Ensure that second bit of the header is always 1

quic/packets.nim

@@ -6,11 +6,18 @@ type
     headerShort
     headerLong
   PacketHeader* = object
-    bytes*: seq[byte]
+    bytes: seq[byte]
   PacketNumber* = range[0'u64..2'u64^62-1]
 
+proc newPacketHeader*(bytes: seq[byte]): PacketHeader =
+  assert bytes[0].bits[1] == 1
+  PacketHeader(bytes: bytes)
+
 proc kind*(header: PacketHeader): PacketHeaderKind =
   PacketHeaderKind(header.bytes[0].bits[0])
 
 proc `kind=`*(header: var PacketHeader, kind: PacketHeaderKind) =
   header.bytes[0].bits[0] = Bit(kind)
+
+proc bytes*(header: PacketHeader): seq[byte] =
+  header.bytes

tests/quic/testPackets.nim

@@ -6,14 +6,18 @@ import quic/bits
 suite "packets":
 
   test "first bit of the header indicates its kind":
-    check PacketHeader(bytes: @[0b00000000'u8]).kind == headerShort
-    check PacketHeader(bytes: @[0b10000000'u8]).kind == headerLong
+    check newPacketHeader(@[0b01000000'u8]).kind == headerShort
+    check newPacketHeader(@[0b11000000'u8]).kind == headerLong
 
   test "header kind can be set":
-    var header = PacketHeader(bytes: @[0b00000000'u8])
+    var header = newPacketHeader(@[0b01000000'u8])
     header.kind = headerLong
     check header.bytes[0].bits[0] == 1
 
+  test "second bit of the header should always be 1":
+    expect Exception:
+      discard newPacketHeader(@[0b00000000'u8])
+
   test "packet numbers are in the range 0 to 2^62-1":
     check PacketNumber.low == 0
     check PacketNumber.high == 2'u64 ^ 62 - 1