Re: [jose] encrypting AND signing a token

["Jim Schaad" <[e-mail]>]

<personal>

I would note that the original PKCS#7 specifications had a mode that
provided a similar sign and encrypt as a single operation mode.  When the
PKCS#7 specifications where adopted by the IETF as part of the CMS work,
this mode was discussed and very deliberately dropped because of numerous
security problems that had been found.  These included (but are not limited
to) the fact that it was signed or who signed it was sometimes a security
leak.  Also there were attacks where the signed and encrypted mode could be
converted to just an encrypted mode.

I would think that there would be a need for a very detailed security
analysis that we are not prepared to do in order to support a signed and
encrypted mode.

Jim


> -----Original Message-----
> From: [e-mail] [mailto:[e-mail]] On Behalf Of
> Dick Hardt
> Sent: Friday, November 02, 2012 12:30 PM
> To: Mike Jones
> Cc: [e-mail]
> Subject: Re: [jose] encrypting AND signing a token
> 
> Not only is my original token increasing in size by 4/3, I am also adding
> another header, payload and signature.
> 
> One of the objectives of JWT was to enabled compact tokens. It would seem
> that we should be able to support both signing and encryption of the same
> token.
> 
> All the encryption use cases I can think of involving asymmetric keys
would
> also require signing with the senders private key.
> 
> My suggestion is to be explicit in what the algorithm etc. is used for:
> 
> Rather than "alg" and "enc", we have:
> 
> "algs" - algorithm for token signing
> "algk" - algorithm for content management key encryption "alge" -
algorithm
> for payload encryption
> 
> Similiarly,
> 
> "kids" - key id for signing
> "kidk" - key id for content managment key encryption
> 
> We could probably make these three or even two letter codes if you want to
> save a couple bytes.
> 
> -- Dick
> 
> On Nov 2, 2012, at 8:46 AM, Mike Jones <[e-mail]>
> wrote:
> 
> > The way you put it brings one straightforward solution to mind.  Solve
1-3
> with a JWE.  Solve 4-5 by signing the JWE as a JWS payload.  Done.
> >
> > I do understand that the 4/3 space blowup-of double base64url encoding
> the JWE motivates your earlier proposal about nested signing.  (See Dick's
> 10/29/12 message "[jose] signing an existing JWT".)  I also understand
that if
> you could do integrity with the asymmetric signature then the integrity
> provided by the JWE itself may be redundant.  I don't have a specific
proposal
> on how to do that.
> >
> > 				-- Mike
> >
> > -----Original Message-----
> > From: [e-mail] [mailto:[e-mail]] On Behalf
> > Of Dick Hardt
> > Sent: Friday, November 02, 2012 8:22 AM
> > To: [e-mail]
> > Subject: [jose] encrypting AND signing a token
> >
> > I am trying to figure out how to implement JWT/JWS/JWE to solve a real
> world problem.
> >
> > 1) Bob sends a token to Charlie via Alice. (Alice gets token from Bob
> > and then Alice gives token to Charlie)
> > 2) Alice must be prevented from reading the token. (token needs to be
> > encrypted)
> > 3) Bob and Charlie can share a symmetric key.
> >
> > I can solve this with JWE.
> >
> > Now let's add another condition.
> >
> > 4) Charlie wants non-repuditation that Bob created the token.
> > 5) Bob has a private key and a public key
> >
> > I don't see how to do this using JWE. It seems I have to sign the same
token
> I had previously with JWS. This seems inefficient since I should be able
to
> replace the JWE integrity computation done with the symmetric key with the
> private key -- but the "alg" parameter is the same in both encrypting and
> signing.
> >
> > Now let's expand this to replacing the symmetric key with a
public/private
> key pair for encryption. Bob encrypts with Charlies public key and signs
with
> Bob's private key (we also need to make sure we are not doing naive
> encryption and signing here, would be a really useful to specify what
needs
> to be done there). Now we need to have parameters for both public/private
> key pairs in the header.
> >
> > Am I missing something here?
> >
> > Seems like we can do this if we change the header parameters to specify
if
> they ("alg", "kid", et.c) are for token signing, payload encryption or
content
> key encryption.
> >
> > -- Dick
> >
> >
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