A Defensive Computing Checklist    by Michael Horowitz
NOTE: I gave a presentation on Defensive Computing at the HOPE conference in July 2022
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For messaging apps, End-to-End encryption is the top of the line. It is offered by Signal, Threema, Wire, Session, WhatsApp and others and is often abbreviated E2EE.

Note that there are limits to the protection offered by end-to-end encrypted apps. The app sending an E2EE message sees the message before encrypting it, so the app could save it or send it or send parts of it in an insecure way. Because messages are sent using end-to-end encryption, does not mean that everything leaving the app is always and only sent that way. Likewise, the app that receives an E2EE message might save the message in an insecure manner.

Even if both the sending and receiving app store messages securely, the app still needs to retrieve messages and if either device is seized, the messages probably can be read (there may be an app configuration setting for this). On mobile devices, messages can also leak if the sender's device was hacked, the recipient's device was hacked or the recipient is simply not trustworthy and leaks messages, either on purpose or by accident. Even with messages that self-destruct, the recipient can take a picture of their screen showing a message.

Taking a step back, Android and iOS are probably not the best place for secure communication. On mobile devices, you can not see the end to end encryption, so you have to take it on faith. In contrast, with secure websites, the browser indicates when encryption is used and assorted websites can test and verify the encryption. Also, when looking at a website, you can tell what computer you are communicating with. In contrast, this is hidden when using mobile apps. On Android, someone could be tricked into installing a hacked app from outside the Play store. Even within the Play store, there may be multiple apps with the exact same name. A scam copy of an app can look exactly like the real thing, do what the real app does, but, also leak messages.


  1. Website: Secure Messaging Apps Comparison by Mark Williams. A detailed evaluation of 13 secure messaging apps. Only 4 are recommended: Signal, Threema, Wire and Session. The last site update was October 2021, so it may no longer be maintained.
  2. Best WhatsApp alternatives that respect your privacy by Douglas Crawford of ProtonMail (Feb. 2021). An overview and comparison of Signal, Telegram, Threema, Wickr Me, Wire, Element and Keybase.
  3. Arguing against three products: How WhatsApp, Signal & Co Threaten Privacy from TU Darmstadt University (Sept 2020). Researchers performed crawling attacks on WhatsApp, Signal, and Telegram. Maybe not the best choices. Quoting: " ... very few users change the default privacy settings, which for most messengers are not privacy-friendly at all." The Telegram contact discovery service exposes sensitive information even about owners of phone numbers who are not registered with the service. More here.
  4. On the Feb. 21, 2020 episode of his Privacy, Security, & OSINT podcast, Michael Bazzell recommended Wickr over Wire and Signal. For Signal, he suggested using it with a Google Voice number that is only used with Signal. He did not look into Threema.


Amongst techies, Signal is well regarded for security and encryption. It fails, however, on anonymity. It is worshiped like a religion despite using phone numbers as Signal userids. It can be critically important to hide who you communicate with and Signal does not do this. This strikes mes as a classic nerd mistake, to focus on technology (encryption in this case) and ignore the human need to be anonymous.

This Oct. 2021 blog by Yael Grauer How To Use Signal Without Giving Out Your Phone Number Using a Chromebook and an Old Phone points out many of the problems with the Signal app. Her solution is ridiculous and it too shows why Signal is a poor choice. Others have written the same article: Using Signal Without Giving Your Phone Number by Martin Shelton on Medium (Sept 2017) and How to Use Signal Without Giving Out Your Phone Number: A Gendered Security Issue by Jillian York for Vice (Aug 2017) and How to use Signal without giving out your phone number by Micah Lee for The Intercept (Sept 2017). All these articles are about hiding your main phone number. That is not the same as being anonymous. Not at all.

Other problems with Signal: you can not put the same account on two phones so all your eggs are in one basket, it supports disappearing messages but this has to be configured separately for each person you communicate with and there is no access to the service through a web browser.

If you do use Signal, there are quite a few dos and don'ts. This May 2017 article by Micah Lee is a good guide: How to keep your chats truly private with Signal.

