RE: Platform Support for JWA Crypto Algorithms

Ease of implementation is relative. Maybe tomorrow I will discover that the needed concat-KDF is supported "natively" by Mozilla's NSS library. Then it is very easy.
Currently it looks difficult on that platform because the documentation says that the digest function "finish" resets the digest, which would probably lead to different results than in the examples in the appendices A4 and A5.
" http://doxygen.db48x.net/mozilla/html/interfacensICryptoHMAC.html#afa7330d1ac9e69aafc30f71ba35da74e
NOTE: This method may be called any time after |init| is called. This call resets the object to its pre-init state.
"
I still think that some thirty bytes is not much and that generating the CIK randomly is much easier. I implemented the concat-kdf in java
https://code.google.com/p/jsoncrypto/source/browse/trunk/src/org/jsoncrypto/crypto/KDFConcatGenerator.java and I know that implementing my proposal below is easier.
Anyway: One could ask why the concat-KDF is not implemented in Bouncycastle while three other KDFs are implemented there.
Mozilla implements KDFs too:
https://mxr.mozilla.org/mozilla-central/source/security/nss/lib/pk11wrap/pk11skey.c#1437
Although there is no indication in the code which KDF this implements. Maybe a Mozillian on these lists can clarify.
If it is a KDF that is implemented in Bouncycastle too then I suggest to change the default KDF in JWE to that one.

The implementation of KDF can lead to buffer overflows is no secret:
https://bugzilla.mozilla.org/show_bug.cgi?id=617565
Which means that KeyDerivationFunction implementation is not as easy as it could be.

Why do you think that the list of NIST's crypto implementations lists all algs except KDF?
http://csrc.nist.gov/groups/STM/cavp/documents/components/componentval.html

NIST defines other KDFs too. Why is JWE using one that is implemented nowhere? Any other choice that is implemented on two platforms is a better choice than concat-kdf I think.

Axel

http://doxygen.db48x.net/mozilla/html/interfacensICryptoHMAC.html#afa7330d1ac9e69aafc30f71ba35da74e
NOTE: This method may be called any time after |init| is called. This call resets the object to its pre-init state.

From: Mike Jones [mailto:Michael.Jones@microsoft.com]
Sent: Monday, October 29, 2012 5:25 PM
To: Nennker, Axel; jose@ietf.org
Cc: public-webcrypto@w3.org
Subject: RE: Platform Support for JWA Crypto Algorithms

No, Concat often isn't natively supported, but it's very easy to implement given implementations of SHA-256 and SHA-512, as shown in http://tools.ietf.org/html/draft-ietf-jose-json-web-encryption-06#appendix-A.4 and http://tools.ietf.org/html/draft-ietf-jose-json-web-encryption-06#appendix-A.5.

When the table was discussed at the WebCrypto F2F, it was pointed out that a shortcoming of the current table is that it doesn't indicate which of the "NO" values are effectively show-stoppers and which are easy to build implementations of, and so not a problem in practice.  As shown in the appendices, I believe that Concat is in the latter category.  Given the ease of implementation, it's certainly not worth adding space to the JWEs to work around.

                                                            -- Mike

From: Axel.Nennker@telekom.de [mailto:Axel.Nennker@telekom.de]
Sent: Monday, October 29, 2012 6:03 AM
To: Mike Jones; jose@ietf.org
Cc: public-webcrypto@w3.org
Subject: RE: Platform Support for JWA Crypto Algorithms

As one can see from this table the KDF is unsupported on all platforms (except one).
http://self-issued.info/presentations/Platform_Support_for_JWA-04_Crypto_Algorithms.xlsx

JWE

kdf

CS256

Concat Key Derivation Function (KDF)

NO

Win7





NO

NO

NO

NO

NO

NO

NO

NO



NO

NO

NO

JWE

kdf

CS384

Concat Key Derivation Function (KDF)

NO

Win7





NO

NO

NO

NO

NO

NO

NO

NO



NO

NO

NO

JWE

kdf

CS512

Concat Key Derivation Function (KDF)

NO

Win7





NO

NO

NO

NO

NO

NO

NO

NO



NO

NO

NO


Isn't this an indication that we should look at alternatives?

e.g.: we could generate the integrity protection key randomly instead of deriving it from the content encryption key.
This would add some more bytes (e.g. about 32) to the jwt but is very easy to implement on all platforms.


One way to do it would be to generate enough bytes "Bytes" in "JWE Encrypted Key" for encryption and integrity.
The CEK is then "Bytes[0 .. cekLength-1]" and the CIK "Bytes[cekLength .. cekLength+cikLength-1]"


Axel

[On some platforms (Firefox/NSS) it might even be nearly impossible to implement (without extending the platform's functions) because the build-in digest function is always reset when finalize (doFinal) is called. The spec of the Concat-KDF says that bytes are generated in a loop but the digest is NOT reset in the loop.]


From: jose-bounces@ietf.org<mailto:jose-bounces@ietf.org> [mailto:jose-bounces@ietf.org] On Behalf Of Mike Jones
Sent: Monday, October 29, 2012 7:28 AM
To: jose@ietf.org<mailto:jose@ietf.org>
Subject: [jose] Platform Support for JWA Crypto Algorithms

FYI, I posted the table describing support for the JWA algorithms in common Web development platforms that we discussed at IETF 84.  See http://self-issued.info/?p=884.

                                                            -- Mike