Signal is a free app for text messages and voice and video calls on Android, iPhone, Windows, Mac and Linux. Signal Messenger LLC, based in California, operates the service, and the Signal Technology Foundation, a US 501(c)(3) nonprofit, holds the trademarks. Signal states that it cannot be acquired by a tech company.

Messaging is free for everyone, and the app shows no ads. Grants and donations pay for the service. The one paid option is longer media history in backups.

Every chat in Signal is end-to-end encrypted by default, group chats and calls included, and the source code of both the apps and the server is public. The weak point is metadata: Signal still needs a phone number to sign up, and its servers see who receives each message and from which IP address.

Cryptography & Protocol

Signal encrypts chats with its own messenger protocol, the Signal Protocol, and publishes its specifications. Encryption is on by default for every chat and every call, and group chats are encrypted the same way.

The protocol’s Double Ratchet provides forward secrecy and post-compromise security, both listed in Signal’s protocol documentation. The privacy policy states that Signal cannot decrypt or otherwise access the content of messages or calls.

In September 2023 Signal announced PQXDH, a key agreement that combines a classical algorithm (X25519) with a post-quantum one (CRYSTALS-Kyber), so an attacker has to break both. Formal verification is described for the later post-quantum ratchet, not for PQXDH.

In October 2025 Signal announced the Sparse Post-Quantum Ratchet, added on top of the Double Ratchet to form what Signal calls the Triple Ratchet. Signal states that the new design provably keeps forward secrecy and post-compromise security against attackers with quantum computers. The design was formally verified with the ProVerif tool, and Cryspen’s hax and F* tools check it again on every automated build.

Metadata Privacy

Signal requires a phone number to register an account; the number is the sign-up identifier. Since February 2024 users can set a username, and the phone number is no longer visible to everyone in Signal by default. People who already have the number saved in their contacts still see it. Signal does not offer a searchable directory of usernames, and a username can be changed at any time.

To find which of your contacts use Signal, the app may send contact information to the server in hashed form, as the privacy policy describes. This contact discovery method still means the server receives data derived from your address book.

Sealed sender hides who is sending a message from Signal’s servers, a form of metadata protection. The servers still see the recipient and the connection’s IP address. Signal’s 2018 announcement of sealed sender described resistance to traffic correlation through timing and IP addresses as ongoing work.

Signal’s page of government requests lists seven, from October 2016 to March 2026. For an October 2016 grand jury subpoena, Signal provided only the date and time the user registered and the last date the user connected to the service.

Signal’s privacy policy, last updated in May 2018, lists no telemetry or crash-log collection.

Transparency & Verifiability

The complete source code of the Signal apps and of the Signal server is open source on GitHub; the server is published under the GNU AGPLv3 license.

The Android app has supported reproducible builds since version 3.15.0. A user can build the app from the public code in Docker and compare the result with the app installed on the phone; a match confirms the installed app was built from the published code. Signal documents this for Android only.

In 2017 five academic researchers published a formal security analysis of the protocol’s key agreement and the Double Ratchet at the IEEE EuroS&P conference, concluding “We have found no major flaws in the design”. The paper is an academic analysis of the protocol, not an audit of the Signal apps or server. The post-quantum ratchet announced in October 2025 was developed with researchers from PQShield, AIST and NYU, with papers at Eurocrypt 2025 and USENIX 2025.

The 2022 Twilio incident

In summer 2022 a phishing attack gave an attacker access to the customer support console of Twilio, the company that provides Signal with phone-number verification. The incident counts as part of Signal’s data breach history because phone numbers or SMS verification codes of Signal users were exposed.

For about 1,900 users, the attacker could have re-registered their number on another device or learned that the number was registered with Signal. The attacker searched explicitly for three numbers, and one of those three users reported that the account had been re-registered. Message history, contact lists and profile data were not accessed.

Signal unregistered the affected accounts, asked those users to register again and notified them by SMS. Signal recommends turning on registration lock in the app settings, a feature it created against this kind of attack.

Usability & Reach

Signal has apps for Android, iPhone, iPad, Windows, Mac and Linux, and supports voice and video calls. The app is free with no ads, affiliate marketing or tracking.

Multi-device support works through a primary phone, and other devices, such as the desktop app and an iPad, link to it. The phone has to come online at least once every 30 days, or linked devices are unlinked; a linked device that is inactive for 45 days is unlinked too. An account can have at most 5 linked devices.

Signal Secure Backups are encrypted with a 64-character recovery key generated on the device and never shared with Signal’s servers, a backup model in which Signal cannot read the backup. If you lose the key, you lose access to the backup permanently, and Signal cannot recover it. The free tier backs up all text messages and the last 45 days of media, up to 100 MiB. For $1.99 a month the paid tier keeps media older than 45 days, up to 100 GB.

Funding & Sustainability

Signal’s funding model is a nonprofit paid for by grants and donations, and it started with a loan from Brian Acton. Signal states that it does not monetize surveillance, shows no ads and cannot be acquired by a tech company.

In November 2023 Signal estimated its running costs at about $50 million a year by 2025, of which infrastructure accounts for about $14 million and staff for about $19 million. Donations and grants have to cover that amount.

Pros and Cons

Every chat, group and call is end-to-end encrypted by default, with no unencrypted mode
Forward secrecy and post-compromise security, plus a post-quantum key agreement since September 2023
Sealed sender, as Signal describes it, hides who sent a message from Signal's servers
The apps and the server are open source, and the Android app can be rebuilt and compared with the installed one
In the October 2016 subpoena Signal provided only the registration time and the last connection date
Backups are locked with a recovery key that never reaches Signal
Free on Android, iPhone, Windows, Mac and Linux, with no ads
Run by a nonprofit that states it cannot be acquired by a tech company

A phone number is required to sign up; usernames only hide it from other users
Signal's servers still see the recipient of each message and the IP address
The app may send hashed contact information to the server to find who else uses Signal
The independent work on record covers the protocol, not the apps or the server, and reproducible builds are documented for Android only
In 2022 a phishing attack on Signal's phone-verification provider exposed about 1,900 accounts to possible re-registration
Linked devices unlink if the primary phone is not online at least once every 30 days
Free backups keep media for 45 days only; longer history costs $1.99 a month

Our Rating

Cryptography & Protocol9.5/10
Every chat, group and call is end-to-end encrypted by default with forward secrecy and post-compromise security, and the key agreement has combined classical and post-quantum algorithms since September 2023.
Metadata Privacy6.5/10
A phone number is required, contacts are sent to the server in hashed form, and the server still sees the recipient and IP address; sealed sender hides the sender from it.
Transparency & Verifiability8.5/10
The apps and the server are open source and the Android app can be rebuilt and compared with the installed one; the independent work on record covers the protocol, not the apps or the server.
Usability & Reach8.0/10
Runs on Android, iPhone, Windows, Mac and Linux with calls and encrypted backups; linked devices need the primary phone online at least once every 30 days.
Funding & Sustainability8.5/10
A nonprofit paid for by donations and grants, with no ads; Signal estimates its running costs at about $50 million a year by 2025.

Overall8.2/10

Cryptography & Protocol 30% · Metadata Privacy 25% · Transparency & Verifiability 20% · Usability & Reach 15% · Funding & Sustainability 10%

See our rating methodology →