Arc’s post-quantum roadmap covers wallet signatures, validator authentication,
private smart contract state, and offchain infrastructure.
SLH-DSA-SHA2-128s signature verification is live on Arc mainnet.
Post-quantum transaction signing is a future milestone and will likely
implement EIP-8141 frame transactions once the EIP is finalized.
Why post-quantum security matters
Most public-key cryptography used today is vulnerable to large-scale quantum
computers. If those computers become practical, blockchains face two risks:
- Signature forgery. A quantum computer that breaks public-key cryptography
can forge signatures that secure wallets, authorize transactions, and
authenticate network participants.
- Harvest-now, decrypt-later attacks. Encrypted data captured today can be
stored and decrypted later when quantum attacks become practical, exposing
private transaction details, balances, and other sensitive data.
Because blockchain data is long-lived, post-quantum protections need to be in
place before quantum attacks are widely available.
Post-quantum roadmap
Arc’s roadmap introduces each layer in a production-aligned sequence.
Post-quantum wallet signatures
Arc mainnet supports onchain verification of SLH-DSA-SHA2-128s signatures. Arc
will likely implement EIP-8141 frame transactions in a future milestone, once
the EIP is finalized. Frame transactions allow wallets to choose their own
transaction signatures, and Arc will offer native support for multiple
post-quantum signatures to allow users to optimize gas costs, security, and
interoperability.
The
PQ Signature Verify precompile
enables SLH-DSA-SHA2-128s as an application-layer authorization primitive. A
contract can accept a message, public key, and signature in calldata, verify
them onchain, and conditionally execute state changes without requiring the
signer to hold an Arc account.
Ecosystem constraints to keep in mind:
- Hardware wallet support will take time to mature.
- Post-quantum standards are still evolving, so long-term signature choices may
change.
Expect a transition period as tooling, wallet support, and integrations
mature.
Post-quantum privacy
Arc Privacy addresses the harvest-now,
decrypt-later threat. Users encrypt their transactions and call queries using
HPKE with X-Wing KEM (combining X25519 and ML-KEM-768), HKDF-SHA256, and
AES-256-GCM. All communication with Arc Privacy nodes occurs over TLS 1.3 with
X25519MLKEM768 (also combining X25519 and ML-KEM-768).
Attackers can capture encrypted data today and attempt to decrypt it later when
quantum attacks become practical. Arc privacy nodes encrypt contract state and
event logs with post-quantum encryption to protect sensitive balances and
transaction details.
Post-quantum validator signatures
Validator authentication also requires post-quantum protection to keep the
ledger resilient. Arc adds post-quantum validator signatures in a later phase.
This sequencing is intentional: validator upgrades must be introduced carefully
to preserve throughput, latency, and operational reliability. Because Arc uses
sub-second finality, the window to exploit validator signatures is narrower than
the wallet-signature risk, making wallets the higher-priority target.
Offchain infrastructure
Quantum resilience extends to the entire blockchain stack and surrounding
infrastructure. Circle’s roadmap includes upgrading node-to-node communication
to TLS 1.3 with X25519MLKEM768 hybrid key agreement and implementing HPKE
encrypted data flows. Arc infrastructure will use post-quantum configurations
for cloud environments and operational systems.
Offchain traffic and stored data face the same post-quantum threat vectors as
onchain data when vulnerable cryptography remains in use. 
