What does Touma say to the "Replica" Sorcerer with the Obsidian Blade? Question for any of you Index/Railgun fans out there.

At the end of A Certain Magical Index I Episode 18 the "Replica" Sorcerer (the one with the Obsidian Blade) asks Touma to make sure he protects Misaka.  When Touma answers, we can only see his lips moving (no sound) and then he smiles.

Misaka is listening from behind one of the iron beams of course.  Normally in Anime it is fairly clear to me what these moments signify, but in this case, both the Sorcerer's and Misaka's response are (in my opinion) ambiguous.

Anyone care to wager what he said?
Manga volume 11, chapter 64, page 15


The full quote was: 


  "That’s right, it's a promise. This promise is to protect Misaka Mikoto and the world around her. This is my agreement with a certain nameless person who fell in love with her so quickly."


He explains this to Shirai Kuroko regarding his promise with Etzali from Toaru Majutsu no Index Light Novel Volume 08. 

source: Touma's quotes on Toaru Majutsu no Index Wikia