Re: [jose] #14: Support longer wrapped keys than OAEP allows

[Mike Jones <[e-mail]>]

Since you message appears to take it as given that “there should be only one way of encrypting keys”, I’ll point out that I don’t think that it’s reasonable to assume that.  JOSE is first and foremost an engineering exercise, where the cost/benefit generality/complexity tradeoffs matter, and the goal is a ubiquitously implemented crypto format for the Web that solves the problems that people actually have, rather than a mathematical exercise, where the goal is completeness and generality.  Complexity is the enemy of adoption.

So it’s fair game to ask “What are the costs and benefits of having only one way to encrypt keys”, versus taking that as a given.

I happen to personally believe that encrypting a bare symmetric ephemeral Content Master Key is sufficiently different than encrypting a key that may be public, private, or symmetric and may have additional attributes, that it’s at least worth asking the engineering question whether special-casing the encryption of this bare symmetric ephemeral key results in engineering benefits.

Encrypting a key with attributes for storage or dissemination is not the same kind of operation as wrapping an ephemeral symmetric key to be used for block encryption.  I’m personally fine with this being done differently.  The engineering benefit if we do it differently in the way that Matt’s draft proposed, at least as I see it, is that we have to invent nothing new.  We already have a great format for encrypting arbitrary data, and keys with attributes are a whole lot like arbitrary data.

I respect that some with disagree with my personal view, but I’d also ask you to respect that the engineering tradeoffs may favor having two ways to do things that on the surface may seem similar, but are actually fairly different in nature.

Best wishes,
-- Mike

From: Richard Barnes
Sent: ‎March‎ ‎18‎, ‎2013 ‎7‎:‎36‎ ‎PM
To: Jim Schaad
CC: Manger, James H, [e-mail]
Subject: Re: [jose] #14: Support longer wrapped keys than OAEP allows

Well, I got to 788 by doing math incorrectly*.

Mike was correct on the other thread that 768 is the right number.  However, that's still too big for a 1024-bit RSA key and SHA1, since 768 + 320 = 1088 > 1024.

Regardless, there is clearly an issue here when wrapping a JWK, which is much larger, possibly containing an RSA key itself.  So if we accept the goal that there should be one way of encrypting keys, then we'll need to deal with getting around the OAEP size limitations.

--Richard

* This is why my degree is in mathematics, and not accounting.


On Mon, Mar 18, 2013 at 8:40 PM, Jim Schaad <[e-mail]<mailto:[e-mail]>> wrote:
Think in terms of encrypting a JWK directly not an intermediate key.

> -----Original Message-----
> From: [e-mail]<mailto:[e-mail]> [mailto:[e-mail]<mailto:[e-mail]>] On Behalf Of
> Manger, James H
> Sent: Monday, March 18, 2013 5:17 PM
> To: [e-mail]; [e-mail]<mailto:[e-mail]>
> Subject: Re: [jose] #14: Support longer wrapped keys than OAEP allows
>
> Richard,
>
> How do you get a 788-bit key length?
>
> draft-mcgrew-aead-aes-cbc-hmac-sha2 defines 5 combinations of AES-
> 128/192/256 and SHA-1/256/384/512. The total key lengths range from 256
> bits to 512 bits.
>
> Keys for two of the algorithms (AEAD_AES_128_CBC_HMAC_SHA_256 and
> AEAD_AES_128_CBC_HMAC_SHA1) fit within OAEP with a 1024-bit RSA key.
>
> Keys for all of the algorithms fit within OAEP with a 2048-bit RSA key.
JWA
> already says RSA key sizes MUST be at least 2048 bits.
>
> This already looks sufficient.
>
> --
> James Manger
>
>
> > From: [e-mail]<mailto:[e-mail]> [mailto:[e-mail]<mailto:[e-mail]>] On Behalf
> > Of Richard Barnes
> > Sent: Tuesday, 19 March 2013 10:25 AM
> > Subject: [jose] WebCrypto feedback on key wrapping
> >
> > 2. Mark Watson (Netflix) noted that if we use RSA directly to encrypt
wrapped
> key objects, then we would need something other than OAEP in order to
carry
> arbitrary-length payloads.  I agreed, and suggested that something like
RSA-
> KEM would be necessary.  Ryan Sleevi (Google) and Vijay observed that KEM
is
> troublesome due to the lack of support by native crypto libraries.
> >
> > Point number 2 likely applies for some scenarios of JWE, especially if
we
> adopt the McGrew approach.  For example, if using HMAC-SHA1 and AES with
> a 256-bit key, the total key length is 788 bits, which is too long to be
encrypted
> with OAEP under a 1,024-bit RSA key.  I'm not sure how to resolve it.  The
best
> idea I've got is to allow wrapped keys to nest, so that you can wrap a key
inside
> of another wrapped key.
> >
> > --Richard
>
>
> >> ----------
> >> Sent: Tuesday, 19 March 2013 10:23 AM
> >> Subject: [jose] #14: Support longer wrapped keys than OAEP allows
> >>
> >> #14: Support longer wrapped keys than OAEP allows
> >>
> >>  The use of RSA-OAEP for key wrapping imposes a limit on the length
> >> of  the key package being wrapped. With SHA1, this length is N-320,
> >> where  N is the length of the RSA modulus.  Especially with larger
> >> hash  functions, and especially for wrapping private keys, the size
> >> of key  packages will be larger than this bound.  We should
> >> incorporate a  mechanism to accommodate these situations.
> >>
> >>
> >> Ticket URL: <http://trac.tools.ietf.org/wg/jose/trac/ticket/14>
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