1. I have a GPG keypair in the same format as the one provided in `test-key.asc`. It is `ed25519`.
2. That key was used to encrypt a file with [age encryption](https://github.com/FiloSottile/age) as shown below
3. I want to decrypt that file, but only have a GPG secret key, as I couldn't
   find out how to derive a SSH or age key from it.

**GOAL**: Derive an age key from the provided GPG key that decrypt the file as
shown below. A SSH key is also enough, since it can be used with `ssh-to-age` to
derive the age key.

##### Notes:

- A tool exists to do this for RSA keys: [openpgp2ssh](https://manpages.ubuntu.com/manpages/xenial/man1/openpgp2ssh.1.html) but it does not seem to support `ed25519` keys
- Work on `gnupg` was started for this feature, but never finished see this
  issue and commit: https://dev.gnupg.org/T6647

## Example

Example key provided in `test-key.asc` to be imported. Use `--homedir` with
`gpg` to set a temporary `.gnupg` directory

```sh
❯ gpg --homedir ./gnupg --import test-key.asc                                                                                                                                                                          impure ❄ ssh-to-age age
gpg: WARNING: unsafe permissions on homedir '/home/pinpox/code/github.com/pinpox/gpg2age/./gnupg'
gpg: key 76188CF30717B54E: public key "test (test) <test@test.com>" imported
gpg: key 76188CF30717B54E: secret key imported
gpg: Total number processed: 1
gpg:               imported: 1
gpg:       secret keys read: 1
gpg:   secret keys imported: 1

❯ gpg --homedir ./gnupg_testkey/ -K
/home/pinpox/code/github.com/pinpox/gpg2age/./gnupg_testkey/pubring.kbx
-----------------------------------------------------------------------
sec   ed25519 2024-03-25 [C]
      9FE4D484B69DB9F5C7AA208E76188CF30717B54E
uid           [ultimate] test (test) <test@test.com>
ssb   ed25519 2024-03-25 [S]
ssb   cv25519 2024-03-25 [E]
ssb   ed25519 2024-03-25 [A]
```

### Get age key and encrypt test file

```sh
❯ gpg --homedir ./gnupg --export-ssh-key 9FE4D484B69DB9F5C7AA208E76188CF30717B54E
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAICYvKXGcG4a19tTT0Rycbn+D0r/YlKltLJ9dY2gR/Fjx openpgp:0x47C9F3FF
```

```sh
❯ gpg --homedir ./gnupg --export-ssh-key 9FE4D484B69DB9F5C7AA208E76188CF30717B54E | ssh-to-age                                                                                                                             impure ❄ ssh-to-age
age18s8m9hvlrwvltgys4lafyyqe356ntc7e06t4kd2nccqm5amsaa2s878mju # saved as age-public-key
```

```sh
❯ age --encrypt -R age-public-key testfile.txt > testfile.txt.age
```

### Get secret age key

```sh
❯ go run main.go                                                                                                                                                                                                       impure ❄ ssh-to-age age
AGE-SECRET-KEY-165W948VSG5QEM0RPEUX8T3K4YXJT2WF83C2GXQH8Q3Q0ZHCTH44SSV0H34 # saved as age-secret-key
```

### Try to decrypt
```sh
❯ age --decrypt --identity age-secret-key --output decrypted testfile.txt.age                                                                                                                                          impure ❄ ssh-to-age age
age: error: no identity matched any of the recipients
age: report unexpected or unhelpful errors at https://filippo.io/age/report
```

FAIL :(