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The Fortary mobile signing app provides a secure, non-custodial signing experience for organizations that want their team members to participate directly in the MPC signing process from a dedicated mobile device. Beyond initial setup, the mobile signing app is a core part of your organization’s security infrastructure. This page describes the key features and security properties of the signing experience.

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MPC Threshold Signing

The mobile signing app holds one of the MPC key shares required to sign transactions. When a transaction requires signing, the mobile device participates in a distributed signing protocol alongside Fortary’s secure infrastructure — the full private key is never assembled on any single device or server.
  • Your key share never leaves the device — The cryptographic material stays within the device’s secure environment at all times
  • No single point of compromise — Even if any other component were compromised, your mobile key share ensures the attacker cannot sign transactions without your device
  • Transparent participation — Your device is only engaged when a transaction requires your organization’s approval

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Biometric Authentication

Every signing operation on the mobile app requires biometric authentication (Face ID or Touch ID) before proceeding. This ensures that physical possession of the device alone is not sufficient — the authorized user must be present to approve any signing request.
Biometric authentication adds a physical security layer on top of the cryptographic protections built into the MPC protocol. Even if a device were lost or stolen, an attacker could not sign transactions without the enrolled biometric.

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Secure Encrypted Connection

The mobile signing app communicates with Fortary’s infrastructure through an encrypted channel:
  • End-to-end encryption — All communication between the device and Fortary’s signing infrastructure is encrypted
  • Device authentication — The mobile device authenticates itself using device certificates before participating in any signing operation
  • No persistent connection required — The app connects on-demand when a signing request is received and disconnects when the operation is complete

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Non-Custodial Signing Flow

When a transaction is submitted for approval and requires mobile signing, the flow works as follows:
1

Signing Request Arrives

The mobile app receives a notification that a transaction is awaiting your signature. The transaction details are displayed for your review.
2

Review Transaction Details

You review the transaction details on the device — including the destination, amount, network, and any risk assessment information.
3

Biometric Confirmation

You authenticate with Face ID or Touch ID to confirm that you are authorizing this transaction.
4

MPC Signing

Your device participates in the distributed MPC signing protocol. Your key share contributes to the transaction signature without ever being exposed or transmitted.
5

Transaction Signed

The signed transaction is returned to Fortary’s infrastructure for submission to the blockchain. Your key share remains securely on the device.
The mobile signing device stores sensitive cryptographic material. Treat it with the same care as a hardware security module. See Mobile Signing App for device security recommendations and Mobile Signing Device for hardware considerations.
Loss or compromise of the mobile device is not a compromise of the whole system. All transactions process through multiple security checks, policy controls, and require other participants whitelisted signatures.

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