Let’s reflect on some of my recent work that started with understanding Trisquel GNU/Linux, improving transparency into apt-archives, working on reproducible builds of Trisquel, strengthening verification of apt-archives with Sigstore, and finally thinking about security device threat models. A theme in all this is improving methods to have trust in machines, or generally any external entity. While I believe that everything starts by trusting something, usually something familiar and well-known, we need to deal with misuse of that trust that leads to failure to deliver what is desired and expected from the trusted entity. How can an entity behave to invite trust? Let’s argue for some properties that can be quantitatively measured, with a focus on computer software and hardware:
Deterministic Behavior – given a set of circumstances, it should behave the same.
Verifiability and Transparency – the method (the source code) should be accessible for understanding (compare scientific method) and its binaries verifiable, i.e., it should be possible to verify that the entity actually follows the intended deterministic method (implying efforts like reproducible builds and bootstrappable builds).
Accountable – the entity should behave the same for everyone, and deviation should be possible prove in a way that is hard to deny, implying efforts such as Certificate Transparency and more generic checksum logs like Sigstore and Sigsum.
Liberating – the tools and documentation should be available as free software to enable you to replace the trusted entity if so desired. An entity that wants to restrict you from being able to replace the trusted entity is vulnerable to corruption and may stop acting trustworthy. This point of view reinforces that open source misses the point; it has become too common to use trademark laws to restrict re-use of open source software (e.g., firefox, chrome, rust).
Essentially, this boils down to: Trust, Verify and Hold Accountable. To put this dogma in perspective, it helps to understand that this approach may be harmful to human relationships (which could explain the social awkwardness of hackers), but it remains useful as a method to improve the design of computer systems, and a useful method to evaluate safety of computer systems. When a system fails some of the criteria above, we know we have more work to do to improve it.
How far have we come on this journey? Through earlier efforts, we are in a fairly good situation. Richard Stallman through GNU/FSF made us aware of the importance of free software, the Reproducible/Bootstrappable build projects made us aware of the importance of verifiability, and Certificate Transparency highlighted the need for accountable signature logs leading to efforts like Sigstore for software. None of these efforts would have seen the light of day unless people wrote free software and packaged them into distributions that we can use, and built hardware that we can run it on. While there certainly exists more work to be done on the software side, with the recent amazing full-source build of Guix based on a 357-byte hand-written seed, I believe that we are closing that loop on the software engineering side.
So what remains? Some inspiration for further work:
Accountable binary software distribution remains unresolved in practice, although we have some software components around (e.g., apt-sigstore and guix git authenticate). What is missing is using them for verification by default and/or to improve the signature process to use trustworthy hardware devices, and committing the signatures to transparency logs.
Trustworthy hardware to run trustworthy software on remains a challenge, and we owe FSF’s Respect Your Freedom credit for raising awareness of this. Many modern devices requires non-free software to work which fails most of the criteria above and are thus inherently untrustworthy.
Verifying rebuilds of currently published binaries on trustworthy hardware is unresolved.
Completing a full-source rebuild from a small seed on trustworthy hardware remains, preferably on a platform wildly different than X86 such as Raptor’s Talos II.
We need improved security hardware devices and improved established practices on how to use them. For example, while Gnuk on the FST enable a trustworthy software and hardware solution, the best process for using it that I can think of generate the cryptographic keys on a more complex device. Efforts like Tillitis are inspiring here.
Onwards and upwards, happy hacking!
Update 2023-05-03: Added the “Liberating” property regarding free software, instead of having it be part of the “Verifiability and Transparency”.
I’d like to describe and discuss a threat model for computational devices. This is generic but we will narrow it down to security-related devices. For example, portable hardware dongles used for OpenPGP/OpenSSH keys, FIDO/U2F, OATH HOTP/TOTP, PIV, payment cards, wallets etc and more permanently attached devices like a Hardware Security Module (HSM), a TPM-chip, or the hybrid variant of a mostly permanently-inserted but removable hardware security dongles.
Our context is cryptographic hardware engineering, and the purpose of the threat model is to serve as as a thought experiment for how to build and design security devices that offer better protection. The threat model is related to the Evil maid attack.
Our focus is to improve security for the end-user rather than the traditional focus to improve security for the organization that provides the token to the end-user, or to improve security for the site that the end-user is authenticating to. This is a critical but often under-appreciated distinction, and leads to surprising recommendations related to onboard key generation, randomness etc below.
The Substitution Attack
An attacker is able to substitute any component of the device (hardware or software) at any time for any period of time.
Your takeaway should be that devices should be designed to mitigate harmful consequences if any component of the device (hardware or software) is substituted for a malicious component for some period of time, at any time, during the lifespan of that component. Some designs protect better against this attack than other designs, and the threat model can be used to understand which designs are really bad, and which are less so.
Terminology
The threat model involves at least one device that is well-behaving and one that is not, and we call these Good Device and Bad Device respectively. The bad device may be the same physical device as the good key, but with some minor software modification or a minor component replaced, but could also be a completely separate physical device. We don’t care about that distinction, we just care if a particular device has a malicious component in it or not. I’ll use terms like “security device”, “device”, “hardware key”, “security co-processor” etc interchangeably.
From an engineering point of view, “malicious” here includes “unintentional behavior” such as software or hardware bugs. It is not possible to differentiate an intentionally malicious device from a well-designed device with a critical bug.
Don’t attribute to malice what can be adequately explained by stupidity, but don’t naïvely attribute to stupidity what may be deniable malicious.
What is “some period of time”?
“Some period of time” can be any length of time: seconds, minutes, days, weeks, etc.
It may also occur at any time: During manufacturing, during transportation to the user, after first usage by the user, or after a couple of months usage by the user. Note that we intentionally consider time-of-manufacturing as a vulnerable phase.
Even further, the substitution may occur multiple times. So the Good Key may be replaced with a Bad Key by the attacker for one day, then returned, and later this repeats a month later.
What is “harmful consequences”?
Since a security key has a fairly well-confined scope and purpose, we can get a fairly good exhaustive list of things that could go wrong. Harmful consequences include:
Attacker learns any secret keys stored on a Good Key.
Attacker causes user to trust a public generated by a Bad Key.
Attacker is able to sign something using a Good Key.
Attacker learns the PIN code used to unlock a Good Key.
