Cryptoloop
Cryptoloop is a Linux kernel's disk encryption module that relies on the Crypto API, which is a cryptography framework introduced in version 2.5.45 of the Linux kernel mainline. Cryptoloop was first introduced in the 2.5.x kernel series; its functionality was later incorporated into the device mapper, a generic framework used to map one block device onto another.
Cryptoloop can create an encrypted file system within a partition or from within a regular file in the regular file system. Once a file is encrypted, it can be moved to another storage device. This is accomplished by making use of a loop device, a pseudo device that enables a normal file to be mounted as if it were a physical device. By encrypting I/O to the loop device, any data being accessed must first be decrypted before passing through the regular file system; conversely, any data being stored will be encrypted.
Cryptoloop is vulnerable to watermarking attacks, making it possible to determine presence of watermarked data on the encrypted filesystem:
Newer versions of cryptoloop's successor, dm-crypt, are less vulnerable to this type of attack if used correctly.