167

Git now has the ability to sign commits with git commit -S, which is great, but sometimes I forget the flag to commit, and sometimes I mail myself patches which I apply with am, and that command doesn't have a flag for signing.

Is there a way to add a signature to an already recorded commit?

3
  • 17
    For the record, you can tell git to always sign commits via configuration: git config commit.gpgsign true.
    – nicooga
    Oct 23 '18 at 12:47
  • @nicooga I wish your comment had more upvotes so I noticed this earlier. I've had to pull up this question at least half a dozen times, and setting that flag would've saved me a bunch of time. Sep 16 '19 at 21:28
  • If the commits have already been published, you should not rewrite them for any purpose (except removing accidental data leaks), as this would change their commit IDs. You don't need to sign those old commits explicitly, at least not for data integrity purposes. Since each commit contains SHA-1-based IDs of its parents, verifying any single commit will also implicitly verify its entire history via the hash chain. superuser.com/questions/1144817/… Nov 25 '20 at 14:08
175
  1. Go into interactive rebase mode.
  2. Add the following line after each commit you want to sign

    exec git commit --amend --no-edit -S

This will run this command after picking each commit.

UPDATE:

Easier way to do this is:

git rebase --exec 'git commit --amend --no-edit -n -S' -i development

This rebases everything till development (or any hash) and you don't have to copy paste after every commit.

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  • 2
    Oh I wish I had found this sooner. I read so many things, even from GitHub themselves, saying that you can't resign old commits. This proves that completely false! I could have saved hundreds of commits, which I have now squashed. Oh well... thanks for sharing! I made an alias out of this. resign = "!re() { git rebase --exec 'git commit --amend --no-edit -n -S' -i $1; }; re" becomes git resign HASH
    – BarryMode
    Jul 6 '17 at 21:16
  • 2
    This should be common knowledge! You have done a great service for humanity (no sarc!)!
    – hopeseekr
    Feb 3 '18 at 20:25
  • 3
    There's no reason to rebase at all. Just run git commit --amend --no-edit -n -S. Mar 6 '18 at 1:51
  • 23
    Doesn’t this change the history, requiring a git push --force?
    – Steve
    Apr 14 '18 at 17:21
  • 3
    @BarryMode people are correct in that you can't resign old commits. Technically this answer does not resign an old commit, but just creates a new commit containing the same content with an additional signature. It is not the same commit in that force push is required.
    – SOFe
    Apr 16 '20 at 1:53
52

Signing a commit changes its contents, so more recent commits depending on it will change their hash.

If you just want to sign the most recent commit, git commit -S --amend will work.

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    Most of the time signing the most recent commit, or even creating an empty signed commit on top of it will be sufficient to guarantee authenticity. Git commits are a Merkle tree, so modification of any ancestor commit requires rewrite of its descendants, thus invalidating the signature.
    – gronostaj
    Jul 22 '20 at 12:38
  • 1
    @gronostaj That is only true though as long as there are no SHA-1 preimage attack is found. This is not the case currently, however git is moving away from SHA-1 already.
    – rugk
    Jan 21 at 13:05
21

I use git rebase -i --root ( see Rewriting History ) and change pick to edit.

Then I use git commit -S --amend --no-edit && git rebase --continue (on Windows) for each commits.

This is manually sign for each commits. I hope we will found better solution.

1
  • I have my home directory as a git repo (for dotfiles). Some programs interactively pick up changes as its rebasing, funny to see the history being replayed live. It's slow enough because signing is slow Jan 22 '17 at 1:33
14

If you need to GPG sign all commits SINCE a particular commit on the current branch, you can use the following instead:

git filter-branch --commit-filter 'git commit-tree -S "$@";' <COMMIT>..HEAD

Where <COMMIT> is the commit id (e.g. abc123e5).

This has the added benefit that it does not disturb the commit metadata (including commit date). The commit hashes will change, though (since it's a digest of the contents of each commit, and a signature is being added to each commit).

If you also would like to stop getting prompted for your GPG passphrase on every commit, also see this answer: https://askubuntu.com/a/805550

NOTE: Switching from gpg to gpg2 for GIT signing will require you to re-import your private key in GPG 2.

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    One can also add --env-filter 'export GIT_COMMITTER_DATE="$GIT_AUTHOR_DATE"' to ensure the commit date is not reset to today's date.
    – Arjan
    Dec 27 '20 at 20:42
  • The downside with using the author date as the committer date is that you'll lose the commit date on rebased/cherry-picked/amended commits.
    – GuyPaddock
    Feb 20 at 14:49
9

I also stumbled on the same problem and here is my solution:

git rebase -i --root --exec 'git commit --amend --no-edit --no-verify -S'

this will sign all of my commits from the first initial commit and also bypass commit hook that I set up using husky. No need to change pick to edit.

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  • 1
    This one modifies the commit date. The commit date is set to the current time.
    – rugk
    Jan 21 at 12:57
6

If you want to filter only specific commits and sign only them you can use filter-branch:

git filter-branch --commit-filter 'if [ "$GIT_COMMITTER_EMAIL" = "user@domain.com" ];
  then git commit-tree -S "$@";
  else git commit-tree "$@";
  fi' HEAD

This is useful if, for some reason, you want to sign only your own commits.

3

If no filtering on commit is needed, then it is preferred to use rebase than filter-branch:

git rebase -i master --exec 'git commit --amend --no-edit --no-verify -S --reset-author'

Else, you can leave untouched the commits you don't own.
Set the following alias in ~/.gitconfig (replace your@address.com with your email address):

resign = "!_() { : git checkout ; [ \"$#\" -eq 0 ] && echo 'Usage: resign <rev-list>' && exit 2; \
                   git filter-branch --commit-filter ' \
                   if [ \"$GIT_COMMITTER_EMAIL\" = \"your@address.com\" ]; then git commit-tree -S \"$@\"; else git commit-tree \"$@\"; fi' $1; }; _"

Then for instance, to resign all your commits in the current branch pulled from master, do:

git resign master..

Credits to previous answers by BarryMode and Roberto Leinardi

1

here's the one I use for all commits, yes it will re-write history:

git rebase --exec 'git commit --amend --no-edit -n -S' -i --root
1
  • Note that this resets the commit date to today's date.
    – Arjan
    Dec 27 '20 at 20:43
0

To sign off last N commits, you can also do:

git rebase HEAD~N --signoff
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  • 4
    Note that --signoff is different than signing (e.g. when using git commit -S). AFAIK signoff just adds a line like Signed-off-by <author> <email> to the commit message.
    – stian
    Sep 18 '20 at 7:33

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