| 1 | // Checkpoint.swift — native tamper-evidence: verify the instance's signed |
| 2 | // checkpoint chain and gossip verified heads between members inside E2EE |
| 3 | // traffic (docs/tamper-evidence.md, #573 slices 1+2). The Swift twin of |
| 4 | // web/static/integrity.js — the pure verification core is nonisolated static |
| 5 | // so it can be unit-tested with `swift test` and pinned to the SAME wire |
| 6 | // vectors the Go and web sides pin (CheckpointTests + checkpoint_test.go + |
| 7 | // integrity.test.mjs must agree, or the platforms disagree about what a valid |
| 8 | // chain is). |
| 9 | // |
| 10 | // Ed25519 signature verification uses CryptoKit's Curve25519.Signing, over the |
| 11 | // base64 payload STRING bytes exactly as the web does (verify-first, decode |
| 12 | // after). The chain of hashes is what holds history together; the signature is |
| 13 | // attribution. A verification failure must always END as a recorded alert, |
| 14 | // never a thrown error — a monitor that dies quietly on one garbled row is an |
| 15 | // alarm-suppression vector. |
| 16 | |
| 17 | import Foundation |
| 18 | import CryptoKit |
| 19 | |
| 20 | // MARK: - Wire types (match /api/checkpoints and checkpoint.go) |
| 21 | |
| 22 | /// One checkpoint row as served by GET /api/checkpoints. |
| 23 | public struct CheckpointRow: Codable, Sendable, Equatable { |
| 24 | public let id: Int64 |
| 25 | public let seq_from: Int64 |
| 26 | public let seq_to: Int64 |
| 27 | public let root: String |
| 28 | public let chain: String |
| 29 | public let ts: Int64 |
| 30 | public let signed: String // JSON: {payload: base64(JSON CkptPayload), sig: base64} |
| 31 | } |
| 32 | |
| 33 | public struct CheckpointsResponse: Codable, Sendable { |
| 34 | public let checkpoints: [CheckpointRow]? |
| 35 | public let public_key: String? |
| 36 | } |
| 37 | |
| 38 | /// The signed wrapper inside `CheckpointRow.signed` (release.go shape). |
| 39 | private struct SignedWrapper: Codable { let payload: String; let sig: String } |
| 40 | |
| 41 | /// The decoded payload the signature covers. |
| 42 | private struct CkptPayload: Codable { |
| 43 | let seq_from: Int64 |
| 44 | let seq_to: Int64 |
| 45 | let root: String |
| 46 | let prev_chain: String |
| 47 | let chain: String |
| 48 | let ts: Int64 |
| 49 | let pub: String // base64 ed25519 public key that signed this checkpoint |
| 50 | } |
| 51 | |
| 52 | /// A member's gossiped head: `[seqTo, chain]`, encoded as a heterogeneous JSON |
| 53 | /// array to match the web's `_ckpt` exactly. Rides inside the encrypted message |
| 54 | /// payload so the server can neither read nor selectively strip it. |
| 55 | public struct CkptGossip: Codable, Sendable, Equatable { |
| 56 | public let seqTo: Int64 |
| 57 | public let chain: String |
| 58 | public init(seqTo: Int64, chain: String) { self.seqTo = seqTo; self.chain = chain } |
| 59 | public init(from decoder: Decoder) throws { |
| 60 | var c = try decoder.unkeyedContainer() |
| 61 | seqTo = try c.decode(Int64.self) |
| 62 | chain = try c.decode(String.self) |
| 63 | } |
| 64 | public func encode(to encoder: Encoder) throws { |
| 65 | var c = encoder.unkeyedContainer() |
| 66 | try c.encode(seqTo) |
| 67 | try c.encode(chain) |
| 68 | } |
| 69 | } |
| 70 | |
| 71 | /// The verified head of the chain this device has confirmed. |
| 72 | public struct CkptHead: Codable, Sendable, Equatable { |
| 73 | public var id: Int64 |
| 74 | public var seqTo: Int64 |
| 75 | public var chain: String |
| 76 | public var pub: String |
| 77 | public var gossip: CkptGossip { CkptGossip(seqTo: seqTo, chain: chain) } |
| 78 | } |
| 79 | |
| 80 | /// An integrity alert. `kind` strings match the web exactly. |
| 81 | public struct CkptAlert: Codable, Sendable, Equatable { |
| 82 | public var kind: String |
| 83 | public var seqTo: Int64? |
| 84 | public var id: Int64? |
