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Post-quantum migration for Bitcoin layer 2s

Originally published in pqc-research, where it is maintained. This page renders that document.

Abstract

A Bitcoin layer 2 has a post-quantum problem that neither Bitcoin nor Ethereum has alone. It settles to a base layer it cannot change, borrows a consensus stack from a third ecosystem, and operates a bridge whose trust root is cryptography of its own choosing. Its migration therefore divides into surfaces the L2 can change, surfaces it inherits from an upstream ecosystem, and surfaces on which it can only bound exposure until the base layer acts.

This paper gives a taxonomy of an L2's cryptographic surfaces keyed on the party that can fix each, and a roadmap that carries every surface through the four stages of a cryptographic migration. It reviews the state of the three base layers an L2 draws on, analyses the migration options for each surface, and examines one deployed L2, GOAT, as a case study. Four findings recur. Symmetric and hash-based layers protect only what they carry, so a bridge built from post-quantum primitives remains exposed when what it carries is an elliptic-curve assertion. Settlement and bridge outputs depend on the base layer's output types, so the L2 can only bound their exposure until the base layer acts. A bridge's pairing-based verifier can be replaced by a hash-based one under garbling but not under witness encryption, and the measured cost of such a verifier is of the order of \(10^8\) gates. Consensus keys are the least costly surface, and their migration costs bandwidth and storage rather than computation.

Contents

  1. Introduction
  2. Surfaces and their owners
  3. A migration roadmap
  4. The base layers
  5. Settlement and bridge outputs
  6. The bridge proof system
  7. Consensus keys
  8. The execution layer
  9. Case study: GOAT
  10. Conclusion

1. Introduction

Most post-quantum analysis of blockchains treats a chain as one cryptographic system: one signature scheme securing funds, one consensus mechanism and one governance process to change them. A Bitcoin L2 is instead a composition of systems designed separately, migrating on separate timelines and owned by separate parties.

  • The L2 cannot fix its base layer. Bitcoin has specified no post-quantum signature scheme: BIP-360 states that it "does not include the introduction of post-quantum signature schemes", and BIP-361 lists Requires: TBD Post Quantum Signature BIP. Value held in classical L1 outputs is on Bitcoin's timeline, not the L2's.
  • The L2 borrows a consensus stack from a third ecosystem, and inherits that ecosystem's migration path and constraints.
  • The L2 owns a bridge whose trust root is cryptography of its own choosing, and which is the highest-value target in the system.

The goal is to make both the L2 and the base layer post-quantum secure, while recognising that the L2 can schedule only part of the work. The contributions of this paper are a taxonomy of the surfaces and their owners (section 2); a roadmap placing each surface against the four stages of a migration (section 3); a review of the base layers (section 4); an analysis of the options for each surface (sections 5 to 8), each with the pitfalls specific to it; and a case study of one deployed L2 (section 9).

Method. The analysis is drawn from primary sources: BIP and EIP texts, the Cosmos SDK and CometBFT release notes, the design documents and security analyses of the protocols discussed, and the published designs and code of the L2 examined. Cost figures for hash-based verifiers are measured on real proofs rather than estimated. One L2 is examined in depth, so the taxonomy rests on structural reasoning supported by a single case rather than on a survey.

2. Surfaces and their owners

A Bitcoin L2 that settles to L1 and runs its own consensus has at least the following cryptographic surfaces, listed in Table 1. The taxonomy makes the ownership question explicit before the question of which scheme to adopt.

Table 1. Cryptographic surfaces of a Bitcoin L2 and the party that can fix each.

# Surface Typical scheme Broken by Shor? Who can fix it
1 L1 settlement outputs secp256k1 / Schnorr yes the base layer only
2 Bridge attestation and covenant keys secp256k1, Schnorr, MuSig2, BLS yes the L2, within the base layer's output types
3 Bridge proof system Groth16, PLONK (pairing-based) or STARK (hash-based) if pairing-based, including a pairing-based wrap of a STARK the L2, with its proof-system provider
4 Bridge data commitments Merkle, Winternitz, Lamport no (hash-based) —
5 L2 consensus keys ed25519, secp256k1, BLS12-381 yes the L2, through its consensus stack
6 L2 execution accounts secp256k1 ECDSA yes the L2, following its execution ecosystem
7 EVM cryptographic precompiles ecrecover, BN254 pairing, BLS12-381, KZG yes upstream, and not at all for immutable callers

Notes. Row numbers are used throughout the paper. "Broken by Shor?" asks whether Shor's algorithm, run on a cryptographically relevant quantum computer, recovers keys or forges proofs; the hash-based schemes of row 4 face only the quadratic speed-up of Grover's algorithm, which parameter choice absorbs. Row 2 has two owners: the L2 chooses the signing scheme and the signer set, while the base layer fixes the output types that hold the value.

