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const std = @import("std");const zat = @import("zat");
const P256 = std.crypto.ecc.P256;const HkdfSha256 = std.crypto.kdf.hkdf.HkdfSha256;const HmacSha256 = std.crypto.auth.hmac.sha2.HmacSha256;
pub const want_mac_header = "atproto-space-want-mac";pub const mac_header = "atproto-space-mac";pub const commit_version = 1;
pub const MacType = enum { commit, record,
pub fn name(self: MacType) []const u8 { return switch (self) { .commit => "atproto-space-repo", .record => "atproto-space-record", }; }
// Commits share their MAC version with the commit format. pub fn version(self: MacType) u8 { return switch (self) { .commit => commit_version, .record => 1, }; }};
pub const Commit = struct { ver: u8 = commit_version, repo: []const u8, space: []const u8, rev: []const u8, data: [32]u8,};
pub const Error = error{ MissingMacHeader, MalformedMacHeader, UnsupportedMacType, UnsupportedMacVersion, MissingKeyShare, InvalidKemKey,};
// ---- KEM ----
/// Repo host side: derive the shared secret for a syncer's `key_share` with the/// author's KEM secret key.pub fn decapsulate(author_secret: [32]u8, key_share: []const u8) Error![32]u8 { var buf: [512]u8 = undefined; var fba = std.heap.FixedBufferAllocator.init(&buf); const syncer = try p256KeyPoint(fba.allocator(), key_share); const author = P256.basePoint.mul(author_secret, .big) catch return error.InvalidKemKey; const shared = syncer.mul(author_secret, .big) catch return error.InvalidKemKey; const dh = shared.affineCoordinates().x.toBytes(.big); return deriveSharedSecret(&dh, &syncer.toUncompressedSec1(), &author.toUncompressedSec1());}
/// Syncer side, for tests: a fresh ephemeral key share against an author key.pub fn encapsulate(io: std.Io, allocator: std.mem.Allocator, author_kem_key: []const u8) !struct { key_share: []const u8, shared_secret: [32]u8 } { const author = try p256KeyPoint(allocator, author_kem_key); const ephemeral = std.crypto.sign.ecdsa.EcdsaP256Sha256.KeyPair.generate(io); const secret = ephemeral.secret_key.toBytes(); const dh = (author.mul(secret, .big) catch return error.InvalidKemKey).affineCoordinates().x.toBytes(.big); const ephemeral_point = ephemeral.public_key.p; return .{ .key_share = try zat.multicodec.formatDidKey(allocator, .p256, &ephemeral_point.toCompressedSec1()), .shared_secret = deriveSharedSecret(&dh, &ephemeral_point.toUncompressedSec1(), &author.toUncompressedSec1()), };}
// "HPKE-v1" || "KEM" || u16be(0x0010), the suite ID for DHKEM(P-256, HKDF-SHA256).const kem_prefix = "HPKE-v1" ++ "KEM" ++ "\x00\x10";
fn deriveSharedSecret(dh: *const [32]u8, syncer_key: *const [65]u8, author_key: *const [65]u8) [32]u8 { var extract = HkdfSha256.extractInit(""); extract.update(kem_prefix ++ "eae_prk"); extract.update(dh); var prk: [HkdfSha256.prk_length]u8 = undefined; extract.final(&prk);
const info = "\x00\x20" ++ kem_prefix ++ "shared_secret"; var info_buf: [info.len + 65 + 65]u8 = undefined; @memcpy(info_buf[0..info.len], info); @memcpy(info_buf[info.len..][0..65], syncer_key); @memcpy(info_buf[info.len + 65 ..][0..65], author_key); var out: [32]u8 = undefined; HkdfSha256.expand(&out, &info_buf, prk); return out;}
fn p256KeyPoint(allocator: std.mem.Allocator, did_key: []const u8) Error!P256 { const parsed = zat.multicodec.parseDidKey(allocator, did_key) catch return error.InvalidKemKey; defer allocator.free(parsed.raw); if (parsed.key_type != .p256) return error.InvalidKemKey; return P256.fromSec1(parsed.raw) catch error.InvalidKemKey;}
// ---- MACs ----
/// HMAC-SHA256(secret, "atproto-space-repo-v1" || lp(space) || lp(repo) || lp(rev) || data)pub fn computeCommitMac(shared_secret: *const [32]u8, commit: Commit) [32]u8 { var hmac = HmacSha256.init(shared_secret); hmac.update("atproto-space-repo-v1"); updateLengthPrefixed(&hmac, commit.space); updateLengthPrefixed(&hmac, commit.repo); updateLengthPrefixed(&hmac, commit.rev); hmac.update(&commit.data); return hmac.finalResult();}
