Re: [TLS] DHE key derivation

Michael D'Errico <mike-list@pobox.com> Fri, 27 September 2013 16:05 UTC

Return-Path: <mike-list@pobox.com>
X-Original-To: tls@ietfa.amsl.com
Delivered-To: tls@ietfa.amsl.com
Received: from localhost (localhost [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id 5910121F9D0E for <tls@ietfa.amsl.com>; Fri, 27 Sep 2013 09:05:49 -0700 (PDT)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -2.574
X-Spam-Level:
X-Spam-Status: No, score=-2.574 tagged_above=-999 required=5 tests=[AWL=0.025, BAYES_00=-2.599]
Received: from mail.ietf.org ([12.22.58.30]) by localhost (ietfa.amsl.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id lhRltl-WErLO for <tls@ietfa.amsl.com>; Fri, 27 Sep 2013 09:05:44 -0700 (PDT)
Received: from sasl.smtp.pobox.com (a-pb-sasl-quonix.pobox.com [208.72.237.25]) by ietfa.amsl.com (Postfix) with ESMTP id 8D36721F8A38 for <tls@ietf.org>; Fri, 27 Sep 2013 09:05:44 -0700 (PDT)
Received: from sasl.smtp.pobox.com (unknown [127.0.0.1]) by a-pb-sasl-quonix.pobox.com (Postfix) with ESMTP id DC0CFDD26; Fri, 27 Sep 2013 12:05:43 -0400 (EDT)
DKIM-Signature: v=1; a=rsa-sha1; c=relaxed; d=pobox.com; h=message-id :date:from:mime-version:to:cc:subject:references:in-reply-to :content-type:content-transfer-encoding; s=sasl; bh=5k49+22qL4DC 9o3z7MFqqztu2mo=; b=INzU0OS+LGCDc5/pY3UMuYIE2B7myi4YekCtdUWmONMj WQ0M+YafvTMW381Li4wHFe0JUI2Hoq9cIYkyzflqSCadzVJ8qk/8RBkcDM4kWbNM 1NHMkuefb8epllgMIy6tR8194DHKylk4s95cTfkL5xHfFh1Jyulz8qhfq8+Kh+U=
DomainKey-Signature: a=rsa-sha1; c=nofws; d=pobox.com; h=message-id:date :from:mime-version:to:cc:subject:references:in-reply-to :content-type:content-transfer-encoding; q=dns; s=sasl; b=i9FCee JgQjPwMTVL8tieSsE2aAV4kzyBgkvG+2R9HcB9L/lU0igSWfPTyAFVdzn0jfzNb5 Nct/rKiG7kHA+HtbozenBujRKf3j8CW63zSuhjI26Xkbpzxk/TaIevaLJkYWzxhK Mdu1JoPXgM7IJ9iltOUBVUm/JQcEGHR9bhTCA=
Received: from a-pb-sasl-quonix.pobox.com (unknown [127.0.0.1]) by a-pb-sasl-quonix.pobox.com (Postfix) with ESMTP id A3F11DD22; Fri, 27 Sep 2013 12:05:19 -0400 (EDT)
Received: from iMac.local (unknown [24.234.153.62]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by a-pb-sasl-quonix.pobox.com (Postfix) with ESMTPSA id 0D1C1DD12; Fri, 27 Sep 2013 12:04:57 -0400 (EDT)
Message-ID: <5245ACA8.3020504@pobox.com>
Date: Fri, 27 Sep 2013 09:04:56 -0700
From: Michael D'Errico <mike-list@pobox.com>
User-Agent: Thunderbird 2.0.0.24 (Macintosh/20100228)
MIME-Version: 1.0
To: mrex@sap.com
References: <20130927153439.54C8A1A9B4@ld9781.wdf.sap.corp>
In-Reply-To: <20130927153439.54C8A1A9B4@ld9781.wdf.sap.corp>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
X-Pobox-Relay-ID: 8E96F5EA-278E-11E3-95F6-CE710E5B5709-38729857!a-pb-sasl-quonix.pobox.com
Cc: tls@ietf.org
Subject: Re: [TLS] DHE key derivation
X-BeenThere: tls@ietf.org
X-Mailman-Version: 2.1.12
Precedence: list
List-Id: "This is the mailing list for the Transport Layer Security working group of the IETF." <tls.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/options/tls>, <mailto:tls-request@ietf.org?subject=unsubscribe>
List-Archive: <http://www.ietf.org/mail-archive/web/tls>
List-Post: <mailto:tls@ietf.org>
List-Help: <mailto:tls-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/tls>, <mailto:tls-request@ietf.org?subject=subscribe>
X-List-Received-Date: Fri, 27 Sep 2013 16:05:49 -0000

Martin Rex wrote:
>> It is not recommended to use the same RSA key for both signing and
>> key encipherment.
> 
> Isn't that (using the same RSA key for both signing and key encipherment)
> what pretty much all TLS Servers do when they support both,
> traditional RSA ciphersuites plus DHE_RSA and/or ECDHE_RSA?

I believe that many servers do use the same key for both, though
they don't have to.  This practice is likely due to cost (buy one
certificate instead of two) and software limitations.

Mike