[TLS] About the key block generation for different ciphers.

nilesh <nilesh.tayade@netscout.com> Tue, 04 October 2011 11:35 UTC

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Date: Tue, 04 Oct 2011 17:08:37 +0530
From: nilesh <nilesh.tayade@netscout.com>
Organization: NetScout
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Subject: [TLS] About the key block generation for different ciphers.
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Hi,

In case of SSL/TLS we generate the master_secret and then the key_block.
The size of the key block depends upon the cipher suit that is negotiated.

Do we have any reference where I can understand the fields in key_block?

E.g. I could identify that
For AES_256_CBC_SHA we need - 2 * 32 bytes of keys and 2 * 20 bytes of MAC.
For 3DES_EDE_CBC_SHA we need - 2 * 24bytes of keys, 2 * 20bytes of MAC 
and 2 * 8bytes of IV.

So based on the above information I can generate 104bytes of key_block 
for these ciphers and extract the required keys.
So for AES_256_CBC_SHA-
key_block[0..19] - MAC for client, key_block[20..39] - MAC for server.
key_block[40..71] - client write key and key_block[72..104] - server 
write key.

Could someone please advice any reference on how to get the key_block 
details for a give cipher suit?


-- 
Thanks,
Nilesh

[TLS] About the key block generation for differen… nilesh
Re: [TLS] About the key block generation for diff… Marsh Ray
Re: [TLS] About the key block generation for diff… nilesh
Re: [TLS] About the key block generation for diff… Marsh Ray