pub fn verifyCommitMac(shared_secret: *const [32]u8, commit: Commit, mac: []const u8) bool { if (mac.len != 32) return false; return std.crypto.timing_safe.eql([32]u8, computeCommitMac(shared_secret, commit), mac[0..32].*);}
/// HMAC-SHA256(secret, "atproto-space-record-v1" || lp(uri) || cid_bytes)pub fn computeRecordMac(shared_secret: *const [32]u8, uri: []const u8, cid_bytes: []const u8) [32]u8 { var hmac = HmacSha256.init(shared_secret); hmac.update("atproto-space-record-v1"); updateLengthPrefixed(&hmac, uri); hmac.update(cid_bytes); return hmac.finalResult();}
fn updateLengthPrefixed(hmac: *HmacSha256, value: []const u8) void { var len: [2]u8 = undefined; std.mem.writeInt(u16, &len, @intCast(value.len), .big); hmac.update(&len); hmac.update(value);}
// ---- Headers ----
/// `type="atproto-space-repo", ver=1, mac=:<padded base64>:`pub fn formatMacHeader(allocator: std.mem.Allocator, mac_type: MacType, mac: *const [32]u8) ![]const u8 { var encoded: [std.base64.standard.Encoder.calcSize(32)]u8 = undefined; _ = std.base64.standard.Encoder.encode(&encoded, mac); return std.fmt.allocPrint(allocator, "type=\"{s}\", ver={d}, mac=:{s}:", .{ mac_type.name(), mac_type.version(), encoded });}
/// `type="atproto-space-repo", ver=1, key_share="did:key:..."`pub fn formatWantMacHeader(allocator: std.mem.Allocator, mac_type: MacType, key_share: []const u8) ![]const u8 { return std.fmt.allocPrint(allocator, "type=\"{s}\", ver={d}, key_share=\"{s}\"", .{ mac_type.name(), mac_type.version(), key_share });}
/// Parse a request header and return its `key_share`.pub fn parseWantMacHeader(allocator: std.mem.Allocator, value: ?[]const u8, mac_type: MacType) Error![]const u8 { const fields = try parseMacDictionary(allocator, value, mac_type); return switch (fields.key_share orelse return error.MissingKeyShare) { .string => |key_share| key_share, else => error.MissingKeyShare, };}
/// Parse a response header and return its MAC.pub fn parseMacHeader(allocator: std.mem.Allocator, value: ?[]const u8, mac_type: MacType) Error![]const u8 { const fields = try parseMacDictionary(allocator, value, mac_type); return switch (fields.mac orelse return error.MalformedMacHeader) { .bytes => |mac| mac, else => error.MalformedMacHeader, };}
const MacFields = struct { key_share: ?Item = null, mac: ?Item = null,};
fn parseMacDictionary(allocator: std.mem.Allocator, value: ?[]const u8, mac_type: MacType) Error!MacFields { var parser: DictionaryParser = .{ .input = value orelse return error.MissingMacHeader, .allocator = allocator }; var type_item: ?Item = null; var ver_item: ?Item = null; var fields: MacFields = .{}; while (try parser.next()) |member| { const slot: *?Item = if (std.mem.eql(u8, member.key, "type")) &type_item else if (std.mem.eql(u8, member.key, "ver")) &ver_item else if (std.mem.eql(u8, member.key, "key_share")) &fields.key_share else if (std.mem.eql(u8, member.key, "mac")) &fields.mac else continue; slot.* = member.item; } const type_ok = if (type_item) |item| item == .string and std.mem.eql(u8, item.string, mac_type.name()) else false; if (!type_ok) return error.UnsupportedMacType; const ver_ok = if (ver_item) |item| switch (item) { .integer => |ver| ver == mac_type.version(), .decimal => |ver| ver == @as(f64, @floatFromInt(mac_type.version())), else => false, } else false; if (!ver_ok) return error.UnsupportedMacVersion; return fields;}
const Item = union(enum) { integer: i64, decimal: f64, string: []const u8, token: []const u8, bytes: []const u8, boolean: bool, inner_list,};
/// RFC 9651 dictionary parsing, sufficient to read the bare items these headers/// carry. Parameters are parsed and dropped; a later duplicate key wins.const DictionaryParser = struct { input: []const u8, allocator: std.mem.Allocator, pos: usize = 0, started: bool = false,
