Re: [TF-A] [RFC] New feature in TF-A to load encrypted FIP payloads - TF-A

18 Feb 2020

      On 18/02/2020 05:52, Sumit Garg wrote:
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On Tue, 18 Feb 2020 at 00:44, Soby Mathew Soby.Mathew@arm.com wrote:
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Hi Everyone,
I have confirmation from the TBFU architect that both decrypt-then-verify and verify-then-decrypt is acceptable and neither case affects boot integrity.
Thanks for this confirmation.
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So I think the flexibility should be given to the platform to choose this.
@Sumit Garg, could you please rework the patches as I mentioned in my previous feedback and submit again?
Sure I will try to address implementation specific concerns. Actually
earlier I was waiting for above design specific confirmation.
-Sumit
[Adding back the tf-a list]
OK, Thanks Sumit.
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Best Regards
Soby Mathew
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-----Original Message-----
From: TF-A tf-a-bounces@lists.trustedfirmware.org On Behalf Of Raghupathy
Krishnamurthy via TF-A
Sent: 28 January 2020 14:58
To: Sumit Garg sumit.garg@linaro.org
Cc: Daniel Thompson daniel.thompson@linaro.org; Miklos Balint
Miklos.Balint@arm.com; Kiyoshi Owada owada.kiyoshi@socionext.com; tf-
a@lists.trustedfirmware.org; Joakim Bech joakim.bech@linaro.org; nd
nd@arm.com; Sandrine Bailleux Sandrine.Bailleux@arm.com
Subject: Re: [TF-A] [RFC] New feature in TF-A to load encrypted FIP payloads
Hi Sumit,
I completely agree with you! All i'm asking for is that what you are proposing is
ratified in the spec clearly, without any ambiguities and that we don't
implement what we *think* is correct, but implement the spec. If the spec
specifies encryption, we should implement encryption, not authenticated
encryption. One is not a direct substitute for the other and requires careful
thinking. Similarly, the order of signing, encryption, decryption and
authentication must be specified and explained clearly, specifically to avoid
these kinds of discussions.
Thanks
-Raghu
On January 27, 2020 at 10:49 PM, Sumit Garg sumit.garg@linaro.org wrote:
On Mon, 27 Jan 2020 at 22:54, Raghupathy Krishnamurthy
raghu.ncstate@icloud.com wrote:
Sumit,
Great point. This perhaps needs to be added to the list of things that need
clarification(Sandrine can you help with this too?) in the PSA-TBFU . I believe
the answer to your concern lies in the PSA-TBFU in section 3.5, where it talks
about optimizing the trusted boot process. To overcome the problem you're
talking about, you would:

Verify asymmetric signature.
Decrypt firmware using SSK on successful signature verification.
Rekey the firmware using BSSK(or as the PSA specifies, a key derived from

the HUK using a KDF).
You will only verify the asymmetric signature on every firmware update, and
use the rekeyed firmware(encrypted and mac;d with device specific key) on
normal boot.
Following is the quote from PSA-TBFU spec:
"An implementation **can** optimize the trusted boot process at the expense
of **simplicity**"
It doesn't seems to be a recommended practice from spec. And especially for
DRM use-cases, this approach is NOT recommended due to following concerns
raised by DRM vendors (original concerns were with respect to TAs but will
equally apply for firmware as well):

allows the device to self sign code authorized to run on the device.
increase the attack surface by having two different ways to load firmware.
allow a break once break forever situation, if you defeat the RSA 'install' once,

no matter how hard it is, now your firmware is nicely transformed in a secure
firmware and can be reused.
Whereas on the other hand, considering
"sign-then-encrypt-then-authenticate", it provides two mutually exclusive
crypto layers (signature layer and authenticated encryption
layer) which in turns provides implementation flexibility as follows:
Firmware update:

Require both layers.

