beyond the number of octets.  For a 16-byte block, this is equivalent to adding 2^128 to the number.  For the shorter block, it can be 2^120, 2^112, or any power of two that is evenly divisible by 8, all the way down to 2^8.


o  If the block is not 17 bytes long (the last block), pad it with zeros.  This is meaningless if you are treating the blocks as numbers.