Three parties hold the fixes. The base layer alone can change rows 1 and part of row 2; the L2 controls rows 2, 3 and 5; the execution ecosystem, Ethereum for an EVM-compatible L2, determines rows 6 and 7.

3. A migration roadmap

A cryptographic migration passes through four stages, as formulated in NIST CSWP 39: identify dependencies; introduce and test the replacement; move users and data; and retire old paths, disabling the vulnerable authorisation path so that it cannot be forced back into use. The roadmap applies the four stages to each surface of Table 1. Figure 1 gives its shape, and Table 2 states each step.

The migration roadmap: six surfaces in order against the four NIST CSWP 39 stages, coloured by the party that gates each row

Figure 1. The migration roadmap. Rows are surfaces in migration order and columns are the four stages; the fill marks the party that gates the row. Dashed cells await a party other than the L2, and the hatched cell marks the one step that cannot be completed, the retirement of immutable precompile callers.

Table 2. The four migration stages applied to each surface.

Order Surface Identify dependencies Introduce and test Move users and data Retire old paths Gated by
0 All surfaces Inventory every signature and proof verification path, including fixed-length signature checks — — — the L2
1 Bridge attestation keys (row 2) Operator and committee signing keys A post-quantum signature for attestations Rotate the operator set, with dual signing Stop accepting classical attestations the L2
2 Bridge proof system (row 3) Every layer that asserts a pairing relation: wrap, verifier, witness encryption A hash-based verifier, in script or garbled (section 6) Move the dispute path to the new verifier Disable the pairing-based dispute path the L2, with its prover
3 Consensus keys (row 5) Validator key types A post-quantum key type in the consensus stack Per-validator key rotation Disallow new classical consensus keys the L2, with its consensus stack
4 Execution accounts (row 6) Accounts whose public keys are revealed; signature authority accepted by contracts Post-quantum transactions and verification Each holder migrates its own account Revoke both the transaction and the ecrecover powers of the old key (section 8) the execution ecosystem
4 EVM precompiles (row 7) Callers of pairing and KZG precompiles, and their upgradeability Additive post-quantum precompiles Upgradeable callers move Immutable callers cannot be retired the execution ecosystem
5 Settlement and bridge outputs (rows 1–2) Outputs that commit a public key on chain A post-quantum output type and signature Move value into the new output type Sunset of legacy spends the base layer

Notes. Row references are to Table 1; row 4, the hash-based data commitments, needs no migration and is omitted. Order 0 is a precondition for every other row, and rows sharing an order number can proceed in parallel. A dash marks a stage that does not apply. "Gated by" names the party whose action the row waits on, which is not always the party that performs the work: the holders of execution accounts perform stage 3 themselves.

The order follows ownership and severity rather than novelty.

  1. Bridge attestation keys carry the highest value per unit of control: the operator set is the bridge's trust root, the change is entirely the L2's, and dual signing gives rollback at every step.
  2. The bridge proof system is the most severe exposure, since its failure breaks soundness rather than individual keys, and the hardest, since it means replacing the verifier the bridge is built around.
  3. Consensus keys often have the mechanism upstream already, so the work is a dependency upgrade rather than cryptographic design.
  4. Execution accounts and precompiles follow the execution ecosystem; an L2 that diverges ahead of it breaks wallet and tooling compatibility, which for an L2 is usually the product. The precompile inventory is the exception and can be taken now, since an immutable caller can be identified before it matters but never migrated after.
  5. Settlement outputs cannot be fixed by the L2. Exposure is bounded instead (section 5) until the base layer provides an output type and a signature.

