Signal App Review: Security & Privacy

Signal has earned a reputation as one of the most privacy-focused messaging apps on the market. It’s free, open source, and built around end-to-end encryption by default.

This review looks at Signal’s security features, privacy posture, and tradeoffs. Then we’ll answer a question we hear a lot from regulated teams: is Signal appropriate for medical device cybersecurity workflows (PSIRT, incident response, supplier coordination), and what should your policy say about it?

Official site: signal.org

Understanding the Signal App

What is Signal?

Signal is a free messaging app developed by the Signal Foundation. You can send encrypted messages, make voice/video calls, create group chats, and share photos/files. Signal is available on:

Signal at a glance

End-to-end encryption: On by default for messages and calls
Open source: Code is publicly available and routinely scrutinized
Privacy-first design: Built to minimize stored data and reduce metadata exposure where possible
Useful safety controls: disappearing messages, registration lock/PIN, verification via safety numbers

Key Features of Signal

End-to-end encryption: Messages are encrypted from sender to recipient so intermediaries can’t read the content.
Disappearing messages: Optional timers to reduce how long messages remain on devices.
Screen/privacy controls: Options to reduce notification previews and limit casual shoulder-surfing risk.
Safety numbers: A way to verify you’re really talking to who you think you’re talking to (useful for high-stakes conversations).

None of these features are “magic,” but together they create a messaging experience that’s explicitly designed to reduce exposure compared to typical consumer messaging platforms.

Diving into Signal’s Security Measures

End-to-end encryption (the foundation)

Signal encrypts messages, calls, and media directly between sender and recipient. That means the content isn’t readable by network intermediaries—and it’s not readable by Signal itself in the normal course of operation.

Sealed Sender (metadata hardening)

Signal is also known for “Sealed Sender,” which aims to reduce how much sender information is revealed to the service. This doesn’t eliminate metadata in the real world (no messaging app can fully do that), but it’s a meaningful design goal when compared to platforms that collect and monetize user data.

Reference: Signal blog: Sealed Sender

Disappearing messages (useful, but not a compliance control)

Disappearing messages can reduce the lifetime of sensitive conversations on devices. That said, treat it as a risk reducer—not a guarantee. Screenshots, external cameras, device backups, and endpoint compromise can still preserve information.

Open-source transparency

Signal’s open-source approach makes it easier for independent researchers to review how the app works. That tends to build trust over time because flaws are more likely to be discovered and discussed publicly.

Registration lock / PIN

Signal supports a PIN and registration lock features that help protect against account takeover scenarios (for example, if someone attempts to re-register your number elsewhere).

Updates and operational hygiene

Even the best crypto won’t help if devices are outdated. If Signal is used inside an organization, consistent patching and basic mobile security hygiene (screen locks, OS updates, managed devices where appropriate) matter just as much as the app’s design.

Evaluating Signal’s Privacy Policies

Data collection and usage

Signal’s public posture is “collect as little as possible.” In general, the app is designed to function without storing message contents on servers, and without building an advertising profile around your activity.

Reference: Signal Terms & Privacy

User anonymity and confidentiality

Signal is popular with privacy-conscious users because it’s built to protect message content by default and minimize unnecessary data retention. That said, it still relies on phone numbers for registration (a point some users dislike).

Third-party sharing

Signal’s model is fundamentally different from ad-funded platforms. The practical takeaway for most organizations is that Signal is designed to avoid turning messaging into a data product.

Comparing Signal with Other Messaging Apps

Signal vs WhatsApp (security + data ecosystem)

WhatsApp uses end-to-end encryption for message content, which is a positive. The bigger concern for many organizations is the broader data ecosystem around the platform, including metadata and how the parent company’s business model relates to user data.

Signal vs Telegram (default encryption differences)

Telegram is often described as “secure,” but it’s important to distinguish between marketing and defaults. Telegram’s end-to-end encryption is not enabled by default for all chats, whereas Signal’s model is “end-to-end by default.”

