feat(sdk): add verifyMessage and make signMessage compatible with Dash Core signed messages

[thephez]

Summary

WasmSdk.signMessage (wasm-sdk) and wallet.signMessage (js-evo-sdk) exist, but there is no verifyMessage counterpart. Worse, the signature format produced is nonstandard, so nothing else in the ecosystem (Dash Core verifymessage, other wallets) can verify it either.

Current behavior

sign_message in packages/wasm-sdk/src/wallet/key_generation.rs:

• hashes the raw message bytes with a single SHA-256 (no \x19DarkCoin Signed Message:\n magic prefix, no length varint)
• signs with plain (non-recoverable) ECDSA
• returns the 64-byte compact signature as hex

Dash Core's signmessage / verifymessage use the magic prefix, double SHA-256, and a 65-byte recoverable signature encoded as base64. The SDK output is incompatible with that, and because it has no recovery byte, a verifier would need the public key passed explicitly rather than recovering it from an address.

The only tests (packages/wasm-sdk/tests/unit/key-generation.spec.ts) check that the output is a string and is deterministic. No test round-trips a signature, which is how the gap went unnoticed.

History

• signMessage was added in feat(sdk): wasm sdk core and test suite #2709 as part of the initial wasm-sdk wallet helpers.
• feat(sdk): add more SDK methods #2784 re-exported it through the js-evo-sdk wallet facade.
• Neither PR added verification or discussed the format.

Proposed fix

1. Reimplement signMessage on top of the dashcore::sign_message module the SDK already depends on (signed_msg_hash, MessageSignature, base64 helpers) so output matches Dash Core's signmessage.
2. Add verifyMessage(message, signature, address, network) using MessageSignature::is_signed_by_address / recover_pubkey, and expose it in js-evo-sdk's wallet facade.
3. Add round-trip tests, plus a fixture generated by Dash Core signmessage to prove interoperability.

Breaking change

Changing the output format of signMessage is a breaking change to the wasm-sdk and js-evo-sdk wallet API. It should be flagged with ! in the commit and noted in the migration docs.

[thephez]

Additional context after a wider search of the repo:

A correct implementation already exists in rs-platform-wallet. Commit 92f9452 (2026-08-06) added CoreWallet::sign_message in packages/rs-platform-wallet/src/wallet/core/sign_message.rs, exposed via rs-platform-wallet-ffi, rs-unified-sdk-jni, Kotlin ManagedCoreWallet.signMessage, and Swift ManagedCoreWallet.signMessage. It uses dashcore::sign_message::signed_msg_hash (magic prefix + SHA256d), produces a 65-byte recoverable signature, and returns base64, so it is interoperable with Dash Core verifymessage and dashj. The wasm-sdk signMessage is the only outlier.

No SDK surface exposes verification. is_signed_by_address is only called from rs-platform-wallet unit tests. Rust, FFI, Swift, Kotlin, wasm-sdk, and js-evo-sdk all lack a public verifyMessage.

Suggested adjustment to the proposal: align wasm-sdk signMessage with the rs-platform-wallet format (same dashcore primitives), and add verifyMessage(message, signature, address) across the SDK surfaces rather than only in JS.