This article, by Vladimir Katalov of Elcomsoft, shows the security is not perfect: How to Extract and Decrypt Signal Conversation History from the iPhone. The article is from August 2019, perhaps things have changed? I don't know.

FYI: Two Signal accounts on one device: There is an Android app called Molly that is a fork of Signal. On the September 16, 2022 episode of his Privacy, Security & OSINT podcast, Michael Bazzell discussed how he uses it so that there can be two different signal accounts on the same Android device. Molly is not in the Play Store and even installing it from F-Droid is not standard. On Linux, he uses SNAP to have multiple copies of Signal and multiple interdependent Signal accounts on the same copy of Linux. On macOS, the regular and Beta copies of Signal are separate, so installing each lets you have two Signal accounts on a single copy of macOS.

In August 2022, Twilio was hacked and Signal depends on Twilio to validate phone numbers. See their account: Twilio Incident: What Signal Users Need to Know of the incident. Phone numbers of roughly 1,900 Signal users were exposed to the Twilio breach attacker, who could have attempted to register them to another device. One solution is to on a Signal PIN and enable Register Lock. I read the doc and have no idea what this is or does. I have to wonder if Signal is too big and complicated for non techies to use safely. As for a checklist, in Signal do: Settings -> Account -> Registration Lock and verify that it is enabled.

In October 2022, Graphene OS tweeted about why they do not include Signal in their operating system (a clone of Android without anything from Google included). On a technical level, their points are over my head, except for the fact that Signal is dropping support for SMS/MMS. Even without fully understanding it, they make the point that Signal is far from perfect. Quoting "Signal is now dropping support for SMS/MMS. They also don't care much about keeping their dependencies patched, reducing attack surface or internal sandboxing. It would be an issue for GrapheneOS ... They've made many decisions we disagree with including replacing registration lock PIN with a sync PIN, depending on SGX for security, using SGX as a replacement for the previous private contact discovery and making the secure local backup system in the Android app less useful."

Finally, the Signal website is miserable to useless for a newbie to the software. I see this sort of thing all the time, experts can not understand the perspective of someone new to the subject. When experts write documentation, we get a cheat sheet for experts. We do not get anything that helps a newbie get up to speed.


INTRO: WhatsApp is owned by Facebook which should never be trusted. WhatsApp messages are end-to-end encrypted by default. It supports disappearing chats.

Your WhatsApp userid is your cell phone number. The service is unusable without a functioning mobile number, and you cannot hide your number from your WhatsApp contacts. If you swap the SIM card on your phone, and thus start using a new number, will have to change the number associated with your WhatsApp account.

To limit who can add you to groups and who can see information, such as your status and personal information, go to Settings -> Account -> Privacy

Articles from WhatsApp about Privacy and Security. None are dated.

  1. Privacy good starting point
  2. About two-step verification
  3. Account security tips
  4. About end-to-end encrypted backup
  5. About end-to-end encryption

April 13, 2023: A blog from WhatsApp about three upcoming security features: New Security Features: Account Protect, Device Verification, Automatic Security Codes. End-to-end encryption alone is not enough to protect you from account hijacking, device malware, or impersonation.

  1. Account Protect: When you move your WhatsApp account to a new device, they will double check that it’s really you.
  2. Device Verification: Malware can use WhatsApp to send unwanted messages. To help prevent this, they have added checks to authenticate your account. You do not need to do anything to get this feature. More: Device Verification
  3. Automatic Security Codes: The security code verification feature helps to ensure that you are actually communicating with the person you think you are. You can check this manually by going to the encryption tab under a contact's info. Too hard? OK, they will make this easier with a new feature that allows you to automatically verify that you have a secure connection. What a secure connection means, they don't say in the blog. It surely has nothing to do with verifying the person on the other end. With the new feature, click on the encryption tab and you will be able to verify that your personal conversation is secured. More: Key Transparancy

January 19, 2023: Whatsapp accounts are being hacked using the phone number. If the voicemail system for your Whatsapp phone number uses a default pin code, you are at risk. This from a twitter thread by @ihackbanme. In brief:
  You're sleeping. A bad guy tries to login to your account via WhatsApp.
  You get a text message with a pincode
  The attacker clicks on the option that the SMS didn't arrive and asks for a verification by phone.
  WhatsApp calls you. You're sleeping. It goes to Voicemail.
  The voicemail stores the automated voice with the pincode
  The attackers check your voicemail by trying the default pincode which may be the last four digits of your cellphone number
  Then they can log in to YOUR WhatsApp.
  After getting in, bad guys setup a 2FA pincode that prevents you from logging back in

