Re: Signing before Encryption and Signing after Encryption

["Gregory Rubin" <grrubin () gmail com>]

David,

Encrypting with a private key is equivalent to signing it (and is
actually how signing usually takes place).  So in your scenario (using
solely asymmetric keys):

Alice encrypts the message first with Bob's public key and then with
Alice's private key.
Mallory decrypts the message with Alice's public key (since the public
key is the inverse of the private key) and is left with just the
message encrypted to Bob's public key.

The attack proceeds as before.

Signing a message usually consists of encrypting a secure hash of the
message with your private key.  Using a hash (even one such as MD5 or
SHA1) does not add to the security at all, but instead weakens the
security of the signature.  The only reason for the hash is
efficiency.  As already noted in this thread, asymmetric encryption is
slow, so by only signing the hash, it is much faster.  Also, if
instead of signing the hash you were to sign the message, you'd double
the size of the message that needs to be transmitted.  Using hashes
introduces all of the risks associated with hash collisions, a risk
that would never occur if you were to sign the message directly.
(Please correct me if I'm wrong.  It's been a while since I've worked
on the underlying math and strategies.)

On 3/22/06, David Gillett <gillettdavid () fhda edu> wrote:
>   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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