Two questions regarding Diffie-Hellman key exchange

oliver.hanka at oliver.hanka at
Fri Sep 14 23:11:46 WST 2007


I am currently working on my master-thesis, which involves implementing 
the SSH protocol on a smart-card. Therefore I am using dropbear as a non 
cpu and memory intensiv blueprint.

I am currently stucked with two questions regarding the Diffie-Hellman key 
exchange (SSH_MSG_KEXDH_INIT message). First of all, can you point me to a 
document where the prime number p (128Byte) is defined? Unfortunatly the 
RFC 4253 (SSH Transport Layer) doesn't give a hint.

The next question I am puzzled with: How come the result (e) of the client 
side 'e = g^x mod p' calculation is a 133 Byte value? At least, that's 
what it looks like when I sniff the packet with wireshark (formaly 
ethereal). From my understanding, a modulo calculation with a 128 byte 
value should produce a result equal or less than 128 byte. Am I wrong?
Are there additional bytes added to e, which the RFC 4253 doesn't mention? 
 (the message is described in section 8, RFC 4252, jan 2006)

It would be really nice, if someone could help me out with those 

Thanks in advance and have a nice weekend!

Mit freundlichen Grüssen / Best regards
Oliver Hanka

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