Add ChaChaPoly AEAD-4 decryption support (Phase 1)

[weebl2000]

Summary

Adds ChaCha20-Poly1305 (AEAD-4) encryption alongside the existing AES-128-ECB + HMAC-2 scheme. Updated nodes send AEAD-4 to peers that advertise support and fall back to ECB for legacy peers. All nodes can decode both formats. Old nodes continue to work unchanged.

Relates to #259.

What This Means in Practical Terms

The current encryption has a few weaknesses that this PR begins to address:

• Message tampering is too easy to attempt. The existing 2-byte authentication code means an attacker only needs about 65,000 guesses to forge a valid-looking message. At LoRa speeds that's roughly 9 hours of continuous attempts. The new 4-byte tag raises this to over 4 billion guesses — at LoRa rates, that would take over a century.

• Identical messages look identical on the air. The current block cipher (ECB mode) produces the same ciphertext for the same plaintext, which can reveal patterns — for example, you could tell when someone sends the same message twice. The new scheme produces completely different ciphertext every time, even for identical messages.

• Addressing fields are now protected. Currently, only the message body is authenticated. With AEAD, the payload type and addressing hashes (which identify sender and recipient) are included in the authentication check, so an attacker cannot swap or modify them without detection. Outer routing fields like TTL and hop path are intentionally left unauthenticated so repeaters can still forward packets through the mesh.

• Messages get slightly smaller. ECB pads every message up to a 16-byte boundary, wasting airtime. The new scheme has no padding, so most messages shrink by a few bytes on the wire.

• Nothing breaks. Updated nodes send AEAD-4 to peers that advertise support, and fall back to ECB for legacy peers. Old nodes are completely unaffected — they never receive AEAD-4 messages because the sender checks their capability first.

• Nodes advertise their capabilities. Updated nodes include a flag in their advertisements saying "I understand the new encryption." When two updated nodes discover each other, they automatically start using AEAD-4 for their communication.

Wire Format

Current ECB:

[HMAC:2] [ECB_ciphertext:N×16]     (padded to block boundary)

New AEAD-4 (same position in payload):

[nonce:2] [ciphertext:M] [tag:4]    (exact plaintext length, no padding)

Average overhead: ~6 bytes (AEAD) vs ~9.5 bytes (ECB). Most messages get smaller.

Cryptographic Design

Per-message key derivation (eliminates nonce-reuse catastrophe):

msg_key[32] = HMAC-SHA256(shared_secret, nonce || dest_hash || src_hash)

Including dest_hash || src_hash makes keys direction-dependent — Alice→Bob and Bob→Alice derive different keys even with the same nonce value (for 255/256 peer pairs; the 1/256 where dest_hash == src_hash is a residual limitation of 1-byte hashes).

IV construction (12 bytes, from on-wire fields):

iv[12] = { nonce_hi, nonce_lo, dest_hash, src_hash, 0, 0, 0, 0, 0, 0, 0, 0 }

Associated data (authenticated but not encrypted):

• Peer messages: header || dest_hash || src_hash
• Anonymous requests: header || dest_hash
• Group messages: header || channel_hash

Nonce management: 16-bit counter per peer, seeded from hardware RNG on boot and on contact load. Not persisted to flash — always fresh on each boot cycle.

Security Comparison

Property	ECB + HMAC-2 (current)	AEAD-4 (new)
Confidentiality	Identical blocks → identical ciphertext	Unique keystream per message
Forgery resistance	1/65K (~9 hours at LoRa rates)	1/4.3B (~136 years)
Key usage	16 of 32 bytes (AES-128)	Full 32 bytes (ChaCha20-256)
Addressing authentication	None	Payload type & address hashes authenticated via AAD
MAC timing	memcmp (timing side-channel)	secure_compare (constant-time)
Padding waste	0-15 bytes per message	None

