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82 lines
3.5 KiB
Swift
82 lines
3.5 KiB
Swift
#if canImport(CryptoKit)
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import CryptoKit
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#endif
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import Foundation
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/// Why a push envelope could not be opened.
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public enum PushEnvelopeError: Error, Equatable, Sendable {
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/// Not base64, or shorter than nonce + tag — nothing to decrypt.
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case malformedEnvelope
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/// The key is not exactly ``PushEnvelope/keyLength`` bytes.
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case invalidKeyLength
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/// AES-GCM authentication failed: wrong key, tampered ciphertext, or a
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/// nonce/AAD mismatch. Indistinguishable by design.
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case authenticationFailed
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/// No AES-GCM implementation on this platform (Linux test builds; the
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/// dependency-free rule forbids swift-crypto). Production targets are
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/// all Darwin, where CryptoKit is always available.
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case cryptoUnavailable
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}
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/// The end-to-end push envelope (`hapi-push-v1`): the hub encrypts the FCM
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/// data-contract JSON with the device's per-install `pushKey` so APNs (and
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/// any relay in between) only ever sees an opaque blob plus a generic alert.
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///
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/// Wire format: `base64(nonce || ciphertext || tag)` where the cipher is
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/// AES-256-GCM, the nonce is 12 bytes, the tag 16 bytes, and the additional
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/// authenticated data is the ASCII string `hapi-push-v1`.
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///
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/// The hub-side producer implements the same construction; the Notification
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/// Service Extension carries a small self-contained copy of ``decrypt`` (see
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/// `ios/HapiNotificationService/NotificationService.swift`) because an appex
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/// should not link HapiKit.
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public enum PushEnvelope {
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/// AAD binding the ciphertext to this envelope version.
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public static let aad = "hapi-push-v1"
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/// AES-256 key size in bytes.
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public static let keyLength = 32
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/// GCM nonce prefix length in bytes.
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public static let nonceLength = 12
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/// GCM authentication tag suffix length in bytes.
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public static let tagLength = 16
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/// Opens `base64(nonce || ciphertext || tag)` with `key`, returning the
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/// plaintext (the FCM data-contract JSON, see ``PushPayload``).
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///
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/// Structural problems throw ``PushEnvelopeError/malformedEnvelope`` /
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/// ``PushEnvelopeError/invalidKeyLength`` on every platform; the AES-GCM
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/// open itself requires CryptoKit and throws
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/// ``PushEnvelopeError/cryptoUnavailable`` where that is missing.
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public static func decrypt(envelopeBase64: String, key: Data) throws -> Data {
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guard key.count == keyLength else {
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throw PushEnvelopeError.invalidKeyLength
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}
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guard let envelope = Data(base64Encoded: envelopeBase64),
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envelope.count >= nonceLength + tagLength else {
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throw PushEnvelopeError.malformedEnvelope
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}
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#if canImport(CryptoKit)
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// Data slices keep the parent's indices; work with explicit ranges.
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let nonceBytes = envelope.subdata(in: 0..<nonceLength)
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let ciphertext = envelope.subdata(in: nonceLength..<(envelope.count - tagLength))
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let tag = envelope.subdata(in: (envelope.count - tagLength)..<envelope.count)
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do {
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let box = try AES.GCM.SealedBox(
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nonce: AES.GCM.Nonce(data: nonceBytes),
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ciphertext: ciphertext,
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tag: tag
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)
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return try AES.GCM.open(
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box,
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using: SymmetricKey(data: key),
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authenticating: Data(aad.utf8)
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)
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} catch {
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throw PushEnvelopeError.authenticationFailed
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}
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#else
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throw PushEnvelopeError.cryptoUnavailable
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#endif
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}
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}
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