Use the gui to mount the encrypted directory, then login to the synology as root over ssh and type mount. You will see a line like
/volume1/@mycryptdir@ on /volume1/mycryptdir type ecryptfs (rw,relatime,ecryptfs_fnek_sig=88...,ecryptfs_sig=88...,ecryptfs_cipher=aes,ecryptfs_key_bytes=32)
This shows your directory /volume1/mycryptdir is implemented on an underlying /volume1/@mycryptdir@ directory using ecryptfs. Unmount the directory with the gui, then try the following command:
# ecryptfs-add-passphrase Passphrase:
Type in the cleartext passphrase you originally used (not the .key file). It will reply
Inserted auth tok with sig [88...] into the user session keyring
Now type the mount command using the options you saw before. You will need to create the mount point directory:
# mkdir /volume1/mycryptdir # mount /volume1/\@mycryptdir\@/ /volume1/mycryptdir/ -t ecryptfs -o rw,relatime,ecryptfs_fnek_sig=88...,ecryptfs_sig=88...,ecryptfs_cipher=aes,ecryptfs_key_bytes=32
Your filesystem should now be mounted and useable. You should now clear the password from the in-memory keyring:
# keyctl clear @u
When you have finished, unmount the directory with umount /volume1/mycryptdir.