Unsigned Commits

I’m not going to cryptographically sign my git commits, and you shouldn’t either.

I am going to tell you why I don’t think you should sign your Git commits, even though doing so with SSH keys is now easier than ever. But first, to contextualize my objection, I have a brief hypothetical for you, and then a bit of history from the evolution of security on the web.


paper reading “Sign Here:” with a pen poised over it


It seems like these days, everybody’s signing all different kinds of papers.

Bank forms, permission slips, power of attorney; it seems like if you want to securely validate a document, you’ve gotta sign it.

So I have invented a machine that automatically signs every document on your desk, just in case it needs your signature. Signing is good for security, so you should probably get one, and turn it on, just in case something needs your signature on it.

We also want to make sure that verifying your signature is easy, so we will have them all notarized and duplicates stored permanently and publicly for future reference.

No? Not interested?


Hopefully, that sounded like a silly idea to you.

Most adults in modern civilization have learned that signing your name to a document has an effect. It is not merely decorative; the words in the document being signed have some specific meaning and can be enforced against you.

In some ways the metaphor of “signing” in cryptography is bad. One does not “sign” things with “keys” in real life. But here, it is spot on: a cryptographic signature can have an effect.

It should be an input to some software, one that is acted upon. Software does a thing differently depending on the presence or absence of a signature. If it doesn’t, the signature probably shouldn’t be there.


Consider the most venerable example of encryption and signing that we all deal with every day: HTTPS. Many years ago, browsers would happily display unencrypted web pages. The browser would also encrypt the connection, if the server operator had paid for an expensive certificate and correctly configured their server. If that operator messed up the encryption, it would pop up a helpful dialog box that would tell the user “This website did something wrong that you cannot possibly understand. Would you like to ignore this and keep working?” with buttons that said “Yes” and “No”.

Of course, these are not the precise words that were written. The words, as written, said things about “information you exchange” and “security certificate” and “certifying authorities” but “Yes” and “No” were the words that most users read. Predictably, most users just clicked “Yes”.

In the usual case, where users ignored these warnings, it meant that no user ever got meaningful security from HTTPS. It was a component of the web stack that did nothing but funnel money into the pockets of certificate authorities and occasionally present annoying interruptions to users.

In the case where the user carefully read and honored these warnings in the spirit they were intended, adding any sort of transport security to your website was a potential liability. If you got everything perfectly correct, nothing happened except the browser would display a picture of a small green purse. If you made any small mistake, it would scare users off and thereby directly harm your business. You would only want to do it if you were doing something that put a big enough target on your site that you became unusually interesting to attackers, or were required to do so by some contractual obligation like credit card companies.

Keep in mind that the second case here is the best case.

In 2016, the browser makers noticed this problem and started taking some pretty aggressive steps towards actually enforcing the security that HTTPS was supposed to provide, by fixing the user interface to do the right thing. If your site didn’t have security, it would be shown as “Not Secure”, a subtle warning that would gradually escalate in intensity as time went on, correctly incentivizing site operators to adopt transport security certificates. On the user interface side, certificate errors would be significantly harder to disregard, making it so that users who didn’t understand what they were seeing would actually be stopped from doing the dangerous thing.

Nothing fundamental1 changed about the technical aspects of the cryptographic primitives or constructions being used by HTTPS in this time period, but socially, the meaning of an HTTP server signing and encrypting its requests changed a lot.


Now, let’s consider signing Git commits.

You may have heard that in some abstract sense you “should” be signing your commits. GitHub puts a little green “verified” badge next to commits that are signed, which is neat, I guess. They provide “security”. 1Password provides a nice UI for setting it up. If you’re not a 1Password user, GitHub itself recommends you put in just a few lines of configuration to do it with either a GPG, SSH, or even an S/MIME key.