Attacker learns data that is decrypted by a Good Key.
Thin vs Deep solutions
One approach to mitigate many issues arising from device substitution is to have the host (or remote site) require that the device prove that it is the intended unique device before it continues to talk to it. This require an authentication/authorization protocol, which usually involves unique device identity and out-of-band trust anchors. Such trust anchors is often problematic, since a common use-case for security device is to connect it to a host that has never seen the device before.
A weaker approach is to have the device prove that it merely belongs to a class of genuine devices from a trusted manufacturer, usually by providing a signature generated by a device-specific private key signed by the device manufacturer. This is weaker since then the user cannot differentiate two different good devices.
In both cases, the host (or remote site) would stop talking to the device if it cannot prove that it is the intended key, or at least belongs to a class of known trusted genuine devices.
Upon scrutiny, this “solution” is still vulnerable to a substitution attack, just earlier in the manufacturing chain: how can the process that injects the per-device or per-class identities/secrets know that it is putting them into a good key rather than a malicious device? Consider also the consequences if the cryptographic keys that guarantee that a device is genuine leaks.
The model of the “thin solution” is similar to the old approach to network firewalls: have a filtering firewall that only lets through “intended” traffic, and then run completely insecure protocols internally such as telnet.
The networking world has evolved, and now we have defense in depth: even within strongly firewall’ed networks, it is prudent to run for example SSH with publickey-based user authentication even on locally physical trusted networks. This approach requires more thought and adds complexity, since each level has to provide some security checking.
I’m arguing we need similar defense-in-depth for security devices. Security key designs cannot simply dodge this problem by assuming it is working in a friendly environment where component substitution never occur.
Example: Device authentication using PIN codes
To see how this threat model can be applied to reason about security key designs, let’s consider a common design.
Many security keys uses PIN codes to unlock private key operations, for example on OpenPGP cards that lacks built-in PIN-entry functionality. The software on the computer just sends a PIN code to the device, and the device allows private-key operations if the PIN code was correct.
Let’s apply the substitution threat model to this design: the attacker replaces the intended good key with a malicious device that saves a copy of the PIN code presented to it, and then gives out error messages. Once the user has entered the PIN code and gotten an error message, presumably temporarily giving up and doing other things, the attacker replaces the device back again. The attacker has learnt the PIN code, and can later use this to perform private-key operations on the good device.
This means a good design involves not sending PIN codes in clear, but use a stronger authentication protocol that allows the card to know that the PIN was correct without learning the PIN. This is implemented optionally for many OpenPGP cards today as the key-derivation-function extension. That should be mandatory, and users should not use setups that sends device authentication in the clear, and ultimately security devices should not even include support for that. Compare how I build Gnuk on my PGP card with the kdf_do=required option.
Example: Onboard non-predictable key-generation
Many devices offer both onboard key-generation, for example OpenPGP cards that generate a Ed25519 key internally on the devices, or externally where the device imports an externally generated cryptographic key.
Let’s apply the subsitution threat model to this design: the user wishes to generate a key and trust the public key that came out of that process. The attacker substitutes the device for a malicious device during key-generation, imports the private key into a good device and gives that back to the user. Most of the time except during key generation the user uses a good device but still the attacker succeeded in having the user trust a public key which the attacker knows the private key for. The substitution may be a software modification, and the method to leak the private key to the attacker may be out-of-band signalling.
This means a good design never generates key on-board, but imports them from a user-controllable environment. That approach should be mandatory, and users should not use setups that generates private keys on-board, and ultimately security devices should not even include support for that.
Example: Non-predictable randomness-generation
Many devices claims to generate random data, often with elaborate design documents explaining how good the randomness is.
Let’s apply the substitution threat model to this design: the attacker replaces the intended good key with a malicious design that generates (for the attacker) predictable randomness. The user will never be able to detect the difference since the random output is, well, random, and typically not distinguishable from weak randomness. The user cannot know if any cryptographic keys generated by a generator was faulty or not.
This means a good design never generates non-predictable randomness on the device. That approach should be mandatory, and users should not use setups that generates non-predictable randomness on the device, and ideally devices should not have this functionality.
Case-Study: Tillitis
I have warmed up a bit for this. Tillitis is a new security device with interesting properties, and core to its operation is the Compound Device Identifier (CDI), essentially your Ed25519 private key (used for SSH etc) is derived from the CDI that is computed like this:
cdi = blake2s(UDS, blake2s(device_app), USS)
Let’s apply the substitution threat model to this design: Consider someone replacing the Tillitis key with a malicious key during postal delivery of the key to the user, and the replacement device is identical with the real Tillitis key but implements the following key derivation function:
cdi = weakprng(UDS’, weakprng(device_app), USS)
Where weakprng is a compromised algorithm that is predictable for the attacker, but still appears random. Everything will work correctly, but the attacker will be able to learn the secrets used by the user, and the user will typically not be able to tell the difference since the CDI is secret and the Ed25519 public key is not self-verifiable.
Conclusion
Remember that it is impossible to fully protect against this attack, that’s why it is merely a thought experiment, intended to be used during design of these devices. Consider an attacker that never gives you access to a good key and as a user you only ever use a malicious device. There is no way to have good security in this situation. This is not hypothetical, many well-funded organizations do what they can to derive people from having access to trustworthy security devices. Philosophically it does not seem possible to tell if these organizations have succeeded 100% already and there are only bad security devices around where further resistance is futile, but to end on an optimistic note let’s assume that there is a non-negligible chance that they haven’t succeeded. In these situations, this threat model becomes useful to improve the situation by identifying less good designs, and that’s why the design mantra of “mitigate harmful consequences” is crucial as a takeaway from this. Let’s improve the design of security devices that further the security of its users!
The absolute number may not be impressive, but what I hope is at least a useful contribution is that there actually is a number on how much of Trisquel is reproducible. Hopefully this will inspire others to help improve the actual metric.
When I set about to understand how Trisquel worked, I identified a number of things that would improve my confidence in it. The lowest hanging fruit for me was to manually audit the package archive, and I wrote a tool called debdistdiff to automate this for me. That led me to think about apt archive transparency more in general. I have made some further work in that area (hint: apt-verify) that deserve its own blog post eventually. Most of apt archive transparency is futile if we don’t trust the intended packages that are in the archive. One way to measurable increase trust in the package are to provide reproducible builds of the packages, which should by now be an established best practice. Code review is still important, but since it will never provide positive guarantees we need other processes that can identify sub-optimal situations automatically. The way reproducible builds easily identify negative results is what I believe has driven much of its success: its results are tangible and measurable. The field of software engineering is in need of more such practices.