The fourth stage is the one most often omitted. A surface on which a new algorithm has been introduced but the old path has not been retired is no more secure than before, since an adversary uses the old path: an account that adds a post-quantum key still answers to its ECDSA key until both of that key's powers are revoked (section 8.1), and a chain that admits ML-DSA-65 consensus keys still accepts secp256k1 ones until they are disallowed (section 7). The inventory of stage 0 should also include fixed-length signature checks: code that validates a 64-byte signature rejects a 3,309-byte ML-DSA-65 signature, and such checks sit in transaction decoding, gossip limits and commit validation, where they surface only at activation.

4. The base layers

An L2 draws on up to three ecosystems: Bitcoin for settlement, Ethereum for execution and Cosmos for consensus. Table 3 compares their positions.

Table 3. The post-quantum position of the three base layers, as of October 2026.

Bitcoin Ethereum Cosmos
Core problem public-key exposure on an immutable ledger signature size at validator scale key-type negotiation across chains
Layer addressed first output type and script consensus signatures, and accounts separately validator consensus keys
Post-quantum scheme chosen none hash-based, aggregated ML-DSA-65 (FIPS 204)
Furthest artifact draft BIPs prototypes shipped, opt-in
Governance soft fork, rough consensus coordinated upgrades per-chain opt-in through genesis parameters

Notes. "Post-quantum scheme chosen" records a scheme adopted by the protocol, not a candidate: SHRINCS is a draft proposal for Bitcoin (section 4.1), and Ethereum's hash-based direction rests on prototypes (section 4.2). "Furthest artifact" is the most advanced stage reached, from specification through prototype to release. Cosmos's "shipped" means available in a Cosmos SDK release for chains that opt in; no chain is required to adopt it.

4.1 Bitcoin

Two draft BIPs address the problem; neither is activated. BIP-360 defines an output type that commits no public key on chain, and BIP-361 proposes a pre-announced sunset of legacy ECDSA and Schnorr spends. BIP-361 depends on a post-quantum signature BIP that does not yet exist. The first candidate, SHRINCS, posted as a draft in August 2026, is hash-based: a stateful signature from 548 bytes with an SLH-DSA fallback of 5,777 bytes under one public key. It defines no output type, and it supports no multi-signature aggregation, which bears directly on a bridge whose covenant is emulated by a multi-signature (section 5).

4.2 Ethereum

Ethereum treats consensus migration as an aggregation problem: hash-based signatures rest only on hash security but are too large to place on chain one per validator, so recursive aggregation in a hash-based zkVM, leanVM, compresses them, with leanSig as the prototype signature scheme. Accounts follow a separate path. Account abstraction is the vehicle: EIP-8141 (frame transactions) admits an arbitrary post-quantum signature scheme, and EIP-7885 exposes the number-theoretic transform as a precompile rather than welding one scheme into the protocol. Section 8 takes up the consequences for an EVM-compatible L2. Nothing post-quantum has shipped to Ethereum mainnet; the proposals cited are drafts.

4.3 Cosmos

Cosmos SDK v0.55 ships ML-DSA-65 as a validator consensus key type, opt-in through the chain's permitted key types, together with validator key rotation, so an existing chain can move its validators without a new genesis. Public keys grow from 32 to 1,952 bytes and signatures from 64 to 3,309. CometBFT does not aggregate commit signatures: a commit carries one signature per voting validator, so its size is linear in the validator set (Table 4).

Table 4. Signature bytes in one CometBFT commit, by validator count and key type.

Validators ed25519 ML-DSA-65
50 3.2 KB 165 KB
100 6.4 KB 331 KB
150 9.6 KB 496 KB

Notes. Computed from signature sizes of 64 bytes for ed25519 and 3,309 bytes for ML-DSA-65, assuming that every validator votes; 1 KB is 1,000 bytes. Vote metadata, which is the same under both key types, is excluded.

One hazard generalises beyond Cosmos: an IBC light client verifies a counterparty's commit signatures with its own compiled-in cryptography, so enabling a key type before counterparties can verify it stops packet flow and eventually expires the client. Migration in an interoperable ecosystem is a coordination problem before it is a cryptographic one.

5. Settlement and bridge outputs

A bridge holds deposited value in base-layer outputs, and the post-quantum security of those outputs is set by the output types and signatures the base layer offers. On Bitcoin none is post-quantum yet: the outputs a BitVM-style bridge uses commit a public key on chain, and only a new output type, as BIP-360 proposes, together with a post-quantum signature removes that exposure. Both are on the base layer's timeline.