Normal boot:

DRM use-case, require both layers.
Boot time optimization required, can use only authenticated encryption.
Platform provides secure on-chip NVM and boot time optimization required,

can use only signature verification (or simply hash stored in secure on-chip
NVM memory).
-Sumit
Thanks
Raghu
On January 26, 2020 at 10:34 PM, Sumit Garg sumit.garg@linaro.org wrote:
On Fri, 24 Jan 2020 at 16:36, Sumit Garg sumit.garg@linaro.org wrote:
On Fri, 24 Jan 2020 at 04:02, Raghupathy Krishnamurthy
raghu.ncstate@icloud.com wrote:
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I also just realized that both the TBBR and ARM PSA only talk about
encryption of the image, and not authenticated encryption. The guarantees
provided by both are completely different. Your
review(https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/2495/)
talks about the requirement R060_TBBR_FUNCTION being implemented, which
is technically not true(and potentially misleading). We must make a note of this
difference and use the appropriate terminology, without mixing the two, in the
documentation, commit messages, source code comments and error prints.
The tool is also called 'encrypt_fw ' but should maybe be named appropriately
to indicate it is doing authenticated encryption.
I wouldn't call it misleading. Since firmware encryption feature
essentially provides confidentiality protection and authenticated
encryption is the type of crypto algorithm which we have used to
implement it.
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BTW, ARM PSA(file:///home/raghu/repos/fvp/DEN0072-PSA_TBFU_1-0-
REL.pdf) expects that the image manifest(X509 certificate) contain the hash of
the ENCRYPTED image(Table 2 and as described in my answer to your question
"How would this ensure integrity of ciphertext"). The TBBR spec completely
misses this fact, and is a crucial detail if we only implement encryption(as
opposed to authenticated encryption).Build_macros.mk, in your change, passes
the un-encrypted image to cert-tool. You can get away with it in your
implementation, since you are using authenticated encryption, not if you were
only implementing firmware encryption.
I have already highlighted the issue with signing the ciphertext in my
previous reply which deviates from security properties provided by
signature verification of plain firmware. So I think we need to
revisit ARM PSA TBFU spec.
In addition to this, there are implementation specific issues with "signing the
ciphertext" too. It simply makes the ciphertext immutable for device and
disallows meeting following firmware re-encryption requirement as per TBBR
spec:
R070_TBBR_PROTECTION. The Trusted boot firmware may do the binding of
software image updates at run- time by decrypting the updated SoC certificates
and software images using the OTP/Fuse Secret Symmetric Key (SSK), followed
by the re-encrypting these SoC certificates and software images using a
reproducible secret unique per device symmetric key (BSSK), and then updating
the ToC correspondingly.
Also, externally signing every firmware image encrypted with BSSK doesn't
seem scalable as well. It also hampers the case where encryption key is never
exposed out from device eg. encryption key is only accessible to hardware
crypto engine etc.
-Sumit
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Is it possible for somebody from ARM to have the TBBR spec updated to
reflect this? Also perhaps talk to the spec writers about incorporating
authenticated encryption into TBBR and PSA? This patch set is somewhat
trailblazing in this regard.
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-Raghu
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On January 23, 2020 at 12:08 PM, Raghupathy Krishnamurthy via TF-A <tf-
a@lists.trustedfirmware.org> wrote:
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Hi Sumit,
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Thanks for your response.
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So firstly I would suggest you to revisit TBBR spec [1],
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[RK] I'm very familiar with the TBBR spec and the requirements. Note
that not all SoC's adhere perfectly to the TBBR spec, since it does
not apply to devices in all market segments. However, these devices do
use arm trusted firmware and TBBR CoT in a slightly modified form,
which is still perfectly valid. Also, the TBBR spec can be changed if
required :)
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Why would one use authenticated decryption only to establish TBBR
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Chain of Trust providing device the capability to self sign its firmware?
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[RK] Fair point. However, you may have devices that don't have the
processing power or hardware budget or cost factors(paying for HSM's to store
private asymmetric keys), to implement asymmetric verification, in which case
using authenticated decryption to verify firmware authenticity and integrity is
perfectly valid. The attacks on devices that use symmetric keys to verify
firmware authenticity and integrity are usually related to exploiting firmware
flaws that leak the key or insiders leaking keys, but that is a different problem
and requires different solutions. Fundamentally, there is nothing wrong with
using symmetric keys for this purpose, so long as the key is well protected. Also
note, security requirements and guarantees are different for different systems.
The risk is taken by the system designer and should not be imposed by
framework code. I don't advocate doing this but it is an option that your
implementation does not provide(and perhaps rightly so).
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How would this ensure integrity of ciphertext?
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[RK] You sign the ciphertext. In your design, you pass bl31_enc.bin to
cert_tool to sign. You don't decrypt the encrypted cipher text until you have
verified the asymmetric signature(which provides integrity). As far as signature
verification is concerned, whether you sign the plain text or ciphertext is
immaterial, since you are simply verifying that the absolute bits you loaded
have not been modified(assuming you use a secure signature scheme).
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Have a look at some defective sign and encrypt techniques here [2]
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[RK] Again, very familiar with [2]. In the S/MIME case, you have multiple
parties. With secure boot, you have one party, effectively verifying its own