| 85 | public var at: Int64 |
| 86 | /// Investigable but not fatal (a key change) — the walk continues past it. |
| 87 | public var keep: Bool? |
| 88 | var key: String { "\(kind):\(seqTo.map(String.init) ?? id.map(String.init) ?? "")" } |
| 89 | } |
| 90 | |
| 91 | /// The result of verifying one served page of checkpoints. |
| 92 | public struct CkptPageResult: Sendable { |
| 93 | public var head: CkptHead? |
| 94 | public var hist: [CkptGossip] // verified (seqTo, chain) boundary pairs |
| 95 | public var roots: [String] // verified checkpoint roots (inclusion-proof trust) |
| 96 | public var alerts: [CkptAlert] |
| 97 | public var fatal: Bool // a non-`keep` alert stopped the walk |
| 98 | } |
| 99 | |
| 100 | /// One journal event proven to be in a committed checkpoint's Merkle tree |
| 101 | /// (docs/tamper-evidence.md client sampling). The wire shape from |
| 102 | /// GET /api/checkpoint-proof (checkpoint.go inclusionProof). The client |
| 103 | /// recomputes the leaf from the row fields, walks the audit path, and checks |
| 104 | /// the root against a checkpoint it already trusts via the signed chain. |
| 105 | public struct InclusionProof: Codable, Sendable { |
| 106 | public let seq: Int64 |
| 107 | public let ts: Int64 |
| 108 | public let kind: String |
| 109 | public let msg_id: Int64 |
| 110 | public let thread_id: String |
| 111 | public let blob_hash: String |
| 112 | public let checkpoint_id: Int64 |
| 113 | public let root: String |
| 114 | public let leaf_index: Int |
| 115 | public let tree_size: Int |
| 116 | public let audit_path: [String] // hex sibling hashes, bottom-up |
| 117 | } |
| 118 | |
| 119 | public struct CheckpointProofResponse: Codable, Sendable { |
| 120 | public let proofs: [InclusionProof]? |
| 121 | public let committed_to: Int64? |
| 122 | } |
| 123 | |
| 124 | // MARK: - Pure verification core (nonisolated, vector-pinned) |
| 125 | |
| 126 | public enum Checkpoint { |
| 127 | static let chainDomain = "lchat-ckpt-chain\n" |
| 128 | |
| 129 | /// chainHashHex mirrors checkpoint.go's chainHash and integrity.js's |
| 130 | /// chainHashHex — a checkpoint's commitment to its predecessor. `prev` is |
| 131 | /// "" for the genesis checkpoint. This is wire format: it MUST match the Go |
| 132 | /// and web sides byte for byte (pinned by CheckpointTests). |
| 133 | public static func chainHashHex(prev: String, root: String, seqTo: Int64) -> String { |
| 134 | let s = chainDomain + prev + "\n" + root + "\n" + String(seqTo) |
| 135 | let digest = SHA256.hash(data: Data(s.utf8)) |
| 136 | return digest.map { String(format: "%02x", $0) }.joined() |
| 137 | } |
| 138 | |
| 139 | /// ed25519Verify checks one signature over the base64 payload STRING bytes, |
| 140 | /// returning false (never throwing) on any malformed input — a garbled row |
| 141 | /// is a recorded failure, not a crash. |
| 142 | public static func ed25519Verify(pubBase64: String, payload: String, sigBase64: String) -> Bool { |
| 143 | guard let pubData = Data(base64Encoded: pubBase64), |
| 144 | let sigData = Data(base64Encoded: sigBase64), |
| 145 | let key = try? Curve25519.Signing.PublicKey(rawRepresentation: pubData) |
| 146 | else { return false } |
| 147 | return key.isValidSignature(sigData, for: Data(payload.utf8)) |
| 148 | } |
| 149 | |
| 150 | /// verifyPage walks one page of served checkpoints, extending `prior` (the |
| 151 | /// last verified head, or nil on first sync). Each payload names its own |
| 152 | /// signing key, so a mid-chain key change alerts (investigable) but the |
| 153 | /// walk continues; broken hashes/links/signatures stop it. A faithful port |
| 154 | /// of integrity.js verifyCheckpointPage. |
| 155 | public static func verifyPage(_ rows: [CheckpointRow], prior: CkptHead?, checkSigs: Bool) -> CkptPageResult { |
| 156 | var alerts: [CkptAlert] = [] |
| 157 | var hist: [CkptGossip] = [] |
