No, this is not possible to do without information about the intended size of each of the strings. As long as you pad with a pattern that is also valid potential input, you are going to run into problems if the valid input happens to end with that pattern. If you pad with 'ab' pairs then run into problems with (for example) 'nice grab' .
To get around this you either need to keep track of the length of each string, or you need to pad with something that is defined as not being valid input. For example you might be able to get away with padding with char(65535)