_posts: add git signing post

Signed-off-by: William Woodruff <[email protected]>

Author: woodruffw

_posts/2025-02-03-Homebrew-git-signing.md

@@ -0,0 +1,43 @@
+---
+title: "Homebrew's new git signing key"
+author: woodruffw
+---
+
+Over the next few days, Homebrew's repositories will begin to transition
+from PGP-based signing to SSH-based signing for [@BrewTestBot] commits.
+
+As part of this change, we will be rotating our [@BrewTestBot]'s key.
+This rotation should not affect most users, but you may notice
+it if you currently manually verify `git` commits from
+[Homebrew/brew], [Homebrew/homebrew-core], or similar.
+
+Once all repositories have been transitioned, we will revoke
+the old PGP key to prevent unintended future use:
+
+* Main key: `3C76C3F1E573FA9E`
+* Signing subkey: `82D7D104050B0F0F`
+
+The new SSH signing key has the following public half:
+
+```
+ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIE0QzQJ6gl6Yxru0QrSaDRNatiHajcKxDu9lxQrFl8Nw
+```
+
+Users can also discover this signing key programmatically through GitHub's
+REST API:
+
+```
+$ gh api /users/BrewTestBot/ssh_signing_keys
+[
+  {
+    "id": 475371,
+    "key": "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIE0QzQJ6gl6Yxru0QrSaDRNatiHajcKxDu9lxQrFl8Nw",
+    "title": "BREWTESTBOT_SSH_SIGNING_KEY",
+    "created_at": "2025-02-03T17:50:27.377+01:00"
+  }
+]
+```
+
+[@BrewTestBot]: https://github.com/BrewTestBot
+[Homebrew/brew]: https://github.com/Homebrew/brew
+[Homebrew/homebrew-core]: https://github.com/Homebrew/homebrew-core