> For the complete documentation index, see [llms.txt](https://docs.vida.id/identity-stack/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.vida.id/identity-stack/authentication/authentication-factors/cryptographic-token-pki.md).

# Cryptographic Token (PKI)

The **VIDA Cryptographic Token** leverages **Public Key Infrastructure (PKI)** to provide strong, tamper-proof authentication and transaction security. At the heart of this system is the **Phone Token** , a device-bound credential that uses cryptographic signing to ensure that only a user’s registered mobile device can authorise sensitive actions such as logins, payments, or high-risk operations.

Cryptographic signing ensures that a transaction request is **authentic, tamper-proof, and verifiable**. It uses **public-key cryptography (asymmetric encryption)**, where a **Private Key (held by the partner server)** signs the transaction, and a **Public Key (stored by VIDA)** verifies it.

### **Key Pair Generation (During Registration)**

*  The VIDA CDK in the mobile app generates a **key pair**:
  * **Private Key** : Stored securely on the device .
  * **Public Key** : Sent to VIDA’s server for verification purposes.
* VIDA **links the Credential ID** with the **Phone Token** and stores the Public Key securely.

### **Transaction Signing Process**

**Step 1: User Initiates a Transaction**

* The user tries to perform an action (e.g., login, payment, high-risk operation).
* The app collects transaction details (e.g., amount, recipient, timestamp).

**Step 2: Hashing the Transaction Data**

* The app **creates a hash** (fingerprint) of the transaction data using a secure hashing algorithm (e.g., SHA-256).
  * Example:

    ```json
    Transaction Data: {"amount": 100, "to": "XYZ", "timestamp": 1700000000}
    Hash Output: 4b227777d4dd1fc61c6f884f48641d02b07a7fc1
    ```

**Step 3: Signing with the Private Key**

* The **Private Key** (stored on the device) is used to encrypt the hash, creating a **digital signature**.
* The signature is unique to both:
  * The **transaction data**
  * The **Private Key** (ensuring only the authorized device can sign it)

### **Transaction Verification by VIDA**

**Step 1: Receiving the Signed Request**

* The signed transaction (hash + signature)  is sent to VIDA.

**Step 2: Hash Recalculation**

* VIDA **recalculates the hash** from the received transaction data to check for tampering.

**Step 3: Signature Verification**

* VIDA **decrypts the digital signature** using the **Public Key** (stored from registration).
* If the decrypted value **matches the recalculated hash**, it proves:\
  \- The transaction was signed by the registered device.\
  \- The data wasn’t tampered with.
* If verification passes, VIDA **approves the transaction**. If not, it is **rejected**.

***

### **Why is This Secure?**

**Only the authorized device** can sign transactions (Private Key never leaves the device).\
Even if the **transaction data is intercepted**, it cannot be altered (hash mismatch would fail verification).\
The **signature is unique** for each transaction, preventing replay attacks.

This process ensures that **only the user’s registered device can authorize the transactions.**