From 2b7e7634e62512e26b76deaabe86bd3a582e4b9a Mon Sep 17 00:00:00 2001
From: Sean Gillespie <sean@mistersg.net>
Date: Tue, 19 Dec 2023 16:45:54 -0500
Subject: [PATCH] feature: Add a script that opens the luks device

---
 default.nix     | 22 +++++-------
 yk-luks-open.sh | 90 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 99 insertions(+), 13 deletions(-)
 create mode 100755 yk-luks-open.sh

diff --git a/default.nix b/default.nix
index 47da38a..8cb0c4a 100644
--- a/default.nix
+++ b/default.nix
@@ -2,7 +2,14 @@
 
 let
   inherit (nixpkgs) callPackage pkgs stdenv;
+
   pbkdf2Sha512 = callPackage ./pbkdf2-sha512 { };
+  rbtohex = pkgs.writeShellScriptBin
+    "rbtohex"
+    ''( od -An -vtx1 | tr -d ' \n' )'';
+  hextorb = pkgs.writeShellScriptBin
+    "hextorb"
+    ''( tr '[:lower:]' '[:upper:]' | sed -e 's/\([0-9A-F]\{2\}\)/\\\\\\x\1/gI'| xargs printf )'';
 in
   stdenv.mkDerivation {
     name = "yubikey-luks-setup";
@@ -12,18 +19,7 @@ in
       parted
       pbkdf2Sha512
       yubikey-personalization
+      rbtohex
+      hextorb
     ];
-    
-    shellHook = ''
-      rbtohex() {
-        ( od -An -vtx1 | tr -d ' \n' )
-      }
-
-      hextorb() {
-        ( tr '[:lower:]' '[:upper:]' | sed -e 's/\([0-9A-F]\{2\}\)/\\\\\\x\1/gI'| xargs printf )
-      }
-    '';
-
-    
-    inherit (pkgs) cryptsetup openssl yubikey-personalization;
   }
diff --git a/yk-luks-open.sh b/yk-luks-open.sh
new file mode 100755
index 0000000..617bb57
--- /dev/null
+++ b/yk-luks-open.sh
@@ -0,0 +1,90 @@
+#!/usr/bin/env bash
+
+function usage {
+    cat >&2 <<EOF
+Usage: yk-luks-open.sh [OPTIONS] DEVICE
+
+Mount a LUKS encrypted filesystem with Yubikey on NixOS
+
+Options:
+
+  -c, --storage=file       Path of the salt on and iterations on the unencrypted device
+  -l, --key-length=number  Length of the LUKS slot key
+  -p, --passphrase         Prompt for 2FA passphrase
+  -s, --slot=number        Which slot on the YubiKey to challenge.
+  -h, --help               Show this help
+EOF
+}
+
+# Get CLI options
+options=$(getopt --options "c:l:ps:h" --long "key-length:,passphrase,slot:,storage:,help" -- "$@")
+
+# Inspect CLI options
+eval set -- "$options"
+while true; do
+    case $1 in
+        -c|--storage)
+            STORAGE=$2
+            shift 2
+            ;;
+        -l|--key-length)
+            KEY_LENGTH=$2
+            shift 2
+            ;;
+        -p|--passphrase)
+            PROMPT_PHRASE=
+            shift
+            ;;
+        -s|--slot)
+            SLOT=$2
+            shift 2
+            ;;
+        -h|--help)
+            usage
+            exit 0
+            ;;
+        --)
+            shift
+            break
+            ;;
+        *)
+            echo -e "Unhandled option '$1'"
+            exit 2
+    esac
+done
+
+# Inspect the device
+DEVICE=$1
+if [[ -z "$DEVICE" ]]; then
+    echo -e "Missing required option: DEVICE"
+
+    usage
+    exit 1
+fi
+
+# Set defaults from specified options
+: ${STORAGE:=/mnt/boot/crypt-storage/default}
+: ${KEY_LENGTH:=512}
+: ${SLOT:=1}
+
+# Prompt for the passphrase
+if [[ "${PROMPT_PHRASE+DEFINED}" ]]; then
+    read -s -p "Passphrase: " USER_PASSPHRASE
+    echo
+else
+    USER_PASSPHRASE=
+fi
+
+# Look up salt and iterations
+SALT=$(awk 'NR == 1 { print }' < "$STORAGE")
+ITERATIONS=$(awk 'NR == 2 { print }' < "$STORAGE")
+
+# Calculate LUKS key
+CHALLENGE=$(echo -n $SALT | openssl dgst -binary -sha512 | rbtohex)
+RESPONSE=$(ykchalresp -2 -x $CHALLENGE 2>/dev/null)
+LUKS_KEY="$(echo "$USER_PASSPHRASE" | pbkdf2-sha512 $(($KEY_LENGTH / 8)) $ITERATIONS $RESPONSE | rbtohex)"
+
+# Open the LUKS device
+echo -n "$LUKS_KEY" \
+    | hextorb \
+    | cryptsetup open "$DEVICE" encrypted --key-file=-