But while GitHub’s documentation quite lucidly tells you how to sign your commits, its explanation of why is somewhat less clear. Their purse is the word “Verified”; it’s still green. If you enable “vigilant mode”, you can make the blank “no verification status” option say “Unverified”, but not much else changes.

This is like the old-style HTTPS verification “Yes”/“No” dialog, except that there is not even an interruption to your workflow. They might put the “Unverified” status on there, but they’ve gone ahead and clicked “Yes” for you.

It is tempting to think that the “HTTPS” metaphor will map neatly onto Git commit signatures. It was bad when the web wasn’t using HTTPS, and the next step in that process was for Let’s Encrypt to come along and for the browsers to fix their implementations. Getting your certificates properly set up in the meanwhile and becoming familiar with the tools for properly doing HTTPS was unambiguously a good thing for an engineer to do. I did, and I’m quite glad I did so!

However, there is a significant difference: signing and encrypting an HTTPS request is ephemeral; signing a Git commit is functionally permanent.

This ephemeral nature meant that errors in the early HTTPS landscape were easily fixable. Earlier I mentioned that there was a time where you might not want to set up HTTPS on your production web servers, because any small screw-up would break your site and thereby your business. But if you were really skilled and you could see the future coming, you could set up monitoring, avoid these mistakes, and rapidly recover. These mistakes didn’t need to badly break your site.

We can extend the analogy to HTTPS, but we have to take a detour into one of the more unpleasant mistakes in HTTPS’s history: HTTP Public Key Pinning, or “HPKP”. The idea with HPKP was that you could publish a record in an HTTP header where your site commits2 to using certain certificate authorities for a period of time, where that period of time could be “forever”. Attackers gonna attack, and attack they did. Even without getting attacked, a site could easily commit “HPKP Suicide” where they would pin the wrong certificate authority with a long timeline, and their site was effectively gone for every browser that had ever seen those pins. As a result, after a few years, HPKP was completely removed from all browsers.

Git commit signing is even worse. With HPKP, you could easily make terrible mistakes with permanent consequences even though you knew the exact meaning of the data you were putting into the system at the time you were doing it. With signed commits, you are saying something permanently, but you don’t really know what it is that you’re saying.


Today, what is the benefit of signing a Git commit? GitHub might present it as “Verified”. It’s worth noting that only GitHub will do this, since they are the root of trust for this signing scheme. So, by signing commits and registering your keys with GitHub, you are, at best, helping to lock in GitHub as a permanent piece of infrastructure that is even harder to dislodge because they are not only where your code is stored, but also the arbiters of whether or not it is trustworthy.

In the future, what is the possible security benefit? If we all collectively decide we want Git to be more secure, then we will need to meaningfully treat signed commits differently from unsigned ones.

There’s a long tail of unsigned commits several billion entries long. And those are in the permanent record as much as the signed ones are, so future tooling will have to be able to deal with them. If, as stewards of Git, we wish to move towards a more secure Git, as the stewards of the web moved towards a more secure web, we do not have the option that the web did. In the browser, the meaning of a plain-text HTTP or incorrectly-signed HTTPS site changed, in order to encourage the site’s operator to change the site to be HTTPS.

In contrast, the meaning of an unsigned commit cannot change, because there are zillions of unsigned commits lying around in critical infrastructure and we need them to remain there. Commits cannot meaningfully be changed to become signed retroactively. Unlike an online website, they are part of a historical record, not an operating program. So we cannot establish the difference in treatment by changing how unsigned commits are treated.

That means that tooling maintainers will need to provide some difference in behavior that provides some incentive. With HTTPS where the binary choice was clear: don’t present sites with incorrect, potentially compromised configurations to users. The question was just how to achieve that. With Git commits, the difference in treatment of a “trusted” commit is far less clear.

If you will forgive me a slight straw-man here, one possible naive interpretation is that a “trusted” signed commit is that it’s OK to run in CI. Conveniently, it’s not simply “trusted” in a general sense. If you signed it, it’s trusted to be from you, specifically. Surely it’s fine if we bill the CI costs for validating the PR that includes that signed commit to your GitHub account?

