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

by Raghupathy Krishnamurthy

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(a)linaro.org> wrote: On Mon, 27 Jan 2020 at 22:54, Raghupathy Krishnamurthy <raghu.ncstate(a)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: 1) Verify asymmetric signature. 2) Decrypt firmware using SSK on successful signature verification. 3) 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(a)linaro.org> wrote: On Fri, 24 Jan 2020 at 16:36, Sumit Garg <sumit.garg(a)linaro.org> wrote: On Fri, 24 Jan 2020 at 04:02, Raghupathy Krishnamurthy <raghu.ncstate(a)icloud.com> wrote: > > 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. > > 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 > > 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. > > -Raghu > > > On January 23, 2020 at 12:08 PM, Raghupathy Krishnamurthy via TF-A <tf-a(a)lists.trustedfirmware.org> wrote: > > Hi Sumit, > > > Thanks for your response. > > > So firstly I would suggest you to revisit TBBR spec [1], > > > > [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 :) > > > Why would one use authenticated decryption only to establish TBBR > > Chain of Trust providing device the capability to self sign its firmware? > > [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). > > > How would this ensure integrity of ciphertext? > > > > [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). > > > Have a look at some defective sign and encrypt techniques here [2] > > > > [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. > > > Why would one not use TBBR CoT here? > > > > [RK] see above. Not all systems are designed equal. > > > > and why would one like to hardcode in a device during > > provisioning to boot only either an encrypted or a plain firmware > image? > > > [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. > > > How would one handle a case where BL31 is in plain format and BL32 is in encrypted format? > > > > [RK]TBBR CoT is equipped to do this. The table is defined on a per image basis. > > > If you are really paranoid about authentication of FIP header... > > [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? > > > Thanks > -Raghu > > > On January 23, 2020 at 12:38 AM, Sumit Garg <sumit.garg(a)linaro.org> wrote: > > > Hi Raghu, > > I guess you have completely misunderstood this feature. This is an > optional feature which allows to load encrypted FIP payloads using > authenticated decryption which MUST be used along with signature > verification (or TBBR CoT). > > So firstly I would suggest you to revisit TBBR spec [1], especially > requirements: R040_TBBR_TOC, R060_TBBR_FUNCTION etc. > > On Thu, 23 Jan 2020 at 00:14, Raghupathy Krishnamurthy > <raghu.ncstate(a)icloud.com> wrote: > > > > Hello, > > > 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. > > It looks like you have mixed both TBBR CoT and this authenticated > decryption feature. They both are completely different and rather > complement each other where TBBR CoT establishes > secure/signed/verified boot and this authenticated decryption feature > provides confidentiality protection for FIP payloads. > > > 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. > > Why would one use authenticated decryption only to establish TBBR > Chain of Trust providing device the capability to self sign its > firmwares? We must use signature verification for TBBR CoT (see > section: 2.1 Authentication of Code Images by Certificate in TBBR spec > [1]). > > > The authentication framework is already well designed and well equipped to handle these types of extensions. > 1) This would make the change simpler, since you would not require changes to the FIP tool and the FIP layer. > 2) 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). > > How would this ensure integrity of ciphertext? This approach may be > vulnerable to Chosen Ciphertext Attacks (CCAs). Authentication tag as > part of AES-GCM provides integrity protection for ciphertext. > > > 3) 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. > > > > Have a look at some defective sign and encrypt techniques here [2]. > The order can't be any arbitrary one, we need to be careful about > this. > > > 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. > > Why would one not use TBBR CoT here? > > > 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). > > Again you are confusing TBBR CoT with authenticated decryption > feature. And why would one like to hardcode in a device during > provisioning to boot only either an encrypted or a plain firmware > image? > > > There seems to be no benefit to having a flag in the FIP header. > > How would one handle a case where BL31 is in plain format and BL32 is > in encrypted format? > > > 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. > > > > If you are really paranoid about authentication of FIP header then you > should look at implementing optional requirement: R010_TBBR_TOC as per > TBBR spec [1]. > > [1] https://developer.arm.com/docs/den0006/latest/trusted-board-boot-requiremen… > [2] http://world.std.com/~dtd/sign_encrypt/sign_encrypt7.html > > -Sumit > > > -Raghu > > > On January 22, 2020 at 3:51 AM, Sumit Garg via TF-A <tf-a(a)lists.trustedfirmware.org> wrote: > > > Hi Sandrine, > > > On Wed, 22 Jan 2020 at 15:43, Sandrine Bailleux > <Sandrine.Bailleux(a)arm.com> wrote: > > > > > Hello Sumit, > > > > > Thank you for reworking the patches and addressing all of my review > > > comments. I am happy with the latest version of these and consider them > > > ready to go. I plan to leave them in Gerrit for another week to give > > > extra time for other potential reviewers to have a look and comment. > > > > > > > Thanks for your review. > > > To everyone on the list: Please raise any concerns you may have about > > > these patches in the coming week. If I don't hear anything by 29th > > > January 2020, I will merge these patches. > > > > > @Sumit: One of the next actions for this patch stack would be to have > > > some level of testing in the CI system to detect any potential > > > regressions. We (at Arm) can quite easily add a few build tests but then > > > testing the software stack on QEMU is a bit more involved for various > > > reasons (first instance of QEMU testing, dependencies on OPTEE, UEFI, > > > ...) so this might have to wait for some time. > > > > > > > Okay, will wait for CI testing. > > > -Sumit > > > Regards, > > > Sandrine > > > -- > TF-A mailing list > TF-A(a)lists.trustedfirmware.org > https://lists.trustedfirmware.org/mailman/listinfo/tf-a > -- > TF-A mailing list > TF-A(a)lists.trustedfirmware.org > https://lists.trustedfirmware.org/mailman/listinfo/tf-a