The Pros and Cons of Using Signal

Advantages

Strong privacy posture: Designed around minimizing exposure, not monetizing conversations
End-to-end encryption by default: Less chance of “someone forgot to turn on secure mode”
Good safety features: safety numbers, disappearing messages, registration lock/PIN
Usable: Clean interface, calls work well, and it’s easy to adopt

Potential drawbacks

Requires a phone number: A sticking point for anonymity-focused users and some enterprise policies
Not an enterprise compliance platform: It’s not designed for central retention, eDiscovery, or corporate archiving
User base variance: Your contacts may not already be on Signal

Signal for Medical Device Cybersecurity Teams: Where It Fits (and Where It Doesn’t)

If you’re in a medical device company, the question usually isn’t “Is Signal secure?” It’s: is this the right tool for specific cybersecurity workflows, and what are the boundaries?

Where Signal can make sense

Signal can be useful for time-sensitive, security-focused coordination where you want a privacy-forward channel and you’re not relying on enterprise retention:

PSIRT coordination: triage, reproduction notes, timelines, and mitigation planning
Incident response comms: out-of-band coordination if your primary collaboration tools are disrupted
Supplier coordination: quick back-and-forth during CVE impact analysis and patch planning
Security leadership escalation: brief, high-sensitivity conversations where minimizing exposure matters

Where Signal usually does not fit

Signal is generally not the right choice when you need enterprise compliance controls or when your messages include regulated data types.

Don’t use it for ePHI/patient communications unless your compliance team has explicitly approved it for that purpose.
Don’t treat it as a system of record for regulated documentation, CAPA, or quality records.
Don’t assume disappearing messages equals “no retention” (screenshots and endpoints still exist).

Practical policy line that works well: “Signal is permitted for PSIRT/incident-response coordination that does not include ePHI. It is not approved for patient messaging or regulated record retention.”

A simple hardening checklist (good enough to paste into a policy)

Enable registration lock / PIN
Disable message previews on the lock screen
Use disappearing messages for high-sensitivity operational chatter (with realistic expectations)
Verify safety numbers for high-stakes conversations (vendor disclosures, active incidents)
Clarify “no ePHI” and “not a system of record” in your internal guidance

Conclusion

Signal is a strong contender if your priority is privacy-forward messaging with end-to-end encryption by default. It’s not an enterprise compliance platform, but it can be a very practical tool for security teams—especially for PSIRT and incident response coordination—when used with clear boundaries.

If you want help building a medical device cybersecurity program where communication, triage, and response are structured (and defensible), these may be useful:

Messaging Apps Cybersecurity FAQs

How secure is Signal compared to other messaging apps?

Signal is widely regarded as one of the stronger defaults because end-to-end encryption is on by default, it’s open source, and it’s designed to minimize stored data. “More secure” still depends on your threat model and how devices are managed.

Can Telegram chats be intercepted by third parties?

Telegram’s default chats are not end-to-end encrypted. Telegram offers “Secret Chats” for end-to-end encryption, but they are not the default experience. Always confirm what mode you’re actually using.

What are the cybersecurity risks with WhatsApp even with end-to-end encryption?

Encryption protects message content in transit, but risks can still come from compromised endpoints (malware/spyware), phishing, and cloud backups. Metadata and broader data ecosystem concerns are also common discussion points.

How can we keep messaging private during a security incident?

Use end-to-end encrypted apps, keep devices updated, avoid clicking unknown links, enable strong device locks, and define clear boundaries on what data can be shared. For regulated teams, make sure your incident communications policy aligns with your compliance requirements.

Should medical device teams use Signal for PSIRT or incident response?

It can be a good “out-of-band” coordination channel for PSIRT/IR as long as your policy is clear (for example: no ePHI, not a system of record) and your device hygiene is solid. When in doubt, run it through your compliance and security leadership.