Original: If a block cipher ( not operating in a stream cipher mode ) were to be used in this type of application , the designer would need to choose either transmission efficiency or implementation complexity , since block ciphers can not directly work on blocks shorter than their block size .
Simplified: If a block cipher were to be used in this type of application , the designer would need to choose either transmission efficiency or implementation complexity , since block ciphers can not directly work on blocks shorter than their block size .