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

by Sumit Garg

On Mon, 27 Jan 2020 at 22:43, Raghupathy Krishnamurthy via TF-A <tf-a(a)lists.trustedfirmware.org> wrote: > > Thanks Soby, Sumit, Sandrine and Joakim! > > > Agree with Joakim/Soby about encrypt then sign and sign then encrypt being okay, when authenticated encryption is used. I'd like to point out again, that the link provided by Sumit talks about encrypt-then-sign and sign-then-encrypt, using asymmetric encryption and asymmetric signing only, and as Joakim rightly pointed out(and as i did in my earlier email), does not necessarily apply here. They are not talking about symmetric encryption with asymmetric signing, which is what PSA-TBFU and TBBR are talking about. > I usually don't like signing plain text and encrypt(not authenticated encryption, just encryption like aes-cbc etc) plain text due to https://moxie.org/blog/the-cryptographic-doom-principle/(written by the creator of the the WhatsApp end-to-end encryption protocol, this applies to symmetric encryption and symmetric signing/MAC's, but it applies to any ciphertext that is decrypted without verifying its integrity). If we were to sign the plain text then encrypt the firmware, the size of the encrypted file needs to be integrity protected as well, not just the bit that indicates that the file is encrypted. If not, when we decrypt the firmware image, we can perform some creative attacks on symmetric encryption, as specified in the link, and requires careful implementation of error handling/reporting on decryption errors. Just to clarify here further that authenticated encryption (aes-gcm) follows “encrypt-then-authenticate” principal only, which is mentioned optimal as per https://moxie.org/blog/the-cryptographic-doom-principle/. -Sumit > > > I also like Soby's approach of having an attribute EP_ENCRYPTED and relying on platform_pre_image_load() and platform_post_image_load() to do the decryption and that addresses my non-spec related concerns. It also allows for decrypt-then-authenticate(which i wouldn't use) and authenticate-then-decrypt. It also puts the attribute that indicates firmware encryption in the image descriptor table which is integrity protected by virtue of it being in the ROM or being signed, and also leaves the FIP layer unaltered. > > > Thanks > -Raghu > > On January 27, 2020 at 6:42 AM, Soby Mathew <Soby.Mathew(a)arm.com> wrote: > > > On 27/01/2020 12:34, Joakim Bech via TF-A wrote: > > on raw binaries are there so we can be sure that > we're loading unmodified firmware coming from the one owning a private key > corresponding to the public key hardcoded into the device (or via a hash of > the public key). I think we all can agree > > OK, I have finally managed to catch up on this thread. Apologies for the > delayed response. > > As Joakim mentions, I think both the mentioned cases ie. encrypt plain > text -> sign and sign -> encrypt are valid and it depend on the threat > model and security requirement. > > I have had a brief look at the patch stack and coupling the feature to > the FIP does not seem like a good idea to me( + the added complexity > protecting the ToC). Currently meta data of the firmware images is > passed OOB via the `bl_mem_params_node_t` descriptor to the BL images > whereas this patch breaks that convention. It is better to follow the > set convention and avoid dependency on FIP format (btw platforms need > not use FIP format and can use other packaging formats). > > The iv data to decrypt seems to be prepended to the encrypted file in > the fip which is making custom manipulations file pointer manipulations > which is raising some red flags. > > https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/2495/8/drive… > > IMO, this should not visible to be FIP driver. I am not sure of the best > practice for sending iv data but if it has to be prepended, it should be > FIP agnostic. > > The TBBR assumes a single owner for all images whereas the TBFU > supercedes this spec by allowing multiple owners for different images. > We are in the process of enhancing the CoT for different Root of Trusts > for different images and there needs to be capability to encrypt with > different keys for different images based on ownership. The current > implementation has some limitations like introducing a platform API > invocation within the driver layer to get the single key which is not > ideal IMO. > > I haven't fully flushed out my ideas, but thoughts are based on > enhancements done by Masahiro for adding decompression support for BL > images. See https://github.com/ARM-software/arm-trusted-firmware/pull/1224 . > > Basically, this patch series allows any filter to be setup before/after > the images are loaded. It relies on pre-load and post load hooks which > are platform specific to perform the filter operation. So, if a new > image attribute `EP_ENCRYPTED` is added ep_info_exp.h, then BL2 needs to > do the following in bl2_plat_handle_pre_image_load() > { > if EP_ENCRYPTED is set : > load the image to SRAM and decrypt using crypto module > } > > This will cater for `decrypt-then-authenticate` flow. > > The fip_tool no longer needs to be aware of encryption and the build > process just needs to pipe the encrypted binaries to the fip_tool. > > Similarly if `authenticate-then-decrypt` needs to be supported, then all > that the platform needs to do is implement decrypt in > bl2_plat_handle_post_image_load(). > > The platform can now use different keys to use for different BL images > if it needs to do so. > > Some usecases require the firmware images to be re-encrypted using HUK > after firmware update (aka firmware binding) and following this design > will allow this to be done as well. > > > Best Regards > Soby Mathew > -- > TF-A mailing list > TF-A(a)lists.trustedfirmware.org > https://lists.trustedfirmware.org/mailman/listinfo/tf-a

