What is the JMCX file format?
A .jmcx file is an encrypted file created by JM-Crypt, a paid privacy tool from Belgian developer E. Bourlard. It wraps a single original file, such as a document or photo, in ciphertext and locks it behind a password. Without the right password and the JM-Crypt app, the contents stay unreadable.
JM-Crypt ships as two matching apps: JM-Crypt PC for Windows and JM-Crypt Mobile for Android. Both use the same format, so a file encrypted on a phone opens on a PC and the reverse. The vendor states the cipher is AES-256 in CBC mode with PKCS padding, encrypt-then-MAC authentication, and a random IV, with SHA3-256 handling hashing and HMAC. Everything runs on-device with no server and no backdoor, so keys never leave your hardware.
The format is aimed at people who handle sensitive material, including lawyers and doctors, and is marketed as protection against theft and ransomware. JM-Crypt also uses the related extensions .jmc, .jmck, .jmcp, and .jmcr for other encrypted and key files in the suite.
Security & safety
RISK: LOWA .jmcx file is inert encrypted data and cannot run on its own, so the file itself poses little direct risk. The real risk is data loss: if you forget the password or lose access to JM-Crypt, the contents cannot be recovered, since the vendor keeps no keys and there is no backdoor. Only decrypt .jmcx files you created or received from someone you trust, and keep a secure backup of the password.
Format details
in a nutshellPrograms that open JMCX files
Windows1 app
JM-Crypt PC
Paid Install JM-Crypt PC from the Microsoft Store, open the app, load the .jmcx file, and enter the password used at encryption to decrypt it back to the original file. 
Android1 app
JM-Crypt Mobile
Paid Install JM-Crypt Mobile from Google Play, select the .jmcx file, and enter its password to decrypt and view the original content on your phone or tablet. Technical details
deep spec| Encoding | Binary |
| Container | Proprietary JM-Crypt encrypted container |
| Compression | None documented |
| Encryption | AES-256 in CBC mode with PKCS padding, encrypt-then-MAC authentication, and a random initialization vector (IV). Integrity and key derivation use SHA3-256 hashing and HMAC. |
| Typical size | Slightly larger than the original plaintext file (IV, padding, and MAC overhead) |
| Structure | Ciphertext produced by JM-Crypt's two-pass scheme: the file is first encrypted with key material drawn from the vendor's proprietary random key generator, then password-protected. Each file carries its own random IV and authentication tag. |
| Integrity | HMAC/SHA3-256 authentication tag (encrypt-then-MAC) verifies integrity before decryption |
| Encryption Note | The vendor markets the key material as one-time-pad-style output from a homemade fast Fourier transform (FFT) key generator; the actual on-disk cipher is standard AES-256-CBC. |
| Platforms | Windows, Android |
| Notes | Files are portable between JM-Crypt PC (Windows) and JM-Crypt Mobile (Android). Decryption requires the JM-Crypt app plus the password used at encryption time. No cloud server or backdoor is involved; keys never leave the device. |
| Released | 2017 |
| Latest version | 1.8.4.0 (JM-Crypt Mobile, Android) |
JMCX conversions
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