K1 - Secp256k1 Elliptic Curve in Swift

Alexander Cyon

Last update: Oct 25, 2022

Related tags

Security ecdsa ecdh secp256k1 schnorr schnorr-signatures

Overview

K1 🏔

Safer than K2

K1 is Swift wrapper around libsecp256k1 (bitcoin-core/secp256k1), offering ECDSA, Schnorr (BIP340) and ECDH features.

Features

ECDSA Signatures

let alice = try K1.PrivateKey.generateNew()
let message = "Send Bob 3 BTC".data(using: .utf8)!
let signature = try alice.ecdsaSign(unhashed: message)
let isSignatureValid = try alice.publicKey.isValidECDSASignature(signature, unhashed: message)
assert(isSignatureValid, "Signature should be valid.")

Schnorr Signatures

let alice = try K1.PrivateKey.generateNew()
let message = "Send Bob 3 BTC".data(using: .utf8)!
let signature = try alice.schnorrSign(unhashed: message)
let isSignatureValid = try alice.publicKey.isValidSchnorrSignature(signature, unhashed: message)
assert(isSignatureValid, "Signature should be valid.")

Schnorr Scheme

The Schnorr signature implementation is BIP340, since we use libsecp256k1 which only provides the BIP340 Schnorr scheme.

It is worth noting that some Schnorr implementations are incompatible with BIP340 and thus this library, e.g. Zilliqa's (kudelski report, libsecp256k1 proposal, Twitter thread).

ECDH

let alice = try K1.PrivateKey.generateNew()
let bob = try K1.PrivateKey.generateNew()

let ab = try alice.sharedSecret(with: bob.publicKey)
let ba = try bob.sharedSecret(with: alice.publicKey)
assert(ab == ba, "Alice and Bob should be able to agree on the same secret")

Alternatives

GigaBitcoin/secp256k1.swift (also using libsecp256k1, ❌ No Schnorr)
KevinVitale/WalletKit (also using libsecp256k1, ❌ No Schnorr)
yenom/BitcoinKit ( 💀 Discontinued, also using libsecp256k1, ❌ No Schnorr)
oleganza/CoreBitcoin (ObjC + Swift, ❌ No Schnorr)
Sajjon/EllipticCurveKit (mine, ☣️ unsafe, ✅ Schnorr support)

Non-Swift but SPM support

greymass/secp256k1 (Fork of libsecp256k1)

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Comments

Rationale for ecdh_skip_hash_extract_x_and_y

This libarary is great, I really like the minimalist API but I'm having interoperability issues using it for ECDH.

I believe it is due to PrivateKey.sharedSecret using ecdh_skip_hash_extract_x_and_y

https://github.com/Sajjon/K1/blob/89cac3e2f7765f4a782ae71d2de231c08dcfaa53/Sources/K1/K1/Keys/PrivateKey/PrivateKey/PrivateKey%2BBridge%2BTo%2BC.swift#L144

The result is Data that when put through SHA256.hash produces a inconsistent result from libraries like this:

https://github.com/bitcoin-core/secp256k1/blob/185a6af22792531a629959834fff9257e396abb5/src/modules/ecdh/main_impl.h#L13-L24

I was wondering if this was an intentional design decision and what the rationale was?

opened by mikekelly 2