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

by Raghupathy Krishnamurthy

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: 1) Verify asymmetric signature. 2) Decrypt firmware using SSK on successful signature verification. 3) 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. Thanks Raghu On January 26, 2020 at 10:34 PM, Sumit Garg <sumit.garg(a)linaro.org> wrote: On Fri, 24 Jan 2020 at 16:36, Sumit Garg <sumit.garg(a)linaro.org> wrote: On Fri, 24 Jan 2020 at 04:02, Raghupathy Krishnamurthy <raghu.ncstate(a)icloud.com> wrote: > > 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. > > 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 > > 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. > > -Raghu > > > On January 23, 2020 at 12:08 PM, Raghupathy Krishnamurthy via TF-A <tf-a(a)lists.trustedfirmware.org> wrote: > > Hi Sumit, > > > Thanks for your response. > > > So firstly I would suggest you to revisit TBBR spec [1], > > > > [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 :) > > > Why would one use authenticated decryption only to establish TBBR > > Chain of Trust providing device the capability to self sign its firmware? > > [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). > > > How would this ensure integrity of ciphertext? > > > > [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). > > > Have a look at some defective sign and encrypt techniques here [2] > > > > [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. > > > Why would one not use TBBR CoT here? > > > > [RK] see above. Not all systems are designed equal. > > > > and why would one like to hardcode in a device during > > provisioning to boot only either an encrypted or a plain firmware > image? > > > [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. > > > How would one handle a case where BL31 is in plain format and BL32 is in encrypted format? > > > > [RK]TBBR CoT is equipped to do this. The table is defined on a per image basis. > > > If you are really paranoid about authentication of FIP header... > > [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? > > > Thanks > -Raghu > > > On January 23, 2020 at 12:38 AM, Sumit Garg <sumit.garg(a)linaro.org> wrote: > > > Hi Raghu, > > I guess you have completely misunderstood this feature. This is an > optional feature which allows to load encrypted FIP payloads using > authenticated decryption which MUST be used along with signature > verification (or TBBR CoT). > > So firstly I would suggest you to revisit TBBR spec [1], especially > requirements: R040_TBBR_TOC, R060_TBBR_FUNCTION etc. > > On Thu, 23 Jan 2020 at 00:14, Raghupathy Krishnamurthy > <raghu.ncstate(a)icloud.com> wrote: > > > > Hello, > > > 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. > > It looks like you have mixed both TBBR CoT and this authenticated > decryption feature. They both are completely different and rather > complement each other where TBBR CoT establishes > secure/signed/verified boot and this authenticated decryption feature > provides confidentiality protection for FIP payloads. > > > 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. > > Why would one use authenticated decryption only to establish TBBR > Chain of Trust providing device the capability to self sign its > firmwares? We must use signature verification for TBBR CoT (see > section: 2.1 Authentication of Code Images by Certificate in TBBR spec > [1]). > > > The authentication framework is already well designed and well equipped to handle these types of extensions. > 1) This would make the change simpler, since you would not require changes to the FIP tool and the FIP layer. > 2) 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). > > How would this ensure integrity of ciphertext? This approach may be > vulnerable to Chosen Ciphertext Attacks (CCAs). Authentication tag as > part of AES-GCM provides integrity protection for ciphertext. > > > 3) 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. > > > > Have a look at some defective sign and encrypt techniques here [2]. > The order can't be any arbitrary one, we need to be careful about > this. > > > 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. > > Why would one not use TBBR CoT here? > > > 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). > > Again you are confusing TBBR CoT with authenticated decryption > feature. And why would one like to hardcode in a device during > provisioning to boot only either an encrypted or a plain firmware > image? > > > There seems to be no benefit to having a flag in the FIP header. > > How would one handle a case where BL31 is in plain format and BL32 is > in encrypted format? > > > 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. > > > > If you are really paranoid about authentication of FIP header then you > should look at implementing optional requirement: R010_TBBR_TOC as per > TBBR spec [1]. > > [1] https://developer.arm.com/docs/den0006/latest/trusted-board-boot-requiremen… > [2] http://world.std.com/~dtd/sign_encrypt/sign_encrypt7.html > > -Sumit > > > -Raghu > > > On January 22, 2020 at 3:51 AM, Sumit Garg via TF-A <tf-a(a)lists.trustedfirmware.org> wrote: > > > Hi Sandrine, > > > On Wed, 22 Jan 2020 at 15:43, Sandrine Bailleux > <Sandrine.Bailleux(a)arm.com> wrote: > > > > > Hello Sumit, > > > > > Thank you for reworking the patches and addressing all of my review > > > comments. I am happy with the latest version of these and consider them > > > ready to go. I plan to leave them in Gerrit for another week to give > > > extra time for other potential reviewers to have a look and comment. > > > > > > > Thanks for your review. > > > To everyone on the list: Please raise any concerns you may have about > > > these patches in the coming week. If I don't hear anything by 29th > > > January 2020, I will merge these patches. > > > > > @Sumit: One of the next actions for this patch stack would be to have > > > some level of testing in the CI system to detect any potential > > > regressions. We (at Arm) can quite easily add a few build tests but then > > > testing the software stack on QEMU is a bit more involved for various > > > reasons (first instance of QEMU testing, dependencies on OPTEE, UEFI, > > > ...) so this might have to wait for some time. > > > > > > > Okay, will wait for CI testing. > > > -Sumit > > > Regards, > > > Sandrine > > > -- > TF-A mailing list > TF-A(a)lists.trustedfirmware.org > https://lists.trustedfirmware.org/mailman/listinfo/tf-a > -- > TF-A mailing list > TF-A(a)lists.trustedfirmware.org > https://lists.trustedfirmware.org/mailman/listinfo/tf-a

