Messages in this thread | | | Subject | Re: [RFC PATCH 03/20] X.509: Allow X.509 certs to be blacklisted [ver #2] | From | Mimi Zohar <> | Date | Fri, 05 Feb 2016 11:16:19 -0500 |
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Hi David,
On Wed, 2016-02-03 at 15:46 +0000, David Howells wrote: > Mimi Zohar <zohar@linux.vnet.ibm.com> wrote: > > > > Allow X.509 certs to be blacklisted based on their TBS hash. > > > > What is the TBS hash? This doesn't seem to be the key identifier. > > It's the TBSCertificate hash (I'll change to calling it that in the patch > description). "TBS" stands for "To Be Signed". This is what is hashed for > the signature to be generated upon - so it's something we have to expend > resources calculating anyway. > > The reason I'm using this is this is what UEFI puts into its blacklist. See: > > http://uefi.blogspot.co.uk/2013/09/uefi-24-review-part-13-hash-of.html > > To quote: > > Now, in the UEFI 2.4 specification, three new types of signatures for > the black list were added. Specifically, the various recommended forms > of the To-Be-Signed hash value (160, 256 and 512-bits) which are > created during the creation of a certificate could be added to the > black list instead of the certificate itself. > > > The cert associated with this key identifier is loaded onto the .ima > > keyring. > > We could also check that. There's no requirement that we only check the TBS - > but the TBS hash is something we must check.
Ok, but before "IMA: Use the system blacklist keyring" patch, this needs to be addressed.
> I wonder if I should mark the blacklist key as to what the value it holds > should be checked against. There's a number of different things we could put > in there: > > (1) TBSCertificate hash. > > (2) Subject Key Identifier content. > > (3) Authenticode Binary hash (for PE files). > > (4) PKCS#7 component digest.
Ok
> > eg: openssl x509 -in <pathname> -inform DER -notext -out > > > > <snip> > > > > X509v3 extensions: > > X509v3 Subject Key Identifier: > > > > 71:12:39:B3:AB:E6:8D:BF:70:E7:26:DE:C8:4A:3F:5F:17:EF:00:6C > > Note that this isn't a mandatoryy field and if it's not present, we have no way > to calculate it as there's no one standard-defined method.
The hex string appears when displaying the keys. For example, keyctl show %keyring:.ima shows:
734429129 --als--v 0 0 \_ asymmetric: : local: signing key: 711239b3abe68dbf70e726dec84a3f5f17ef006c
Mimi
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