Now, someone can piggy-back off a 1-line typo fix that you made on top of an unsigned commit to some large repo, making you implicitly responsible for transitively signing all unsigned parent commits, even though you haven’t looked at any of the code.

Remember, also, that the only central authority that is practically trustable at this point is your GitHub account. That means that if you are using a third-party CI system, even if you’re using a third-party Git host, you can only run “trusted” code if GitHub is online and responding to requests for its “get me the trusted signing keys for this user” API. This also adds a lot of value to a GitHub credential breach, strongly motivating attackers to sneakily attach their own keys to your account so that their commits in unrelated repos can be “Verified” by you.

Let’s review the pros and cons of turning on commit signing now, before you know what it is going to be used for:

Pro Con
Green “Verified” badge Unknown, possibly unlimited future liability for the consequences of running code in a commit you signed
Further implicitly cementing GitHub as a centralized trust authority in the open source world
Introducing unknown reliability problems into infrastructure that relies on commit signatures
Temporary breach of your GitHub credentials now lead to potentially permanent consequences if someone can smuggle a new trusted key in there
New kinds of ongoing process overhead as commit-signing keys become new permanent load-bearing infrastructure, like “what do I do with expired keys”, “how often should I rotate these”, and so on

I feel like the “Con” column is coming out ahead.


That probably seemed like increasingly unhinged hyperbole, and it was.

In reality, the consequences are unlikely to be nearly so dramatic. The status quo has a very high amount of inertia, and probably the “Verified” badge will remain the only visible difference, except for a few repo-specific esoteric workflows, like pushing trust verification into offline or sandboxed build systems. I do still think that there is some potential for nefariousness around the “unknown and unlimited” dimension of any future plans that might rely on verifying signed commits, but any flaws are likely to be subtle attack chains and not anything flashy and obvious.

But I think that one of the biggest problems in information security is a lack of threat modeling. We encrypt things, we sign things, we institute rotation policies and elaborate useless rules for passwords, because we are looking for a “best practice” that is going to save us from having to think about what our actual security problems are.

I think the actual harm of signing git commits is to perpetuate an engineering culture of unquestioningly cargo-culting sophisticated and complex tools like cryptographic signatures into new contexts where they have no use.

Just from a baseline utilitarian philosophical perspective, for a given action A, all else being equal, it’s always better not to do A, because taking an action always has some non-zero opportunity cost even if it is just the time taken to do it. Epsilon cost and zero benefit is still a net harm. This is even more true in the context of a complex system. Any action taken in response to a rule in a system is going to interact with all the other rules in that system. You have to pay complexity-rent on every new rule. So an apparently-useless embellishment like signing commits can have potentially far-reaching consequences in the future.

Git commit signing itself is not particularly consequential. I have probably spent more time writing this blog post than the sum total of all the time wasted by all programmers configuring their git clients to add useless signatures; even the relatively modest readership of this blog will likely transfer more data reading this post than all those signatures will take to transmit to the various git clients that will read them. If I just convince you not to sign your commits, I don’t think I’m coming out ahead in the felicific calculus here.

What I am actually trying to point out here is that it is useful to carefully consider how to avoid adding junk complexity to your systems. One area where junk tends to leak in to designs and to cultures particularly easily is in intimidating subjects like trust and safety, where it is easy to get anxious and convince ourselves that piling on more stuff is safer than leaving things simple.

If I can help you avoid adding even a little bit of unnecessary complexity, I think it will have been well worth the cost of the writing, and the reading.

Acknowledgments

Thank you to my patrons who are supporting my writing on this blog. If you like what you’ve read here and you’d like to read more of it, or you’d like to support my various open-source endeavors, you can support me on Patreon as well! I am also available for consulting work if you think your organization could benefit from expertise on topics such as “What else should I not apply a cryptographic signature to?”.