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

by Raghupathy Krishnamurthy

Thanks Soby, Sumit, Sandrine and Joakim! Agree with Joakim/Soby about encrypt then sign and sign then encrypt being okay, when authenticated encryption is used. I'd like to point out again, that the link provided by Sumit talks about encrypt-then-sign and sign-then-encrypt, using asymmetric encryption and asymmetric signing only, and as Joakim rightly pointed out(and as i did in my earlier email), does not necessarily apply here. They are not talking about symmetric encryption with asymmetric signing, which is what PSA-TBFU and TBBR are talking about. I usually don't like signing plain text and encrypt(not authenticated encryption, just encryption like aes-cbc etc) plain text due to https://moxie.org/blog/the-cryptographic-doom-principle/(written by the creator of the the WhatsApp end-to-end encryption protocol, this applies to symmetric encryption and symmetric signing/MAC's, but it applies to any ciphertext that is decrypted without verifying its integrity). If we were to sign the plain text then encrypt the firmware, the size of the encrypted file needs to be integrity protected as well, not just the bit that indicates that the file is encrypted. If not, when we decrypt the firmware image, we can perform some creative attacks on symmetric encryption, as specified in the link, and requires careful implementation of error handling/reporting on decryption errors. I also like Soby's approach of having an attribute EP_ENCRYPTED and relying on platform_pre_image_load() and platform_post_image_load() to do the decryption and that addresses my non-spec related concerns. It also allows for decrypt-then-authenticate(which i wouldn't use) and authenticate-then-decrypt. It also puts the attribute that indicates firmware encryption in the image descriptor table which is integrity protected by virtue of it being in the ROM or being signed, and also leaves the FIP layer unaltered. Thanks -Raghu On January 27, 2020 at 6:42 AM, Soby Mathew <Soby.Mathew(a)arm.com> wrote: On 27/01/2020 12:34, Joakim Bech via TF-A wrote: on raw binaries are there so we can be sure that we're loading unmodified firmware coming from the one owning a private key corresponding to the public key hardcoded into the device (or via a hash of the public key). I think we all can agree OK, I have finally managed to catch up on this thread. Apologies for the delayed response. As Joakim mentions, I think both the mentioned cases ie. encrypt plain text -> sign and sign -> encrypt are valid and it depend on the threat model and security requirement. I have had a brief look at the patch stack and coupling the feature to the FIP does not seem like a good idea to me( + the added complexity protecting the ToC). Currently meta data of the firmware images is passed OOB via the `bl_mem_params_node_t` descriptor to the BL images whereas this patch breaks that convention. It is better to follow the set convention and avoid dependency on FIP format (btw platforms need not use FIP format and can use other packaging formats). The iv data to decrypt seems to be prepended to the encrypted file in the fip which is making custom manipulations file pointer manipulations which is raising some red flags. https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/2495/8/drive… IMO, this should not visible to be FIP driver. I am not sure of the best practice for sending iv data but if it has to be prepended, it should be FIP agnostic. The TBBR assumes a single owner for all images whereas the TBFU supercedes this spec by allowing multiple owners for different images. We are in the process of enhancing the CoT for different Root of Trusts for different images and there needs to be capability to encrypt with different keys for different images based on ownership. The current implementation has some limitations like introducing a platform API invocation within the driver layer to get the single key which is not ideal IMO. I haven't fully flushed out my ideas, but thoughts are based on enhancements done by Masahiro for adding decompression support for BL images. See https://github.com/ARM-software/arm-trusted-firmware/pull/1224 . Basically, this patch series allows any filter to be setup before/after the images are loaded. It relies on pre-load and post load hooks which are platform specific to perform the filter operation. So, if a new image attribute `EP_ENCRYPTED` is added ep_info_exp.h, then BL2 needs to do the following in bl2_plat_handle_pre_image_load() { if EP_ENCRYPTED is set : load the image to SRAM and decrypt using crypto module } This will cater for `decrypt-then-authenticate` flow. The fip_tool no longer needs to be aware of encryption and the build process just needs to pipe the encrypted binaries to the fip_tool. Similarly if `authenticate-then-decrypt` needs to be supported, then all that the platform needs to do is implement decrypt in bl2_plat_handle_post_image_load(). The platform can now use different keys to use for different BL images if it needs to do so. Some usecases require the firmware images to be re-encrypted using HUK after firmware update (aka firmware binding) and following this design will allow this to be done as well. Best Regards Soby Mathew

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Re: [TF-A] What prevents BL1 FWU SMC's from being called on multiple cores ?

