Security Considerations

Understand the security properties of P256K and how to avoid common cryptographic pitfalls.

Overview

P256K layers a Swift API over bitcoin-core/secp256k1, which is engineered for constant-time execution and side-channel resistance on the private-key paths. The sections below cover the areas where application code can still make mistakes that weaken those guarantees: context randomization lifecycle, nonce hygiene, comparison timing, secret zeroization, and ECDSA signature malleability.

Context Randomization

Every cryptographic operation in P256K depends on a secp256k1 context managed by P256K.Context. The shared context is created and randomized once at process startup using OS-provided entropy.

Randomization seeds a blinding factor that protects ECDSA signing, Schnorr signing, and public key generation against timing and power analysis attacks. The blinding factor is applied to the base point multiplication, making the internal computation pattern independent of the secret key.

Note

ECDH key agreement uses a different kind of elliptic curve point multiplication and does not currently benefit from context randomization.

Nonce Reuse

The most dangerous mistake in elliptic curve cryptography is reusing a nonce (random value) across two different signing operations with the same private key. If the same nonce k is used to sign two different messages, an attacker can compute the private key from the two signatures using simple algebra.

MuSig2 SecureNonce

P256K uses Swift’s ~Copyable (non-copyable) type for P256K.Schnorr.SecureNonce to prevent accidental nonce reuse at the type-system level. Once a SecureNonce is consumed by a signing operation, it cannot be used again:

let nonce = try P256K.MuSig.Nonce.generate(
    secretKey: privateKey,
    publicKey: privateKey.publicKey,
    msg32: messageBytes
)

// The secnonce is consumed here -- it cannot be reused
let partial = try privateKey.partialSignature(
    for: digest,
    pubnonce: nonce.pubnonce,
    secureNonce: nonce.secnonce,  // consumed, cannot use again
    publicNonceAggregate: aggregateNonce,
    publicKeyAggregate: aggregate
)

ECDSA and Schnorr Nonces

For standard (non-MuSig) ECDSA and Schnorr signing, P256K uses deterministic nonce generation (RFC 6979 for ECDSA, BIP-340 for Schnorr) by default, which eliminates the risk of random nonce reuse entirely.

Constant-Time Comparison

When comparing secret values (shared secrets, keys, signatures), use constant-time comparison to avoid timing side-channels. P256K’s SharedSecret type uses constant-time equality internally:

let secret1 = alice.sharedSecretFromKeyAgreement(with: bobPublicKey)
let secret2 = bob.sharedSecretFromKeyAgreement(with: alicePublicKey)

// This comparison is constant-time
secret1 == secret2

Never compare secret bytes using standard == on Data or [UInt8], as this may short-circuit on the first differing byte, leaking information about which bytes match.

Secret Key Zeroization

P256K uses SecureBytes internally for private key storage. When a SecureBytes value is deallocated, its memory is overwritten with zeros using memset_s (or an equivalent that the compiler cannot optimize away). This prevents secret key material from lingering in memory after the key object is destroyed.

ECDSA Signature Malleability

An ECDSA signature (r, s) has a counterpart (r, n - s) that is also valid for the same message and public key. This malleability can cause problems in systems that use the signature as a unique transaction identifier (e.g., Bitcoin before SegWit).

P256K enforces lower-S normalization (BIP-62 rule 6): secp256k1_ecdsa_verify only accepts signatures where s is in the lower half of the curve order. The signature(for:) overloads on P256K.Signing.PrivateKey always produce normalized signatures, and the normalize property on a recoverable signature (P256K.Recovery.ECDSASignature) converts it to the canonical form.