[PATCH v3 0/4] fit: allow signing with an OpenSSL engine
Quentin Schulz
foss+uboot at 0leil.net
Fri Nov 21 18:14:56 CET 2025
I have a couple of products whose U-Boot FIT is signed via a proprietary
OpenSSL engine which only expects the name of a "slot" to select the key
to sign data with.
Currently mkimage fit support expects either a key-dir (-k) or a
key-file (-G) as a toggle for signing, however this doesn't apply to our
usecase because we use an OpenSSL engine (so no key-file to provide)
which doesn't mimic a directory layout like key-dir implies. Moreover,
binman really expects private keys (.key extension) to be available in
this key-dir directory, which we of course cannot provide.
This series allows to sign a FIT image with mkimage (and binman) with
an OpenSSL engine, including PKCS11 and custom engines. If a key-dir
needs to be passed (which is typical for PKCS11), one can do so by using
fit,engine-keydir.
Note that the public key (.crt extension) still needs to be available if
one wants to embed it for signature verification (which is probably what
one wants to do :) ). It is probably possible to use the engine for
getting the public key instead of storing it on disk, but this needs to
be added to fdt_add_pubkey and then binman, through a mechanism
different from fit,engine*.
One issue though is that since binman resolves key paths absolutely and
that I don't believe an OpenSSL engine would happen to have the exact
same key_id value than a local absolute path, fit,encrypt and
fit,engine cannot cohabit. An issue for the next person who wants
an OpenSSL engine AND encrypt the same FIT image, I don't.
Note that LibreSSL supports neither engines nor providers as far as I
could tell (engine support has been explicitly removed).
Note that OpenSSL engines have been deprecated since 3.0 (Q3-2021),
however note that OpenSSL 3.5 still seems to support engines (git grep)
and is EOL end of Q1 2030.
+Cc Eddie who's working on OpenSSL provider support, maybe we can work
together on supporting both engines and providers via the same API we
expose to the user?
+Cc Wolfgang who seems interested in signing with PKCS11 engine which
should now be supported.
+Cc Peter who seems interested according to v1
If anyone has an idea on how to test PKCS11 with SOftHSMv2 with id=
passed in fit,engine-keydir, I'm all ears.
I'm also wondering if the explanation around fit,engine-keydir aren't
too much. After all, they are passed verbatim to mkimage as -k argument
and the special cases are all specific to mkimage and not binman.
This depends on https://lore.kernel.org/all/20251121-pkcs11-ci-v1-1-8ba6da56c2f5@cherry.de/T/#u
Signed-off-by: Quentin Schulz <quentin.schulz at cherry.de>
---
Changes in v3:
- did NOT add T-b and R-B on patch 3 and 4 since there's been some
rework,
- added R-b on patch 2,
- reworded commit log of patch 3,
- reworded documentation of fit,engine and the special case of pkcs11 to
hopefully be easier to digest,
- put fit,engine and fit,engine-keydir extraction into a separate
function,
- fixed issues due to modification of the environment in tests failing
other tests, by using unittest.mock.patch.dict() on os.environ as
suggested by the unittest.mock doc,
- added test for fit,encrypt + fit,engine raising an issue to keep 100%
coverage for fit etype,
- fixed issue when running binman with only one process (-T or -P 1) by
renaming uniquely files and directories created in the input dir in
individual tests as it is shared among all tests run by a process,
- removed apt-get install of the lib packages in the test as
Bintool.apt_install() is racey (apt itself is...) and would fail when
running binman with multiple processes. The package is now part of the
CI container (well, in a different series). If it's not detected, the
test is skipped.
- Link to v2: https://patch.msgid.link/20251117-binman-engine-v2-0-e053f683c0b8@cherry.de
Changes in v2:
- added R-b on patch 1,
- did NOT add T-b and R-B on patch 3 since there's been some rework,
- added binman test for signing with dummy RSA engine,
- added binman test for signing with SoftHSMv2 PKCS11 engine,
- added binman test for signing with SoftHSMv2 PKCS11 engine and
specific keydir (which contains an object= to tackle that special
case)
- added dummy RSA engine for binman test,
- renamed property to fit,engine as my hunch is that it could be used
for encrypting as well (from the same engine with the same keydir but
likely a different key-name-hint so that a different keypair is used)
- added fit,engine-keydir to easily support PKCS11 with arguments other
than object= taken from key-name-hint,
- Link to v1: https://patch.msgid.link/20251031-binman-engine-v1-0-c13c1b5dac43@cherry.de
---
Quentin Schulz (4):
fit: support signing with only an engine_id
tools: binman: mkimage: add support for passing the engine
tools: binman: fit: add support for OpenSSL engines
tools: binman: fit: add tests for signing with an OpenSSL engine
tools/binman/btool/mkimage.py | 5 +-
tools/binman/btool/softhsm2_util.py | 21 ++
tools/binman/entries.rst | 54 ++++-
tools/binman/etype/fit.py | 93 ++++++++-
tools/binman/ftest.py | 223 +++++++++++++++++++++
tools/binman/test/340_dummy-rsa4096.crt | 31 +++
tools/binman/test/340_fit_signature_engine.dts | 99 +++++++++
.../test/340_fit_signature_engine_encrypt.dts | 100 +++++++++
.../test/340_fit_signature_engine_pkcs11.dts | 99 +++++++++
.../340_fit_signature_engine_pkcs11_object.dts | 100 +++++++++
tools/binman/test/340_openssl.conf | 10 +
tools/binman/test/340_softhsm2.conf | 16 ++
tools/binman/test/Makefile | 6 +-
tools/binman/test/dummy-rsa-engine.c | 149 ++++++++++++++
tools/fit_image.c | 3 +-
tools/image-host.c | 4 +-
16 files changed, 1001 insertions(+), 12 deletions(-)
---
base-commit: a642f401d700034c82128defc2d6f9c18065de1f
change-id: 20251030-binman-engine-e349b02696d0
Best regards,
--
Quentin Schulz <quentin.schulz at cherry.de>
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