by Soby Mathew

On 24/01/2020 20:20, Raghupathy Krishnamurthy via TF-A wrote: > It appears that the BL1 FWU SMC's are written under the assumption that only one core can call the SMC's at any given time but i don't see anything enforcing it. What prevent's this ? > > > -Raghu > Hi Raghu The BL1 itself is uni-processor [except the early assembly code which differentiates primary core from secondaries]. Hence it makes no attempt to provide protection for the SMCs from multiple cores. Best Regards Soby Mathew

6 years

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

by Raghupathy Krishnamurthy

Thanks Sumit. We are beginning to go off topic and i don't think you and i agree entirely. I'll lay out my concerns, to see if others on the list share my opinion/concerns, so i hope you can let them respond. If nobody else shares my concerns or responds, the patch will be merged soon anyway, so no point dragging this out :) I apologize for the long email. I don't know how to keep this topic short and detailed. :) Here are my concerns: 1) The patch set claims to implement R060_TBBR_FUNCTION from the TBBR spec. I don't agree with this. The patch is using authenticated encryption(which does provide confidentiality) but is different from implementing firmware encryption as described in the TBBR(and PSA) in my view. If we are implementing R060_TBBR_FUNCTION, in my view, this should be implemented such that the firmware is first encrypted(symmetric), and the encrypted firmware should be asymmetrically signed(as per PSA). Also, the order for verification should be asymmetric signature verification and then decryption(symmetric). In this patch, plain text firmware is asymmetrically signed, plain text firmware is encrypted and authenticated with a symmetric key using authenticated encryption. The order for verification in this patch is authenticate and decrypt using the symmetric key and THEN asymmetric signature verification on the decryted(and now plain text) firmware. The ordering of verification and decryption are subtly different from what the spec(TBBR and PSA) expects. Security does not *seem* to be broken as far as i can tell, but the patch is not strictly an implementation of the spec, as it claims. This can throw off people reading the spec and trying to match code to spec. This is not good when someone wants to audit and gain confidence in the security of the implementation. At a minimum, we need someone writing the spec to address this. 2) The TBBR spec does not talk about authenticated encryption of firmware and only talks about encryption of firmware. It also does not specify if the encrypted firmware needs to be signed or the plain text need to be signed. This is critical detail. It also does not specify the order in which encryption and signing, decryption and verification must be done. These are important details and need to be addressed explicitly(perhaps in both TBBR and PSA). Use of authenticated encryption needs to be explicitly discussed as well. 3) TBBR specifies that the TOC(table of contents) *may* be authenticated. The TOC contains bits that dictate security policy, in this case, if the firmware is encrypted or not. I don't think it is good to consume security policy from unauthenticated data. In my view, this information must be in the signed image manifest(certificate in ATF) since an attacker can at-least cause simple DoS attacks by flipping bits in the FIP header, undetected. If the bit is flipped, a firmware image going out to a million devices may not boot, since an attacker decided to flip a bit, and signature verification fails, since the boot loader decided not decrypt the firmware based in an unsigned bit of information. PSA appears to have a better approach and makes no provision such data to be outside the image manifest, which is always signed. 4) This last one is a matter of opinion and can live with the current design. The FIP layer is now aware of the crypto module. FIP has also become coupled with authenticated encryption(If we do this, why not have the FIP layer call auth_mod_verify_img() and why stop at cyrpto_mod_auth_decrypt()) . FIP is security aware. I'd like to keep things the way they are today, where IO and security are separate modules and glued together by another layer(load_auth_image() for example). This is cleaner in my view since security policy(should i decrypt? should i verify signatures? should i apply any security at all?) is separate from IO. Thanks Raghu On January 24, 2020 at 3:06 AM, Sumit Garg <sumit.garg(a)linaro.org> wrote: On Fri, 24 Jan 2020 at 04:02, Raghupathy Krishnamurthy <raghu.ncstate(a)icloud.com> wrote: 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. 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. -Sumit 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. -Raghu On January 23, 2020 at 12:08 PM, Raghupathy Krishnamurthy via TF-A <tf-a(a)lists.trustedfirmware.org> wrote: Hi Sumit, Thanks for your response. So firstly I would suggest you to revisit TBBR spec [1], [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 :) Why would one use authenticated decryption only to establish TBBR Chain of Trust providing device the capability to self sign its firmware? [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). How would this ensure integrity of ciphertext? [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). Have a look at some defective sign and encrypt techniques here [2] [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. Why would one not use TBBR CoT here? [RK] see above. Not all systems are designed equal. and why would one like to hardcode in a device during provisioning to boot only either an encrypted or a plain firmware image? [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. How would one handle a case where BL31 is in plain format and BL32 is in encrypted format? [RK]TBBR CoT is equipped to do this. The table is defined on a per image basis. If you are really paranoid about authentication of FIP header... [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? Thanks -Raghu On January 23, 2020 at 12:38 AM, Sumit Garg <sumit.garg(a)linaro.org> wrote: Hi Raghu, I guess you have completely misunderstood this feature. This is an optional feature which allows to load encrypted FIP payloads using authenticated decryption which MUST be used along with signature verification (or TBBR CoT). So firstly I would suggest you to revisit TBBR spec [1], especially requirements: R040_TBBR_TOC, R060_TBBR_FUNCTION etc. On Thu, 23 Jan 2020 at 00:14, Raghupathy Krishnamurthy <raghu.ncstate(a)icloud.com> wrote: Hello, 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. It looks like you have mixed both TBBR CoT and this authenticated decryption feature. They both are completely different and rather complement each other where TBBR CoT establishes secure/signed/verified boot and this authenticated decryption feature provides confidentiality protection for FIP payloads. 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. Why would one use authenticated decryption only to establish TBBR Chain of Trust providing device the capability to self sign its firmwares? We must use signature verification for TBBR CoT (see section: 2.1 Authentication of Code Images by Certificate in TBBR spec [1]). The authentication framework is already well designed and well equipped to handle these types of extensions. 1) This would make the change simpler, since you would not require changes to the FIP tool and the FIP layer. 2) 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). How would this ensure integrity of ciphertext? This approach may be vulnerable to Chosen Ciphertext Attacks (CCAs). Authentication tag as part of AES-GCM provides integrity protection for ciphertext. 3) 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. Have a look at some defective sign and encrypt techniques here [2]. The order can't be any arbitrary one, we need to be careful about this. 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. Why would one not use TBBR CoT here? 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). Again you are confusing TBBR CoT with authenticated decryption feature. And why would one like to hardcode in a device during provisioning to boot only either an encrypted or a plain firmware image? There seems to be no benefit to having a flag in the FIP header. How would one handle a case where BL31 is in plain format and BL32 is in encrypted format? 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. If you are really paranoid about authentication of FIP header then you should look at implementing optional requirement: R010_TBBR_TOC as per TBBR spec [1]. [1] https://developer.arm.com/docs/den0006/latest/trusted-board-boot-requiremen… [2] http://world.std.com/~dtd/sign_encrypt/sign_encrypt7.html -Sumit -Raghu On January 22, 2020 at 3:51 AM, Sumit Garg via TF-A <tf-a(a)lists.trustedfirmware.org> wrote: Hi Sandrine, On Wed, 22 Jan 2020 at 15:43, Sandrine Bailleux <Sandrine.Bailleux(a)arm.com> wrote: Hello Sumit, Thank you for reworking the patches and addressing all of my review comments. I am happy with the latest version of these and consider them ready to go. I plan to leave them in Gerrit for another week to give extra time for other potential reviewers to have a look and comment. Thanks for your review. To everyone on the list: Please raise any concerns you may have about these patches in the coming week. If I don't hear anything by 29th January 2020, I will merge these patches. @Sumit: One of the next actions for this patch stack would be to have some level of testing in the CI system to detect any potential regressions. We (at Arm) can quite easily add a few build tests but then testing the software stack on QEMU is a bit more involved for various reasons (first instance of QEMU testing, dependencies on OPTEE, UEFI, ...) so this might have to wait for some time. Okay, will wait for CI testing. -Sumit Regards, Sandrine -- TF-A mailing list TF-A(a)lists.trustedfirmware.org https://lists.trustedfirmware.org/mailman/listinfo/tf-a -- TF-A mailing list TF-A(a)lists.trustedfirmware.org https://lists.trustedfirmware.org/mailman/listinfo/tf-a

