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Contact Exchange

Exchange contact cards securely by scanning QR codes in person.

How It Works

Vauchi uses in-person exchange to establish contact relationships. Both parties must be physically present to complete an exchange.

sequenceDiagram
    participant A as You
    participant B as Contact

    Note over A,B: Meet in person

    A->>B: Show QR code
    B->>A: Scan QR code
    Note over A,B: Proximity verified
    A->>B: Scan their QR code

    Note over A,B: Exchange complete!
    Note over A,B: Both have each other's cards

Why In-Person?

The in-person requirement is a privacy and security feature:

Threat How In-Person Prevents It
Spam Can't be added by strangers
Impersonation You verify identity yourself
Man-in-the-middle Direct device communication
Screenshot attacks Proximity verification

Exchange Methods

QR Code (Primary)

The main method for exchanging contacts:

  1. Open the Exchange tab
  2. Show your QR code
  3. Have the other person scan it
  4. Scan their QR code
  5. Exchange complete

QR codes expire after 5 minutes for security.

One-Way Sharing

Share your card without receiving theirs:

  1. Enable Share Only mode
  2. Show your QR code
  3. They scan and receive your card
  4. You don't receive their card

Useful for sharing business cards at events.

Video Call Exchange

For contacts you can't meet in person:

  1. Start a video call
  2. Share your screen showing the QR code
  3. They scan it from the video
  4. Switch roles
  5. Exchange complete

Note

Video call exchange is less secure than in-person because you can't verify physical proximity.

Proximity Verification

On mobile devices, Vauchi verifies physical proximity using ultrasonic audio:

  • Both phones emit and listen for an audio handshake (18-20 kHz)
  • Range: approximately 2 meters
  • If verification fails, exchange is blocked
  • This prevents screenshot attacks

Troubleshooting Proximity

If proximity verification fails:

  1. Ensure both phones have working speakers/microphones
  2. Move closer together (within 1-2 meters)
  3. Reduce background noise
  4. Disable any audio-blocking apps
  5. Try again

On desktop, proximity verification is skipped — manual confirmation is required instead.

After Exchange

Once exchange completes:

  • The new contact appears in your Contacts list
  • You can see their contact card (fields they've shared)
  • They can see your contact card (fields you've shared)
  • Future updates sync automatically

Security Properties

Property Mechanism
Proximity required Ultrasonic audio handshake
No man-in-the-middle X3DH key agreement with identity keys
Forward secrecy Ephemeral keys discarded after exchange
Replay prevention One-time token, 5-minute expiry
Card authenticity Ed25519 signature on contact card