  1. Yes yes I know about heartbleed and Bleichenbacher attacks and adoption of forward-secret ciphers and CRIME and BREACH and none of that is relevant here, okay? Jeez. 

  2. Do you see what I did there. 

Photo Flow

How do you edit and share photos when more than one person is involved?

Hello, the Internet. If you don’t mind, I’d like to ask you a question about photographs.

My spouse and I both take pictures. We both anticipate taking more pictures in the near future. No reason, just a total coincidence.

We both have iPhones, and we both have medium-nice cameras that are still nicer than iPhones. We would both like to curate and touch up these photos and actually do something with them; ideally we would do this curation collaboratively, whenever either of us has time.

This means that there are three things we want to preserve:

  1. The raw, untouched photographs, in their original resolution,
  2. The edits that have been made to them, and
  3. The “workflow” categorization that has been done to them (minimally, “this photo has not been looked at”, “this photo has been looked at and it’s not good enough to bother sharing”, “this photo has been looked at and it’s good enough to be shared if it’s touched up”, and “this has been/should be shared in its current state”). Generally speaking this is a “which album is it in” categorization.

I like Photos. I have a huge photo library with years of various annotations in it, including faces (the only tool I know of that lets you do offline facial recognition so you can automatically identify pictures of your relatives without giving the police state a database to do the same thing).

However, iCloud Photo Sharing has a pretty major issue; it downscales photographs to “up to 2048 pixels on the long edge”, which is far smaller even than the 12 megapixels that the iPhone 7 sports; more importantly it’s lower resolution than our television, so the image degradation is visible. This is fine for sharing a pic or two on a small phone screen, but not good for a long-term archival solution.

To complicate matters, we also already have an enormous pile of disks in a home server that I have put way too much energy into making robust; a similarly-sized volume of storage would cost about $1300 a year with iCloud (and would not fit onto one account, anyway). I’m not totally averse to paying for some service if it’s turnkey, but something that uses our existing pile of storage would definitely get bonus points.

Right now, my plan is to dump all of our photos into a shared photo library on a network drive, only ever edit them at home, try to communicate carefully about when one or the other of us is editing it so we don’t run into weird filesystem concurrency issues, and hope for the best. This does not strike me as a maximally robust solution. Among other problems, it means the library isn’t accessible to our mobile devices. But I can’t think of anything better.

Can you? Email me. If I get a really great answer I’ll post it in a followup.

Sourceforge Update

Authenticate downloaded binaries from sourceforge a little more.

When I wrote my previous post about Sourceforge, things were looking pretty grim for the site; I (rightly, I think) slammed them for some pretty atrocious security practices.

I invited the SourceForge ops team to get in touch about it, and, to their credit, they did. Even better, they didn't ask for me to take down the article, or post some excuse; they said that they knew there were problems and they were working on a long-term plan to address them.

This week I received an update from said ops, saying:

We have converted many of our mirrors over to HTTPS and are actively working on the rest + gathering new ones. The converted ones happen to be our larger mirrors and are prioritized.

We have added support for HTTPS on the project web. New projects will automatically start using it. Old projects can switch over at their convenience as some of them may need to adjust it to properly work. More info here:

https://sourceforge.net/blog/introducing-https-for-project-websites/

Coincidentally, right after I received this email, I installed a macOS update, which means I needed to go back to Sourceforge to grab an update to my boot manager. This time, I didn't have to do any weird tricks to authenticate my download: the HTTPS project page took me to an HTTPS download page, which redirected me to an HTTPS mirror. Success!

(It sounds like there might still be some non-HTTPS mirrors in rotation right now, but I haven't seen one yet in my testing; for now, keep an eye out for that, just in case.)

If you host a project on Sourceforge, please go push that big "Switch to HTTPS" button. And thanks very much to the ops team at Sourceforge for taking these problems seriously and doing the hard work of upgrading their users' security.