fn next(self: *DictionaryParser) Error!?struct { key: []const u8, item: Item } { if (!self.started) { self.started = true; self.skipSp(); if (self.pos == self.input.len) return error.MalformedMacHeader; } else { self.skipOws(); if (self.pos == self.input.len) return null; if (self.input[self.pos] != ',') return error.MalformedMacHeader; self.pos += 1; self.skipOws(); if (self.pos == self.input.len) return error.MalformedMacHeader; } const key = try self.parseKey(); var item: Item = .{ .boolean = true }; if (self.peek() == '=') { self.pos += 1; item = if (self.peek() == '(') try self.parseInnerList() else try self.parseBareItem(); } try self.parseParameters(); return .{ .key = key, .item = item }; }
fn peek(self: *const DictionaryParser) ?u8 { return if (self.pos < self.input.len) self.input[self.pos] else null; }
fn skipSp(self: *DictionaryParser) void { while (self.peek() == ' ') self.pos += 1; }
fn skipOws(self: *DictionaryParser) void { while (self.peek()) |c| { if (c != ' ' and c != '\t') break; self.pos += 1; } }
fn parseKey(self: *DictionaryParser) Error![]const u8 { const start = self.pos; const first = self.peek() orelse return error.MalformedMacHeader; if (!std.ascii.isLower(first) and first != '*') return error.MalformedMacHeader; self.pos += 1; while (self.peek()) |c| { if (!(std.ascii.isLower(c) or std.ascii.isDigit(c) or c == '_' or c == '-' or c == '.' or c == '*')) break; self.pos += 1; } return self.input[start..self.pos]; }
fn parseParameters(self: *DictionaryParser) Error!void { while (self.peek() == ';') { self.pos += 1; self.skipSp(); _ = try self.parseKey(); if (self.peek() == '=') { self.pos += 1; _ = try self.parseBareItem(); } } }
fn parseInnerList(self: *DictionaryParser) Error!Item { self.pos += 1; while (true) { self.skipSp(); if (self.peek() == ')') { self.pos += 1; try self.parseParameters(); return .inner_list; } _ = try self.parseBareItem(); try self.parseParameters(); const c = self.peek() orelse return error.MalformedMacHeader; if (c != ' ' and c != ')') return error.MalformedMacHeader; } }
fn parseBareItem(self: *DictionaryParser) Error!Item { const c = self.peek() orelse return error.MalformedMacHeader; return switch (c) { '-', '0'...'9' => self.parseNumber(), '"' => self.parseString(), ':' => self.parseBytes(), '?' => self.parseBoolean(), '*', 'a'...'z', 'A'...'Z' => self.parseToken(), else => error.MalformedMacHeader, }; }
fn parseNumber(self: *DictionaryParser) Error!Item { const start = self.pos; if (self.peek() == '-') self.pos += 1; const digits_start = self.pos; var dot: ?usize = null; while (self.peek()) |c| { if (std.ascii.isDigit(c)) { self.pos += 1; } else if (c == '.' and dot == null) { if (self.pos - digits_start > 12) return error.MalformedMacHeader; dot = self.pos; self.pos += 1; } else break; } const text = self.input[start..self.pos]; if (self.pos == digits_start) return error.MalformedMacHeader; if (dot) |dot_pos| { const fraction = self.pos - dot_pos - 1; if (fraction == 0 or fraction > 3) return error.MalformedMacHeader; return .{ .decimal = std.fmt.parseFloat(f64, text) catch return error.MalformedMacHeader }; } if (self.pos - digits_start > 15) return error.MalformedMacHeader; return .{ .integer = std.fmt.parseInt(i64, text, 10) catch return error.MalformedMacHeader }; }
fn parseString(self: *DictionaryParser) Error!Item { self.pos += 1; var out: std.ArrayList(u8) = .empty; while (self.peek()) |c| { self.pos += 1; switch (c) { '"' => return .{ .string = out.items }, '\\' => { const escaped = self.peek() orelse return error.MalformedMacHeader; if (escaped != '"' and escaped != '\\') return error.MalformedMacHeader; self.pos += 1; out.append(self.allocator, escaped) catch return error.MalformedMacHeader; }, 0x20...0x21, 0x23...0x5b, 0x5d...0x7e => out.append(self.allocator, c) catch return error.MalformedMacHeader, else => return error.MalformedMacHeader, } } return error.MalformedMacHeader; }