The design of my setup to build Trisquel reproducible are as follows.
The project debdistget is responsible for downloading Release/Packages files (which are the most relevant files from dists/) from apt archives, and works by commiting them into GitLab-hosted git-repositories. I maintain several such repositories for popular apt-archives, including for Trisquel and its upstream Ubuntu. GitLab invokes a schedule pipeline to do the downloading, and there is some race conditions here.
The project debdistdiff is used to produce the list of added and modified packages, which are the input to actually being able to know what packages to reproduce. It publishes human readable summary of difference for several distributions, including Trisquel vs Ubuntu. Early on I decided that rebuilding all of the upstream Ubuntu packages is out of scope for me: my personal trust in the official Debian/Ubuntu apt archives are greater than my trust of the added/modified packages in Trisquel.
The final project reproduce-trisquel puts the pieces together briefly as follows, everything being driven from its .gitlab-ci.yml file.
There is a (manually triggered) job generate-build-image to create a build image to speed up CI/CD runs, using a simple Dockerfile.
There is a (manually triggered) job generate-package-lists that uses debdistdiff to generate and store package lists and puts its output in lists/. The reason this is manually triggered right now is due to a race condition.
There is a (scheduled) job that does two things: from the package lists, the script generate-ci-packages.sh builds a GitLab CI/CD instruction file ci-packages.yml that describes jobs for each package to build. The second part is generate-readme.sh that re-generate the project’s README.md based on the build logs and diffoscope outputs that stored in the git repository.
Through the ci-packages.yml file, there is a large number of jobs that are dynamically defined, which currently are manually triggered to not overload the build servers. The script build-package.sh is invoked and attempts to rebuild a package, and stores build log and diffoscope output in the git project itself.
I did not expect to be able to use the GitLab shared runners to do the building, however they turned out to work quite well and I postponed setting up my own runner. There is a manually curated lists/disabled-aramo.txt with some packages that all required too much disk space or took over two hours to build. Today I finally took the time to setup a GitLab runner using podman running Trisquel aramo, and I expect to complete builds of the remaining packages soon — one of my Dell R630 server with 256GB RAM and dual 2680v4 CPUs should deliver sufficient performance.
Current limitations and ideas on further work (most are filed as project issues) include:
We don’t support *.buildinfo files. As far as I am aware, Trisquel does not publish them for their builds. Improving this would be a first step forward, anyone able to help? Compare buildinfo.debian.net. For example, many packages differ only in their NT_GNU_BUILD_ID symbol inside the ELF binary, see example diffoscope output for libgpg-error. By poking around in jenkins.trisquel.org I managed to discover that Trisquel built initramfs-utils in the randomized path /build/initramfs-tools-bzRLUp and hard-coding that path allowed me to reproduce that package. I expect the same to hold for many other packages. Unfortunately, this failure turned into success with that package moved the needle from 42% reproducibility to 43% however I didn’t let that stand in the way of a good headline.
The mechanism to download the Release/Package-files from dists/ is not fool-proof: we may not capture all ever published such files. While this is less of a concern for reproducibility, it is more of a concern for apt transparency. Still, having Trisquel provide a service similar to snapshot.debian.org would help.
Having at least one other CPU architecture would be nice.
Due to lack of time and mental focus, handling incremental updates of new versions of packages is not yet working. This means we only ever build one version of a package, and never discover any newly published versions of the same package. Now that Trisquel aramo is released, the expected rate of new versions should be low, but still happens due to security or backports.
Porting this to test supposedly FSDG-compliant distributions such as PureOS and Gnuinos should be relatively easy. I’m also looking at Devuan because of Gnuinos.
The elephant in the room is how reproducible Ubuntu is in the first place.
Happy Easter Hacking!
Update 2023-04-17: The original project “reproduce-trisquel” that was announced here has been archived and replaced with two projects, one generic “debdistreproduce” and one with results for Trisquel: “reproduce/trisquel“.
I’ve used hardware-backed OpenPGP keys since 2006 when I imported newly generated rsa1024 subkeys to a FSFE Fellowship card. This worked well for several years, and I recall buying more ZeitControl cards for multi-machine usage and backup purposes. As a side note, I recall being unsatisfied with the weak 1024-bit RSA subkeys at the time – my primary key was a somewhat stronger 1280-bit RSA key created back in 2002 — but OpenPGP cards at the time didn’t support more than 1024 bit RSA, and were (and still often are) also limited to power-of-two RSA key sizes which I dislike.
I had my master key on disk with a strong password for a while, mostly to refresh expiration time of the subkeys and to sign other’s OpenPGP keys. At some point I stopped carrying around encrypted copies of my master key. That was my main setup when I migrated to a new stronger RSA 3744 bit key with rsa2048 subkeys on a YubiKey NEO back in 2014. At that point, signing other’s OpenPGP keys was a rare enough occurrence that I settled with bringing out my offline machine to perform this operation, transferring the public key to sign on USB sticks. In 2019 I re-evaluated my OpenPGP setup and ended up creating a offline Ed25519 key with subkeys on a FST-01G running Gnuk. My approach for signing other’s OpenPGP keys were still to bring out my offline machine and sign things using the master secret using USB sticks for storage and transport. Which meant I almost never did that, because it took too much effort. So my 2019-era Ed25519 key still only has a handful of signatures on it, since I had essentially stopped signing other’s keys which is the traditional way of getting signatures in return.
None of this caused any critical problem for me because I continued to use my old 2014-era RSA3744 key in parallel with my new 2019-era Ed25519 key, since too many systems didn’t handle Ed25519. However, during 2022 this changed, and the only remaining environment that I still used my RSA3744 key for was in Debian — and they require OpenPGP signatures on the new key to allow it to replace an older key. I was in denial about this sub-optimal solution during 2022 and endured its practical consequences, having to use the YubiKey NEO (which I had replaced with a permanently inserted YubiKey Nano at some point) for Debian-related purposes alone.