In a federated bridge these outputs are held in custody by a threshold of signers. A BitVM-style bridge holds no custody: Bitcoin has no covenant opcode, so the bridge emulates one. An N-of-N MuSig2 key of the signer set presigns the transaction graph that may spend a deposit, and each signer deletes its share, so that a single honest deletion confines the deposit to the presigned graph. The emulation rests on the deleted key being unrecoverable, which is the assumption a quantum adversary removes: the discrete logarithm of the aggregated public key is a key that signs any transaction, and the presigned graph then no longer constrains the deposit. A post-quantum emulation needs a post-quantum multi-signature to presign with, which SHRINCS does not provide, or a script that checks N individual post-quantum signatures, at a size linear in N. A native covenant opcode, such as OP_CHECKTEMPLATEVERIFY (BIP-119), would enforce the graph without a signing key, and so without the exposure, wherever the graph can be expressed as templates. Each option is on the base layer's timeline.

What an L2 can do unilaterally is bound the exposure. It grows with the time value remains in such outputs, so a policy that rotates outputs on a schedule, caps the value in any one output and shortens the lifetime of connector outputs reduces it without waiting on the base layer. Such a policy bounds the risk rather than removing it.

6. The bridge proof system

6.1 The wrapper pattern

A BitVM-style bridge convinces Bitcoin script that a proof of the L2's state transition verified. In current designs a zkVM produces a hash-based STARK, the STARK is wrapped in a constant-size Groth16 proof over BN254, and the Groth16 verifier is what the dispute protocol evaluates. Hash-based layers surround the wrap: the STARK below it, and the Winternitz or Lamport commitments that carry values into script above it. Every layer between them asserts, or is keyed to, the BN254 pairing relation, and a forged pairing relation is a forged peg-out: the failure is one of soundness rather than of an individual key.

The pattern is as easily introduced during a migration as inherited from an existing design. Wrapping a classical proof in a post-quantum one leaves the classical assertion in place: a STARK proving that a Groth16 proof verified, or a zkVM proving that an ECDSA signature verified, is forged as soon as the inner pairing or signature is. A migration step helps only if it changes what is asserted.

6.2 Generations of the on-chain verifier

The designs that bring a SNARK verifier to Bitcoin form one line of work. Each answers the question how is a Bitcoin script convinced that a Groth16 proof verified, and each moves more of the pairing arithmetic off chain. What varies across generations is the execution of verification; the relation verified is invariant (Table 5).

Table 5. Generations of the Groth16 verifier on Bitcoin.

Generation Mechanism Cost addressed Relation asserted
BitVM2 Groth16 verifier compiled to Bitcoin script on-chain: a Disprove script of several hundred KB, over $14,000 in a reported unhappy-path experiment Groth16 / BN254
BitVM3 verifier circuit garbled; the dispute evaluates the garbling on-chain cost collapses, but a garbled circuit is 42 GiB Groth16 / BN254
BABE witness encryption for linear pairing relations, with a garbled scalar multiplication for the non-linear part off-chain storage and setup, ~1000× lower than BitVM3 Groth16 / BN254
Deferred Binding BABE extended so that part of the public input may be fixed after garbling usable when L2 state is known only at withdrawal Groth16 / BN254

Notes. Costs are those reported for each design and are not measured on a common workload. The BitVM2 and BitVM3 figures are as quoted by the BABE authors, and the BABE figure is their reduction relative to BitVM3. The last column is constant across the generations, so none of them changes the quantum exposure of row 3 of Table 1.

BABE (Garg, Kolonelos, Sergeevitch, Sridhar and Tse) keeps BitVM3's on-chain savings and "reduces its off-chain storage and setup costs by three orders of magnitude", using "a witness encryption scheme for linear pairing relations to verify Groth16 proofs" together with "a very efficient garbled circuit for scalar multiplication on elliptic curves", which builds on Argo MAC (Eagen and Lai); Duty-Free Bits (Khambhati, Bhattacharya and Heath) improves both encodings further. Deferred Binding, with its formal primitive partial-binding witness encryption, lets the dynamic part of the public input (L2 state, sequencer commitments) be bound at withdrawal rather than at garbling.

