Code example of signing using hashed message? about secp256k1.swift

[piersonleo]

Thanks for the input, Chris!

After looking through the library, I think that the java library is producing the signature in form of compact representation, with the recovery id appended at the end to form a 65 bytes signature.

So I managed to reproduce the same signature as the java library using this

let compactSign = try privateKey.ecdsa.recoverableSignature(for: hashDigest).compactRepresentation

[csjones]

Hey @piersonleo 👋 Is this the kind of example you're looking for?

let privateKey = try! secp256k1.Signing.PrivateKey()
let preHashedData = try! "f08a78cbbaee082b052ae0708f32fa1e50c5c421aa772ba5dbb406a2ea6be342".bytes

let hashDigest = HashDigest(preHashedData)

let signature = try! privateKey.ecdsa.signature(for: hashDigest)

[piersonleo]

Hi Chris, ah yes this is the example I'm looking for. Thank you.

Kinda unrelated, but I've also been using a secp256k1 library in java, but it seems that swift and java libraries returns a different signature result. Do you perhaps know the reason why it could result to that difference?

Here's a sample of the data and the results I got using both the java library and this library

privateKey string = "187dc06196cb99b99ee19afb876b79defd246cc123fb9e0c617345f77d3737df"
preHashedData = "34b90c9d26d0b19bdb348b31570d39bcda88bac94249f7219444a55bcaef1908"


java public key = "b6f264cc07e01b3e48a3ae3557ed752bc5caae20d6cd048af59d7065b47bada4056ee891d570b926c12b5e19af3c6e071b2d0b029f9dc4061391604178235fcf"
java signature result = "54c1d0eb6babd0124987d69d1870efaefbda5abb1ce10cd225a9aacab1b59de641f2c815a7a98db233ad3570a078c4d17a43f0ebb3f04e83e1c16ba260fbecc501"

swift public key = "03b6f264cc07e01b3e48a3ae3557ed752bc5caae20d6cd048af59d7065b47bada4"
swift signature result = "e69db5b1caaaa925d20ce11cbb5adafbaeef70189dd6874912d0ab6bebd0c154c5ecfb60a26bc1e1834ef0b3ebf0437ad1c478a07035ad33b28da9a715c8f241"

When i try to recover the public key and verify both of the signature result using the golang ecdsa library the java signature passed the verification while the swift signature fails.

[csjones]

You're welcome Pierson 😁

Looking at the java library, my understanding is it leverages libsodium (this package uses libsecp256k1) and I'm not exactly sure how make these ends meet because subtle differences could end up with completely different outputs.

For example, the java public key you posted is uncompressed but doesn't include 04 which denotes key format. If we switch the swift public key format from compressed, 03, to uncompressed - we will get the expected public key.

let expectedPrivateKey = "187dc06196cb99b99ee19afb876b79defd246cc123fb9e0c617345f77d3737df"
let privateKeyBytes = try! expectedPrivateKey.bytes
let privateKey = try! secp256k1.Signing.PrivateKey(rawRepresentation: privateKeyBytes, format: .uncompressed)
        
print(String(bytes: Array(privateKey.publicKey.rawRepresentation))) // 04b6f264cc07e01b3e48a3ae3557ed752bc5caae20d6cd048af59d7065b47bada4056ee891d570b926c12b5e19af3c6e071b2d0b029f9dc4061391604178235fcf

After completing the sanity check, we would want to start debugging the signatures themselves. First thing to notice is that the signatures are different character lengths. This makes me think of questions like, "What are the steps the java library used to create the signature?" and whether you'd get the result you're looking for by using the recovery signature API.

let recoverableSignature = try! privateKey.ecdsa.recoverableSignature(for: hashDigest)

Hope that helps!