In December 2022 I bought a new laptop and setup a FST-01SZ with my Ed25519 key, and while I have taken a vacation from Debian, I continue to extend the expiration period on the old RSA3744-key in case I will ever have to use it again, so the overall OpenPGP setup was still sub-optimal. Having two valid OpenPGP keys at the same time causes people to use both for email encryption (leading me to have to use both devices), and the WKD Key Discovery protocol doesn’t like two valid keys either. At FOSDEM’23 I ran into Andre Heinecke at GnuPG and I couldn’t help complain about how complex and unsatisfying all OpenPGP-related matters were, and he mildly ignored my rant and asked why I didn’t put the master key on another smartcard. The comment sunk in when I came home, and recently I connected all the dots and this post is a summary of what I did to move my offline OpenPGP master key to a Nitrokey Start.
First a word about device choice, I still prefer to use hardware devices that are as compatible with free software as possible, but the FST-01G or FST-01SZ are no longer easily available for purchase. I got a comment about Nitrokey start in my last post, and had two of them available to experiment with. There are things to dislike with the Nitrokey Start compared to the YubiKey (e.g., relative insecure chip architecture, the bulkier form factor and lack of FIDO/U2F/OATH support) but – as far as I know – there is no more widely available owner-controlled device that is manufactured for an intended purpose of implementing an OpenPGP card. Thus it hits the sweet spot for me.
Nitrokey Start
The first step is to run latest firmware on the Nitrokey Start – for bug-fixes and important OpenSSH 9.0 compatibility – and there are reproducible-built firmware published that you can install using pynitrokey. I run Trisquel 11 aramo on my laptop, which does not include the Python Pip package (likely because it promotes installing non-free software) so that was a slight complication. Building the firmware locally may have worked, and I would like to do that eventually to confirm the published firmware, however to save time I settled with installing the Ubuntu 22.04 packages on my machine:
$ sha256sum python3-pip*
ded6b3867a4a4cbaff0940cab366975d6aeecc76b9f2d2efa3deceb062668b1c python3-pip_22.0.2+dfsg-1ubuntu0.2_all.deb
e1561575130c41dc3309023a345de337e84b4b04c21c74db57f599e267114325 python3-pip-whl_22.0.2+dfsg-1ubuntu0.2_all.deb
$ doas dpkg -i python3-pip*
...
$ doas apt install -f
...
$
Installing pynitrokey downloaded a bunch of dependencies, and it would be nice to audit the license and security vulnerabilities for each of them. (Verbose output below slightly redacted.)
jas@kaka:~$ pip3 install --user pynitrokey
Collecting pynitrokey
Downloading pynitrokey-0.4.34-py3-none-any.whl (572 kB)
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Building wheels for collected packages: nrfutil, crcmod, sly, tlv8, commentjson, hexdump, pyspinel, fire, intervaltree, lark-parser, naturalsort, future
Building wheel for nrfutil (setup.py) ... done
Created wheel for nrfutil: filename=nrfutil-6.1.7-py3-none-any.whl size=898520 sha256=de6f8803f51d6c26d24dc7df6292064a468ff3f389d73370433fde5582b84a10
Stored in directory: /home/jas/.cache/pip/wheels/39/2b/9b/98ab2dd716da746290e6728bdb557b14c1c9a54cb9ed86e13b
Building wheel for crcmod (setup.py) ... done
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Stored in directory: /home/jas/.cache/pip/wheels/85/4c/07/72215c529bd59d67e3dac29711d7aba1b692f543c808ba9e86
Building wheel for sly (setup.py) ... done
Created wheel for sly: filename=sly-0.4-py3-none-any.whl size=27352 sha256=f614e413918de45c73d1e9a8dca61ca07dc760d9740553400efc234c891f7fde
Stored in directory: /home/jas/.cache/pip/wheels/a2/23/4a/6a84282a0d2c29f003012dc565b3126e427972e8b8157ea51f
Building wheel for tlv8 (setup.py) ... done
Created wheel for tlv8: filename=tlv8-0.10.0-py3-none-any.whl size=11266 sha256=3ec8b3c45977a3addbc66b7b99e1d81b146607c3a269502b9b5651900a0e2d08
Stored in directory: /home/jas/.cache/pip/wheels/e9/35/86/66a473cc2abb0c7f21ed39c30a3b2219b16bd2cdb4b33cfc2c
Building wheel for commentjson (setup.py) ... done
Created wheel for commentjson: filename=commentjson-0.9.0-py3-none-any.whl size=12092 sha256=28b6413132d6d7798a18cf8c76885dc69f676ea763ffcb08775a3c2c43444f4a
Stored in directory: /home/jas/.cache/pip/wheels/7d/90/23/6358a234ca5b4ec0866d447079b97fedf9883387d1d7d074e5
Building wheel for hexdump (setup.py) ... done
Created wheel for hexdump: filename=hexdump-3.3-py3-none-any.whl size=8913 sha256=79dfadd42edbc9acaeac1987464f2df4053784fff18b96408c1309b74fd09f50
Stored in directory: /home/jas/.cache/pip/wheels/26/28/f7/f47d7ecd9ae44c4457e72c8bb617ef18ab332ee2b2a1047e87
Building wheel for pyspinel (setup.py) ... done
Created wheel for pyspinel: filename=pyspinel-1.0.3-py3-none-any.whl size=65033 sha256=01dc27f81f28b4830a0cf2336dc737ef309a1287fcf33f57a8a4c5bed3b5f0a6
Stored in directory: /home/jas/.cache/pip/wheels/95/ec/4b/6e3e2ee18e7292d26a65659f75d07411a6e69158bb05507590
Building wheel for fire (setup.py) ... done