| 158 | var roots: [String] = [] // root is chain-hash-bound below, so trusting it is sound |
| 159 | var head = prior |
| 160 | var pub = prior?.pub |
| 161 | let now = Int64(Date().timeIntervalSince1970) |
| 162 | |
| 163 | for cp in rows { |
| 164 | // Parse: signed wrapper → base64 payload → CkptPayload. Any failure |
| 165 | // is a "malformed" alert, never a throw. |
| 166 | guard let signedData = cp.signed.data(using: .utf8), |
| 167 | let wrapper = try? JSONDecoder().decode(SignedWrapper.self, from: signedData), |
| 168 | let payloadData = Data(base64Encoded: wrapper.payload), |
| 169 | let payload = try? JSONDecoder().decode(CkptPayload.self, from: payloadData), |
| 170 | !payload.pub.isEmpty |
| 171 | else { |
| 172 | alerts.append(CkptAlert(kind: "malformed", seqTo: nil, id: cp.id, at: now)) |
| 173 | break |
| 174 | } |
| 175 | if checkSigs && !ed25519Verify(pubBase64: payload.pub, payload: wrapper.payload, sigBase64: wrapper.sig) { |
| 176 | alerts.append(CkptAlert(kind: "bad-signature", seqTo: nil, id: cp.id, at: now)) |
| 177 | break |
| 178 | } |
| 179 | if chainHashHex(prev: payload.prev_chain, root: payload.root, seqTo: payload.seq_to) != payload.chain { |
| 180 | alerts.append(CkptAlert(kind: "bad-chain-hash", seqTo: nil, id: cp.id, at: now)) |
| 181 | break |
| 182 | } |
| 183 | if payload.prev_chain != (head?.chain ?? "") { |
| 184 | alerts.append(CkptAlert(kind: "broken-link", seqTo: payload.seq_to, id: cp.id, at: now)) |
| 185 | break |
| 186 | } |
| 187 | if let h = head, payload.seq_from != h.seqTo + 1 { |
| 188 | alerts.append(CkptAlert(kind: "gap", seqTo: payload.seq_to, id: cp.id, at: now)) |
| 189 | break |
| 190 | } |
| 191 | if cp.seq_to != payload.seq_to || cp.chain != payload.chain { |
| 192 | alerts.append(CkptAlert(kind: "row-drift", seqTo: nil, id: cp.id, at: now)) |
| 193 | break |
| 194 | } |
| 195 | if let p = pub, payload.pub != p { |
| 196 | // Legitimate-but-investigable (restore from a pre-key backup, |
| 197 | // deliberate rotation) — alert once, keep verifying. |
| 198 | alerts.append(CkptAlert(kind: "key-change", seqTo: payload.seq_to, id: cp.id, at: now, keep: true)) |
| 199 | } |
| 200 | pub = payload.pub |
| 201 | head = CkptHead(id: cp.id, seqTo: payload.seq_to, chain: payload.chain, pub: payload.pub) |
| 202 | hist.append(CkptGossip(seqTo: payload.seq_to, chain: payload.chain)) |
| 203 | roots.append(payload.root) |
| 204 | } |
| 205 | let fatal = alerts.contains { $0.keep != true } |
| 206 | return CkptPageResult(head: head, hist: hist, roots: roots, alerts: alerts, fatal: fatal) |
| 207 | } |
| 208 | |
| 209 | // MARK: - Inclusion proofs (mirror checkpoint.go / integrity.js) |
| 210 | |
| 211 | static let leafDomain = "lchat-ckpt-leaf\n" |
| 212 | static let nodeDomain = "lchat-ckpt-node\n" |
| 213 | |
| 214 | private static func hex(_ d: [UInt8]) -> String { d.map { String(format: "%02x", $0) }.joined() } |
| 215 | private static func fromHex(_ s: String) -> [UInt8]? { |
| 216 | guard s.count % 2 == 0 else { return nil } |
| 217 | var out = [UInt8](); out.reserveCapacity(s.count / 2) |
| 218 | var i = s.startIndex |
| 219 | while i < s.endIndex { |
| 220 | let j = s.index(i, offsetBy: 2) |
| 221 | guard let b = UInt8(s[i..<j], radix: 16) else { return nil } |
| 222 | out.append(b); i = j |
| 223 | } |
| 224 | return out |
| 225 | } |
| 226 | |
| 227 | /// leafHashHex mirrors journalLeafHash — the domain-separated hash of a |
| 228 | /// journal row (the tree's leaf). Field order is wire format (checkpoint.go). |
| 229 | public static func leafHashHex(seq: Int64, ts: Int64, kind: String, msgID: Int64, blobHash: String, threadID: String) -> String { |
| 230 | let enc = leafDomain + String(seq) + "\n" + String(ts) + "\n" + kind + "\n" |
| 231 | + String(msgID) + "\n" + blobHash + "\n" + threadID |
| 232 | return hex(Array(SHA256.hash(data: Data(enc.utf8)))) |
| 233 | } |