messages across time. There is only one key used to verify signatures. 1.1 and
1.2 does not apply. Also you are encrypting and signing with completely
different keys and algorithms. Section 1.2 applies when you use RSA/El-gamal
encryption. Here you use symmetric encryption and asymmetric signing.
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Why would one not use TBBR CoT here?
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[RK] see above. Not all systems are designed equal.
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and why would one like to hardcode in a device during
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provisioning to boot only either an encrypted or a plain firmware
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image?
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[RK] Why would you not? You typically want to have the same security policy
for a class of devices and not be modifiable by an attacker. It isn't common for
the same class of devices to use encrypted firmware some times, and un-
encrypted firmware other times. If it is common, there is no problem with
setting the bit in the FIP header, as long as verified boot is mandatory. The only
concern(as my original email said) is the coupling of the FIP layer and the
crypto module, in the implementation. I still don't like that fact that the bit
saying the file is encrypted is not signed and this may require talking to the
TBBR spec writers. Page 22 of the TBBR spec calls out ToC as "Trusted Table of
Contents". The FIP header cannot be "trusted", if it is not in ROM or its integrity
is verified by some means! R010_TBBR_TOC should perhaps be mandatory
then. Also see R080_TBBR_TOC that says the TOC MUST be ROM'ed or tied by
hardware in readable registers. This requirement seems contradictory to
R010_TBBR_TOC, given that the FIP header(TOC) is copied from mutable NVM
by ROM or some boot stage and then ROM'd or loaded into registers. I may be
misunderstanding R080_TBBR_TOC, but i'd interpret it as the TOC(FIP header
in ATF implementation of TBBR) as being in ROM or integrity verified.
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How would one handle a case where BL31 is in plain format and BL32 is in
encrypted format?
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[RK]TBBR CoT is equipped to do this. The table is defined on a per image
basis.
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If you are really paranoid about authentication of FIP header...
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[RK] I don't mean to pontificate but there are real world customers buying
real hardware, running ATF code, who care about such details and ask about
such things routinely. It is not just me being paranoid and is definitely not a
minor matter to think of such details. We should discuss more and consider the
implications of R080_TBBR_TOC and R010_TBBR_TOC, perhaps on a different
thread, without blocking your code review. Can somebody from ARM clarify
these requirements with the spec writers?
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Thanks
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-Raghu
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On January 23, 2020 at 12:38 AM, Sumit Garg sumit.garg@linaro.org
wrote:
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Hi Raghu,
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I guess you have completely misunderstood this feature. This is an
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optional feature which allows to load encrypted FIP payloads using
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authenticated decryption which MUST be used along with signature
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verification (or TBBR CoT).
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So firstly I would suggest you to revisit TBBR spec [1], especially
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requirements: R040_TBBR_TOC, R060_TBBR_FUNCTION etc.
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On Thu, 23 Jan 2020 at 00:14, Raghupathy Krishnamurthy
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raghu.ncstate@icloud.com wrote:
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Hello,
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The patch stack looks good. The only comment i have is that the FIP layer has
now become security aware and supports authenticated decryption(only). This
is a deviation from the secure/signed/verified boot design, where we use the
TBBR COT to dictate the security operations on the file. This is nice, because
file IO is decoupled from the security policy. This may be a big deviation(i
apologize if this was considered and shot down for some other reason), but it
may be worthwhile to consider making authenticated decryption a part of the
authentication framework as opposed to coupling it with the FIP layer.
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It looks like you have mixed both TBBR CoT and this authenticated
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decryption feature. They both are completely different and rather
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complement each other where TBBR CoT establishes
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secure/signed/verified boot and this authenticated decryption feature
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provides confidentiality protection for FIP payloads.
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At a high level, this would mean adding a new authentication
method(perhaps AUTH_METHOD_AUTHENTICATED_DECRYPTION), and having
the platform specify that the image is using authenticated encryption in the
TBBR COT.
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Why would one use authenticated decryption only to establish TBBR
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Chain of Trust providing device the capability to self sign its
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firmwares? We must use signature verification for TBBR CoT (see
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section: 2.1 Authentication of Code Images by Certificate in TBBR spec
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[1]).
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The authentication framework is already well designed and well equipped to
handle these types of extensions.
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This would make the change simpler, since you would not require changes