6.3 The relation and the security reduction

The witness encryption is keyed to the Groth16 verification equation on BN254,

\[e(\pi_1, \pi_2) = e(\alpha, \beta) \cdot e(\mathit{vk}_x, \gamma) \cdot e(\pi_3, \delta_2)\]

so the witness that decrypts the payout secret is a valid Groth16 proof, and the scheme's correctness guarantees that anyone holding one can decrypt. The on-chain side is hash-based: Bitcoin script performs hash commitments, garbled-gate unlocking and a hashlock on the payout secret, and "No pairings or target-group arithmetic are evaluated on chain." The security analysis of partial-binding witness encryption states the reduction directly:

\[\mathrm{Adv}_{\mathrm{pbWE}}(\lambda) \le \mathrm{Adv}_{\mathrm{BABE}}(\lambda) + \mathrm{Adv}_{\mathrm{DLog}[\mathbb{G}_1]}(\lambda)\]

and concludes that it "stays within standard BABE assumptions plus GC-security and Lamport one-wayness". Against a quantum adversary \(\mathrm{Adv}_{\mathrm{DLog}[\mathbb{G}_1]}\) on BN254 goes to 1 and \(\mathrm{Adv}_{\mathrm{BABE}}\) rests on the same curve, so the bound becomes vacuous: the adversary forges a Groth16 proof for a withdrawal that never occurred, decrypts the payout secret through the scheme's own correctness, and spends the hashlocked output, without breaking any garbling or inverting any hash.

This is the wrapper pattern in its least visible form. Witness encryption, garbled circuits, hashlocks and Lamport commitments are post-quantum primitives, and three of the four on-chain primitives are post-quantum; the pairing persists in the relation being encrypted against. The garbling layer has a parameter of its own: 128-bit wire labels under an AES-128 PRF place it at NIST post-quantum category 1, the floor of the approved range. Where garbling gates decryption of a payout secret rather than serving as an optimisation, the label width warrants an explicit decision; 256-bit labels restore category-5 margin at roughly twice the garbling cost.

6.4 Replacing the pairing-based verifier

Witness encryption does not transfer to a hash-based verifier. BABE encrypts against a linear pairing relation; a STARK verifier has no pairing relation, no target group and no short algebraic acceptance predicate, so there is nothing for a ciphertext to be keyed to, and witness encryption for hash-based proof systems is an open problem. Garbling does transfer: a garbled circuit requires only that the verifier be a Boolean circuit, which a hash-based verifier is. BABE thereby creates a tension between cost and security: it lowers the cost of verifying Groth16 on Bitcoin, and with it the pressure to replace Groth16, whereas replacement is the post-quantum requirement.

A hash-based verifier can reach Bitcoin by two routes.

In script. Bitcoin script has no multiplication and no concatenation opcodes; multiplication is emulated, as BitVM already does at 254 bits, but a script cannot assemble the two-child preimage a SHA-256 Merkle step hashes without OP_CAT (BIP-347, specified but not activated). The dependency belongs to the hash rather than to the proof system: an algebraic hash such as Poseidon2 over a 31-bit field is field arithmetic, which script emulates, so a FRI or WHIR verifier over such a hash needs no soft fork. Its cost is the hash: a Poseidon2 permutation is about 572 KB of script, a Merkle path costs one permutation per level, and a full verifier at 100-bit security is several hundred megabytes of script, about 97% of it query work. Divided into transaction-sized segments for a disprove protocol, of which a dispute executes one, it is of the order of two thousand segments, the same order as BitVM2's own. Lattice-based arguments avoid the per-level hash but pay in verifier work and require changing the prover's commitment scheme; their cost in script is unmeasured.

Garbled. A binary-field STARK verifier, WHIR over \(\mathrm{GF}(2^{128})\) with Blake3, garbled as in BitVM3, has \(9.4 \times 10^7\) non-free gates and garbles to 1.5 GB for a \(2^{18}\)-row trace at 104 bits of soundness, about thirty times fewer non-free gates than the garbled Groth16 verifier (Garbling a Hash-Based STARK Verifier for Bitcoin). The field and the hash, more than the proof system, determine the cost of a garbled verifier: as a Boolean circuit, a Poseidon2 permutation over a 31-bit prime field costs about 120 Blake3 compressions, so a proof committed with an algebraic hash over a prime field is better wrapped into a binary-field proof than garbled directly. Ziren, the zkVM in the case study, defines exactly this route alongside its pairing-based wrap: a binary stage over \(\mathrm{GF}(2^{128})\) with Blake3 and a narrow recursion reduce its proof to one whose verifier is a compact Boolean circuit (Duan et al.).

