how to decrypt ecryptfs file with private key instead of passphrase

Question

I have an encrypted share folder on my synology NAS DS413 (which uses ecryptfs). I can manually mount the encrypted folder and read the decrypted files without issue, using synologies GUI. For some reason, I have never been able to mount the encrypted folder using my passphrase . But I can always do it by using the private key generated during ecryptfs setup.

So I have since been doing some research on decrypting the encrypted files without a synology (for example if this thing catches fire or is stolen and I need to restore from backup). I've read several threads and howto's on decrypting synology/ecryptfs encrypted shares using linux and encryptfs-utils. But the howto always tells you to provide the passphrase and never mention the use of the key for decryption. So my question is how do I decrypt using the key (which works to mount and decrypt with synology's software)? The key I have is 80 bytes and is binary. The first 16 bytes are integers only and the remaining bytes appear to be random hex.

Thanks for any tips!

Answer 1

See by "How To Recover Synology encrypted folders in Linux" by Robert Castle. Summary:

MOUNTOPTIONS=""
for option in                           \
  "key=passphrase"                      \
  "ecryptfs_cipher=aes"                 \
  "ecryptfs_key_bytes=32"               \
  "ecryptfs_passthrough=no"             \
  "ecryptfs_enable_filename_crypto=yes" \
; do
  MOUNTOPTIONS="${MOUNTOPTIONS}${MOUNTOPTIONS:+,}$option"
done
sudo mount -t ecryptfs -o $MOUNTOPTIONS,passwd=$PASSWORD $CRYPTDIR $TARGET