Try thinking just as much of how you present as much as what you present

I'm going to present my argument with a few attention grabbing words here... but in truth what you need to do is start a discussion much life we do here. A good way is to grab their attention.

If a knife is used to kill a man, is that knife evil?

Obviously the answer to that is no. However what is a knife more than a tool? What is to kill more than a harmful action? What is evil more than a moral view? If we rewrite this question very generically it becomes

If a tool is used to do harm to people, is that tool bad?

The answer to this question is obviously No. A tool is nothing more than a medium to perform an action. Much like paint can be used for art, it can also be used for protest, defacing, or even destruction. What matters here is not the use of the tool itself but of the desire of the operator. If an operator intends to do harm, there will always be tools to do harm. If an operator intends to do good, there will be the same tools to do goo.

The duality of it all is that a tool is a tool. It is in the hands of the person that determines whether the action performed is for good or for evil. Those actions can only be judged by others and thus it is only through our eyes that we can pass judgement. Because we are the ones to judge your friends statements aren't wrong, but they show ignorance of what cryptography really is, and how they should not be the ones to judge it. If they can not explain it without mentioning the word itself, then they do not understand it enough to speak on it.

A good basis for discussion and not argument/bandwagon

"If you are to judge cryptography you must know what it is and what it does. If you can answer that to me I will listen to your stance, so please explain to me what cryptography is so we are on the same page." The statement not only shows you have an understanding yourself, but it asks if they have an understanding in you asking for them to clarify it to you. If they are wrong, they have already opened themselves to a debate about if they really know what it is.

At this point if you can prove them wrong, or raise sufficient doubt in their argument, the group mentality is shifted onto you as the default authority. Remember you don't have to disprove their arguments, you must just prove them wrong. A good example of this comes from the movie "Thank You for Smoking"(Rated R) in the following scene:

Joey Naylor: ...so what happens when you're wrong?

Nick Naylor: Whoa, Joey I'm never wrong.

Joey Naylor: But you can't always be right...

Nick Naylor: Well, if it's your job to be right, then you're never wrong.

Joey Naylor: But what if you are wrong?

Nick Naylor: OK, let's say that you're defending chocolate, and I'm defending vanilla. Now if I were to say to you: 'Vanilla is the best flavour ice-cream', you'd say...

Joey Naylor: No, chocolate is.

Nick Naylor: Exactly, but you can't win that argument... so, I'll ask you: so you think chocolate is the end all and the all of ice-cream, do you?

Joey Naylor: It's the best ice-cream, I wouldn't order any other.

Nick Naylor: Oh! So it's all chocolate for you is it?

Joey Naylor: Yes, chocolate is all I need.

Nick Naylor: Well, I need more than chocolate, and for that matter I need more than vanilla. I believe that we need freedom. And choice when it comes to our ice-cream, and that Joey Naylor, that is the defintion of liberty.

Joey Naylor: But that's not what we're talking about

Nick Naylor: Ah! But that's what I'm talking about.

Joey Naylor: ...but you didn't prove that vanilla was the best...

Nick Naylor: I didn't have to. I proved that you're wrong, and if you're wrong I'm right.

Joey Naylor: But you still didn't convince me

Nick Naylor: It's that I'm not after you. I'm after them.

Here you can see he is clearly explaining to his son that to win the majority you must not target the argument, but the audience. Once the audience knows that someone here is that authority then they are much more likely to listen.