Implement blackbox in Golang (#250)

* Initial release

pkg/crypters/gnupg/keychain.go

@@ -0,0 +1,107 @@
+package gnupg
+
+/*
+
+# How does Blackbox manage key rings?
+
+Blackbox uses the user's .gnupg directory for most actions, such as decrypting data.
+Decrypting requires the user's private key, which is stored by the user in their
+home directory (and up to them to store safely).
+Black box does not store the user's private key in the repo.
+
+When encrypting data, black needs the public key of all the admins, not just the users.
+To assure that the user's `.gnupg` has all these public keys, prior to
+encrypting data the public keys are imported from .blackbox, which stores
+a keychain that stores the public (not private!) keys of all the admins.
+
+FYI: v1 does this import before decrypting, because I didn't know any better.
+
+# Binary compatibility:
+
+When writing v1, we didn't realize that the pubkey.gpg file is a binary format
+that is not intended to be portable. In fact, it is intentionally not portable.
+This means that all admins must use the exact same version of GnuPG
+or the files (pubring.gpg or pubring.kbx) may get corrupted.
+
+In v2, we store the public keys in the portable ascii format
+in a file called `.blackbox/public-keys-db.asc`.
+It will also update the binary files if they exist.
+If `.blackbox/public-keys-db.asc` doesn't exist, it will be created.
+
+Eventually we will stop updating the binary files.
+
+# Importing public keys to the user
+
+How to import the public keys to the user's GPG system:
+
+If pubkeyring-ascii.txt exists:
+	gpg --import pubkeyring-ascii.asc
+Else if pubring.kbx
+	gpg --import pubring.kbx
+Else if pubring.gpg
+	gpg --import pubring.gpg
+
+This is what v1 does:
+  #if gpg2 is installed next to gpg like on ubuntu 16
+  if [[ "$GPG" != "gpg2" ]]; then
+    $GPG --export --no-default-keyring --keyring "$(get_pubring_path)" >"$keyringasc"
+    $GPG --import "$keyringasc" 2>&1 | egrep -v 'not changed$' >&2
+  Else
+    $GPG --keyring "$(get_pubring_path)" --export | $GPG --import
+  fi
+
+# How to add a key to the keyring?
+
+Old, binary format:
+    # Get the key they want to add:
+        FOO is a user-specified directory, otherwise $HOME/.gnupg:
+	    $GPG --homedir="FOO" --export -a "$KEYNAME" >TEMPFILE
+	# Import into the binary files:
+	    KEYRINGDIR is .blackbox
+        $GPG --no-permission-warning --homedir="$KEYRINGDIR" --import TEMPFILE
+	# Git add any of these files if they exist:
+	    pubring.gpg pubring.kbx trustdb.gpg blackbox-admins.txt
+	# Tell the user to git commit them.
+
+New, ascii format:
+	# Get the key to be added.  Write to a TEMPFILE
+        FOO is a user-specified directory, otherwise $HOME/.gnupg:
+	    $GPG --homedir="FOO" --export -a "$KEYNAME" >TEMPFILE
+	# Make a tempdir called TEMPDIR
+	# Import the pubkeyring-ascii.txt to TEMPDIR's keyring. (Skip if file not found)
+	# Import the temp1 data to TEMPDIR
+	# Export the TEMPDIR to create a new .blackbox/pubkeyring-ascii.txt
+	    PATH_TO_BINARY is the path to .blackbox/pubring.gpg; if that's not found then pubring.kbx
+        $GPG --keyring PATH_TO_BINARY --export -a --output .blackbox/pubkeyring-ascii.txt
+	# Git add .blackbox/pubkeyring-ascii.txt and .blackbox/blackbox-admins.txt
+	# Tell the user to git commit them.
+	# Delete TEMPDIR
+
+# How to remove a key from the keyring?
+
+Old, binary format:
+    # Remove key from the binary file
+    $GPG --no-permission-warning --homedir="$KEYRINGDIR" --batch --yes --delete-key "$KEYNAME" || true
+	# Git add any of these files if they exist:
+	    pubring.gpg pubring.kbx trustdb.gpg blackbox-admins.txt
+	# Tell the user to git commit them.
+
+New, ascii format:
+	# Make a tempdir called TEMPDIR
+	# Import the pubkeyring-ascii.txt to TEMPDIR's keyring. (Skip if file not found)
+    # Remove key from the ring file
+    $GPG --no-permission-warning --homedir="$KEYRINGDIR" --batch --yes --delete-key "$KEYNAME" || true
+	# Export the TEMPDIR to create a new .blackbox/pubkeyring-ascii.txt
+	    PATH_TO_BINARY is the path to .blackbox/pubring.gpg; if that's not found then pubring.kbx
+        $GPG --keyring PATH_TO_BINARY --export -a --output .blackbox/pubkeyring-ascii.txt
+	# Git add .blackbox/pubkeyring-ascii.txt and .blackbox/blackbox-admins.txt
+	# Update the .blackbox copy of pubring.gpg, pubring.kbx, or trustdb.gpg (if they exist)
+	#     with copies from TEMPDIR (if they exist).  Git add any files that are updated.
+	# Tell the user to git commit them.
+	# Delete TEMPDIR
+
+*/
+
+//func prepareUserKeychain() error {
+//	return nil
+//}