Created wheel for fire: filename=fire-0.5.0-py2.py3-none-any.whl size=116951 sha256=3d288585478c91a6914629eb739ea789828eb2d0267febc7c5390cb24ba153e8
Stored in directory: /home/jas/.cache/pip/wheels/90/d4/f7/9404e5db0116bd4d43e5666eaa3e70ab53723e1e3ea40c9a95
Building wheel for intervaltree (setup.py) ... done
Created wheel for intervaltree: filename=intervaltree-3.1.0-py2.py3-none-any.whl size=26119 sha256=5ff1def22ba883af25c90d90ef7c6518496fcd47dd2cbc53a57ec04cd60dc21d
Stored in directory: /home/jas/.cache/pip/wheels/fa/80/8c/43488a924a046b733b64de3fac99252674c892a4c3801c0a61
Building wheel for lark-parser (setup.py) ... done
Created wheel for lark-parser: filename=lark_parser-0.7.8-py2.py3-none-any.whl size=62527 sha256=3d2ec1d0f926fc2688d40777f7ef93c9986f874169132b1af590b6afc038f4be
Stored in directory: /home/jas/.cache/pip/wheels/29/30/94/33e8b58318aa05cb1842b365843036e0280af5983abb966b83
Building wheel for naturalsort (setup.py) ... done
Created wheel for naturalsort: filename=naturalsort-1.5.1-py3-none-any.whl size=7526 sha256=bdecac4a49f2416924548cae6c124c85d5333e9e61c563232678ed182969d453
Stored in directory: /home/jas/.cache/pip/wheels/a6/8e/c9/98cfa614fff2979b457fa2d9ad45ec85fa417e7e3e2e43be51
Building wheel for future (setup.py) ... done
Created wheel for future: filename=future-0.18.3-py3-none-any.whl size=492037 sha256=57a01e68feca2b5563f5f624141267f399082d2f05f55886f71b5d6e6cf2b02c
Stored in directory: /home/jas/.cache/pip/wheels/5e/a9/47/f118e66afd12240e4662752cc22cefae5d97275623aa8ef57d
Successfully built nrfutil crcmod sly tlv8 commentjson hexdump pyspinel fire intervaltree lark-parser naturalsort future
Installing collected packages: tlv8, sortedcontainers, sly, pyserial, pyelftools, piccata, naturalsort, libusb1, lark-parser, intelhex, hexdump, fastjsonschema, crcmod, asn1crypto, wrapt, urllib3, typing_extensions, tqdm, termcolor, ruamel.yaml.clib, python-dateutil, pyspinel, pypemicro, pycryptodome, psutil, protobuf, prettytable, oscrypto, milksnake, libusbsio, jinja2, intervaltree, humanfriendly, future, frozendict, fido2, ecdsa, deepmerge, commentjson, click-option-group, click-command-tree, capstone, astunparse, argparse-addons, ruamel.yaml, pyocd-pemicro, pylink-square, pc_ble_driver_py, fire, cmsis-pack-manager, bincopy, pyocd, nrfutil, nkdfu, spsdk, pynitrokey
WARNING: The script nitropy is installed in '/home/jas/.local/bin' which is not on PATH.
Consider adding this directory to PATH or, if you prefer to suppress this warning, use --no-warn-script-location.
Successfully installed argparse-addons-0.12.0 asn1crypto-1.5.1 astunparse-1.6.3 bincopy-17.10.3 capstone-4.0.2 click-command-tree-1.1.0 click-option-group-0.5.5 cmsis-pack-manager-0.2.10 commentjson-0.9.0 crcmod-1.7 deepmerge-0.3.0 ecdsa-0.18.0 fastjsonschema-2.16.3 fido2-1.1.0 fire-0.5.0 frozendict-2.3.5 future-0.18.3 hexdump-3.3 humanfriendly-10.0 intelhex-2.3.0 intervaltree-3.1.0 jinja2-3.0.3 lark-parser-0.7.8 libusb1-1.9.3 libusbsio-2.1.11 milksnake-0.1.5 naturalsort-1.5.1 nkdfu-0.2 nrfutil-6.1.7 oscrypto-1.3.0 pc_ble_driver_py-0.17.0 piccata-2.0.3 prettytable-2.5.0 protobuf-3.20.3 psutil-5.9.4 pycryptodome-3.17 pyelftools-0.29 pylink-square-0.11.1 pynitrokey-0.4.34 pyocd-0.31.0 pyocd-pemicro-1.1.5 pypemicro-0.1.11 pyserial-3.5 pyspinel-1.0.3 python-dateutil-2.7.5 ruamel.yaml-0.17.21 ruamel.yaml.clib-0.2.7 sly-0.4 sortedcontainers-2.4.0 spsdk-1.7.1 termcolor-2.2.0 tlv8-0.10.0 tqdm-4.65.0 typing_extensions-4.3.0 urllib3-1.26.15 wrapt-1.15.0
jas@kaka:~$
Then upgrading the device worked remarkable well, although I wish that the tool would have printed URLs and checksums for the firmware files to allow easy confirmation.
jas@kaka:~$ PATH=$PATH:/home/jas/.local/bin
jas@kaka:~$ nitropy start list
Command line tool to interact with Nitrokey devices 0.4.34
:: 'Nitrokey Start' keys:
FSIJ-1.2.15-5D271572: Nitrokey Nitrokey Start (RTM.12.1-RC2-modified)
jas@kaka:~$ nitropy start update
Command line tool to interact with Nitrokey devices 0.4.34
Nitrokey Start firmware update tool
Platform: Linux-5.15.0-67-generic-x86_64-with-glibc2.35
System: Linux, is_linux: True
Python: 3.10.6
Saving run log to: /tmp/nitropy.log.gc5753a8
Admin PIN:
Firmware data to be used:
- FirmwareType.REGNUAL: 4408, hash: ...b'72a30389' valid (from ...built/RTM.13/regnual.bin)
- FirmwareType.GNUK: 129024, hash: ...b'25a4289b' valid (from ...prebuilt/RTM.13/gnuk.bin)
Currently connected device strings:
Device:
Vendor: Nitrokey
Product: Nitrokey Start
Serial: FSIJ-1.2.15-5D271572
Revision: RTM.12.1-RC2-modified
Config: *:*:8e82
Sys: 3.0
Board: NITROKEY-START-G
initial device strings: [{'name': '', 'Vendor': 'Nitrokey', 'Product': 'Nitrokey Start', 'Serial': 'FSIJ-1.2.15-5D271572', 'Revision': 'RTM.12.1-RC2-modified', 'Config': '*:*:8e82', 'Sys': '3.0', 'Board': 'NITROKEY-START-G'}]
Please note:
- Latest firmware available is:
RTM.13 (published: 2022-12-08T10:59:11Z)
- provided firmware: None
- all data will be removed from the device!
- do not interrupt update process - the device may not run properly!
- the process should not take more than 1 minute
Do you want to continue? [yes/no]: yes
...