Don’t Trust Sourceforge, Ever

Authenticate downloaded binaries from sourceforge. A little.

Update: please see my more recent post about updates in the interim.

If you use a computer and you use the Internet, chances are you’ll eventually find some software that, for whatever reason, is still hosted on Sourceforge. In case you’re not familiar with it, Sourceforge is a publicly-available malware vector that also sometimes contains useful open source binary downloads, especially for Windows.


In addition to injecting malware into their downloads (a practice they claim, hopefully truthfully, to have stopped), Sourceforge also presents an initial download page over HTTPS, then redirects the user to HTTP for the download itself, snatching defeat from the jaws of victory. This is fantastically irresponsible, especially for a site offering un-sandboxed binaries for download, especially in the era of Let’s Encrypt where getting a TLS certificate takes approximately thirty seconds and exactly zero dollars.

So: if you can possibly find your downloads anywhere else, go there.


But, rarely, you will find yourself at the mercy of whatever responsible stewards1 are still operating Sourceforge if you want to get access to some useful software. As it happens, there is a loophole that will let you authenticate the binaries that you download from them so you won’t be left vulnerable to an evil barista: their “file release system”, the thing you use to upload your projects, will allow you to download other projects as well.

To use it, first, make yourself a sourceforge account. You may need to create a dummy project as well. Sourceforge maintains an HTTPS-accessible list of key fingerprints for all the SSH servers that they operate, so you can verify the public key below.

Then you’ll need to connect to their upload server over SFTP, and go to the path /home/frs/project/<the project’s name>/.../ to get the file.

I have written a little Python script2 that automates the translation of a Sourceforge file-browser download URL, one that you can get if you right-click on a download in the “files” section of a project’s website, and runs the relevant scp command to retrieve the file for you. This isn’t on PyPI or anything, and I’m not putting any effort into polishing it further; the best possible outcome of this blog post is that it immediately stops being necessary.


  1. Are you one of those people? I would prefer to be lauding your legacy of decades of valuable contributions to the open source community instead of ridiculing your dangerous incompetence, but repeated bug reports and support emails have gone unanswered. Please get in touch so we can discuss this. 

  2. Code:

     1
     2
     3
     4
     5
     6
     7
     8
     9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    20
    21
    22
    23
    24
    25
    26
    27
    28
    29
    30
    31
    32
    33
    34
    35
    36
    37
    38
    39
    40
    41
    42
    43
    44
    45
    46
    #!/usr/bin/env python2
    
    import sys
    import os
    
    sfuri = sys.argv[1]
    
    # for example,
    # http://sourceforge.net/projects/refind/files/0.9.2/refind-bin-0.9.2.zip/download
    
    import re
    matched = re.match(
        r"https://sourceforge.net/projects/(.*)/files/(.*)/download",
        sfuri
    )
    
    if not matched:
        sys.stderr.write("Not a SourceForge download link.\n")
        sys.exit(1)
    
    project, path = matched.groups()
    
    sftppath = "/home/frs/project/{project}/{path}".format(project=project, path=path)
    
    def knows_about_web_sf_net():
        with open(
                os.path.expanduser("~/.ssh/known_hosts"), "rb"
        ) as read_known_hosts:
            data = read_known_hosts.read().split("\n")
            for line in data:
                if 'web.sourceforge.net' in line.split()[0]:
                    return True
        return False
    
    sfkey = """
    web.sourceforge.net ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEA2uifHZbNexw6cXbyg1JnzDitL5VhYs0E65Hk/tLAPmcmm5GuiGeUoI/B0eUSNFsbqzwgwrttjnzKMKiGLN5CWVmlN1IXGGAfLYsQwK6wAu7kYFzkqP4jcwc5Jr9UPRpJdYIK733tSEmzab4qc5Oq8izKQKIaxXNe7FgmL15HjSpatFt9w/ot/CHS78FUAr3j3RwekHCm/jhPeqhlMAgC+jUgNJbFt3DlhDaRMa0NYamVzmX8D47rtmBbEDU3ld6AezWBPUR5Lh7ODOwlfVI58NAf/aYNlmvl2TZiauBCTa7OPYSyXJnIPbQXg6YQlDknNCr0K769EjeIlAfY87Z4tw==
    """
    
    if not knows_about_web_sf_net():
        with open(
                os.path.expanduser("~/.ssh/known_hosts"), "ab"
        ) as append_known_hosts:
            append_known_hosts.write(sfkey)
    cmd = "scp web.sourceforge.net:{sftppath} .".format(sftppath=sftppath)
    print(cmd)
    os.system(cmd)
    

The Most Important Thing You Will Read On This Blog All Year

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512

I have two PGP keys.

One, 16F13480, is signed by many people in the open source community.  It is a
4096-bit RSA key.

The other, 0FBC4A07, is superficially worse.  It doesn't have any signatures on
it.  It is only a 3072-bit RSA key.

However, I would prefer that you all use 0FBC4A07.

16F13480 lives encrypted on disk, and occasionally resident in memory on my
personal laptop.  I have had no compromises that I'm aware of, so I'm not
revoking the key - I don't want to lose all the wonderful trust I build up.  In
order to avoid compromising it in the future, I would really prefer to avoid
decrypting it any more often than necessary.

By contrast, aside from backups which I have not yet once had occasion to
access, 0FBC4A07 exists only on an OpenPGP smart card, it requires a PIN, it is
never memory resident on a general purpose computer, and is only plugged in
when I'm actively Doing Important Crypto Stuff.  Its likelyhood of future
compromise is *extremely* low.

If said smart card had supported 4096-bit keys I probably would have just put
the old key on the more secure hardware and called it a day.  Sadly, that is
not the world we live in.

Here's what I'd like you to do, if you wish to interact with me via GnuPG:

    $ gpg --recv-keys 0FBC4A07 16F13480
    gpg: requesting key 0FBC4A07 from hkp server keys.gnupg.net
    gpg: requesting key 16F13480 from hkp server keys.gnupg.net
    gpg: key 0FBC4A07: "Matthew "Glyph" Lefkowitz (OpenPGP Smart Card) <glyph@twistedmatrix.com>" 1 new signature
    gpg: key 16F13480: "Matthew Lefkowitz (Glyph) <glyph@twistedmatrix.com>" not changed
    gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
    gpg: depth: 0  valid:   2  signed:   0  trust: 0-, 0q, 0n, 0m, 0f, 2u
    gpg: next trustdb check due at 2015-08-18
    gpg: Total number processed: 2
    gpg:              unchanged: 1
    gpg:         new signatures: 1
    $ gpg --edit-key 16F13480
    gpg (GnuPG/MacGPG2) 2.0.22; Copyright (C) 2013 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.


    gpg: checking the trustdb
    gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
    gpg: depth: 0  valid:   2  signed:   0  trust: 0-, 0q, 0n, 0m, 0f, 2u
    gpg: next trustdb check due at 2015-08-18
    pub  4096R/16F13480  created: 2012-11-16  expires: 2016-04-12  usage: SC
                         trust: unknown       validity: unknown
    sub  4096R/0F3F064E  created: 2012-11-16  expires: 2016-04-12  usage: E
    [ unknown] (1). Matthew Lefkowitz (Glyph) <glyph@twistedmatrix.com>

    gpg> disable

    gpg> save
    Key not changed so no update needed.
    $

If you're using keybase, "keybase encrypt glyph" should be pointed at the
correct key.

Thanks for reading,

- -glyph
-----BEGIN PGP SIGNATURE-----
Version: GnuPG/MacGPG2 v2.0.22 (Darwin)
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=k0u9
-----END PGP SIGNATURE-----