November 26, 2022: WhatsApp data leak: 500 million user records for sale by Jurgita Lapienytė for Cybernews. Someone is selling up-to-date mobile phone numbers of nearly 500 million WhatsApp users. A data sample investigated by Cybernews likely confirms this to be true. It is not known how the data was obtained. Defenses that I read about:
 --Beware of unknown numbers trying to message you or call you on WhatsApp
 --If you get a message from an unknown WhatsApp number, block the number and do not click on any links in the message
 --Configuration suggestions: Settings -> Privacy. Change "last seen and online" and "profile photo" and "about" to "contacts only"

How to Use WhatsApp Privacy Settings by Yael Grauer for Consumer Reports. Published January 8, 2021. Last Updated August 16, 2022.
Despite the end-to-end message encryption ... "when you use the app, you may be sharing more information than you realize with your contacts, anyone else with your phone number, and parent company Meta, which also owns Facebook and Instagram." Very long article. Some of the topics covered:

WhatsApp can make encrypted backups. See this feature description from WhatsApp: About end-to-end encrypted backup (undated)

From I Accidentally Hacked a Peruvian Crime Ring by Albert Fox Cahn for Wired (Dec 2021). The article makes a strong case for securing an account with an optional PIN or two factor authentication. And, despite the WhatsApp end-to-end encryption, Facebook knows who your contacts are, what groups you belong to, and when and to whom you send messages. Quoting: "With a simple subpoena ... they can get much of your account information. With a full warrant, the platforms can provide records on every aspect of your digital network (apart from the message itself). They can record who we communicate with, how often, the groups we're part of, and the identity of every member, along with your full contacts list. Even worse, WhatsApp can do this in nearly real time, transforming a 'privacy-protective platform' into a government tracking tool."

September 2022: WhatsApp will soon let users hide their online status from their friends. From: Some lucky WhatsApp users can now hide their online status by Chandraveer Mathur for Android Police.

Private WhatsApp groups are not very private. See Google Is Letting People Find Invites to Some Private WhatsApp Groups by Joseph Cox of Vice (Feb 2020)

How to minimise targeted ads on social media: WhatsApp from Privacy International (May 2019)

Upgrading WhatsApp Security by Martin Shelton on Medium (Feb. 2017)

You are safer when WhatsApp does not automatically download stuff (pictures, audio, video, documents) because you never know if the file is malicious. To prevent automatic downloads:

  1. iPhone: Configuring auto-download from WhatsApp. By default, it automatically downloads images over a cellular connection. Audio and video will automatically download on Wi-Fi. To change this: WhatsApp -> Settings -> Data and Storage Usage. Tap on photos, audio, videos and documents and choose Never, Wi-Fi, or Wi-Fi and Cellular.
  2. Android: Configuring auto-download from WhatsApp. By default, it automatically downloads images over your cellular connection. Other types of files? Doesn't say. To configure: WhatsApp -> More options -> Settings -> Data and storage usage -> Media auto-download. There is no Never option, instead you have uncheck a bunch of checkboxes as per the video.


From my research, Threema seems to be the best encrypted communication app. Steve Gibson, of the Security Now podcast, prefers it. The Mozilla foundation gave Threema an excellent rating on their Privacy Not Included website where they considered it a Best Of product. Sven Taylor of Restore Privacy also liked it. The app is developed in Switzerland, and has more than 10 million users, including the Swiss government and the Swiss army. It has passed two independent security audits.

Threema does text and voice messages, voice and video calls, groups, distribution lists and file sharing. Users are identified in the system with a randomly generated 8-digit Threema ID. Users must create a username and password to log into the app. Optionally, users can link their Threema account to an email address or a phone number and give it access to their contacts. Again, this is optional. The mobile app costs $5 in the US, a one-time charge. There is a version for Windows, macOS and Linux, but the mobile app is still required. There is also a web interface, but it too requires the mobile app.

