Synology Archive Decryption

Decrypt Synology PAT (system update) and SPK (application) encrypted archives to recover the underlying TAR contents.

When to Use This Skill

Use this skill when:

• The user has a

  .pat

  or

  .spk

  file and wants to extract its contents
• The user needs to analyze Synology firmware or applications
• The user mentions Synology NAS, DSM, BSM, or firmware extraction
• The user wants to inspect what's inside a Synology package

Quick Start

# Decrypt an SPK or PAT file
python3 scripts/decrypt_synology_archive.py <archive-file>

# Extract the resulting TAR
tar xf <output>.tar

How It Works

Synology encrypts their firmware and application packages using hard-coded keys embedded in their extraction libraries. This skill automates the decryption process:

1. Detect archive type - PAT (system) or SPK (application)
2. Verify header signature - Ed25519 signature validation
3. Derive encryption key - Using the hard-coded master key and archive-specific subkey
4. Decrypt with XChaCha20-Poly1305 - Stream decryption of all entries
5. Output clean TAR - Standard tar archive ready for extraction

Archive Structure

PAT Files (System Updates)

.pat (encrypted archive)
├── Header (MessagePack + Ed25519 signature)
├── Encrypted entries (XChaCha20-Poly1305)
└── → .tar (decrypted output)
    ├── hda1.tgz (root filesystem)
    ├── rd.bin (initramfs)
    ├── packages/ (embedded SPKs)
    └── ...

SPK Files (Applications)

.spk (encrypted archive)
├── Header (MessagePack + Ed25519 signature)
├── Encrypted entries (XChaCha20-Poly1305)
└── → .tar (decrypted output)
    ├── app/ (application files)
    ├── conf/ (configuration)
    └── ...

Key Technical Details

Cryptographic Components

Component	Purpose	Algorithm
signature_key	Verify archive header	Ed25519
master_key	Derive per-archive key	Hard-coded 128-bit
subkey_id	Archive-specific identifier	uint64 at offset 0x10
stream_key	Actual encryption key	crypto_kdf_derive_from_key
nonce	Per-chunk IV	18 bytes, prepended to ciphertext
cipher	Data encryption	XChaCha20-Poly1305 secretstream

File Format

1. Magic bytes:

   0xBFBAAD

   or

   0xADBEEF

   (3 bytes)
2. Header length: Little-endian uint32
3. Header data: MessagePack-encoded metadata
4. Signature: 64-byte Ed25519 signature
5. Encrypted data: Chunks of up to 0x400000 + 0x11 bytes

Usage Examples

Example 1: Decrypt a System Update

# Download a PAT file
wget https://archive.synology.com/download/Os/BSM/BSM_BST150-4T_65374.pat

# Decrypt it
python3 scripts/decrypt_synology_archive.py BSM_BST150-4T_65374.pat

# Extract the contents
tar xf BSM_BST150-4T_65374.tar

Example 2: Decrypt an Application Package

# Decrypt an SPK
python3 scripts/decrypt_synology_archive.py SynologyPhotos-rtd1619b-1.7.0-0794.spk

# Inspect the application
tar xf SynologyPhotos-rtd1619b-1.7.0-0794.tar
ls -la app/

Example 3: Inspect PAT Structure First

# Optional: dump PAT structure to see what's inside
python3 scripts/inspect_pat_structure.py BSM_BST150-4T_65374.pat

# Then decrypt specific embedded packages
python3 scripts/decrypt_synology_archive.py packages/SomeApp.spk

Common Issues

"No matching keys found"

• The archive may be from a DSM version not yet in the key database
• Check if the file is actually a valid PAT/SPK:

  file yourfile.pat

• Try extracting from a different DSM version

"Signature verification failed"

• The archive may be corrupted or modified
• Ensure the file was downloaded completely
• Check for network corruption during download

"Invalid archive format"

• Verify the magic bytes:

  xxd -l 4 yourfile.pat

• Should show

  bf ba ad

  or

  ad be ef

• The file may not be a Synology archive

"Decryption failed at offset X"

• The archive may be truncated
• Check file size matches expected download size
• Try re-downloading the archive

Dependencies

Required Python packages:

pip install msgpack pycryptodomex

Optional for PAT inspection:

pip install patology  # or use the bundled script

Security Notes

• This skill uses publicly documented hard-coded keys from Synology's own libraries
• The decryption is performed offline - no network access required
• All keys are embedded in the official Synology extraction tools
• This is for legitimate firmware analysis and security research

References

• Synacktiv Research - Pwn2Own Ireland 2024
• synodecrypt on GitHub
• patology on GitHub
• libsodium documentation