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brontide: exclude MAC length from cipher text packet length prefix

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Pervasively we would include the length of the MAC in the length prefix
for cipher text packets. As a result, the MAC would eat into the total
payload size. To remedy this, we now exclude the MAC from the length
prefix for cipher text packets, and instead account for the length of
the MAC on the packet when reading messages.
This commit is contained in:
Olaoluwa Osuntokun
2017-01-07 19:15:58 -08:00
parent 387d41e5df
commit d046efb502
3 changed files with 18 additions and 17 deletions
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15
brontide/noise.go
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@@ -641,12 +641,13 @@ func (b *BrontideMachine) WriteMessage(w io.Writer, p []byte) error {
// The total length of each message payload including the MAC size
// payload exceed the largest number encodable within a 16-bit unsigned
// integer.
if len(p)+macSize > math.MaxUint16 {
if len(p) > math.MaxUint16 {
return ErrMaxMessageLengthExceeded
}
// The full length of the packet includes the 16 byte MAC.
fullLength := uint16(len(p) + macSize)
// The full length of the packet is only the packet length, and does
// NOT include the MAC.
fullLength := uint16(len(p))
var pktLen [2]byte
binary.BigEndian.PutUint16(pktLen[:], fullLength)
@@ -684,11 +685,11 @@ func (b *BrontideMachine) ReadMessage(r io.Reader) ([]byte, error) {
// Next, using the length read from the packet header, read the
// encrypted packet itself.
pktLen := binary.BigEndian.Uint16(pktLenBytes)
ciperText := make([]byte, pktLen)
if _, err := io.ReadFull(r, ciperText[:]); err != nil {
pktLen := uint32(binary.BigEndian.Uint16(pktLenBytes)) + macSize
cipherText := make([]byte, pktLen)
if _, err := io.ReadFull(r, cipherText[:]); err != nil {
return nil, err
}
return b.recvCipher.Decrypt(nil, nil, ciperText)
return b.recvCipher.Decrypt(nil, nil, cipherText)
}