Starting bootloader upload procedure
Device: Nitrokey Start FSIJ-1.2.15-5D271572
Connected to the device
Running update!
Do NOT remove the device from the USB slot, until further notice
Downloading flash upgrade program...
Executing flash upgrade...
Waiting for device to appear:
Wait 20 seconds.....
Downloading the program
Protecting device
Finish flashing
Resetting device
Update procedure finished. Device could be removed from USB slot.
Currently connected device strings (after upgrade):
Device:
Vendor: Nitrokey
Product: Nitrokey Start
Serial: FSIJ-1.2.19-5D271572
Revision: RTM.13
Config: *:*:8e82
Sys: 3.0
Board: NITROKEY-START-G
device can now be safely removed from the USB slot
final device strings: [{'name': '', 'Vendor': 'Nitrokey', 'Product': 'Nitrokey Start', 'Serial': 'FSIJ-1.2.19-5D271572', 'Revision': 'RTM.13', 'Config': '*:*:8e82', 'Sys': '3.0', 'Board': 'NITROKEY-START-G'}]
finishing session 2023-03-16 21:49:07.371291
Log saved to: /tmp/nitropy.log.gc5753a8
jas@kaka:~$
jas@kaka:~$ nitropy start list
Command line tool to interact with Nitrokey devices 0.4.34
:: 'Nitrokey Start' keys:
FSIJ-1.2.19-5D271572: Nitrokey Nitrokey Start (RTM.13)
jas@kaka:~$
Before importing the master key to this device, it should be configured. Note the commands in the beginning to make sure scdaemon/pcscd is not running because they may have cached state from earlier cards. Change PIN code as you like after this, my experience with Gnuk was that the Admin PIN had to be changed first, then you import the key, and then you change the PIN.
jas@kaka:~$ gpg-connect-agent "SCD KILLSCD" "SCD BYE" /bye
OK
ERR 67125247 Slut på fil <GPG Agent>
jas@kaka:~$ ps auxww|grep -e pcsc -e scd
jas 11651 0.0 0.0 3468 1672 pts/0 R+ 21:54 0:00 grep --color=auto -e pcsc -e scd
jas@kaka:~$ gpg --card-edit
Reader ...........: 20A0:4211:FSIJ-1.2.19-5D271572:0
Application ID ...: D276000124010200FFFE5D2715720000
Application type .: OpenPGP
Version ..........: 2.0
Manufacturer .....: unmanaged S/N range
Serial number ....: 5D271572
Name of cardholder: [not set]
Language prefs ...: [not set]
Salutation .......:
URL of public key : [not set]
Login data .......: [not set]
Signature PIN ....: forced
Key attributes ...: rsa2048 rsa2048 rsa2048
Max. PIN lengths .: 127 127 127
PIN retry counter : 3 3 3
Signature counter : 0
KDF setting ......: off
Signature key ....: [none]
Encryption key....: [none]
Authentication key: [none]
General key info..: [none]
gpg/card> admin
Admin commands are allowed
gpg/card> kdf-setup
gpg/card> passwd
gpg: OpenPGP card no. D276000124010200FFFE5D2715720000 detected
1 - change PIN
2 - unblock PIN
3 - change Admin PIN
4 - set the Reset Code
Q - quit
Your selection? 3
PIN changed.
1 - change PIN
2 - unblock PIN
3 - change Admin PIN
4 - set the Reset Code
Q - quit
Your selection? q
gpg/card> name
Cardholder's surname: Josefsson
Cardholder's given name: Simon
gpg/card> lang
Language preferences: sv
gpg/card> sex
Salutation (M = Mr., F = Ms., or space): m
gpg/card> login
Login data (account name): jas
gpg/card> url
URL to retrieve public key: https://josefsson.org/key-20190320.txt
gpg/card> forcesig
gpg/card> key-attr
Changing card key attribute for: Signature key
Please select what kind of key you want:
(1) RSA
(2) ECC
Your selection? 2
Please select which elliptic curve you want:
(1) Curve 25519
(4) NIST P-384
Your selection? 1
The card will now be re-configured to generate a key of type: ed25519
Note: There is no guarantee that the card supports the requested size.
If the key generation does not succeed, please check the
documentation of your card to see what sizes are allowed.
Changing card key attribute for: Encryption key
Please select what kind of key you want:
(1) RSA
(2) ECC
Your selection? 2
Please select which elliptic curve you want:
(1) Curve 25519
(4) NIST P-384
Your selection? 1
The card will now be re-configured to generate a key of type: cv25519
Changing card key attribute for: Authentication key
Please select what kind of key you want:
(1) RSA
(2) ECC
Your selection? 2
Please select which elliptic curve you want:
(1) Curve 25519
(4) NIST P-384
Your selection? 1
The card will now be re-configured to generate a key of type: ed25519
gpg/card>
jas@kaka:~$ gpg --card-edit
Reader ...........: 20A0:4211:FSIJ-1.2.19-5D271572:0
Application ID ...: D276000124010200FFFE5D2715720000
Application type .: OpenPGP
Version ..........: 2.0
Manufacturer .....: unmanaged S/N range
Serial number ....: 5D271572
Name of cardholder: Simon Josefsson
Language prefs ...: sv
Salutation .......: Mr.
URL of public key : https://josefsson.org/key-20190320.txt
Login data .......: jas
Signature PIN ....: not forced
Key attributes ...: ed25519 cv25519 ed25519
Max. PIN lengths .: 127 127 127
PIN retry counter : 3 3 3
Signature counter : 0
KDF setting ......: on
Signature key ....: [none]
Encryption key....: [none]
Authentication key: [none]
General key info..: [none]
jas@kaka:~$
Once setup, bring out your offline machine and boot it and mount your USB stick with the offline key. The paths below will be different, and this is using a somewhat unorthodox approach of working with fresh GnuPG configuration paths that I chose for the USB stick.
jas@kaka:/media/jas/2c699cbd-b77e-4434-a0d6-0c4965864296$ cp -a gnupghome-backup-masterkey gnupghome-import-nitrokey-5D271572
jas@kaka:/media/jas/2c699cbd-b77e-4434-a0d6-0c4965864296$ gpg --homedir $PWD/gnupghome-import-nitrokey-5D271572 --edit-key B1D2BD1375BECB784CF4F8C4D73CF638C53C06BE
gpg (GnuPG) 2.2.27; Copyright (C) 2021 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Secret key is available.
