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End-to-End Encrypted Communications: Phone Apps

Last modified July 18, 2018
  • English
Duration: 1 hour(s)

Learning Objectives

Learners will:

  • Explore what “private communication” might mean.
  • Be familiar with encryption as an idea.
  • Be able to explain why end-to-end encryption is useful when communicators don’t trust intermediary third-parties and companies.
  • Know where to find more information about end-to-end encrypted communication and downloading related messaging apps, such as Signal.
  • Be able to identify that SMS is an insecure communication method.


  • Participants must have a mobile device with them if doing an install.
  • Trainer has already done threat modeling with this group.


Instructor: Learners
1:5 (One instructor to five students)

Suggested Materials

  • Projector
  • If planning an install, participants should bring their smart phones with them.  

Lesson Content


Facilitator splits up the group into two teams.

Facilitator: “Please introduce yourself to the people next to you by your preferred names. After, I’d like you to address the following prompt over the next five minutes. Your ideas can be as wild and imaginative as you want them to be. I’d like you to come up with as many ideas as you can in the next five minutes.”

To Team 1: “Your team is the Communicators. Let’s say you want to send a secret message to a friend. You don’t mind that others can see the fact that you and your friend are communicating, but you want to make sure the message’s content stays between you and your friend. How would you do it?”

To Team 2: “Your team is the Interceptors. Your group will be trying to intercept the secret message that the communicators are sending. You very badly want to read that secret message. What are the options you have? What kind of capabilities do you have? How might you get around the Communicators’ cleverness?”


Facilitator can encourage wild ideas in their responses. Team 1 Participants may say things like: passing a note that’s carefully folded and written in invisible ink, sending a SMS text message, using an obscure game chat channel to communicate, physically running and meeting to pass the note in person at a different location each time, writing in a made-up language that only you and your friend know, using an encrypted messaging app, etc.

Team 2 Participants may say things like: they imagine they are the government, and they have capabilities and resources like lawyers, system administrators, law enforcement, and three letter agencies. They can hire someone to help them crack the message or put malware on devices. Or, they may imagine they are a team of technically skilled hackers. Or, they might be a cell phone company, and intercept SMS messages. The hope is that they are able to come up with lots of roles of who may be involved in wanting to get this message.

After the five minutes are up, pose questions like the following. Depending on the participants’ responses, the facilitator can spend five minutes having the group go over each answer.

  • What are the benefits of using invisible ink? Could someone crack how to read the invisible ink, once they identify that invisible ink is being used?
  • What are the benefits of sending a SMS? Could someone crack how to read a SMS? How easy or hard might that be?
  • What are the benefits of meeting in person at a secret location? Could someone figure out where you are meeting? How easy or hard might that be?
  • What are the benefits of using an obscure tool, like a video game chat system? Could someone figure out where you are meeting, or are they unlikely to look?
  • What are the benefits of writing in a made-up language? Could someone crack a made up language? What would happen after the made up language is translated and figured out?

Discussion points may vary on participants’ ideas of what might be secure or insecure.

Facilitator can then discuss: “Did anyone bring up using encryption? What examples of encryption do you know? What does encryption allow us to do?”

Facilitator should wait for responses, and guide participants toward an explanation like: “Encryption allows us to scramble messages into meaningless gibberish, and back into readable information.”

Knowledge Share: SMS vs. End-to-End Encrypted Messages

(Facilitator can show the gif of SMS passing through a cell phone network.)

Facilitator: “This is a SMS, or text message, going through a cell phone network. What kinds of computers might have access to this text? How many people might have access to these computers?”

Guide participants to an answer along the lines of: Cell phone companies and others with access to cell towers. And since these companies share their data with each other across borders depending on business partnerships, possibly thousands and thousands of people.

Facilitator can lead a raise of hands and call and response portion.

Call and Response

Q: Who here has an iPhone?

Q: iPhone users, have you noticed that messages you send to other iPhone users are a different color than those you send to non-iPhone users? What color are they?

A: Blue!

Q: Does anybody have any idea what that color might indicate?

A: Apple iPhones use end-to-end-encryption for iMessage.

Q: What happens when you send messages to someone who has an Android phone?

A: Messages are green. They are sent unencrypted.

- Transition -

Q: But what if the person you want to chat with doesn’t have an iPhone, and you still want an end-to-end encrypted communication?

A: (Participants may already know of end-to-end encrypted apps that people can download.)


Talking Points

Facilitator: “One thing you should note is that both parties need to have end-to-end encryption tools downloaded in order to communicate privately with each other.”