6 years

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

by Soby Mathew

On 27/01/2020 12:34, Joakim Bech via TF-A wrote: > on raw binaries are there so we can be sure that > we're loading unmodified firmware coming from the one owning a private key > corresponding to the public key hardcoded into the device (or via a hash of > the public key). I think we all can agree OK, I have finally managed to catch up on this thread. Apologies for the delayed response. As Joakim mentions, I think both the mentioned cases ie. encrypt plain text -> sign and sign -> encrypt are valid and it depend on the threat model and security requirement. I have had a brief look at the patch stack and coupling the feature to the FIP does not seem like a good idea to me( + the added complexity protecting the ToC). Currently meta data of the firmware images is passed OOB via the `bl_mem_params_node_t` descriptor to the BL images whereas this patch breaks that convention. It is better to follow the set convention and avoid dependency on FIP format (btw platforms need not use FIP format and can use other packaging formats). The iv data to decrypt seems to be prepended to the encrypted file in the fip which is making custom manipulations file pointer manipulations which is raising some red flags. https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/2495/8/drive… IMO, this should not visible to be FIP driver. I am not sure of the best practice for sending iv data but if it has to be prepended, it should be FIP agnostic. The TBBR assumes a single owner for all images whereas the TBFU supercedes this spec by allowing multiple owners for different images. We are in the process of enhancing the CoT for different Root of Trusts for different images and there needs to be capability to encrypt with different keys for different images based on ownership. The current implementation has some limitations like introducing a platform API invocation within the driver layer to get the single key which is not ideal IMO. I haven't fully flushed out my ideas, but thoughts are based on enhancements done by Masahiro for adding decompression support for BL images. See https://github.com/ARM-software/arm-trusted-firmware/pull/1224 . Basically, this patch series allows any filter to be setup before/after the images are loaded. It relies on pre-load and post load hooks which are platform specific to perform the filter operation. So, if a new image attribute `EP_ENCRYPTED` is added ep_info_exp.h, then BL2 needs to do the following in bl2_plat_handle_pre_image_load() { if EP_ENCRYPTED is set : load the image to SRAM and decrypt using crypto module } This will cater for `decrypt-then-authenticate` flow. The fip_tool no longer needs to be aware of encryption and the build process just needs to pipe the encrypted binaries to the fip_tool. Similarly if `authenticate-then-decrypt` needs to be supported, then all that the platform needs to do is implement decrypt in bl2_plat_handle_post_image_load(). The platform can now use different keys to use for different BL images if it needs to do so. Some usecases require the firmware images to be re-encrypted using HUK after firmware update (aka firmware binding) and following this design will allow this to be done as well. Best Regards Soby Mathew