sec ed25519/D73CF638C53C06BE
created: 2019-03-20 expired: 2019-10-22 usage: SC
trust: ultimate validity: expired
[ expired] (1). Simon Josefsson <simon@josefsson.org>
gpg> keytocard
Really move the primary key? (y/N) y
Please select where to store the key:
(1) Signature key
(3) Authentication key
Your selection? 1
sec ed25519/D73CF638C53C06BE
created: 2019-03-20 expired: 2019-10-22 usage: SC
trust: ultimate validity: expired
[ expired] (1). Simon Josefsson <simon@josefsson.org>
gpg>
Save changes? (y/N) y
jas@kaka:/media/jas/2c699cbd-b77e-4434-a0d6-0c4965864296$
Don’t forget to change the PIN at this point. At this point it is useful to confirm that the Nitrokey has the master key available and that is possible to sign statements with it, back on your regular machine:
jas@kaka:~$ gpg --card-status
Reader ...........: 20A0:4211:FSIJ-1.2.19-5D271572:0
Application ID ...: D276000124010200FFFE5D2715720000
Application type .: OpenPGP
Version ..........: 2.0
Manufacturer .....: unmanaged S/N range
Serial number ....: 5D271572
Name of cardholder: Simon Josefsson
Language prefs ...: sv
Salutation .......: Mr.
URL of public key : https://josefsson.org/key-20190320.txt
Login data .......: jas
Signature PIN ....: not forced
Key attributes ...: ed25519 cv25519 ed25519
Max. PIN lengths .: 127 127 127
PIN retry counter : 3 3 3
Signature counter : 1
KDF setting ......: on
Signature key ....: B1D2 BD13 75BE CB78 4CF4 F8C4 D73C F638 C53C 06BE
created ....: 2019-03-20 23:37:24
Encryption key....: [none]
Authentication key: [none]
General key info..: pub ed25519/D73CF638C53C06BE 2019-03-20 Simon Josefsson <simon@josefsson.org>
sec> ed25519/D73CF638C53C06BE created: 2019-03-20 expires: 2023-09-19
card-no: FFFE 5D271572
ssb> ed25519/80260EE8A9B92B2B created: 2019-03-20 expires: 2023-09-19
card-no: FFFE 42315277
ssb> ed25519/51722B08FE4745A2 created: 2019-03-20 expires: 2023-09-19
card-no: FFFE 42315277
ssb> cv25519/02923D7EE76EBD60 created: 2019-03-20 expires: 2023-09-19
card-no: FFFE 42315277
jas@kaka:~$ echo foo|gpg -a --sign|gpg --verify
gpg: Signature made Thu Mar 16 22:11:02 2023 CET
gpg: using EDDSA key B1D2BD1375BECB784CF4F8C4D73CF638C53C06BE
gpg: Good signature from "Simon Josefsson <simon@josefsson.org>" [ultimate]
jas@kaka:~$
Finally to retrieve and sign a key, for example Andre Heinecke’s that I could confirm the OpenPGP key identifier from his business card.
jas@kaka:~$ gpg --locate-external-keys aheinecke@gnupg.com
gpg: key 1FDF723CF462B6B1: public key "Andre Heinecke <aheinecke@gnupg.com>" imported
gpg: Total number processed: 1
gpg: imported: 1
gpg: marginals needed: 3 completes needed: 1 trust model: pgp
gpg: depth: 0 valid: 2 signed: 7 trust: 0-, 0q, 0n, 0m, 0f, 2u
gpg: depth: 1 valid: 7 signed: 64 trust: 7-, 0q, 0n, 0m, 0f, 0u
gpg: next trustdb check due at 2023-05-26
pub rsa3072 2015-12-08 [SC] [expires: 2025-12-05]
94A5C9A03C2FE5CA3B095D8E1FDF723CF462B6B1
uid [ unknown] Andre Heinecke <aheinecke@gnupg.com>
sub ed25519 2017-02-13 [S]
sub ed25519 2017-02-13 [A]
sub rsa3072 2015-12-08 [E] [expires: 2025-12-05]
sub rsa3072 2015-12-08 [A] [expires: 2025-12-05]
jas@kaka:~$ gpg --edit-key "94A5C9A03C2FE5CA3B095D8E1FDF723CF462B6B1"
gpg (GnuPG) 2.2.27; Copyright (C) 2021 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
pub rsa3072/1FDF723CF462B6B1
created: 2015-12-08 expires: 2025-12-05 usage: SC
trust: unknown validity: unknown
sub ed25519/2978E9D40CBABA5C
created: 2017-02-13 expires: never usage: S
sub ed25519/DC74D901C8E2DD47
created: 2017-02-13 expires: never usage: A
The following key was revoked on 2017-02-23 by RSA key 1FDF723CF462B6B1 Andre Heinecke <aheinecke@gnupg.com>
sub cv25519/1FFE3151683260AB
created: 2017-02-13 revoked: 2017-02-23 usage: E
sub rsa3072/8CC999BDAA45C71F
created: 2015-12-08 expires: 2025-12-05 usage: E
sub rsa3072/6304A4B539CE444A
created: 2015-12-08 expires: 2025-12-05 usage: A
[ unknown] (1). Andre Heinecke <aheinecke@gnupg.com>
gpg> sign
pub rsa3072/1FDF723CF462B6B1
created: 2015-12-08 expires: 2025-12-05 usage: SC
trust: unknown validity: unknown
Primary key fingerprint: 94A5 C9A0 3C2F E5CA 3B09 5D8E 1FDF 723C F462 B6B1
Andre Heinecke <aheinecke@gnupg.com>
This key is due to expire on 2025-12-05.
Are you sure that you want to sign this key with your
key "Simon Josefsson <simon@josefsson.org>" (D73CF638C53C06BE)
Really sign? (y/N) y
gpg> quit
Save changes? (y/N) y
jas@kaka:~$
This is on my day-to-day machine, using the NitroKey Start with the offline key. No need to boot the old offline machine just to sign keys or extend expiry anymore! At FOSDEM’23 I managed to get at least one DD signature on my new key, and the Debian keyring maintainers accepted my Ed25519 key. Hopefully I can now finally let my 2014-era RSA3744 key expire in 2023-09-19 and not extend it any further. This should finish my transition to a simpler OpenPGP key setup, yay!
