Finding #7: Curve25519 ECDH forward secrecy + fix encryption test suite

[sensei-hacker]

Summary

Implements Finding #7 (Forward Secrecy) from the security audit: replaces the static session key protocol with an ephemeral Curve25519 Diffie-Hellman handshake, and fixes the test_encryption test suite which was always ERRORED.

ECDH Handshake Protocol

TX → RX  MSP_ELRS_INIT_ENCRYPT  { tx_pub[32] | HKDF-SHA256(master, tx_pub, "auth")[0:16] }
RX → TX  MSP_ELRS_DH_RESPONSE   { rx_pub[32] | HKDF-SHA256(master, rx_pub, "auth")[0:16] }

1. TX generates an ephemeral Curve25519 key pair, sends its public key with an HKDF-SHA256 auth tag proving knowledge of the master key
2. RX verifies the tag, generates its own ephemeral key pair, computes the DH shared secret, derives the session key, then replies with its own public key + auth tag
3. TX verifies the RX tag, computes the same shared secret, derives the same session key; both sides immediately erase their private keys

Key security properties:

• Perfect Forward Secrecy: private keys erased after each handshake — past sessions cannot be decrypted even if the master key is later compromised
• Mutual authentication: both sides prove knowledge of the master key via HKDF-based MAC before any secrets are derived
• Session key binding: HKDF salt includes both public keys so neither party can unilaterally control key material

Changes

Implementation:

• src/src/common.cpp — generate_dh_keypair(), compute_dh_mac(), verify_dh_mac(), derive_session_key(), apply_session_key(); moved CollectEntropy() and RandRSSI() here so both TX and RX can use them
• src/src/tx_main.cpp — InitCryptoECDH() (Phase 1) and ProcessDHResponse() (Phase 2); updated state machine and telemetry receiver to intercept DH response
• src/src/rx_main.cpp — CryptoSetKeysECDH() replacing CryptoSetKeys(); DH response queued as priority telemetry
• src/include/encryption.h — ENCRYPTION_STATE_DH_SENT, dh_handshake_t struct, new helper declarations
• src/lib/MSP/msptypes.h — MSP_ELRS_DH_RESPONSE 0x56
• src/lib/Crypto/src/ — vendored Curve25519, SHA256, HKDF, BigNumberUtil from the rweather Crypto library; added RNG.h/cpp stub so Curve25519.cpp compiles without hardware RNG

Tests:

• src/platformio.ini — added USE_ENCRYPTION to native test build so the tests actually run (was silently skipped before)
• src/test/test_encryption/collect_entropy_stub.cpp — provides /dev/urandom-backed CollectEntropy() for native, replacing the hardware implementation in common.cpp
• src/test/test_encryption/test_encryption.cpp — fixed two broken "vulnerability demonstration" tests whose assertions were intentionally wrong; added 6 new Curve25519 ECDH tests

New ECDH Tests

Test	What it verifies
test_ecdh_shared_secret_agreement	DH property: both sides derive the same shared secret
test_ecdh_different_keypairs_different_secrets	Different key pairs give different secrets
test_ecdh_mac_determinism_and_key_binding	HKDF auth tag is deterministic and master-key-bound
test_ecdh_session_key_derivation	Session key derivation is deterministic, salt-dependent
test_ecdh_full_handshake_session_key_agreement	End-to-end: TX and RX derive the same session key
test_ecdh_ephemeral_keys_are_unique	Different private keys produce different public keys

Test Results

112/112 tests pass (was 83/84, with test_encryption always ERRORED)

ESP32 TX (Unified_ESP32_2400_TX_via_ETX): ✅ SUCCESS
ESP32 RX (Unified_ESP32_2400_RX_via_UART): ✅ SUCCESS