24 Byte Crypto

decoder/build.zig

@@ -18,6 +18,7 @@ pub fn build(b: *std.Build) !void {
         .target = target,
         .name = "main",
         .optimize = .ReleaseSafe,
+        .link_libc = true,
     });
 
     const unit_tests = b.addTest(.{

decoder/lib/crypto.zig

@@ -1,8 +1,8 @@
 const std = @import("std");
 
-pub fn decrypt(data: []u8, key: [16]u8) void {
+pub fn decrypt(data: []u8, key: [24]u8) void {
     var extended_key: [32]u8 = undefined;
-    @memcpy(extended_key[0..16], key[0..16]);
-    @memcpy(extended_key[16..32], key[0..16]);
+    @memcpy(extended_key[0..24], key[0..24]);
+    @memcpy(extended_key[24..32], key[0..8]);
     std.crypto.stream.salsa.Salsa20.xor(data, data, 0, extended_key, std.mem.zeroes([8]u8));
 }

decoder/subscribe.zig

@@ -21,6 +21,7 @@ pub fn execute(body: []u8) !void {
     const header: *const SubscribeHeader = @ptrCast(body.ptr);
     const channel_index = header.channel - 1;
 
+    if (root.subscriptions[channel_index]) |*subscription| subscription.deinit();
     root.subscriptions[channel_index] = lib.Subscription.init(header.start, header.end, body[@sizeOf(SubscribeHeader)..]);
 
     try flash.saveSubscriptions(@truncate(channel_index));

design/ectf25_design/encoder.py

@@ -24,7 +24,7 @@
 
 
 def hash(data: bytes):
-    return blake3(data).digest(length=16)
+    return blake3(data).digest(length=24)
 
 
 class Encoder:
@@ -74,7 +74,7 @@ def encode(self, channel: int, frame: bytes, timestamp: int) -> bytes:
                 else:
                     curr = hash(curr + RIGHT_SALT)
 
-            encrypted_frame = Salsa20.new(key=curr+curr, nonce=bytes([0 for _ in range(8)])).encrypt(frame)
+            encrypted_frame = Salsa20.new(key=curr+curr[:8], nonce=bytes([0 for _ in range(8)])).encrypt(frame)
 
             message = struct.pack("<IQ", channel, timestamp) + encrypted_frame
 

design/ectf25_design/gen_secrets.py

@@ -36,7 +36,7 @@ def gen_secrets(channels: list[int]) -> bytes:
     signer = eddsa.new(keypair, "rfc8032")
 
     secrets = {
-        "seeds": {str(channel): os.urandom(16).hex() for channel in channels},
+        "seeds": {str(channel): os.urandom(24).hex() for channel in channels},
         "subscription_salt": os.urandom(32).hex(),
         "public_key": keypair.public_key().export_key(format="raw").hex(),
         "private_key": keypair.export_key(format="PEM"),

design/ectf25_design/gen_subscription.py

@@ -25,7 +25,7 @@
 
 
 def hash(data: bytes):
-    return blake3(data).digest(length=16)
+    return blake3(data).digest(length=24)
 
 
 @dataclass(frozen=True)
@@ -62,11 +62,9 @@ def gen_subscription(
                 curr = hash(curr + LEFT_SALT)
             else:
                 curr = hash(curr + RIGHT_SALT)
-        print(root, curr.hex())
         hashes.append(curr)
 
     subscription_salt = bytes.fromhex(secrets["subscription_salt"])
-    print(f"{subscription_salt.hex()}{device_id:08x}")
     key = blake3(f"{subscription_salt.hex()}{device_id:08x}".encode()).digest()
 
     # Pack the subscription. This will be sent to the decoder with ectf25.tv.subscribe

docs/design.typ

@@ -261,8 +261,8 @@ This aspect of the design secures the system in the following ways:
       [Start Timestamp], [8 bytes], bytesize(8),
       [End Timestamp], [8 bytes], bytesize(8),
       [Packed Subtree Root Hashes],
-      ${16n "bytes" | n in NN, 1 <= n <= 126 }$,
-      box(box(bytesize(16)) + " " + box($...n "times"$)),
+      ${24n "bytes" | n in NN^+, n <= 126 }$,
+      box(box(bytesize(24)) + " " + box($...n "times"$)),
     ),
   ),
   caption: [Structure of a subscribe packet],
@@ -420,7 +420,7 @@ def get_nodes(a: int, b: int) -> list[Node]:
     while a <= b:
         power = min((b - a + 1).bit_length() - 1, ctz(a))
         ranges.append(Node(offset=a, power=power))
-        a += 1 << power
+        a += 2 ** power
 
     return ranges
 ```