.C9S file extension
To open .C9S files on Windows, install Cryptomator (official project) and open Cryptomator.
To open a .c9s file, open the enclosing Cryptomator vault in Cryptomator and unlock it with the correct password/key. A .c9s file is not meant to be opened as a standalone document; it is encrypted vault content and becomes readable only through Cryptomator.
Last updated: April 29, 2026 · Reviewed by Julian Stricker
Open on your device
Choose your operating system for a dedicated step-by-step opening guide.
How to open .C9S files
Use these platform-specific instructions to open .C9S files safely.
Windows
- Install Cryptomator (official project) and open Cryptomator.
- In Cryptomator, add/open the vault folder that contains the .c9s file, then unlock the vault with the correct credentials.
- Access the decrypted files via the unlocked/mounted view; avoid trying to open the .c9s file directly from the raw vault folder.
Mac
- Install Cryptomator (official project) and open Cryptomator.
- Add/open the vault that contains the .c9s file and unlock it.
- Use the decrypted mounted location shown by Cryptomator to open your files (not the .c9s ciphertext files in the vault folder).
Linux
- Install Cryptomator (official project) and start Cryptomator.
- Open the vault directory containing the .c9s file and unlock it.
- Open the decrypted view provided by Cryptomator; do not edit or move individual .c9s files inside the vault directory.
iOS
- If you received a .c9s file by itself, you generally need the whole vault structure; transfer the entire vault to a device where you can open it with Cryptomator, then unlock the vault to access decrypted content.
Android
- If you only have a single .c9s file, it is typically not usable without the full vault; copy the entire vault and open/unlock it with Cryptomator to access the decrypted files.
Security notes
- .c9s files are encrypted data within a Cryptomator vault; the main security risk is accidental data loss from altering, renaming, or moving ciphertext files inside the vault directory. Manage content through Cryptomator’s decrypted view instead.
- Treat a lone .c9s file as incomplete evidence of your data: without the full vault structure and correct credentials, it should not be expected to open. If someone sends you only .c9s files, request the complete vault folder structure.
- When unlocking a vault on a shared or untrusted machine, remember that decrypted files become accessible in the mounted/decrypted view while the vault is unlocked; lock the vault when not in use.
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Can't open this file?
These are the most common causes and fixes when .C9S files fail to open.
Common reasons
- Trying to open a .c9s file directly shows gibberish or fails
- Unlock fails or files look missing after unlocking
- File associations or MIME type recognition is not working
Fix steps
- Locate the vault folder that contains the .c9s file (the whole vault structure matters).
- Open/unlock the vault in Cryptomator and use the decrypted mounted view to access the original files.
OS-specific troubleshooting
What is a .C9S file?
.c9s is a file extension registered with IANA for the media type application/vnd.cryptomator.encrypted, used for Cryptomator-encrypted files within a vault structure. The file content is encrypted and is intended to be accessed via Cryptomator’s vault unlocking workflow. On desktop systems, Cryptomator exposes decrypted files through a mounted virtual drive or filesystem view after you unlock the vault.
Background
Cryptomator is a client-side encryption system for cloud and local storage. Instead of producing a single encrypted container file, it stores many encrypted files and directories inside a vault structure; recognizable encrypted file extensions are used, including .c9r and .c9s (per Cryptomator’s security documentation).
Because these files are encrypted building blocks of a vault, opening a .c9s file in a text editor or viewer will only show unreadable ciphertext. The practical way to use such files is to unlock the vault with Cryptomator, which then provides a decrypted view of your data.
The IANA registration for application/vnd.cryptomator.encrypted explicitly lists .c9s as a related extension. Desktop environments may map the extension to this MIME type using shared MIME-info database rules, but that mapping alone does not provide decryption; it only helps the system identify the file type.
Common MIME types: application/vnd.cryptomator.encrypted
Further reading
Authoritative resources for more details on the .C9S format.
- IANA Media Type Registration: application/vnd.cryptomator.encrypted
- IANA Media Types Registry Index (application tree)
- Cryptomator Security Target (mentions .c9r and .c9s extensions)
- Cryptomator Vault Cryptography (vault internals background)
- Cryptomator for Windows, macOS, and Linux (official GitHub repository)
- Shared MIME-info Database Specification (how desktops map extensions to MIME types)
Common .C9S issues
Trying to open a .c9s file directly shows gibberish or fails
.c9s files are encrypted vault components (ciphertext). They are not intended to be opened as documents outside Cryptomator’s vault unlock process.
- Locate the vault folder that contains the .c9s file (the whole vault structure matters).
- Open/unlock the vault in Cryptomator and use the decrypted mounted view to access the original files.
Unlock fails or files look missing after unlocking
This is often caused by using the wrong vault folder, incomplete vault data (only some files were copied), or a mismatch in vault contents due to partial sync/copy.
- Verify you copied/synced the entire vault folder structure, not individual .c9s files.
- Re-open the correct vault location in Cryptomator and try unlocking again.
File associations or MIME type recognition is not working
Some systems won’t know what to do with .c9s, even though the extension is registered for application/vnd.cryptomator.encrypted; desktop environments may require MIME database updates to recognize it.
- Open the vault from within Cryptomator instead of relying on double-clicking a .c9s file.
- On Linux desktops, ensure your MIME database is up to date per the shared MIME-info mechanism used by your environment.
FAQ
What program opens .c9s files?
.c9s is associated with Cryptomator encrypted vault files (IANA: application/vnd.cryptomator.encrypted). You typically access the underlying data by opening and unlocking the vault in Cryptomator.
Can I convert a .c9s file by renaming it (for example to .txt or .zip)?
No. Renaming does not decrypt or convert it. A .c9s file is encrypted content; you must unlock the vault in Cryptomator to get the original plaintext files.
Why do I have .c9s and .c9r files in my folder?
Cryptomator uses recognizable encrypted file extensions (including .c9r and .c9s) as part of its vault structure. These are normal inside a Cryptomator vault and are not meant to be human-readable.
Is .c9s a single encrypted container with all my files?
No. Cryptomator vaults are stored as a structure of encrypted files and metadata; individual .c9s files are components within that structure rather than a complete archive by themselves.
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