Swift-Sodium中如何实现公私钥标量乘法？求技术指导

Hey there! I totally get the confusion when you're used to the ScalarMultiplication wrapper in the .NET libsodium port but don't see an equivalent in SwiftSodium. The good news is we can still implement this functionality easily—either by calling the underlying libsodium C functions directly, or using existing SwiftSodium helper methods that leverage scalar multiplication under the hood.

Option 1: Directly Call the Underlying libsodium C Function #

SwiftSodium exposes all the core libsodium C functions, so we can use crypto_scalarmult directly to perform the scalar-point multiplication. Here's a clean wrapper function for it:

import SwiftSodium

func performScalarMultiplication(scalar: Bytes, point: Bytes) -> Bytes? {
    let sodium = Sodium.shared
    // Validate input lengths (scalar should be 32 bytes, point should be 32 bytes for Curve25519)
    guard scalar.count == sodium.crypto_scalarmult_scalarbytes,
          point.count == sodium.crypto_scalarmult_bytes else {
        print("Invalid input lengths for scalar multiplication")
        return nil
    }
    
    var result = Bytes(repeating: 0, count: sodium.crypto_scalarmult_bytes)
    // Execute the scalar multiplication
    let success = crypto_scalarmult(&result, scalar, point) == 0
    
    return success ? result : nil
}

// Example Usage
let sodium = Sodium.shared
guard let privateKey = sodium.box.keyPair().secretKey,
      let publicKey = sodium.box.keyPair().publicKey else {
    fatalError("Failed to generate Curve25519 key pair")
}

// Multiply the private key (scalar) with the public key (point)
if let sharedSecret = performScalarMultiplication(scalar: privateKey, point: publicKey) {
    print("Generated shared secret: \(sharedSecret.hexEncodedString())")
}

Key Notes for This Approach: #

• The scalar should be a valid Curve25519 private key (32 bytes), or any 32-byte value (though only values between 1 and the curve's order are mathematically valid for secure operations).
• The point must be a valid Curve25519 public key (32 bytes). You can validate a point using crypto_scalarmult_curve25519_is_valid_point if needed.

Option 2: Use SwiftSodium's Built-in box.before() Method #

If your use case is generating a shared secret (which is the most common scenario for scalar multiplication), SwiftSodium's Box class already has a method that does exactly this: before(). This method internally calls crypto_scalarmult to compute the shared secret from a private key and a public key.

import SwiftSodium

let sodium = Sodium.shared
// Generate key pairs for two parties
guard let aliceKeyPair = sodium.box.keyPair(),
      let bobKeyPair = sodium.box.keyPair() else {
    fatalError("Failed to generate key pairs")
}

// Compute the shared secret using Alice's private key and Bob's public key
if let sharedSecret = sodium.box.before(publicKey: bobKeyPair.publicKey, secretKey: aliceKeyPair.secretKey) {
    print("Shared secret via box.before(): \(sharedSecret.hexEncodedString())")
}

This is the most idiomatic approach if you're working with Curve25519 key pairs for secure communication, as it handles all input validation and low-level logic for you.

内容的提问来源于stack exchange，提问作者GJDK