Signal exchanges private messages and calls between Signal accounts and encrypts their contents before they leave the sending device. The service routes encrypted data, while the participating devices hold the readable conversation. Signal differs from ordinary SMS because the mobile carrier does not receive the message text. It still uses a phone number during account registration, and encryption does not hide every fact about when a device connects to the service.
Local history
Signal keeps conversation history on the user’s devices rather than maintaining a readable cloud inbox that a fresh installation can simply download. Deleting the app without a transfer or supported backup can therefore remove the only accessible copy. The other participant’s device may still retain its copy, but that does not rebuild the local conversation automatically.
Message delivery and history transfer are separate jobs. A registered account can receive new messages after setup without recovering old ones. Direct transfer needs both devices and a supported migration path. Backup formats also have platform limits, so a file created for one mobile system cannot always restore on another.
Linked devices
A QR scan links a desktop, tablet, or secondary phone to the primary account. Signal allows up to five linked devices. During the first link, the user can transfer chats and recent media. Choosing not to transfer at that point is not a temporary display setting; changing the decision later requires reinstalling the linked app and linking it again.
A linked device can continue working when the primary phone is offline, but the phone must connect at least once every thirty days. Signal also unlinks a device after forty-five days without activity. These limits make an unused desktop a poor archival copy because inactivity can break its account relationship even when its local files remain on the computer.
Safety numbers
Each one-to-one chat has a safety number that represents the encryption identity at both ends. Two people can compare the number or scan its QR form through another trusted channel. A matching result confirms that the encrypted chat reaches the expected Signal installations. It does not prove a contact’s legal name or control of another account outside Signal.
A safety number can change after a device or registration change. If the contact was marked as verified, Signal requires manual approval before sending another message. Messages that failed before the change do not resend automatically after approval. The sender needs to send them again if they still matter.
Message timers
Disappearing messages remove messages from participating devices after the selected timer. A new timer affects future messages rather than rewriting the lifetime of every earlier item. Each linked device processes its own local copy, so a device that stays offline may handle expiry when it reconnects and processes the queued state.
The timer is a cleanup control, not protection against a hostile recipient. The recipient can photograph the screen, copy content before it disappears, or use another device to record it. Signal states this limit directly because end-to-end encryption controls the network path, not what an intended recipient does after reading the message.
Encrypted calls
Signal calls use the same end-to-end protection principle as messages. A group call or voice call still depends on microphone, camera, notification, and network permissions on the local device. Blocking one of those permissions can produce a silent microphone or missing video without changing the account’s encryption state. A safety check and a device-permission check answer different questions and should not substitute for one another.
