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

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