Re: [TLS] Query regarding generating client_write_key.

Simon Josefsson <simon@josefsson.org> Tue, 27 September 2011 14:02 UTC

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From: Simon Josefsson <simon@josefsson.org>
To: nilesh <nilesh.tayade@netscout.com>
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Date: Tue, 27 Sep 2011 16:05:21 +0200
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Cc: tls@ietf.org
Subject: Re: [TLS] Query regarding generating client_write_key.
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nilesh <nilesh.tayade@netscout.com>; writes:

> Hi,
>
> I am new to the TLS/Cryptography and started working on decrypting the
> packets over SSL/TLS.
> I have followed the RFC2246, 5246.
...
> Queries:
> 1. The md5 generates 16bytes and SHA1 generates 20bytes so xor will be
> 20bytes.
> But the key we expect should be 32bytes (iterated the MD5/SHA1 twice
> for that). How could this be achieved?
> 2. Any suggestions on if the PRF() is designed somewhere? Pointers on
> how it could be designed?
>
> I tried several ways, but getting the wrong key. Could someone please
> advice?

Did you read section 5 of RFC 5246?  It explains how the PRF function
works.

/Simon