How to encrypt letters less than block n using Hill Cipher I am learning about Hill Cipher and know that it is a block cipher where each block of n letters (considered as an $n$-component vector) is multiplied by an invertible $n \times n$ matrix, again modulus $26$. 

The article did not touch on what happens if the the block of letters is less than length $n$. 

Eg: Assume I am supposed to encrypt 100 letters – how do I encrypt the last 22 letters?

I was thinking of padding similar to MD5, or are there other methods to resolve this issue? If so, what are those methods?
Since it's a linear cipher, you should be wary about a guessable padding, otherwise if your last block is only one char long, you will reveal almost your whole matrix on this last block. If you're too afraid of mangling the last word, use something like 'Z'+(random chars). But I really would not use any predictable padding with such a cipher.