fn parseBytes(self: *DictionaryParser) Error!Item { self.pos += 1; const end = std.mem.indexOfScalarPos(u8, self.input, self.pos, ':') orelse return error.MalformedMacHeader; const encoded = self.input[self.pos..end]; self.pos = end + 1; for (encoded) |c| { if (!(std.ascii.isAlphanumeric(c) or c == '+' or c == '/' or c == '=')) return error.MalformedMacHeader; } const decoder = std.base64.standard.Decoder; const len = decoder.calcSizeForSlice(encoded) catch return error.MalformedMacHeader; const decoded = self.allocator.alloc(u8, len) catch return error.MalformedMacHeader; decoder.decode(decoded, encoded) catch return error.MalformedMacHeader; return .{ .bytes = decoded }; }
fn parseBoolean(self: *DictionaryParser) Error!Item { self.pos += 1; const c = self.peek() orelse return error.MalformedMacHeader; self.pos += 1; return switch (c) { '0' => .{ .boolean = false }, '1' => .{ .boolean = true }, else => error.MalformedMacHeader, }; }
fn parseToken(self: *DictionaryParser) Error!Item { const start = self.pos; self.pos += 1; while (self.peek()) |c| { const tchar = std.ascii.isAlphanumeric(c) or std.mem.indexOfScalar(u8, "!#$%&'*+-.^_`|~:/", c) != null; if (!tchar) break; self.pos += 1; } return .{ .token = self.input[start..self.pos] }; }};
fn hexBytes(comptime hex: []const u8) [hex.len / 2]u8 { var out: [hex.len / 2]u8 = undefined; _ = std.fmt.hexToBytes(&out, hex) catch unreachable; return out;}
test "KEM matches the RFC 9180 A.3.1 vector" { var arena = std.heap.ArenaAllocator.init(std.testing.allocator); defer arena.deinit(); const allocator = arena.allocator(); // pkEm is the syncer's key share and skRm the author's KEM key. const pk_em = hexBytes("04a92719c6195d5085104f469a8b9814d5838ff72b60501e2c4466e5e67b325ac98536d7b61a1af4b78e5b7f951c0900be863c403ce65c9bfcb9382657222d18c4"); const sk_rm = hexBytes("f3ce7fdae57e1a310d87f1ebbde6f328be0a99cdbcadf4d6589cf29de4b8ffd2"); const point = try P256.fromSec1(&pk_em); const key_share = try zat.multicodec.formatDidKey(allocator, .p256, &point.toCompressedSec1()); const shared = try decapsulate(sk_rm, key_share); try std.testing.expectEqualStrings( "c0d26aeab536609a572b07695d933b589dcf363ff9d93c93adea537aeabb8cb8", &std.fmt.bytesToHex(shared, .lower), );}
test "KEM matches the reference implementation and round trips" { var arena = std.heap.ArenaAllocator.init(std.testing.allocator); defer arena.deinit(); const allocator = arena.allocator(); // @atproto/space 0.0.0-spaces-alpha-20261022141228 decapsulate(P256Keypair.import(0x31 * 32), did of 0x32 * 32). const author_secret: [32]u8 = @splat(0x31); const shared = try decapsulate(author_secret, "did:key:zDnaemu5viSrXFCjxg3KigUcZ57xLDhgQBHMCJTZtatnsZjM6"); try std.testing.expectEqualStrings( "629cf3ba48dc1db7eeff7e9af7d1fe838f62d8152e59a142c7f46791f361bcc6", &std.fmt.bytesToHex(shared, .lower), );
const author = try zat.Keypair.fromSecretKey(.p256, author_secret); const encapsulated = try encapsulate(std.testing.io, allocator, try author.did(allocator)); try std.testing.expectEqualSlices(u8, &encapsulated.shared_secret, &try decapsulate(author_secret, encapsulated.key_share)); const other: [32]u8 = @splat(0x33); try std.testing.expect(!std.mem.eql(u8, &encapsulated.shared_secret, &try decapsulate(other, encapsulated.key_share)));}
test "KEM rejects keys that are not P-256" { var arena = std.heap.ArenaAllocator.init(std.testing.allocator); defer arena.deinit(); const k256 = try zat.Keypair.fromSecretKey(.secp256k1, @splat(0x31)); try std.testing.expectError(error.InvalidKemKey, decapsulate(@splat(0x31), try k256.did(arena.allocator()))); try std.testing.expectError(error.InvalidKemKey, decapsulate(@splat(0x31), "did:key:nope"));}
test "commit and record MACs match the reference implementation" { const secret: [32]u8 = @splat(0x42); // Commit for the space-reference-repo fixture, MAC'd by @atproto/space computeCommitMac. const commit: Commit = .{ .repo = "did:plc:refuser", .space = "at://did:plc:refauthority/space/com.example.forum/general", .rev = "3mstarrefrevaa", .data = hexBytes("869574498510cb637faf8de78a9557ea1223e0263ef37d70a07deec680e55c50"), }; const mac = computeCommitMac(&secret, commit); try std.testing.expectEqualStrings("9f90b6801163a1b25654eab6c1781938a55a96f4355b65cc749dd6a52f3537ee", &std.fmt.bytesToHex(mac, .lower)); try std.testing.expect(verifyCommitMac(&secret, commit, &mac)); var tampered = commit; tampered.data[0] ^= 1; try std.testing.expect(!verifyCommitMac(&secret, tampered, &mac));