to the FIP tool and the FIP layer.
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This would also allow for future cases where a platform may want to only

encrypt the file and use public key authentication on the encrypted file(for ex.
the soc does not have a crypto accelerator for aes-gcm but only for AES and
public key verification, for whatever reason).
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How would this ensure integrity of ciphertext? This approach may be
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vulnerable to Chosen Ciphertext Attacks (CCAs). Authentication tag as
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part of AES-GCM provides integrity protection for ciphertext.
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This would let you choose the order in which you want to do the

authenticated decryption(or just decryption) and signature verification, if you
use both, one or the other.
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Have a look at some defective sign and encrypt techniques here [2].
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The order can't be any arbitrary one, we need to be careful about
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this.
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One other thing i'm not entirely comfortable with is that the flag indicating if
there are encrypted files or not in the FIP, is in the *unsigned* portion of the
FIP header. An attacker could simply flip bits that dictate security policy in the
header and avoid detection(in this case, the indication that the file needs
authenticated decryption). If a platform only uses authenticated encryption, but
not verified boot, an attacker could flip the bit in the FIP header and have any
image loaded on the platform.
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Why would one not use TBBR CoT here?
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If authenticated encryption cannot be used without verified boot(which
requires build time flags), having a flag to indicate that there are encrypted
files in the FIP header is moot, since this can come at build time through the
TBBR COT. In any case, it seems like the security policy that firmware images
need to be decrypted or authenticated with authenticated decryption, seems
like a firmware build time or manufacturing time decision(perhaps a bit set in
the e-fuses).
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Again you are confusing TBBR CoT with authenticated decryption
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feature. And why would one like to hardcode in a device during
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provisioning to boot only either an encrypted or a plain firmware
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image?
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There seems to be no benefit to having a flag in the FIP header.
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How would one handle a case where BL31 is in plain format and BL32 is
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in encrypted format?
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Otherwise, I cant think of any attacks due to this and it may be completely
okay, but generally, consuming data that dictates security policy/operations
before verifying its integrity seems like a recipe for disaster.
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If you are really paranoid about authentication of FIP header then you
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should look at implementing optional requirement: R010_TBBR_TOC as per
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TBBR spec [1].
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[1]
https://developer.arm.com/docs/den0006/latest/trusted-board-boot-requi
rements-client-tbbr-client-armv8-a
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[2] http://world.std.com/~dtd/sign_encrypt/sign_encrypt7.html
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-Sumit
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-Raghu
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On January 22, 2020 at 3:51 AM, Sumit Garg via TF-A <tf-
a@lists.trustedfirmware.org> wrote:
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Hi Sandrine,
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On Wed, 22 Jan 2020 at 15:43, Sandrine Bailleux
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Sandrine.Bailleux@arm.com wrote:
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Hello Sumit,
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Thank you for reworking the patches and addressing all of my review
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comments. I am happy with the latest version of these and consider
them
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ready to go. I plan to leave them in Gerrit for another week to give
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extra time for other potential reviewers to have a look and comment.
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Thanks for your review.
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To everyone on the list: Please raise any concerns you may have about
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these patches in the coming week. If I don't hear anything by 29th
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January 2020, I will merge these patches.
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@Sumit: One of the next actions for this patch stack would be to have
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some level of testing in the CI system to detect any potential
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regressions. We (at Arm) can quite easily add a few build tests but
then
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testing the software stack on QEMU is a bit more involved for various
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reasons (first instance of QEMU testing, dependencies on OPTEE, UEFI,
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Okay, will wait for CI testing.
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-Sumit
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Regards,
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Sandrine
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