6 years

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

by Sandrine Bailleux

On Mon, 2020-01-27 at 12:04 +0000, Sandrine Bailleux via TF-A wrote: > I share this concern. I was actually surprised to see that the TBBR > specification advocates putting this security policy bit in the > unencrypted part of the FIP, I do not know the rationale for that. Sorry, I meant: in the *unsigned* part of the FIP. This bit itself cannot be encrypted, as it indicates how to decrypt data!

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

by Sandrine Bailleux

Hi, On Fri, 2020-01-24 at 18:39 +0000, Raghupathy Krishnamurthy via TF-A wrote: > Here are my concerns: > 1) The patch set claims to implement R060_TBBR_FUNCTION from the TBBR > spec. I don't agree with this. The patch is using authenticated > encryption(which does provide confidentiality) but is different from > implementing firmware encryption as described in the TBBR(and PSA) in > my view. If we are implementing R060_TBBR_FUNCTION, in my view, this > should be implemented such that the firmware is first > encrypted(symmetric), and the encrypted firmware should be > asymmetrically signed(as per PSA). Also, the order for verification > should be asymmetric signature verification and then > decryption(symmetric). In this patch, plain text firmware is > asymmetrically signed, plain text firmware is encrypted and > authenticated with a symmetric key using authenticated encryption. > The order for verification in this patch is authenticate and decrypt > using the symmetric key and THEN asymmetric signature verification on > the decryted(and now plain text) firmware. The ordering of > verification and decryption are subtly different from what the > spec(TBBR and PSA) expects. Security does not *seem* to be broken as > far as i can tell, but the patch is not strictly an implementation of > the spec, as it claims. This can throw off people reading the spec > and trying to match code to spec. This is not good when someone wants > to audit and gain confidence in the security of the implementation. > At a minimum, we need someone writing the spec to address this. I do not have a strong opinion on this one. As I mentioned at the start of this email thread, I am quite new to the concept of authenticated decryption and naively thought this would not differ too much from using decryption + signature verification separately. However, if these are fundamentally different things like you say, I get your point and I too am worried about the confusion this could create. > 2) The TBBR spec does not talk about authenticated encryption of > firmware and only talks about encryption of firmware. It also does > not specify if the encrypted firmware needs to be signed or the plain > text need to be signed. This is critical detail. It also does not > specify the order in which encryption and signing, decryption and > verification must be done. These are important details and need to be > addressed explicitly(perhaps in both TBBR and PSA). Use of > authenticated encryption needs to be explicitly discussed as well. TBBR is quite an old specification and it is unlikely that it will ever be updated. PSA-TBFU is meant to supersede it so I think any update/clarification/addition would only apply to TBFU. I agree that it would be good for PSA-TBFU to provide some guidance around encryption and signature together, in which order they should be done and what security properties they bring together. I can raise this to the PSA architects. > 3) TBBR specifies that the TOC(table of contents) *may* be > authenticated. The TOC contains bits that dictate security policy, in > this case, if the firmware is encrypted or not. I don't think it is > good to consume security policy from unauthenticated data. In my > view, this information must be in the signed image > manifest(certificate in ATF) since an attacker can at-least cause > simple DoS attacks by flipping bits in the FIP header, undetected. If > the bit is flipped, a firmware image going out to a million devices > may not boot, since an attacker decided to flip a bit, and signature > verification fails, since the boot loader decided not decrypt the > firmware based in an unsigned bit of information. PSA appears to have > a better approach and makes no provision such data to be outside the > image manifest, which is always signed. I share this concern. I was actually surprised to see that the TBBR specification advocates putting this security policy bit in the unencrypted part of the FIP, I do not know the rationale for that. Given that TBBR will be deprecated in the future, I wonder whether it would be better to adopt PSA's approach here and move this security policy bit in the signed part of the FIP. This will make this part of the implementation non-TBBR compliant but again, I don't see that as a big concern, as we should look at PSA-TBFU as the future and not worry too much about TBBR if we have good reasons to diverge. > 4) This last one is a matter of opinion and can live with the current > design. The FIP layer is now aware of the crypto module. FIP has also > become coupled with authenticated encryption(If we do this, why not > have the FIP layer call auth_mod_verify_img() and why stop at > cyrpto_mod_auth_decrypt()) . FIP is security aware. I'd like to keep > things the way they are today, where IO and security are separate > modules and glued together by another layer(load_auth_image() for > example). This is cleaner in my view since security policy(should i > decrypt? should i verify signatures? should i apply any security at > all?) is separate from IO. I agree with you. I too found it more elegant and cleaner to have the FIP layer and crypto layer as 2 independent modules. I too feel slightly uncomfortable about introducing such a dependency. But I could not think of a way to avoid it while keeping the security policy bit in the FIP ToC header. That said, if the latter is something we're considering to redesign then we might be able to remove this dependency and keep the design as it is. Regards, Sandrine

