Sesame-iOS/Sesame/API/Message+Extensions.swift

import Foundation

#if canImport(CryptoKit)
import CryptoKit
#else
import Crypto
#endif

extension Message {

    static var length: Int {
        SHA256.byteCount + Content.length
    }

    init<T: Sequence>(decodeFrom data: T) where T.Element == UInt8 {
        let count = SHA256.byteCount
        self.mac = Data(data.prefix(count))
        self.content = .init(decodeFrom: Array(data.dropFirst(count)))
    }

    func isValid(using key: SymmetricKey) -> Bool {
         HMAC<SHA256>.isValidAuthenticationCode(mac, authenticating: content.encoded, using: key)
    }
}

extension Message.Content {

    func authenticate(using key: SymmetricKey) -> Message {
        let mac = HMAC<SHA256>.authenticationCode(for: encoded, using: key)
        return .init(mac: Data(mac.map { $0 }), content: self)
    }

    func authenticateAndSerialize(using key: SymmetricKey) -> Data {
        let encoded = self.encoded
        let mac = HMAC<SHA256>.authenticationCode(for: encoded, using: key)
        return Data(mac.map { $0 }) + encoded
    }
}