| 234 | |
| 235 | private static func nodeHash(_ l: [UInt8], _ r: [UInt8]) -> [UInt8] { |
| 236 | var data = Data(nodeDomain.utf8); data.append(contentsOf: l); data.append(contentsOf: r) |
| 237 | return Array(SHA256.hash(data: data)) |
| 238 | } |
| 239 | |
| 240 | /// rootFromAuditPath recomputes the Merkle root from a leaf (hex), its index |
| 241 | /// m in a tree of n leaves, and its audit path (hex siblings, bottom-up) — |
| 242 | /// the exact mirror of checkpoint.go's rootFromAuditPath. Returns the root |
| 243 | /// hex, or nil on a malformed proof. |
| 244 | public static func rootFromAuditPath(leafHex: String, m: Int, n: Int, pathHex: [String]) -> String? { |
| 245 | guard m >= 0, m < n, let leaf = fromHex(leafHex) else { return nil } |
| 246 | var path: [[UInt8]] = [] |
| 247 | for h in pathHex { guard let b = fromHex(h) else { return nil }; path.append(b) } |
| 248 | func recompute(_ leaf: [UInt8], _ m: Int, _ n: Int, _ path: [[UInt8]]) -> [UInt8]? { |
| 249 | if n == 1 { return path.isEmpty ? leaf : nil } |
| 250 | if path.isEmpty { return nil } |
| 251 | var k = 1 |
| 252 | while k * 2 < n { k *= 2 } |
| 253 | let sibling = path[path.count - 1] |
| 254 | let rest = Array(path[0..<(path.count - 1)]) |
| 255 | if m < k { |
| 256 | guard let left = recompute(leaf, m, k, rest) else { return nil } |
| 257 | return nodeHash(left, sibling) |
| 258 | } |
| 259 | guard let right = recompute(leaf, m - k, n - k, rest) else { return nil } |
| 260 | return nodeHash(sibling, right) |
| 261 | } |
| 262 | guard let root = recompute(leaf, m, n, path) else { return nil } |
| 263 | return hex(root) |
| 264 | } |
| 265 | |
| 266 | /// verifyInclusion checks one proof: recompute the leaf from its row fields, |
| 267 | /// walk the audit path, and confirm it reaches the proof's stated root. The |
| 268 | /// caller separately checks proof.root is a root it TRUSTS (via the signed |
| 269 | /// chain); this only proves the leaf is in that root's tree. Returns |
| 270 | /// (ok, recomputed root). |
| 271 | public static func verifyInclusion(_ p: InclusionProof) -> (ok: Bool, root: String?) { |
| 272 | let leaf = leafHashHex(seq: p.seq, ts: p.ts, kind: p.kind, msgID: p.msg_id, blobHash: p.blob_hash, threadID: p.thread_id) |
| 273 | let root = rootFromAuditPath(leafHex: leaf, m: p.leaf_index, n: p.tree_size, pathHex: p.audit_path) |
| 274 | return (root != nil && root == p.root, root) |
| 275 | } |
| 276 | |
| 277 | /// blobHashHex commits to a stored ciphertext string the way the server does |
| 278 | /// (ciphertextHash = sha256 hex of the blob string), so a client sampling a |
| 279 | /// message can match the journal's blob_hash against what it holds. |
| 280 | public static func blobHashHex(_ blob: String) -> String { |
| 281 | hex(Array(SHA256.hash(data: Data(blob.utf8)))) |
| 282 | } |
| 283 | |
| 284 | /// gossipVerdict compares a peer's gossiped head against our verified head + |
| 285 | /// bounded boundary history. Checkpoint boundaries are global (one |
| 286 | /// checkpoints table per instance), so a peer boundary inside our verified |
| 287 | /// window that we don't have at all is a fork. Returns: "match" | "fork" | |
| 288 | /// "ahead" | "unknown-old" | "no-head". Port of integrity.js gossipVerdict. |
| 289 | public static func gossipVerdict(head: CkptHead?, hist: [CkptGossip], gossip: CkptGossip) -> String { |
| 290 | guard let head else { return "no-head" } |
| 291 | let s = gossip.seqTo |
| 292 | if s > head.seqTo { return "ahead" } |
| 293 | if s == head.seqTo { return gossip.chain == head.chain ? "match" : "fork" } |
| 294 | let floor = hist.first?.seqTo ?? head.seqTo |
| 295 | if s < floor { return "unknown-old" } |
| 296 | guard let ours = hist.first(where: { $0.seqTo == s }) else { return "fork" } |
| 297 | return ours.chain == gossip.chain ? "match" : "fork" |
| 298 | } |
| 299 | } |
| 300 | |
amadan