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There appears to be some inconsistencies between the PKCS#11 v2.40 Current Mechanism Specification and the specification for AES Key Wrap Pad as defined in SP800-38F.

First, according to PKCS#11:

The mechanisms will accept an optional mechanism parameter as the Initialization vector which, if present, must be a fixed size array of 8 bytes, and, if NULL, will use the default initial value defined in Section 2.2.3.1 of [AES KEYWRAP].

The type of this parameter is CK_BYTE_PTR and the pointer points to the array of 8 bytes to be used as the initial value. The length shall be either 0 and the pointer NULL, or 8, and the pointer non-NULL.

However, according to SP800-38F, the ICV2 value for AES-Key Wrap PAD must be four bytes in order to correctly pad the input to the cipher routine (section 6.3 algorithm 5).

Secondly, PKCS#11 specifies that CKM_AES_KEY_WRAP_PAD is used for Encryption/Decryption only, not for Wrapping/Unwrapping operations (Table 65, in section 2.14 in the PKCS#11 current mechanism specification). This seems inconsistent with the goals of the AES KEY Wrap Pad algorithm.

So my questions are:

  1. Is this a known inconsistency and has Cryptoki released any updates to how the CKM_AES_KEY_WRAP_PAD algorithm can be used? I cannot find anything on Google but maybe I missed something.
  2. In my PKCS#11 implementation, can my solution be to inform the user (through documentation) that the IV for CKM_AES_KEY_WRAP_PAD will be truncated to 4-bytes?
  3. I know this appears to be out of spec, but can I in my PKCS#11 implementation allow the user to use CKM_AES_KEY_WRAP_PAD for Wrapping/Unwrapping as well as Encryption/Decryption.
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The PKCS#11 technical committee is currently working on standard version 3.0. You can find the latest working draft here: https://www.oasis-open.org/committees/download.php/64968/pkcs11-curr-v3.0-wd09.docx

In fact, reworked section "AES Key Wrap" (now 2.16) solves your problems:

  • It allows all mechanisms to be used with Encrypt&Decrypt and Wrap&Unwrap.
  • It clarifies that CKM_AES_KEY_WRAP and CKM_AES_KEY_WRAP_PAD actually refer to NIST SP800-38F section 6.2 ("KW"), which in fact require an 8 byte IV.
  • It introduces new CKM_AES_KEY_WRAP_KWP, which is algorithm KWP of NIST SP800-38F section 6.3 you are looking for.

Header files for the working draft can be found here: https://github.com/oasis-tcs/pkcs11/tree/master/working/3-00-wd-01

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  • $\begingroup$ Thanks for the answer! v3.0 definitely clarifies those mechanisms. However, it does appear that they changed the meaning of CKM_AES_KEY_WRAP_PAD between v2.40 and v3.0. In 2.4, CKM_AES_KEY_WRAP_PAD refers to RFC5649 which is the KWP in SP800-38F, however they change it in v3.0 to be regular KW with PKCS#7 padding. I think in the end what I'll have to do is be non-compliant with 2.40 in regards to KWP (CKM_AES_KEY_WRAP_PAD) and note it in user documentation. Again, appreciate the answer! $\endgroup$ – Mike Apr 11 at 11:51

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