--- encrypt_hook	2014-12-09 12:47:02.379911026 +0000
+++ encrypt_hook	2014-12-09 12:47:41.416087076 +0000
@@ -7,7 +7,7 @@
     # Get keyfile if specified
     ckeyfile="/crypto_keyfile.bin"
     if [ -n "$cryptkey" ]; then
-        IFS=: read ckdev ckarg1 ckarg2 <<EOF
+        IFS=: read ckdev ckarg1 ckarg2 ckarg3 ckarg4 <<EOF
 $cryptkey
 EOF
 
@@ -18,9 +18,13 @@
                 *[!0-9]*)
                     # Use a file on the device
                     # ckarg1 is not numeric: ckarg1=filesystem, ckarg2=path
+                    #                        ckarg3=offset, ckarg4=length
                     mkdir /ckey
                     mount -r -t "$ckarg1" "$resolved" /ckey
-                    dd if="/ckey/$ckarg2" of="$ckeyfile" >/dev/null 2>&1
+                    [ -n "$ckarg3$ckarg4" ] && ckoffset="bs=1"
+                    [ -n "$ckarg3" ] && ckoffset="$ckoffset skip=$ckarg3"
+                    [ -n "$ckarg4" ] && ckoffset="$ckoffset count=$ckarg4"
+                    dd if="/ckey/$ckarg2" of="$ckeyfile" $ckoffset >/dev/null 2>&1
                     umount /ckey
                     ;;
                 *)