January 10, 2023. Messenger billed as better than Signal is riddled with vulnerabilities by Dan Goodin for Ars Technica. Academic researchers examined Threema and found 7 vulnerabilities. They privately told Threema about this and some/most of the problems were fixed. Is secure enough in January 2023? I don't know, I need to find an independent expert. Quoting: "Matteo Scarlata and Kien Tuong Truong, two of the ETH researchers who co-authored the paper, said that all the flaws stem from a single trait: the use of a custom protocol rather than an established one that has stood the test of time." The researchers also said "... that a company whose main product is based on cryptography, should always have a cryptographer at hand to assess its security and to propose already-existing protocols when possible, for example the battle-tested TLS instead of creating their bespoke client-to-server protocol". In Threema's response they claimed the bugs were in an old outdated protocol. They failed to say that it was only old because it was revised based on this recent research.


Not end-to-end encrypted by default, but it can be enabled. It might be called Vanish Mode or a "secret conversation". Encrypted conversations are not available on the Facebook website.

January 2023: Facebook/Meta is planning to add more users to the end-to-end (read, fully) encrypted version of Facebook Messenger over the next few months. Users will be chosen at random and notified by Facebook. The fully encrypted version of Messenger now supports link previews, chat themes, user active status, and Android floating bubble mode. More.

In August 2022, Facebook turned over their badly encrypted chat logs to the police who arrested a teenager for getting an abortion. See This Is the Data Facebook Gave Police to Prosecute a Teenager for Abortion by Jason Koebler and Anna Merlan of Motherboard. Just after this s--- hit the proverbial fan, Facebook started testing end-to-end encryption for certain Messenger chats. I don't know how they define "certain" Here is their press release about this. Do not trust Facebook.



One problem with any app on a mobile operating system is the operating system itself. Cellphones are spying machines. I think the best security has to reside on a different OS. One aspect of this is that the end user can not see the encryption. With a web browser, there is a lock icon that insures data is encrypted in transit. There is nothing like this with mobile apps. Another aspect is Location tracking. Mobile Operating Systems really want to track their location. Some people know how to deal with this, but many do not.

Another problem with the above Secure messaging apps is that they require software to be installed and learned. For many non techies, this can be too much to deal with.

My suggestion for secure communication is to use plain old simple boring webmail. Anyone can use webmail, even non techies. But not all webmail, of course, just webmail between two users of the same secure email provider. Two good choices would be ProtonMail and Tutanota, there are probably others.

I am out of step here with every techie in the world.

Neither ProtonMail nor Tutanota can read messages sent between their customers. Both offer free and anonymous accounts. Using webmail means that the browser can prove that encryption is being used. Webmail can also be used on a Chromebook running in Guest mode. Guest mode offers a virgin OS, with no information about you at all, and it is guaranteed to leave no trace of your actions. Guest mode insures the only running software is the Chrome browser. It can not be infected with malware.

With ProtonMail, the world can see the FROM and TO address of emails between two ProtonMail users. Since these can be anonymous, no big deal. The world can also see the subject line and the name of any attached file. Forewarned is forearmed. The world can not see the body of emails or the contents of attached files. Not just the world, the Proton company itself has no access.

FYI: ProtonMail includes protection from Homograph attacks and Enhanced tracking protection.

When it comes to erasing messages after you send them, a Chromebook in Guest Mode is your best bet. Guest Mode erases everything when you log out. Everything. At the operating system level. There is no need to worry about how and where sent/received messages are saved. If a Chromebook is seized by law enforcement, there is nothing on the computer to indicate that webmail was used.

The secure Email company knows the pubic IP address that you connect to their service from. If they were compelled, they might have to provide this information to law enforcement. There are three defenses: use a VPN, use Tor or never connect from a place associated with you (home and office, obviously). This needs to be done when creating an account too. If you pay for a VPN, then the VPN provider knows who you are. To combat this, use a free limited account from ProtonVPN or Windscribe or Tunnelbear or another company. Or, pay for the VPN in cash or with a gift card. Or, have someone else pay for the VPN service.

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