An input block of

A block cipher consists of two paired http://en.wikipedia.org/wiki/Algorithm, one for encryption, E, and the other for decryption, D.^{http://en.wikipedia.org/wiki/Block_cipher#cite_note-4} Both algorithms accept two inputs: an input block of size n bits and a http://en.wikipedia.org/wiki/Key_(cryptography) of size k bits;

I'd like to know whether "n bits" modifies "size" or "size n bits" is a compound noun.
Thank you in advance for your help