Cross-Chain Atomic Swaps

Atomic = either both sides settle or neither. Achieved via shared preimage (HTLC) or shared scalar (adaptor signature).

HTLC-based swap (classic)

For BTC ↔ LTC swap:

User generates preimage P, hash H = SHA256(P).
User publishes BTC HTLC: to LTC-side after CLTV, OR claim immediately with P + LTC-side's signature.
LTC-side publishes LTC HTLC: to BTC-side after CLTV, OR claim immediately with P + BTC-side's signature.
User claims LTC HTLC (revealing P).
LTC-side uses P to claim BTC HTLC.

Atomic: P revealed in one chain instantly enables claim on the other.

Adaptor-signature swap (scriptless)

Uses adaptor signatures (see ../../cryptography/adaptor-sigs/):

No HTLC scripts on chain — looks like ordinary tx.
Same shared scalar t unlocks both sides.

Used in:

BTC ↔ Monero swaps (XMR doesn't support HTLCs directly; adaptor sigs are the standard).
BTC ↔ Liquid privacy-preserving swaps.

BTC ↔ Monero (FarCaster, COMIT, Atomic Swap protocol)

XMR uses ring signatures + stealth addresses. Atomic swaps via adaptor signatures + Monero-specific protocols.

Implementations:

FarCaster — multi-network protocol.
COMIT — atomic swap library.
Atomic Swap (XMR ↔ BTC) by COMIT — production.

Trade-offs: complex, slow (often 30-60 minutes per swap), but trustless.

BTC ↔ Liquid

Boltz ran BTC ↔ Liquid swaps via the submarine swap mechanism, but all Boltz swap services (mainchain, Lightning and Liquid) are offline "until further notice" since 2026-08-03, with no restart date announced as of September 2026 — treat Boltz as a protocol reference, not a live endpoint. Blockstream Swaps (announced 2026-08-10, in beta with select participants as of September 2026) offers trustless BTC ↔ L-BTC chain swaps plus submarine swaps into Lightning. See ../../lightning/submarine-swaps/SKILL.md.

BTC ↔ Lightning

Lightning ↔ on-chain BTC swaps (different chain not technically needed; same network):

Lightning Loop (lightninglabs/loop v0.35.0-beta, 2026-08-25) and peerswap (v7.0.1, September 2026) — both actively released as of September 2026.
Boltz — swaps suspended 2026-08-03 (see above); protocol reference only until it returns.

Privacy benefit

Adaptor-sig variants:

Tx looks like normal cooperative spend on chain.
Counterparty's chain analysis can't link the two legs.

HTLC variants:

Hash + CLTV scripts visible on chain.
Both legs share same hash (correlatable).

Use cases

Privacy via diversification: BTC → XMR → BTC = breaks chain analysis.
DEX-like trading without centralized exchange.
Cross-chain liquidity.

Limitations

Liquidity — atomic swaps need a counterparty.
Latency — multi-confirmation waits per chain (BTC = 10 min, XMR = 2 min × multiple blocks).
Implementation complexity — adaptor sigs are subtle.

bitcoin-privacy-atomic-swaps