fix(abstract-substrate): remove double Ed25519 prefix in MPCv2 recovery

[vibhavgo]

Summary

Fixes MPCv2 non-BitGo recovery for Substrate-based EDDSA coins (TAO, POLYX). addSubstrateRecoverySignature was double-prefixing the Ed25519 MultiSignature discriminant, corrupting the on-wire signature by one byte and causing the chain to reject the extrinsic with 1010: Bad signature.

Linear: WCI-1454

Root cause

addSubstrateRecoverySignature wrapped the raw 64-byte MPCv2 signature with 0x00 before calling txBuilder.addSignature:

const substrateSig = Buffer.concat([Buffer.from([ED25519_MULTI_SIGNATURE_PREFIX]), rawSig]);
txBuilder.addSignature({ pub }, substrateSig);

But Transaction.constructSignedPayload (called during build()) already prepends 0x00:

const edSignature = \`0x00\${signature.toString('hex')}\` as HexString;

Result: polkadot-js's MultiSignature decoder consumed 0x00 as the Ed25519 discriminant, then read the next 64 bytes as the signature — which were [0x00][R (32B)][first 31 bytes of sigma]. The last byte of sigma was dropped; the chain re-verified against the true 64-byte signature and rejected.

MPCv1 was unaffected because its branch already passed the raw 64-byte sig through untouched. sdk-coin-dot implements the same helper correctly (see dot.ts L744 — with a comment explicitly warning against double-prefixing).

Changes

• modules/abstract-substrate/src/abstractSubstrateCoin.ts: MPCv2 branch of addSubstrateRecoverySignature now passes rawSig directly, mirroring MPCv1 and sdk-coin-dot. Removed the unused ED25519_MULTI_SIGNATURE_PREFIX constant. Updated the doc comment to explain the invariant.
• modules/abstract-substrate/test/unit/abstractSubstrateCoin.ts: updated the MPCv2 unit test which previously pinned the buggy behavior (sig[0] === 0x00, sig.slice(1) === rawSig) — it now asserts sig.length === 64 and sig === rawSig.

Scope of impact

Coins that extend SubstrateCoin from @bitgo/abstract-substrate:

Coin	Package	Affected
TAO / TTAO	sdk-coin-tao	yes → fixed
POLYX / TPOLYX	sdk-coin-polyx	yes → fixed
DOT / TDOT	sdk-coin-dot	no — has its own addRecoverySignature that was already correct

Test Plan

• Unit test updated and passes locally (yarn nyc mocha in modules/abstract-substrate).
• End-to-end verified on Bittensor testnet (finney) via WRW → MPCv2 recovery → broadcast:
  • Status: Ready → Broadcast → InBlock → Finalized
  • Block 0x3c5605b939d2cc403211f0d32c88fe1bc4125e2632da4ffa4957eff41c8dcf06
• Retest TAO MPCv2 recovery on staging with released package (tracked in WCI-1435).
• Retest POLYX MPCv2 recovery on staging with released package.

Made with Cursor

[linear-code]

WCI-1464

WCI-1454