I am not a CTO, but as a professor I am often approached at conferences and elsewhere by people at earlier stages of their career who are interested in career opportunities and start a conversation with me assuming that I remember them because we met somewhere a year or two before. It is an embarrassing truth that that is not always the case; try as I might, I simply meet too many people in too many places to be able to immediately recognize each and every person without any contextual cues. However, if I am reminded of who they are I usually have no trouble remembering. (I should add that when I was myself more junior I probably made the exact same mistake many times!)
I therefore recommend that in your email you make no assumptions as to whether or how easily the CTO will remember your conversation, and start your email by reminding him/her of it in highly specific detail. For example:
Dear Mr. [or Ms or Dr. as appropriate] CTO,
My name is Johnny Programmer. We met in October 2014 at the Whatchamacallit Technology Conference in Los Angeles. I was the guy with the nerdy glasses and blue sweater (see attached a photo of me from around that period) who approached you around 11:30 a.m. on the first day of the conference as you were standing by the big neon sign near the convention center entrance. As you may recall, I told you that I was working on a Master's in machine learning at Doohickey University, and we briefly discussed the possibility that I might apply for a position at your company at some point. You were very encouraging and told me to feel free to contact you directly when I finish my degree. Well, I'm happy to let you know that I recently graduated and am now on the job market and looking at what opportunities are available.
I've looked at your company's website and see that there are a few positions that may suit my qualifications, such as [add position details]. I know that there is a formal application process and am happy to follow the instructions online without bothering you about it, but I also thought you might be interested to know about my situation in case you have some other suggestions or ideas I may not be aware of. I would really appreciate any thoughts or advice you might have to offer.
[optionally, some more details about yourself, e.g.:] By the way, I thought you might be interested to know that I graduated with a GPA of 3.85. My data mining project won first place in a class competition we had, and I have excellent references. I attach a current CV.
[attach photo] [optionally, attach CV]
The part where you brag about yourself and/or attach a CV is optional, and I would only include that if I was reasonably confident it wouldn't be perceived as presumptuous or annoying. CTO's probably get a lot of emails like this so I have a strong feeling they might prefer the sender not to presume that they care to spend a lot of their time with an in-depth review of the sender's qualifications. So you might want to leave the CV for a follow-up email if and when it becomes applicable, and just mention one or two nice details about yourself like the GPA and the data mining award.