TrueCrypt was a free and open-source disk encryption software for Windows 7/Vista/XP, Mac OS X, and Linux. Its primary function was to create a virtual encrypted disk inside a file and mount it as a real disk. It also allowed encrypting an entire partition or storage device, such as a USB flash drive or hard disk, and encrypting a partition or disk where Windows was installed (pre-boot authentication).

Encryption was automatic, real-time (as it was used), and transparent to the user. In addition, parallelization and chaining operations made it possible to read and write data as fast as if the disk were not encrypted. Encryption could also be hardware accelerated on modern processors.

It offered plausible deniability if an adversary forced you to reveal the password. This was achieved through a hidden volume (steganalysis) and an operating system.

When a user provided the correct password (and key file) and mounted the TrueCrypt volume, they could copy and move files to the volume just as they would any other disk. Files were automatically decrypted as they were read or copied from the TrueCrypt encrypted volume. Likewise, files written or copied to the TrueCrypt volume were automatically encrypted as they were written to the disk.

The software never saved decrypted data to disk but only stored it temporarily in RAM. Even when the volume was mounted, the data stored on it could only be accessed with the correct password. If the volume was unmounted, all data stored on it became inaccessible.

One of the main advantages was that it left no trace on the system disk of the encryption or decryption of the data. This made it difficult for an attacker to determine which files were encrypted and which were not. In addition, it allowed encryption of partitions or entire disks, which meant that all information stored on them was password protected.

It also allowed encryption of boot drives, which meant that the correct password had to be provided before the operating system could boot. This provided an additional layer of security in case the computer was stolen or accessed unauthorizedly.

TrueCrypt was discontinued in May 2014 due to a variety of reasons. Although still available for download, it is no longer being updated or further developed. Despite this, it remains an option for data protection for older operating systems.