Building and requirements

To build the program, simply open the Visual Studio 2015 solution and click Build. The resulting binary cryptpad.exe requires the .NET Framework 4.

Encryption

cryptpad uses AES encryption, supporting key lengths of up to 256 bits, and can generate keys using either SHA-256 (default), SHA-512, RIPEMD-160 (128 bit keys only) or MD5 (128 bit keys only). Only two block cipher modes of operation are supported, CBC (recommended) and ECB.

File format

Encrypted text files begin with a 128 bit IV, followed by any number of encrypted blocks. Therefore, the file size is always a multiple of the block size, which is 128 bits (16 bytes).

Manual encryption and decryption using OpenSSL

You can, in fact, manually encrypt and decrypt text files exactly as cryptpad does, using OpenSSL. The following example uses bash and assumes AES/CBC/PKCS7 with a 256 bit key generated using SHA-256. However, only minimal changes are required to adapt the commands to other configurations.

First, you need to create the key:

key=`echo -n "$password" | sha256sum | head -c 64`

To encrypt content, generate a random initialization vector and its hexadecimal representation, and use it in combination with the computed key in order to produce the output file:

dd if=/dev/random bs=16 count=1 status=none > file.stxt
iv=`xxd -c 32 -p file.stxt`

echo 'Hello world!' | openssl enc -aes-256-cbc -e -iv $iv -K $key >> file.stxt

You can now open file.stxt in cryptpad.

To decrypt the file, we need to read the IV before reading the actual contents:

iv=`dd if=file.stxt bs=16 count=1 status=none | xxd -c 32 -p`

dd if=file.stxt bs=16 skip=1 status=none | openssl enc -aes-256-cbc -d -iv $iv -K $key