On 2/26/26 11:58 AM, Eric Biggers wrote:
On Thu, Feb 26, 2026 at 10:27:43AM -0500, Simo Sorce wrote:
On Thu, 2026-02-26 at 09:16 -0500, Stefan Berger wrote:
On 2/26/26 7:42 AM, Stefan Berger wrote:
On 2/25/26 7:10 PM, Eric Biggers wrote:
On Wed, Feb 25, 2026 at 09:25:43AM -0500, Stefan Berger wrote:
To avoid duplicate work: Is either one of you planning on writing
patches
for IMA to use ML-DSA and convert the current ML-DSA to also support
HashML?
I had done the work on this before and could dig out the patches
again...
IMA already had to add its own digest prefixing support, since it was
needed to disambiguate between full-file digests and fsverity digests.
See 'struct ima_file_id'. Thus the message signed is at most 66 bytes.
The hash there is still only a hash over a file and that hash is signed,
isn't it?
With that being the case, HashML-DSA isn't necessary. It's not even
possible to use here, since there are no OIDs assigned for the fsverity
digests, so it cannot replace the ima_file_id.
For non-fsverify IMA signatures it is 'possible' to use HashML-DSA and
it's 'working' (recycled old patches yesterday):
Linux: https://github.com/stefanberger/linux/commits/
dhmlsa%2Bima.202602025/
ima-evm-utils: https://github.com/linux-integrity/ima-evm-utils/pull/19/
commits
I'll also note that HashML-DSA is controversial (e.g. see
https://keymaterial.net/2024/11/05/hashml-dsa-considered-harmful/),
The problem with this is that NIST would have to react to these
controversies as we race to support PQC. If something is wrong with the
standard then it would be best for NIST to withdraw/modify HashML-DSA
asap. Otherwise it's the best to follow the standard IMO because if you
don't you get criticism otherwise.
What I am not clear about from FIPS-204 is whether availability of
HashML-DSA is a "must-use" or a "may-use". What speaks against it for
our use case is performance. The lookup of a hash's ID (last digit of
OID) and the creation of the 11 byte encoding to prepend before every
digest for every signature takes cycles.
It is a recommendation, but there are plenty of protocols (TLS,
OpenPGP, etc...) where the decision has been made to use "pure" ML-DSA
only, even if what you are signing is not the full data, but something
containing a hash.
Ideally you do not sign *just* a hash, but some structured data, like a
context label that identifies the hash and some other related metadata
for example. In order to make forgeries much harder should the hashing
algorithm used to hash the data weaken over time. But it is not
strictly necessary (NIST mentioned in some forum, sorry I do not have
the message handy for quoting, that a structured packet is perfectly
fine for use with pure ML-DSA, because it does enough to address the
same issues that a separate internal context does with HashML-DSA).
If pure-ML-DSA works better for IMA, just use pure ML-DSA.
Maybe it should explicitly state in FIPS-204 something along the lines
of "with a given hash either ML-DSA or HashML-DSA can be used (for as
long as you use it in the same way from then on)." At least this way
nobody can point out that HashML-DSA should have been used when you didn't.
NIST will not change the standard documents any time soon, but for FIPS
certification there are Implementation Guidelines.
In any case a FIPS module cannot distinguish between data that happens
to be 32 bytes long and a hash of larger data, so the point is kind of
moot. From the FIPS perspective HashML-DSA is just an available
algorithm that protocol implementations can use, or not.
There are additional guidelines on what this may be useful for, but so
far NIST has not objected to the use of pure ML-DSA even where
theoretically HashML-DSA could be used.
I see that IMA indeed never upgraded full file hashes to use
'struct ima_file_id'. Building a new feature that relies on this seems
> like a bad idea though, given that it's a security bug that makes
the> IMA protocol cryptographically ambiguous. I.e., it means that in IMA,
when the contents of some file are signed, that signature is sometimes
also valid for some other file contents which the signer didn't intend.
You mean IMA should not sign the digest in the ima_file_id structure but
hash the ima_file_id structure in which this file digest is written into
(that we currently sign) and sign/verify this digest? And we would do
this to avoid two different files (with presumably different content)
from having the same hashes leading to the same signature? Which hashes
(besides the non-recommended ones) are so weak now that you must not
merely sign a file's hash?
The problem with this is that older kernels (without patching) won't be
able to handle newer signatures.
Just fix that bug first, which has to be done anyway. Then just use
pure ML-DSA to sign and verify the 'struct ima_file_id'.
> > As Simo mentioned, FIPS 204 doesn't require HashML-DSA when signing a
hash. It's there as an *option* to solve a perceived problem, which is
actually solvable in better ways.
NIST doesn't plan to update FIPS 204 until 2029, and most likely the
updates will just be errata in the text (such as the ones I reported to
them), not changes or withdrawals in the algorithms themselves. But
it's irrelevant: just because HashML-DSA is an option doesn't mean it
has to be used. Pure ML-DSA supports arbitrary data, which includes
And I was sure whether it was merely an 'option'. Who would use it then
if it takes more cycles to hash the prepended 11 byte oid in HashML-DSA?
formatted hashes which allow for pre-hashing perfectly well.
- Eric