Both routes are feasible without a soft fork. What remains is integration into the dispute protocol and a choice by cost. The integration is a rebuild rather than the removal of a wrapper, because the garbling stacks in use are organised around the Groth16 verifier: a garbled-circuit library whose gadgets implement BN254 field and pairing arithmetic offers nothing to a hash-based verifier, which needs binary-field multipliers and hash gadgets, so replacing the proof system replaces the circuit library.

7. Consensus keys

Consensus keys admit the least costly migration. Where the consensus stack ships a post-quantum key type, as Cosmos SDK v0.55 does with ML-DSA-65 (section 4.3), the work is a dependency upgrade, a key rotation and a re-tuning of block parameters. Two threats are distinct. Forged votes in live consensus become possible when a cryptographically relevant quantum computer exists, and not before. Forgery of recorded history, an alternative chain assembled from archived commit signatures, is a harvest-now, forge-later threat: blocks signed before a migration remain forgeable after it.

A migration that avoids a network-wide switch rotates validators individually. In the design published for the case-study L2 (GOAT Research, Post-Quantum Consensus Keys on GOAT), each validator keeps a permanent identity mapped to its current consensus key, rotates with a single transaction carrying the new key and a proof of possession, and is limited to one rotation per retention period; the validator set may remain mixed indefinitely, and the classical key is retained as an account key rather than combined with the new one in a hybrid signature. Activation is gated by fork heights, and each validator then rotates at a time of its choosing.

The measured costs, in Table 6, reverse a common expectation.

Table 6. Cost of moving the case-study L2's consensus keys from secp256k1 to ML-DSA-65.

secp256k1 ML-DSA-65
Verification, per signature 173 µs 23 µs
Signing, per signature 46 µs 196 µs
Commit signatures per block, 20 validators 1.3 KB 66 KB
Validator public keys in state, 20 validators 0.7 KB 39 KB

Notes. Measurements as reported in the published design. Timings depend on implementation and hardware and are comparable within a row only. Sizes count signature and key bytes alone; an ML-DSA-65 public key is 1,952 bytes and a signature 3,309 bytes.

Verification becomes faster, so the migration costs bandwidth and storage rather than computation, linearly in the validator count: at 20 validators the commit signatures add about 585 GB per year, and at 100 about 3 TB, so block, gossip and storage budgets must be re-tuned together with the key type. Aggregation of post-quantum consensus signatures, as in Ethereum's recursive designs, would remove the linear term, but no such mechanism exists in CometBFT, where two attempts to add BLS aggregation were closed without merging. Two points of a rotation design need deliberate treatment: the interaction between the evidence window and the retention period, since misbehaviour under a rotated-out key must remain attributable; and the final stage, disallowing new classical consensus keys once rotation is complete.

8. The execution layer

8.1 Accounts

An EVM-compatible L2 inherits Ethereum's account model, and with it two findings of a cost analysis of Ethereum's proposals (AppliedPQC, Post-Quantum Migration in Ethereum's Execution Layer). First, the cost of a post-quantum transaction lies in its bytes rather than its verification: with a verification precompile, verification is a few percent of a transaction's gas, and the signature's size sets the rest. Second, obtaining a post-quantum key does not complete a migration. The old key retains two independent powers, originating transactions from the address and authorising actions through ecrecover in contracts, and each requires its own revocation. Installing code at the account (EIP-8298) brings EIP-3607 into force and ends ECDSA transaction origination, whereas a delegation under EIP-7702 alone does not; EIP-8151 restricts ecrecover for such accounts, closing signature-based authorisation through immutable contracts such as ERC-20 permit. Until both revocations exist, abandoning the address is the only action that removes both powers. For an L2 the consequence is to follow Ethereum's account semantics rather than diverge ahead of them. An execution client forked from upstream inherits changes such as EIP-8141 and EIP-8151 only after rebasing onto the release that ships them, so the fork's divergence sets the earliest date at which they can reach the L2.

8.2 Precompiles

The EVM exposes elliptic-curve and pairing operations as consensus-level precompiles: secp256k1 recovery, BN254 addition, multiplication and pairing, BLS12-381 operations, KZG point evaluation and secp256r1 verification. A precompile cannot be migrated by its users, since it is a consensus rule and its callers are often immutable. What decides whether an exposure can be migrated is the caller set rather than the precompile (Table 7).

