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

git rebase HEAD~N --signoff

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

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 [email protected] 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" = "[email protected]" ]; 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

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.

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.

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

git filter-branch --commit-filter 'if [ "$GIT_COMMITTER_EMAIL" = "[email protected]" ];
  then git commit-tree -S "$@";
  else git commit-tree "$@";
  fi' HEAD

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

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.

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.

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.