What is the ASEC file format?
An .asec file stores encrypted and secured Android application data as an internally formatted, encrypted loopback container image. .asec files are created during the transfer of an application or its installation to an SD card - the .apk package is then encrypted to prevent unauthorized access or modification. .asec files are typically stored in the .android_secure folder on the SD card and mounted at /mnt/asec/ by Android's vold storage daemon. The .asec format was introduced with Android 2.2 'Froyo' (which first allowed installation of apps to external memory storage) in 2010. This technology was implemented to prevent copying of paid applications to other devices, as the encryption key is bound to the originating device.
Moving the app back to the phone's internal memory causes the .asec container to be decrypted and the application restored to a standard .apk file. Because the encryption key is device-specific and managed by Android's vold daemon, .asec files cannot be moved to another Android device and run on it. The format was officially superseded by Adoptable Storage, introduced in Android 6.0 Marshmallow (2015), which encrypts and integrates an SD card directly into the device's internal storage pool - making .asec containers largely obsolete on modern Android versions. .obb files (used for large app expansion data) remain in active use as a companion format on SD cards.
Security & safety
RISK: LOWAn .asec file is harmless as a file - it's an encrypted, device-bound container that can't be executed or opened off the phone, so it poses no direct malware risk on a PC (it's just unreadable data there). The practical risk is data loss: these are system files, and deleting them or reformatting the SD card removes the apps that were moved to the card, and they generally can't be recovered because the encryption key lives on the original device. Don't try to edit or 'crack' .asec containers, and ignore any tool claiming to extract an installable APK from one - those are scams or malware. If you need the app, reinstall it from the Play Store or its APK.
Format details
in a nutshellPrograms that open ASEC files
Android1 app
Android OS (the device itself)
Built-in There is no 'open' action - Android mounts and decrypts the .asec automatically to run the app. The user interacts with the app normally; the container is invisible. Managed under Settings > Apps (move to/from SD card on older versions). Technical details
deep spec| Container type | Encrypted loopback disk image internally formatted with ext4, mounted as a virtual block device by the Android OS |
| Encryption algorithm | AES-128 in CBC mode, applied by Android's `vold` daemon via the Linux dm-crypt subsystem |
| Key management | Per-container key generated at creation time; stored in `/data/misc/systemkeys/` on internal storage - never written to the SD card itself |
| Mount mechanism | `vold` / `MountService` attaches the image via a Linux loop device and device-mapper (`dm-crypt`); the decrypted contents appear as a normal directory to the installed app |
| Mount point | `/mnt/asec/<package-name>-<n>/` - decrypted app content is accessible at this path while the device is unlocked and running |
| SD card location | `/.android_secure/` - a hidden folder at the root of the FAT32-formatted SD card; not visible through standard USB mass-storage mode |
| File naming convention | `<package-name>-<integer>.asec` (e.g. `com.example.app-1.asec`); the integer suffix increments on each reinstall to the SD card |
| MIME type | `application/octet-stream` - no dedicated MIME type registered; treated as an opaque binary blob by host operating systems |
| Magic bytes / file signature | None - the encrypted container has no readable header; identified solely by the `.asec` extension and its location under `/.android_secure/` |
| Portability | Zero - the encryption key is device-bound; an `.asec` file copied to any other device cannot be decrypted or mounted |
| Copy-protection mechanism | Decryption key is derived per-device, preventing redistribution of paid apps by making the container permanently unreadable without the originating device |
| Supported Android versions | Android 2.2 'Froyo' (API 8) through Android 5.1 'Lollipop' (API 22); formally deprecated in Android 6.0 Marshmallow |
| Superseded by | Adoptable Storage (Android 6.0+, API 23), which absorbs an entire SD card into the device's own encrypted internal storage pool |
| Host OS visibility | `.asec` files appear as opaque binary files when the SD card is read via USB on a PC; no host OS can decrypt or inspect their contents without the originating device's key |
| Released | Android 2.2 'Froyo' (2010), as part of Apps2SD / 'move to SD card'; superseded by Adoptable Storage in Android 6.0 (2015) |
| Open standard | Yes · royalty-free |
| Specification | source.android.com |
ASEC conversions
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