Table 7. Migratability of the EVM's elliptic-curve and pairing precompiles, determined by their caller sets.

Precompile Caller set Migratable
ecrecover every account, gated by per-account state yes: narrow it, as EIP-8151 does
KZG point evaluation rollup contracts: bounded, known and maintained plausibly: coordinate the set
BN254 and BLS12-381 pairing every SNARK verifier deployed: unbounded and largely unmaintained no

Notes. A precompile is migratable if its exposure can be removed without breaking a caller that cannot be changed. The rows are representative: the BN254 and BLS12-381 group operations share the caller set of the pairing row.

A precompile bound to an account identity can be narrowed conditionally on that account's state; a stateless mathematical precompile cannot, since there is no account whose migration status could gate it. For the latter, the levers are additive post-quantum precompiles, which solve the forward problem only; repricing, which breaks callers silently through exhaustion of gas; removal on a fixed date, without precedent on Ethereum; or nothing. The question for a given L2 is therefore whether its own verifier contracts are upgradeable. A pairing precompile also appears on the Bitcoin side of the same bridge, through the Groth16 wrap of section 6, so a single broken assumption compromises the bridge from both directions and should be tracked as one dependency.

9. Case study: GOAT

GOAT is a Bitcoin L2 with a CometBFT and Cosmos SDK consensus layer, an EVM execution layer, and a BitVM2-style bridge whose dispute protocol is moving to garbled verification. Table 8 maps its surfaces onto the taxonomy.

Table 8. The surfaces of GOAT against the taxonomy of Table 1 and the roadmap of Table 2.

Row Surface Current scheme Quantum status Roadmap stage Owner
1 Settlement outputs secp256k1 / Schnorr broken bounded only Bitcoin
2 Peg-in covenant MuSig2 N-of-N, presigned broken bounded only GOAT, within Bitcoin's output types
3 Bridge proof system Ziren STARK, Groth16/BN254 wrap, garbled verifier and witness encryption broken at the wrap and the verifiers built on it replacement introduced and measured GOAT, with ZKM
4 Bridge commitments Winternitz one-time signatures post-quantum — —
5 Consensus keys secp256k1 broken until rotated design published GOAT
6 Execution accounts secp256k1 ECDSA broken follows Ethereum Ethereum, inherited
7 EVM precompiles upstream set broken for pairing and curve precompiles inventory Ethereum, inherited

Notes. Row numbers follow Table 1. "Broken" is conditional on a cryptographically relevant quantum computer. "Roadmap stage" is the furthest step of Table 2 reached as of October 2026. "Bounded only" means that no replacement exists yet and exposure is limited by the output policy of section 5.

Three observations follow. The bridge's Bitcoin-side plumbing is already post-quantum while its cryptographic content is not, which reframes the work on row 3 from a rewrite into the replacement of one relation: the bridge carries values into script through Winternitz commitments, but the relation those commitments carry is a pairing equation. Ziren, the bridge's zkVM, already defines the hash-based route that replaces the wrap, so the remaining work on row 3 lies in the garbling stack and the dispute protocol rather than in the prover. And the consensus keys, the only surface with a published migration design, are also the least costly, which makes them a natural first deployment of post-quantum cryptography in the stack even though the bridge is the more severe exposure.

10. Conclusion

A Bitcoin L2's post-quantum migration is a set of migrations owned by different parties. The L2 controls its bridge and its consensus keys and can carry both through every stage of the roadmap; it inherits its execution layer from Ethereum; and on its settlement outputs it can only bound exposure until Bitcoin defines a post-quantum output type and signature. The bridge is the decisive surface. Its post-quantum position is set not by the primitives it is built from, most of which are already post-quantum, but by the relation those primitives carry, and replacing that relation is feasible today under garbling at a measured cost of the order of \(10^8\) gates.

The analysis has three limits. One L2 has been examined in depth, so the taxonomy rests on a single case. The hash-based verifiers are measured on proofs of Plonky3's systems and of Ziren's experimental garbled route, not yet on a deployed dispute protocol. And several designs discussed, among them BIP-360, BIP-361, SHRINCS, Deferred Binding and the Ethereum account proposals, are drafts whose details may change.

References

Base layers

Migration framework

Bridge proof systems

Consensus and execution layers