eCryptfs encrypts and decrypts individual data extents in each file using a unique randomly generated File Encryption Key (FEK). The FEK is encrypted with the File Encryption Key Encryption Key (FEKEK), and the resulting Encrypted File Encryption Key (EFEK) is stored in the header of each lower file.
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source:http://linuxpoison.blogspot.com/2010/10/13578167754312.html