Peer To Peer Transfer

Overview

This feature enables peer-to-peer (P2P) data exchange between devices without internet, leveraging Google Nearby Connections API and Wi-Fi Direct, Bluetooth, or Wi-Fi Hotspot. Data is securely transmitted using compression, AES encryption, and Base64 encoding.

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Architecture Components

Device Communication Layer

Nearby Connections API (Google)

• Discovery/Connection: Uses BLE for discovery, Wi-Fi Direct/Hotspot for transfer, and automatically selects the best medium.

• Roles:

  • Advertiser: Device making itself available

  • Discoverer: Device searching for peers

Supported Topologies

• P2P_CLUSTER: Mesh, several devices, low bandwidth, suitable for multiplayer/gaming.

• P2P_STAR: One hub, others connect to it, ideal for content sharing.

• P2P_POINT_TO_POINT: Direct 1-to-1, highest speed, best for large files.

• Wi_Fi_P2P: Wi-Fi Direct, long-range, for games or large files.

Connection Selection Logic

• On connection attempt, use:

  • BLE for initial discovery

  • Wi-Fi Direct/Hotspot for large/high-speed transfers

  • Fallback to Bluetooth if Wi-Fi is not available

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Secure Payload: Compression & Encryption

Compression (zlib/GZipEncoder)

• Input: JSON-encoded Dart Map

• Process:

  • Convert JSON string to UTF-8 bytes

  • Compress using GZipEncoder

• Output: Compressed byte array

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Encryption (AES-CBC via encrypt package)

• Key Generation: 32-byte random key (AES-256)

• IV Generation: 16-byte random IV

• Algorithm: AES in CBC mode

• Process:

  • Encrypt compressed bytes with AES-CBC using key & IV

• Security Notes:

  • Unique IV per encryption for semantic security

  • Key+IV must be securely stored or sent with the payload

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Base64 Encoding

• Purpose: Convert binary (key, IV, encrypted data) to URL/text/QR-compatible string

• Process:

  • Encode each part (key, IV, data) as Base64 URL-safe

  • Wrap into a JSON:

    {
      "key": "<Base64-encoded AES key>",
      "iv": "<Base64-encoded IV>",
      "data": "<Base64-encoded encrypted data>"
    }

  • Encode the full JSON as a final Base64 URL-safe string

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Decryption & Decompression Process

1. Decode Base64:

   • Outer string → JSON

2. Extract Payload:

   • Decode key, IV, data fields from JSON (Base64 → bytes)

3. Decrypt:

   • Use AES-CBC with key & IV to decrypt data → compressed bytes

4. Decompress:

   • GZipDecoder on decrypted bytes → UTF-8 JSON

5. Deserialize:

   • Parse JSON to Dart Map

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Key Classes

P2P Communication

• NearbyConnectionManager

  • Handles advertising, discovery, connection, and data transfer

• ConnectionStrategy

  • Enum: P2P_CLUSTER, P2P_STAR, P2P_POINT_TO_POINT, WI_FI_P2P

• ConnectionEventListener

  • Handles state changes, errors, and received bytes

Secure Payload Handling

• SecurePayloadCompressor

  • compress(Map data): List<int>

  • decompress(List<int> compressed): Map

• AESCipher

  • encrypt(List<int> data, key, iv): List<int>

  • decrypt(List<int> encrypted, key, iv): List<int>

• Base64PayloadEncoder

  • encode(key, iv, data): String

  • decode(String): (key, iv, data)

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Data Flow: Send/Receive Sequence

1. Sender:

   • Serialize Map → JSON → UTF-8 bytes

   • Compress (GZip) → compressed bytes

   • Generate AES key & IV

   • Encrypt (AES-CBC) compressed bytes → encrypted bytes

   • Base64-encode (key, IV, encrypted) → JSON → Base64-encode JSON

   • Send over a P2P connection

2. Receiver:

   • Receive Base64-encoded string

   • Base64-decode → JSON → extract key, IV, encrypted data

   • AES decrypt → compressed bytes

   • GZip decompress → JSON string

   • Parse JSON → Map

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Sequence Diagram