Scope

Payload type	AEAD-4 decode	AEAD-4 send	Notes
TXT_MSG, REQ, RESPONSE, PATH	Yes	Yes (if peer advertises AEAD)	Per-peer ECDH secret, no collision risk
ANON_REQ	Yes	No (no prior capability exchange)	Ephemeral ECDH secret
GRP_TXT, GRP_DATA	Yes	No (see group considerations)	Shared channel key

Group Message Considerations

Group channels share a single key among all members. With a 2-byte nonce and multiple senders, cross-sender nonce collisions follow the birthday bound (~300 messages for 50% probability on an active channel). A collision leaks P1 ⊕ P2 for that specific message pair via crib-dragging, but:

• No key recovery — per-message key derivation via HMAC-SHA256 is one-way
• No cascade — each collision is isolated, doesn't affect other messages
• Bounded threat model — the attacker must not have the channel PSK (if they do, they can already read everything)

This is mainly beneficial for public/hashtag channels where the PSK is already widely known and the ECB pattern leakage and weak MAC are a greater concern than the bounded nonce collision risk.

Potential future mitigations explored and deferred:

• Per-sender derived keys (HMAC(channel_secret, sender_pub_key)) — eliminates cross-sender collisions but requires receivers to know all senders' public keys, changing the group security model from "know the PSK = full access" to "know the PSK + sender discovery = access." Ruled out as a usability regression.
• Expanded nonce (4 bytes instead of 2) — pushes birthday bound to ~65,000 messages (~2 years at 100 msgs/day). Costs 2 extra bytes of airtime and creates a different wire format for groups vs peers.
• Sender hash byte on wire — differentiates senders for key derivation at 1 byte cost, but leaks sender identity metadata (traffic correlation, identification via adverts) that is currently hidden inside the encrypted payload.

Decode Order

Adaptive per-peer: for peers with CONTACT_FLAG_AEAD set, try AEAD-4 first then ECB fallback. For unknown/legacy peers, try ECB first then AEAD-4 fallback. This avoids the 1/65536 ECB false-positive rate on AEAD packets (nonce bytes matching truncated HMAC) for known AEAD peers, while minimizing wasted CPU for legacy peers.

Capability Advertisement

• feat1 bit 0 (FEAT1_AEAD_SUPPORT) is set in adverts for all node types (chat, repeater, room, sensor)
• Receivers record peer capability in ContactInfo.flags bit 1 (CONTACT_FLAG_AEAD)
• Old nodes parse feat1 but ignore the value (forward-compatible via existing AdvertDataParser)

Files Changed

• src/MeshCore.h — AEAD constants (AEAD_TAG_SIZE, AEAD_NONCE_SIZE, CONTACT_FLAG_AEAD, FEAT1_AEAD_SUPPORT)
• src/Utils.h / src/Utils.cpp — aeadEncrypt() and aeadDecrypt() using ChaChaPoly
• src/Mesh.h — getPeerFlags(), getPeerNextAeadNonce() virtuals; aead_nonce param on createDatagram/createPathReturn
• src/Mesh.cpp — AEAD send path in createDatagram/createPathReturn; adaptive try-both decode order per peer
• src/helpers/ContactInfo.h — uint16_t aead_nonce field, nextAeadNonce() helper
• src/helpers/BaseChatMesh.h / BaseChatMesh.cpp — Advertise AEAD, track peer capability, seed nonce, AEAD send for all peer message types
• src/helpers/CommonCLI.cpp — Advertise AEAD for repeaters/rooms/sensors

Build Verification

• ESP32 (Heltec_v3_companion_radio_ble): builds successfully
• NRF52 (Xiao_nrf52_companion_radio_ble): builds successfully

Future Work

• Group messages: send AEAD-4 (all updated nodes can already decode it)
• Repeater/room server/sensor firmware: add AEAD send support (currently decode-only, examples/ callers still use ECB)
• ANON_REQ: remain ECB (no prior capability exchange possible)