I’ve always found the operation of apt software package repositories to be a mystery. There appears to be a lack of transparency into which people have access to important apt package repositories out there, how the automatic non-human update mechanism is implemented, and what changes are published. I’m thinking of big distributions like Ubuntu and Debian, but also the free GNU/Linux distributions like Trisquel and PureOS that are derived from the more well-known distributions.
As far as I can tell, anyone who has the OpenPGP private key trusted by a apt-based GNU/Linux distribution can sign a modified Release/InRelease file and if my machine somehow downloads that version of the release file, my machine could be made to download and install packages that the distribution didn’t intend me to install. Further, it seems that anyone who has access to the main HTTP server, or any of its mirrors, or is anywhere on the network between them and my machine (when plaintext HTTP is used), can either stall security updates on my machine (on a per-IP basis), or use it to send my machine (again, on a per-IP basis to avoid detection) a modified Release/InRelease file if they had been able to obtain the private signing key for the archive. These are mighty powers that warrant overview.
I’ve always put off learning about the processes to protect the apt infrastructure, mentally filing it under “so many people rely on this infrastructure that enough people are likely to have invested time reviewing and improving these processes”. Simultaneous, I’ve always followed the more free-software friendly Debian-derived distributions such as gNewSense and have run it on some machines. I’ve never put them into serious production use, because the trust issues with their apt package repositories has been a big question mark for me. The “enough people” part of my rationale for deferring this is not convincing. Even the simple question of “is someone updating the apt repository” is not easy to understand on a running gNewSense system. At some point in time the gNewSense cron job to pull in security updates from Debian must have stopped working, and I wouldn’t have had any good mechanism to notice that. Most likely it happened without any public announcement. I’ve recently switched to Trisquel on production machines, and these questions has come back to haunt me.
The situation is unsatisfying and I looked into what could be done to improve it. I could try to understand who are the key people involved in each project, and may even learn what hardware component is used, or what software is involved to update and sign apt repositories. Is the server running non-free software? Proprietary BIOS or NIC firmware? Are the GnuPG private keys on disk? Smartcard? TPM? YubiKey? HSM? Where is the server co-located, and who has access to it? I tried to do a bit of this, and discovered things like Trisquel having a DSA1024 key in its default apt trust store (although for fairness, it seems that apt by default does not trust such signatures). However, I’m not certain understanding this more would scale to securing my machines against attacks on this infrastructure. Even people with the best intentions, and the state of the art hardware and software, will have problems.
To increase my trust in Trisquel I set out to understand how it worked. To make it easier to sort out what the interesting parts of the Trisquel archive to audit further were, I created debdistdiff to produce human readable text output comparing one apt archive with another apt archive. There is a GitLab CI/CD cron job that runs this every day, producing output comparing Trisquel vs Ubuntu and PureOS vs Debian. Working with these output files has made me learn more about how the process works, and I even stumbled upon something that is likely a bug where Trisquel aramo was imported from Ubuntu jammy while it contained a couple of package (e.g., gcc-8, python3.9) that were removed for the final Ubuntu jammy release.
After working on auditing the Trisquel archive manually that way, I realized that whatever I could tell from comparing Trisquel with Ubuntu, it would only be something based on a current snapshot of the archives. Tomorrow it may look completely different. What felt necessary was to audit the differences of the Trisquel archive continously. I was quite happy to have developed debdistdiff for one purpose (comparing two different archives like Trisquel and Ubuntu) and discovered that the tool could be used for another purpose (comparing the Trisquel archive at two different points in time). At this time I realized that I needed a log of all different apt archive metadata to be able to produce an audit log of the differences in time for the archive. I create manually curated git-repositories with the Release/InRelease and the Packages files for each architecture/component of the well-known distributions Trisquel, Ubuntu, Debian and PureOS. Eventually I wrote scripts to automate this, which are now published in the debdistget project.
At this point, one of the early question about per-IP substitution of Release files were lingering in my mind. However with the tooling I now had available, coming up with a way to resolve this was simple! Merely have apt compute a SHA256 checksum of the just downloaded InRelease file, and see if my git repository had the same file. At this point I started reading the Apt source code, and now I had more doubts about the security of my systems than I ever had before. Oh boy how the name Apt has never before felt more… Apt?! Oh well, we must leave some exercises for the students. Eventually I realized I wanted to touch as little of apt code basis as possible, and noticed the SigVerify::CopyAndVerify function called ExecGPGV which called apt-key verify which called GnuPG’s gpgv. By setting Apt::Key::gpgvcommand I could get apt-key verify to call another tool than gpgv. See where I’m going? I thought wrapping this up would now be trivial but for some reason the hash checksum I computed locally never matched what was on my server. I gave up and started working on other things instead.
Today I came back to this idea, and started to debug exactly how the local files looked that I got from apt and how they differed from what I had in my git repositories, that came straight from the apt archives. Eventually I traced this back to SplitClearSignedFile which takes an InRelease file and splits it into two files, probably mimicking the (old?) way of distributing both Release and Release.gpg. So the clearsigned InRelease file is split into one cleartext file (similar to the Release file) and one OpenPGP signature file (similar to the Release.gpg file). But why didn’t the cleartext variant of the InRelease file hash to the same value as the hash of the Release file? Sadly they differ by the final newline.
Having solved this technicality, wrapping the pieces up was easy, and I came up with a project apt-canary that provides a script apt-canary-gpgv that verify the local apt release files against something I call a “apt canary witness” file stored at a URL somewhere.
I’m now running apt-canary on my Trisquel aramo laptop, a Trisquel nabia server, and Talos II ppc64el Debian machine. This means I have solved the per-IP substitution worries (or at least made them less likely to occur, having to send the same malicious release files to both GitLab and my system), and allow me to have an audit log of all release files that I actually use for installing and downloading packages.
What do you think? There are clearly a lot of work and improvements to be made. This is a proof-of-concept implementation of an idea, but instead of refining it until perfection and delaying feedback, I wanted to publish this to get others to think about the problems and various ways to resolve them.
Btw, I’m going to be at FOSDEM’23 this weekend, helping to manage the Security Devroom. Catch me if you want to chat about this or other things. Happy Hacking!
