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Security & Cryptographic Engine Specification

RapiDrop uses standard, platform-native cryptographic primitives for high performance, zero-dependency security, and perfect forward secrecy.

1. Cryptographic Primitives

Parameter Specification Standard
Key Agreement X25519 Montgomery Curve25519 RFC 7748
Symmetric Cipher AES-256 in Galois/Counter Mode (GCM) NIST SP 800-38D
Key Size 256 bits (32 bytes) FIPS 197
Initialization Vector (IV) 96 bits (12 bytes) CSPRNG per frame RFC 5116
Authentication Tag 128 bits (16 bytes) Galois MAC RFC 5116
Key Derivation Function HKDF-SHA256 (Extract-and-Expand) RFC 5869
Short Authentication String 6-Digit Decimal Code ($\text{UInt32}(K_{auth}[0..3]) \bmod 10^6$) Protocol Specification
File Integrity Digest SHA-256 FIPS 180-4

2. Key Derivation Pipeline

Initiator Keypair (a, A) ──┐
                          ├──> X25519 Shared Secret (S)
Receiver Keypair  (b, B) ──┘
                                   │
                       Nonces Salt (N_A || N_B)
                                   │
                                   ▼
                   HKDF-Extract(Salt, S) ──> PRK (32 Bytes)
                                              │
                      ┌───────────────────────┼───────────────────────┐
                      │                       │                       │
                      ▼                       ▼                       ▼
            HKDF-Expand (Session)   HKDF-Expand (Auth)      HKDF-Expand (Pair)
                      │                       │                       │
                      ▼                       ▼                       ▼
               K_session (32B)          K_auth (32B)            K_pair (32B)
                                              │
                                              ▼
                                         SAS (6 Digits)

Canonical Handshake Test Vectors

All platforms execute identical deterministic test vectors:

  • Alice Private Key: 77076d0a7318a57d3c16c17251b26645df4c2f87ebc0992ab177fba51db92c2a
  • Bob Public Key: de9edb7d7b7dc1b4d35b61c2ece435373f8343c85b78674dadfc7e146f882b4f
  • Derived Shared Secret: 4a5d9d5ba4ce2de1728e3bf480350f25e07e21c947d19e3376f09b3c1e161742
  • Transcript Hash: 1aace1004eca802c7cc3ee4e78a1e2f4da275e0d77998193e85f19d217d90fbc
  • Session Key ($K_{session}$): 02bbd12373e37ce395ea08c747c5296ba9dc881586e84efeb0ec8cdefe7d7127
  • Authentication Token ($K_{auth}$): 17d4384a1b5e20a8268ac2d1e3e28b4aa7281f37c0be20951381ac95432279b5
  • Persistent Pair Record Key ($K_{pair}$): 50b4fa871f3bbab60481054fdd55277c7c69f2efc3f76fca4fbf59c53005b583
  • Numeric SAS Code: 784010

3. Platform Native Implementations

Platform Subsystem Implementation Class
macOS Apple CryptoKit Curve25519.KeyAgreement, AES.GCM, HKDF<SHA256>, SHA256
Android Kotlin & javax.crypto com.prabotics.rapidrop.security.Curve25519, Cipher("AES/GCM/NoPadding"), Mac("HmacSHA256")

4. Threat Model & Asset Protection

Protected Asset Threat Mitigation
Clipboard Content Passive LAN eavesdropping AES-256-GCM encryption with unique random nonces per frame.
Pairing Integrity Man-in-the-Middle (MITM) X25519 key agreement with Short Authentication String (SAS) verification.
Handshake Replay Replaying captured pairing frames 60-second timestamp freshness window (`
Session Keys Key compromise / replay Fresh ephemeral keypairs generated per connection; nonces never reused.
Credentials / Passwords Unintended clipboard sync Automatic suppression of transient password manager clips on macOS (org.nspasteboard.TransientType, org.nspasteboard.ConcealedType, org.nspasteboard.AutoGeneratedType).
File Storage Path traversal attacks Strict sanitizeRelativePath and directory containment checks.
File Data Chunk tampering / corruption Mandatory SHA-256 digest verification before finalizing downloads.

5. Secure Credential Storage at Rest

Platform Storage Mechanism Protected Material
macOS Apple Keychain (kSecClassGenericPassword) Paired session keys, SAS PIN codes
Android Hardware-backed Android Keystore (KeyGenParameterSpec AES-256-GCM) Paired session keys, SAS PIN codes