Comments on the checksum vulnerabilities
hartmans at MIT.EDU
Fri Dec 3 13:37:55 EST 2010
>>>>> "Greg" == Greg Hudson <ghudson at MIT.EDU> writes:
Greg> 2. Enforcement
Greg> Right now, the crypto layer is responsible for ensuring that a
Greg> keyed cksumtype isn't being misapplied to the wrong kind of
Greg> key, and the protocol layer is responsible for ensuring that a
Greg> checksum is keyed when it has to be.
Greg> Since not all checksums need to be keyed (some are contained
Greg> within an encrypted blob), we cannot suddenly start enforcing
Greg> keyed checksums for every krb5_[ck]_verify_checksum operation.
Or if we do we'd need to handle the cases where unkeyed checksums are
permitted. However that would probably be too big of a break for
potential non-tree uses of the API.
I understand we have a lot of verify_checksum APIs. However, I am also
concerned about the fact that we've made this mistake multiple times.
We might be able to get away with changing behavior for the
krb5_k_interface and adding a way to set on a krb5 key object whether
unkeyed checksums are permitted. That's probably more ugly than a new
We could potentially have a flag to or-in with keyusages. Or have a set
of key usages for which unkeyed checksums are permitted.
The last is actually something that seems worth considering.
The other options are kind of ugly.
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