Facilitator: “Telecommunication networks and the Internet have made communicating with people easier than ever, but have also made surveillance more prevalent than ever. Without taking extra steps to protect your privacy, every phone call, text message, email, instant message, voice over IP (VoIP) call, video chat, and social media message may be vulnerable to eavesdroppers.

Often the safest way to communicate with others is in person, without computers or phones being involved at all. Because this isn’t always possible, the next best thing is to use end-to-end encryption.”

Facilitator: “What might this phrase, end-to-end encryption, mean?”

(Facilitator should guide participants toward the answer of: “End-to-end encryption ensures that a message is turned into a unreadable, scrambled message by its original sender, and decoded only by its final recipient,” or “It scrambles messages between devices into gibberish, and is useful when you don’t trust the service providers or companies who are helping to pass along the message.”)

Facilitator: “What are some examples of third parties be that may interact with your SMSes and voice calls?”

(Participants may say some company names like ISPs or cell carriers, or the names of the phone manufacturers themselves.)


Knowledge share: Things to know about end-to-end encrypted apps


Talking Points

  • Most end-to-end encrypted messaging apps we recommend are free!
  • They’re great for use over Wi-Fi and over data plans.
  • Many work internationally.
  • Many allow you to do video chats, group chats, disappearing messages.


You can also share some more in-depth points:

Phone service carriers know that you are using an end-to-end encrypted messaging app because they have access to your phone's’ metadata. Companies that host app download stores (e.g. the Apple App Store or Google Play Store) also know you are using these apps. They are able to see the information about your messages, or metadata, and that includes details like when you downloaded the app. Depending on the app, metadata may be limited to this fact, or can be far more detailed. For instance, an app may also log who you have been contacting, when, and for how long, though the contents of the message itself aren’t seen except by you and your intended recipients.

  • Before choosing a phone app, check what user information the app developers keep.

Picking the right tool for you depends on your level of comfort.

  • Check that the end-to-end encrypted messaging app that you are using is end-to-end encrypted by default.

  • Know the app’s way of notifying you about an unencrypted message.

There’s a risk of someone misunderstanding a message as encrypted, because they may be confused about the app interface. For example, some apps will let you know when you are contacting someone who is not on the end-to-end encrypted app, and there will be some text that says something like: “Unencrypted SMS” or “Unencrypted text”. Look out for this text!

  • Many encrypted messaging and phone apps require you to give your phone number and this may be a concern for some people.

There are also ways to tie your account to a proxy phone number for apps like Signal and WhatsApp.

There are alternative apps that let you choose a username instead, like Wire. If being identified easily is a concern of yours, pick a username that’s not the same as your usernames for other accounts or affiliated with your actual interests or identity.

  • Your country might not allow some encrypted messaging apps. Follow current events about restrictions on the Google Play Store and the Apple App Store.
  • Many end-to-end encrypted messaging apps let you verify the identity of the device of your friend, which mitigates against eavesdroppers who may try to pretend to be your friend:

This practice is called “key verification”, which is also known as checking the “safety number” in an app like Signal.

This helps you really make sure that you are talking to who you think you are! When sending messages, there’s a risk of something called a “machine-in-the-middle attack,” or MitM attack which is just when someone pretends they’re the person you’re trying to communicate with and is able to intercept your messages. The way you can prevent this is through a practice called “key verification.”

By verifying keys, you and the person with whom you’re communicating add another layer of protection to your conversation by verifying each other’s identities, allowing you to be that much more certain that you’re talking to the right person. Key verification allows your contact to confirm that the person they’re communicating with is really you, and you to confirm that the other person is really them, by checking with each other in-person or over a different, secure channel.

In practice, it usually looks like using your phone’s camera to scan your friend’s phone’s QR code, and having them do the same for you. That way their phone is confirmed as theirs.

The facilitator can then ask for questions before moving onto a guided install.


Optional Closing Activities

Survey the audience and check for understanding:

  • What information can companies and service providers and telephone companies see when you communicate?
  • What can a company see when you are using encryption on their platform? (The answer is not “nothing.” The answer is gibberish or scrambled content, and they still see you communicating with someone.)

  • What information can service providers and telephone companies see when you communicate with someone using end-to-end encryption? (Guide answers toward metadata.)

  • What are some popular services that don’t use end-to-end encryption?
  • What does end-to-end encrypted communication help you do?
  • What is a man/machine-in-the-middle attack?
  • Let’s say you have malicious software on your device. Are your end-to-end encrypted communications still private? (The answer is along the lines of “not necessarily.” If learners are confused on this point, Illustrate the difference between encryption at rest and encryption in transit.)