[Freeipa-devel] DNSSEC key wrapping: cryptographer needed

[Petr Spacek]

Hello list,

I'm working on key wrapping mechanism described in thread "LDAP schema for 
DNSSEC keys" [0] and I'm really puzzled from the maze of crypto here. I would 
really appreciate any suggestions or comments on this.

- I have difficulties to pick and use proper wrapping mechanisms and their 
configuration/usage patterns.

- Rick van Rein from OpenDNSSEC-user list noted that we are defeating purpose 
of PKCS#11 by exporting raw keys from HSM. I can see his point and we should 
be able to do all crypto operations inside SoftHSM.

I have asked him to describe his doubts on freeipa-devel list - stay tuned.


We need to wrap
===============
- asymmetric key (zone key)   with symmetric key  (master key)
- symmetric key  (master key) with asymmetric key (replica key)

LDAP schema prescribes some storage formats but my question is about crypto 
algorithms (and padding!) used for key wrapping.


Asymmetric keys
===============
According to schema [1] wrapped asymmetric (i.e. private) key must be stored 
in 'ipaPrivateKey' attribute as DER-encoded EncryptedPrivateKeyInfo structure 
according to RFC 5958 [2].

It seem that SoftHSM's C_WrapKey PKCS#11 call will give us encryptedData part 
of EncryptedPrivateKeyInfo structure so we only have to add algorithm 
identifier, DER-encode the whole thing, and we could be done.

I'm not sure where should I stick IV or what happens if default (fixed) IV 
value is left in place...?


Technically SoftHSM's C_WrapKey call produces PKCS#8 structure encrypted with 
AES according to RFC 3394 (no padding) or RFC 5649 (with padding).

RFC 3394 works only on blocks of 64 bits, which is not our case because 
EncryptedPrivateKeyInfo has no padding. So we should use RFC 5649 to get 
proper padding etc.

And here is the problem: SoftHSM can use RFC 5649 using OpenSSL functions but 
OpenSSL doesn't support RFC 5649! The patch with this functionality was 
submitted back in 2010 to OpenSSL bug tracker [3] but the ticket is in state 
"new" and there was no reply to the e-mail in the original thread [4].

What do we do?
- Convince OpenSSL to review and accept the patch?
- Hack it around #0: Use something else as key wrapping method? Other wrapping 
methods would require SoftHSM modification because SoftHSM supports only AES 
mechanisms now.
- Hack it around #1: Implement the RFC 5649 wrapping algorithm in a downstream 
patch in SoftHSM? Do we have enough expertise to implement it correctly?
- Hack it around #2: Implement raw key extraction from SoftHSM and do key 
wrapping somehow somewhere? (Please nooooooo.)


Symmetric keys
==============
should be stored in 'ipaSecretKey' attribute but detailed storage format 
definition is missing from schema [1] right now.

Unfortunately, SoftHSM doesn't support C_WrapKey PKCS#11 function with 
asymmetric key as *wrapping* key so we need to add this functionality.

The first question is:
What algorithm and padding scheme should we use?

Something like OpenSSL's RSA_PKCS1_OAEP_PADDING [6]? That would be relatively 
easy to implement to SoftHSM because it is already there, just not handled in 
C_WrapKey (so we need to add appropriate 'glue' there).

Another question is storage format of the wrapped key.
PKCS#11 2.40d2 [7] contains this comment about "PKCS #1 RSA OAEP" algorithm:
"For wrapping, the “input” to the encryption operation is the value of the 
CKA_VALUE attribute of the key that is wrapped; similarly for unwrapping. The 
mechanism does not wrap the key type or any other information about the key, 
except the key length; the application must convey these separately."

AFAIK it means that it wraps raw secret key and nothing else. If I'm not wrong 
we should be able to take resulting blob and stick it into OneSymmetricKey 
structure from RFC 6031 [5]. Honza, is it correct assumption/idea?


I would really appreciate any suggestions or comments on this.

Thank you for your time.

[0] https://www.redhat.com/archives/freeipa-devel/2014-April/msg00565.html
[1] http://www.freeipa.org/page/V4/PKCS11_in_LDAP/Schema#Encoded_key_data_2
[2] http://tools.ietf.org/html/rfc5958#section-3
[3] http://rt.openssl.org/Ticket/Display.html?id=2204
[4] http://marc.info/?l=openssl-dev&m=126953517430167&w=2
[5] https://tools.ietf.org/html/rfc6031#section-2
[6] https://www.openssl.org/docs/crypto/RSA_public_encrypt.html
[7] 
http://docs.oasis-open.org/pkcs11/pkcs11-curr/v2.40/csprd02/pkcs11-curr-v2.40-csprd02.html#_Toc387327841

-- 
Petr^2 Spacek