silc packet protocol questions

[Pekka Riikonen]

On Thu, 4 May 2006, mailmaus at lewser.de wrote:

: in "draft-riikonen-silc-pp-08.txt": "2.1 SILC Packet" you say that "the
: padding is always encrypted". by that you mean it is always included in
: the encryption process in case enccyption takes place? i ask because we
:
Yes.

: the padding has at least a length of 1 byte. this means, if there is no
: half-empty block, we add a whole block even if we don't have to?
: 
Actually it's a mistake in the specs, the minimum padding length is 8 
bytes.  There are security reasons for adding padding even when "it's not 
needed".  One good example is for example authentication packets 
(passwords) in which case by adding maximum lenght of padding to the 
packet it makes it difficult to approximate the length of the password 
from the encrypted packet and thus prevents simple attacks against the 
password.

	Pekka
: 
: Pekka Riikonen schrieb:
: > : what is the use of padding in a packet? so far i found out it has
: > : something to do with encryption.
: > : why are packets, that are not encrypted (like the key exchange start
: > : payload) also using it?
: > : 
: > The padding in encrypted packets is used so that the packet length would 
: > be multiple by the block length of the cipher, so that the last block to 
: > be encrypted would not have missing bits.
: >
: > The padding is added to non-ecnrypted packets also for consistency sake; 
: > it's not dependent of the packet type but is part of SILC Packet in 
: > general.  Some old version of SILC protocol used to allow zero length 
: > padding also, but it was removed.  There are only few non-enrypted packets 
: > in SILC protocol, and when performing rekey also all SKE packets are 
: > encrypted.
: >
: > 	Pekka
: > ________________________________________________________________________
: >  Pekka Riikonen                                 priikone at silcnet.org
: >  Secure Internet Live Conferencing (SILC)       http://silcnet.org/
: >   
: