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Question about git filter-branch with signed commits

Subject: Question about git filter-branch with signed commits

## tl;dr

3 messages between Jul 25, 2015 and Jul 26, 2015.

replies: 2people: 2as markdown or json

Adam Liter· Jul 25, 2015, 20:17 UTC · lore
Hello,

I'm trying to clean up the commit history of a git repository by removing certain files. The repository contains signed commits, and when I try to use git filter-branch, the PGP signature is converted to the first line of the commit message.

Is this the expected behavior? If so, is there something that I can do to prevent this from happening?

I'm using git v. 2.4.6. I tried reducing the behavior to a simple case. I asked about this on Stack Overflow (http://stackoverflow.com/q/31552774/2571049), but the question hasn't really gotten any uptake, so I thought I would try asking here too.

In trying to reproduce the behavior in a minimal way, I did the following:

mkdir pgp-git-test cd pgp-git-test git init touch a.txt git add a.txt git commit -m "Add a.txt" -S touch b.txt git add b.txt git commit -m "Add b.txt" -S git filter-branch --index-filter 'git rm --cached --ignore-unmatch a.txt' --prune-empty HEAD git log --oneline --decorate b4efdf0 (HEAD -> master) iQIcBAABCgAGBQJVrvqHAAoJ [...] -----END PGP SIGNATURE----- 9f82e63 iQIcBAABCgAGBQJVrvp7AAoJ [...] -----END PGP SIGNATURE-----

As can be seen after running git log --oneline --decorate, the PGP signature has become the first line of the commit. The rest of the commit message is still there, but it's on a line below the PGP signature.

How can I prevent this from happening when using git filter-branch?

Also, one thing that I was a bit surprised about when trying to create this minimal example is that the first commit is still there. Does anyone know why the commit history still contains the first commit, since the first commit only involved the file a.txt and since I passed --prune-empty to git filter-branch? Shouldn't this commit have been eliminated from the log?

Thanks in advance for the help! I'm relatively new to git, so apologies for bugging you all if this is a trivial error on my part. Thanks!

Best, Adam Liter

Jacob Keller· Jul 25, 2015, 22:50 UTC · re: Adam Liter · lore

Re: Question about git filter-branch with signed commits

On Sat, Jul 25, 2015 at 1:17 PM, Adam Liter <adam.liter@gmail.com> wrote:
Show 7 quoted lines
> Hello,
>
> I'm trying to clean up the commit history of a git repository by removing
> certain files. The repository contains signed commits, and when I try to use
> git filter-branch, the PGP signature is converted to the first line of the
> commit message.
>

It should be possible to craft a filter that removes the signatures entirely. It is not possible to craft a filter-branch which retains the signatures in any *valid* form, though you could re-create new signatures assuming you have all the private keys (ie: you're the one who signed all the commits).

You can't keep the signatures alone, because then they wouldn't be valid, since the data they signed is not the same now. Normally signatures are used so that you can verify "yes, so-and-so is the person who did this thing and no one else could have faked it).

> Is this the expected behavior? If so, is there something that I can do to
> prevent this from happening?
I'm not sure why it's putting the first line at the top of the commit.
Show 29 quoted lines
>
> I'm using git v. 2.4.6. I tried reducing the behavior to a simple case. I
> asked about this on Stack Overflow
> (http://stackoverflow.com/q/31552774/2571049), but the question hasn't
> really gotten any uptake, so I thought I would try asking here too.
>
> In trying to reproduce the behavior in a minimal way, I did the following:
>
> mkdir pgp-git-test
> cd pgp-git-test
> git init
> touch a.txt
> git add a.txt
> git commit -m "Add a.txt" -S
> touch b.txt
> git add b.txt
> git commit -m "Add b.txt" -S
> git filter-branch --index-filter 'git rm --cached --ignore-unmatch a.txt'
> --prune-empty HEAD
> git log --oneline --decorate
> b4efdf0 (HEAD -> master)  iQIcBAABCgAGBQJVrvqHAAoJ [...] -----END PGP
> SIGNATURE-----
> 9f82e63  iQIcBAABCgAGBQJVrvp7AAoJ [...]  -----END PGP SIGNATURE-----
>
> As can be seen after running git log --oneline --decorate, the PGP signature
> has become the first line of the commit. The rest of the commit message is
> still there, but it's on a line below the PGP signature.
>
> How can I prevent this from happening when using git filter-branch?

You probably need to run all the commits through a filter first which strips out all signature info, and then re-write the commits to remove the file. You can't leave the signatures in there alone, as they would no longer be valid, and people who try to verify them would fail.

Show 7 quoted lines
>
> Also, one thing that I was a bit surprised about when trying to create this
> minimal example is that the first commit is still there. Does anyone know
> why the commit history still contains the first commit, since the first
> commit only involved the file a.txt and since I passed --prune-empty to git
> filter-branch? Shouldn't this commit have been eliminated from the log?
>
I'm not sure about this part, sorry.
Show 5 quoted lines
> Thanks in advance for the help! I'm relatively new to git, so apologies for
> bugging you all if this is a trivial error on my part. Thanks!
>
> Best,
> Adam Liter

Regards, Jake

Adam Liter· Jul 26, 2015, 01:24 UTC · re: Jacob Keller · lore

Re: Question about git filter-branch with signed commits

Show 18 quoted lines
>> I'm trying to clean up the commit history of a git repository by 
>> removing
>> certain files. The repository contains signed commits, and when I try 
>> to use
>> git filter-branch, the PGP signature is converted to the first line 
>> of the
>> commit message.
>
> It should be possible to craft a filter that removes the signatures
> entirely. It is not possible to craft a filter-branch which retains
> the signatures in any *valid* form, though you could re-create new
> signatures assuming you have all the private keys (ie: you're the one
> who signed all the commits).
>
> You can't keep the signatures alone, because then they wouldn't be
> valid, since the data they signed is not the same now. Normally
> signatures are used so that you can verify "yes, so-and-so is the
> person who did this thing and no one else could have faked it).
Ah, okay, that makes sense.
Show 45 quoted lines
>> Is this the expected behavior? If so, is there something that I can 
>> do to
>> prevent this from happening?
>
> I'm not sure why it's putting the first line at the top of the commit.
>
>> I'm using git v. 2.4.6. I tried reducing the behavior to a simple 
>> case. I
>> asked about this on Stack Overflow
>> (http://stackoverflow.com/q/31552774/2571049), but the question 
>> hasn't
>> really gotten any uptake, so I thought I would try asking here too.
>>
>> In trying to reproduce the behavior in a minimal way, I did the 
>> following:
>>
>> mkdir pgp-git-test
>> cd pgp-git-test
>> git init
>> touch a.txt
>> git add a.txt
>> git commit -m "Add a.txt" -S
>> touch b.txt
>> git add b.txt
>> git commit -m "Add b.txt" -S
>> git filter-branch --index-filter 'git rm --cached --ignore-unmatch 
>> a.txt'
>> --prune-empty HEAD
>> git log --oneline --decorate
>> b4efdf0 (HEAD -> master)  iQIcBAABCgAGBQJVrvqHAAoJ [...] -----END PGP
>> SIGNATURE-----
>> 9f82e63  iQIcBAABCgAGBQJVrvp7AAoJ [...]  -----END PGP SIGNATURE-----
>>
>> As can be seen after running git log --oneline --decorate, the PGP 
>> signature
>> has become the first line of the commit. The rest of the commit 
>> message is
>> still there, but it's on a line below the PGP signature.
>>
>> How can I prevent this from happening when using git filter-branch?
>
> You probably need to run all the commits through a filter first which
> strips out all signature info, and then re-write the commits to remove
> the file. You can't leave the signatures in there alone, as they would
> no longer be valid, and people who try to verify them would fail.

Do you know of a good way to do this? I'm pretty new to git, and I haven't had much luck searching online or looking through the documentation for ideas of how to even filter out the PGP signatures before filtering out a certain file from the commit history.

If you could even point me in the right direction, I would really appreciate it. Do you think there is a way to do this all automatically, so that it would be possible to keep track of which commits have had their signatures removed and then resign them after filtering out the file (I am the one who signed the commits originally), or would that be impossible since the SHA-1 hashes of the commits will change?

Show 12 quoted lines
>> Also, one thing that I was a bit surprised about when trying to 
>> create this
>> minimal example is that the first commit is still there. Does anyone 
>> know
>> why the commit history still contains the first commit, since the 
>> first
>> commit only involved the file a.txt and since I passed --prune-empty 
>> to git
>> filter-branch? Shouldn't this commit have been eliminated from the 
>> log?
>
> I'm not sure about this part, sorry.

No worries. Thanks for your help and your initial response. I appreciate it!

Show 9 quoted lines
>> Thanks in advance for the help! I'm relatively new to git, so 
>> apologies for
>> bugging you all if this is a trivial error on my part. Thanks!
>>
>> Best,
>> Adam Liter
>
> Regards,
> Jake

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