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RE: Signing before Encryption and Signing after Encryption
From: "David Gillett" <gillettdavid () fhda edu>
Date: Wed, 22 Mar 2006 09:17:01 -0800

  You're quite correct.  I can think of cases where I'd want to
have the encrypted text signed (more often by a machine than by
a human...), but you've reminded me why that's not an acceptable
substitute for signing the plaintext.

  IFF you do the encryption with Asymmetric keys, Alice can encrypt
the message with both Bob's public key and her own private key, 
but we've already talked about why Symmetric keys are more commonly 
used for encryption.
  "Sign after encryption" allows for a message to be encrypted by 
anyone who has the symmetric key, and signed by someone else; "sign 
before encryption" confirms that Alice signed the text and we don't 
care who encrypted it.
  And actually, "sign before encryption" happens *invisibly* all the
time, as signed messages may travel across encrypted links and be
automatically decrypted before delivery.

  I stand corrected.

David Gillett


-----Original Message-----
From: Gregory Rubin [mailto:grrubin () gmail com] 
Sent: Tuesday, March 21, 2006 6:53 PM
To: gillettdavid () fhda edu; security-basics () securityfocus com
Subject: Re: Signing before Encryption and Signing after Encryption

The problem with signing after encryption is that it is 
possible to strip the signature off and claim a message as 
your own.  Consider the following situation (encrypted text 
is signed, not the plain text).

Alice wants to authenticate herself to Bob, so she composes a 
message with secret information, encrypts it to Bob's public 
key, and then signs it with her private key and sends it off.

Mallory intercepts the message. She can't read the message, 
but she can strip off the signature so she now just has the 
message encrypted to Bob's key.  She now signs the message 
with her key and sends it on.

Bob now receives the secret information in an encrypted 
message with a valid signature by Mallory.  Ergo, he believes 
that Mallory knows this secret information.  This is a problem.

If the signature is on the plain text as opposed to the 
private text, this attack fails.

Greg Rubin


On 3/21/06, David Gillett <gillettdavid () fhda edu> wrote:
  Signing requires a private key -- therefore, it *must* be 
Asymmetric.  Asymmetric is typically much slower than Symmetric, so 
you get things like SSL that use Asymmetric to protect the 
exchange of 
the Symmetric key used for actual payload encryption.

  Signing after encryption allows the signature to be verified 
before/without decrypting the payload.  There are a variety of 
circumstances in which that could be useful, which are 
blocked if the 
signing is done first.  I can't think of any where the opposite is 
true.

David Gillett, CISSP


-----Original Message-----
From: shyaam () gmail com [mailto:shyaam () gmail com]
Sent: Tuesday, March 21, 2006 9:28 AM
To: security-basics () securityfocus com
Subject: Signing before Encryption and Signing after Encryption

Hello All,
I was asked a question in an interview. I would like to know more 
about this. I am sorry if it is really basic question.

What are the tradeoffs between Signing before Encryption 
and Signing 
after Encryption? Please do let me know on either case 
when you use 
a Symmetric Key and an Asymmetric key.

I am sure that this is a very basic question. I appologize again.

Kind Regards,
Shyaam

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----------------------------------------------------------------------
----- EARN A MASTER OF SCIENCE IN INFORMATION ASSURANCE - 
ONLINE The 
Norwich University program offers unparalleled Infosec management 
education and the case study affords you unmatched 
consulting experience.
Tailor your education to your own professional goals with degree 
customizations including Emergency Management, Business Continuity 
Planning, Computer Emergency Response Teams, and Digital 
Investigations.

http://www.msia.norwich.edu/secfocus

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---------------------------------------------------------------------------
EARN A MASTER OF SCIENCE IN INFORMATION ASSURANCE - ONLINE
The Norwich University program offers unparalleled Infosec management 
education and the case study affords you unmatched consulting experience. 
Tailor your education to your own professional goals with degree 
customizations including Emergency Management, Business Continuity Planning, 
Computer Emergency Response Teams, and Digital Investigations. 

http://www.msia.norwich.edu/secfocus
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