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

by Sumit Garg

On Fri, 24 Jan 2020 at 04:02, Raghupathy Krishnamurthy <raghu.ncstate(a)icloud.com> wrote: > > 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. > > 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. -Sumit > > 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. > > -Raghu > > > On January 23, 2020 at 12:08 PM, Raghupathy Krishnamurthy via TF-A <tf-a(a)lists.trustedfirmware.org> wrote: > > Hi Sumit, > > > Thanks for your response. > > > So firstly I would suggest you to revisit TBBR spec [1], > > > > [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 :) > > > Why would one use authenticated decryption only to establish TBBR > > Chain of Trust providing device the capability to self sign its firmware? > > [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). > > > How would this ensure integrity of ciphertext? > > > > [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). > > > Have a look at some defective sign and encrypt techniques here [2] > > > > [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. > > > Why would one not use TBBR CoT here? > > > > [RK] see above. Not all systems are designed equal. > > > > and why would one like to hardcode in a device during > > provisioning to boot only either an encrypted or a plain firmware > image? > > > [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. > > > How would one handle a case where BL31 is in plain format and BL32 is in encrypted format? > > > > [RK]TBBR CoT is equipped to do this. The table is defined on a per image basis. > > > If you are really paranoid about authentication of FIP header... > > [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? > > > Thanks > -Raghu > > > On January 23, 2020 at 12:38 AM, Sumit Garg <sumit.garg(a)linaro.org> wrote: > > > Hi Raghu, > > I guess you have completely misunderstood this feature. This is an > optional feature which allows to load encrypted FIP payloads using > authenticated decryption which MUST be used along with signature > verification (or TBBR CoT). > > So firstly I would suggest you to revisit TBBR spec [1], especially > requirements: R040_TBBR_TOC, R060_TBBR_FUNCTION etc. > > On Thu, 23 Jan 2020 at 00:14, Raghupathy Krishnamurthy > <raghu.ncstate(a)icloud.com> wrote: > > > > Hello, > > > 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. > > It looks like you have mixed both TBBR CoT and this authenticated > decryption feature. They both are completely different and rather > complement each other where TBBR CoT establishes > secure/signed/verified boot and this authenticated decryption feature > provides confidentiality protection for FIP payloads. > > > 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. > > Why would one use authenticated decryption only to establish TBBR > Chain of Trust providing device the capability to self sign its > firmwares? We must use signature verification for TBBR CoT (see > section: 2.1 Authentication of Code Images by Certificate in TBBR spec > [1]). > > > The authentication framework is already well designed and well equipped to handle these types of extensions. > 1) This would make the change simpler, since you would not require changes to the FIP tool and the FIP layer. > 2) 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). > > How would this ensure integrity of ciphertext? This approach may be > vulnerable to Chosen Ciphertext Attacks (CCAs). Authentication tag as > part of AES-GCM provides integrity protection for ciphertext. > > > 3) 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. > > > > Have a look at some defective sign and encrypt techniques here [2]. > The order can't be any arbitrary one, we need to be careful about > this. > > > 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. > > Why would one not use TBBR CoT here? > > > 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). > > Again you are confusing TBBR CoT with authenticated decryption > feature. And why would one like to hardcode in a device during > provisioning to boot only either an encrypted or a plain firmware > image? > > > There seems to be no benefit to having a flag in the FIP header. > > How would one handle a case where BL31 is in plain format and BL32 is > in encrypted format? > > > 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. > > > > If you are really paranoid about authentication of FIP header then you > should look at implementing optional requirement: R010_TBBR_TOC as per > TBBR spec [1]. > > [1] https://developer.arm.com/docs/den0006/latest/trusted-board-boot-requiremen… > [2] http://world.std.com/~dtd/sign_encrypt/sign_encrypt7.html > > -Sumit > > > -Raghu > > > On January 22, 2020 at 3:51 AM, Sumit Garg via TF-A <tf-a(a)lists.trustedfirmware.org> wrote: > > > Hi Sandrine, > > > On Wed, 22 Jan 2020 at 15:43, Sandrine Bailleux > <Sandrine.Bailleux(a)arm.com> wrote: > > > > > Hello Sumit, > > > > > Thank you for reworking the patches and addressing all of my review > > > comments. I am happy with the latest version of these and consider them > > > ready to go. I plan to leave them in Gerrit for another week to give > > > extra time for other potential reviewers to have a look and comment. > > > > > > > Thanks for your review. > > > To everyone on the list: Please raise any concerns you may have about > > > these patches in the coming week. If I don't hear anything by 29th > > > January 2020, I will merge these patches. > > > > > @Sumit: One of the next actions for this patch stack would be to have > > > some level of testing in the CI system to detect any potential > > > regressions. We (at Arm) can quite easily add a few build tests but then > > > testing the software stack on QEMU is a bit more involved for various > > > reasons (first instance of QEMU testing, dependencies on OPTEE, UEFI, > > > ...) so this might have to wait for some time. > > > > > > > Okay, will wait for CI testing. > > > -Sumit > > > Regards, > > > Sandrine > > > -- > TF-A mailing list > TF-A(a)lists.trustedfirmware.org > https://lists.trustedfirmware.org/mailman/listinfo/tf-a > -- > TF-A mailing list > TF-A(a)lists.trustedfirmware.org > https://lists.trustedfirmware.org/mailman/listinfo/tf-a

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