var arena = std.heap.ArenaAllocator.init(std.testing.allocator); defer arena.deinit(); const cid = try zat.cbor.Cid.fromString(arena.allocator(), "bafyreihtk4vh7vfupnk2ad5q2yceepsqftci5vija3tsrtuuga7shg56eu"); const record_mac = computeRecordMac( &secret, "at://did:plc:refauthority/space/com.example.forum/general/did:plc:refuser/com.example.post/aaa", cid.raw, ); try std.testing.expectEqualStrings("f775baac9f49eb70ce2836c41d85ecb1e1361bbe05f22d3438ae5d00109f0ad4", &std.fmt.bytesToHex(record_mac, .lower));}
test "commit MAC length-prefixes each field" { const secret: [32]u8 = @splat(7); const a: Commit = .{ .repo = "did:example:alice", .space = "s", .rev = "3kbc", .data = @splat(1) }; const b: Commit = .{ .repo = "did:example:alice3", .space = "s", .rev = "kbc", .data = @splat(1) }; try std.testing.expect(!std.mem.eql(u8, &computeCommitMac(&secret, a), &computeCommitMac(&secret, b)));}
test "MAC headers format and parse like the reference implementation" { var arena = std.heap.ArenaAllocator.init(std.testing.allocator); defer arena.deinit(); const allocator = arena.allocator(); var mac: [32]u8 = undefined; for (&mac, 0..) |*byte, idx| byte.* = @intCast(idx); const header = try formatMacHeader(allocator, .commit, &mac); try std.testing.expectEqualStrings("type=\"atproto-space-repo\", ver=1, mac=:AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8=:", header); try std.testing.expectEqualSlices(u8, &mac, try parseMacHeader(allocator, header, .commit));
const want = try formatWantMacHeader(allocator, .commit, "did:key:zDnaExample"); try std.testing.expectEqualStrings("type=\"atproto-space-repo\", ver=1, key_share=\"did:key:zDnaExample\"", want); try std.testing.expectEqualStrings("did:key:zDnaExample", try parseWantMacHeader(allocator, want, .commit)); try std.testing.expectEqualStrings("did:key:zDnaX", try parseWantMacHeader(allocator, "key_share=\"did:key:zDnaX\";p=1,ver=1, type=\"atproto-space-record\", extra=(1 2)", .record));}
test "want-mac headers reject missing, malformed, and mismatched values" { var arena = std.heap.ArenaAllocator.init(std.testing.allocator); defer arena.deinit(); const allocator = arena.allocator(); try std.testing.expectError(error.MissingMacHeader, parseWantMacHeader(allocator, null, .commit)); try std.testing.expectError(error.MalformedMacHeader, parseWantMacHeader(allocator, "???", .commit)); try std.testing.expectError(error.MalformedMacHeader, parseWantMacHeader(allocator, "", .commit)); try std.testing.expectError(error.MalformedMacHeader, parseWantMacHeader(allocator, "type=\"atproto-space-repo\", ver=1,", .commit)); try std.testing.expectError(error.MissingKeyShare, parseWantMacHeader(allocator, "type=\"atproto-space-repo\", ver=1", .commit)); try std.testing.expectError(error.MissingKeyShare, parseWantMacHeader(allocator, "type=\"atproto-space-repo\", ver=1, key_share=did", .commit)); try std.testing.expectError(error.UnsupportedMacType, parseWantMacHeader(allocator, "type=atproto-space-repo, ver=1, key_share=\"x\"", .commit)); try std.testing.expectError(error.UnsupportedMacType, parseWantMacHeader(allocator, "type=\"atproto-space-repo\", ver=1, key_share=\"x\"", .record)); try std.testing.expectError(error.UnsupportedMacVersion, parseWantMacHeader(allocator, "type=\"atproto-space-repo\", ver=2, key_share=\"x\"", .commit)); try std.testing.expectError(error.UnsupportedMacVersion, parseWantMacHeader(allocator, "type=\"atproto-space-repo\", key_share=\"x\"", .commit)); try std.testing.expectError(error.MalformedMacHeader, parseMacHeader(allocator, "type=\"atproto-space-repo\", ver=1, mac=\"abc\"", .commit));}