For algorithms requiring a fixed block size, we use a value of 256 bytes. This choice is arbitrary. Normally, algorithms that provide a CBC mode also include a non-secret value known as the initialization vector. This prevents identical blocks from appearing to be identical in the encrypted file content.
When calling CNG encryption methods that require an initialization vector (IV), FESF generates this from the key material using a technique known as the Encrypted Salt-Sector Initialization Vector (ESSIV). More details about how FESF generates the IV can be found in the FESFSa Function Reference section, elsewhere in this document.