eleven / messenger Public

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1	import CryptoKit
2	import Foundation
3
4	// 🤖 The home-server backup crypto, byte-compatible with home/web/static/crypto.js
5	// so a blob written by the web (or another device) opens here and vice-versa.
6	// The passkey's PRF output derives a per-credential KEK (HKDF); a random DEK
7	// encrypts the padded blob (AES-GCM); the DEK is wrapped to each enrolled
8	// passkey's KEK. The server only ever stores the opaque envelope.
9	//
10	// Wire formats mirrored EXACTLY (see crypto.js): encryptBytes = iv(12)||ct||tag,
11	// KEK = HKDF-SHA256(prf, salt:∅, info:"eleven-home-kek"), DEK wrap =
12	// b64url(encryptBytes(kek, rawDEK)), plaintext padded to 16 KiB with a 4-byte
13	// big-endian length prefix, envelope = "EHB1"||u32 headerLen||headerJSON||body.
14	public enum HomeCrypto {
15	public static let blobPad = 16384
16
17	// The fixed, public PRF eval input — secrecy lives in the passkey, not here.
18	public static var prfEvalInput: Data { Data("eleven-home-kek-v1".utf8) }
19
20	// ---- primitives (iv(12)||ct||tag, matching WebCrypto AES-GCM) -----------
21
22	static func encryptBytes(key: SymmetricKey, _ plain: Data) throws -> Data {
23	let iv = AES.GCM.Nonce() // 12 random bytes
24	let sealed = try AES.GCM.seal(plain, using: key, nonce: iv)
25	// WebCrypto appends the 16-byte tag to the ciphertext; CryptoKit keeps
26	// them apart. Concatenate to match: iv || ciphertext || tag.
27	return Data(iv) + sealed.ciphertext + sealed.tag
28	}
29
30	static func decryptBytes(key: SymmetricKey, _ combined: Data) throws -> Data {
31	let iv = combined.prefix(12)
32	let rest = combined.dropFirst(12)
33	let ct = rest.dropLast(16)
34	let tag = rest.suffix(16)
35	let box = try AES.GCM.SealedBox(nonce: AES.GCM.Nonce(data: iv),
36	ciphertext: ct, tag: tag)
37	return try AES.GCM.open(box, using: key)
38	}
39
40	// ---- PRF → KEK ----------------------------------------------------------
41
42	public static func kekFromPRF(_ prfOutput: Data) -> SymmetricKey {
43	HKDF<SHA256>.deriveKey(
44	inputKeyMaterial: SymmetricKey(data: prfOutput),
45	salt: Data(),
46	info: Data("eleven-home-kek".utf8),
47	outputByteCount: 32)
48	}
49
50	// ---- DEK wrap / unwrap --------------------------------------------------
51
52	public static func wrapDEK(_ dek: SymmetricKey, kek: SymmetricKey) throws -> String {
53	let raw = dek.withUnsafeBytes { Data($0) }
54	return try encryptBytes(key: kek, raw).base64urlEncodedString()
55	}
56
57	public static func unwrapDEK(_ wrappedB64: String, kek: SymmetricKey) throws -> SymmetricKey {
58	guard let wrapped = Data(base64urlEncoded: wrappedB64) else { throw HomeCryptoError.badBlob }
59	return SymmetricKey(data: try decryptBytes(key: kek, wrapped))
60	}
61
62	// ---- fixed-size padding (length leaks nothing) --------------------------
63
64	static func pad(_ bytes: Data) throws -> Data {
65	guard bytes.count <= blobPad - 4 else { throw HomeCryptoError.tooLarge }
66	var out = Data(count: blobPad)
67	let len = UInt32(bytes.count).bigEndian
68	withUnsafeBytes(of: len) { out.replaceSubrange(0..<4, with: $0) }
69	out.replaceSubrange(4..<(4 + bytes.count), with: bytes)
70	return out
71	}
72
73	static func unpad(_ bytes: Data) throws -> Data {
74	guard bytes.count >= 4 else { throw HomeCryptoError.badBlob }
75	let len = bytes.prefix(4).reduce(UInt32(0)) { ($0 << 8) | UInt32($1) }
76	let start = 4, end = 4 + Int(len)
77	guard end <= bytes.count else { throw HomeCryptoError.badBlob }
78	return bytes.subdata(in: start..<end)
79	}
80
81	// ---- the envelope: "EHB1" || u32 headerLen || header || iv||ct||tag ------
82
83	private static let magic = Data("EHB1".utf8)
84
85	/// Split an envelope into (wrapped-key header, encrypted body) without
86	/// decrypting — to find our wrapped DEK and to re-seal preserving others'.
87	public static func unpack(_ bytes: Data) throws -> (keys: [String: String], body: Data) {
88	guard bytes.count >= 8, bytes.prefix(4) == magic else { throw HomeCryptoError.badBlob }
89	let headerLen = bytes.subdata(in: 4..<8).reduce(UInt32(0)) { ($0 << 8) | UInt32($1) }
90	let headerEnd = 8 + Int(headerLen)
91	guard headerEnd <= bytes.count else { throw HomeCryptoError.badBlob }
92	let headerData = bytes.subdata(in: 8..<headerEnd)
93	struct Header: Decodable { let keys: [String: String]? }
94	let header = try JSONDecoder().decode(Header.self, from: headerData)
95	return (header.keys ?? [:], bytes.subdata(in: headerEnd..<bytes.count))
96	}
97
98	/// Decrypt an envelope's body with an already-unwrapped DEK → the JSON obj.
99	public static func openBlob(_ bytes: Data, dek: SymmetricKey) throws -> (obj: Data, keys: [String: String]) {
100	let (keys, body) = try unpack(bytes)
101	let obj = try unpad(decryptBytes(key: dek, body))
102	return (obj, keys)
103	}
104
105	/// Seal a JSON obj (raw bytes) under the DEK, packed with the wrapped-key
106	/// header. `keys` is [credentialId: wrappedDEK].
107	public static func sealBlob(objJSON: Data, dek: SymmetricKey, keys: [String: String]) throws -> Data {
108	let body = try encryptBytes(key: dek, pad(objJSON))
109	let header = try JSONSerialization.data(withJSONObject: ["keys": keys],
110	options: [.sortedKeys])
111	var out = magic
112	var headerLen = UInt32(header.count).bigEndian
113	out.append(Data(bytes: &headerLen, count: 4))
114	out.append(header)
115	out.append(body)
116	return out
117	}
118
119	public static func generateDEK() -> SymmetricKey { SymmetricKey(size: .bits256) }
120	}
121
122	public enum HomeCryptoError: Error { case badBlob, tooLarge }
123