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284 • Linux Symposium 2004 • Volume One file. Each bit would correspond with a block of the file; a “1” might indicate that the block is a “hole” and should be zero’d out rather than decrypted, and a “0” might indicate that the block should be normally decrypted. Our proposed extensions to Cryptfs in the near future do not currently address the issues of directory structure and file size secrecy. We recognize that this type of confidentiality is important to many, and we plan to explore ways to integrate such features into Cryptfs, possibly by employing extra filesystem layers to aid in the process. Extended Attribute content can also be sensitive. Technically, only enough information to retrieve the symmetric decryption key need be accessible by authorized individuals; all other attributes can be encrypted with that key, just as the contents of the file are encrypted. Processes that are not authorized to access the decrypted content will either be denied access to the file or will receive the encrypted content, depending on how the Cryptfs layer is parameterized. This behavior permits incremental backup utilities to function properly, without requiring access to the unencrypted content of the files they are backing up. At some point, we would like to include file integrity information in the Extended Attributes. As previously mentioned, this can be accomplished via sets of keyed hashes over extents within the file: H 0 = H{O 0 , D 0 , K s } H 1 = H{O 1 , D 1 , K s } . . . H n = H{O n , D n , K s } H f = H{H 0 , H 1 , . . . , H n , n, s, K s } where n is the number of extents in the file, s is the extent size (also contained as another EA), O i is the offset number i within the file, D i is the data from offset O i to O i + s, K s is the key that one must possess in order to make authorized changes to the file, and H f is the hash of the hashes, the number of extents, the extent size, and the secret key, to help detect when an attacker swaps around extents or alters the extent size. Keyed hashes prove that whoever modified the data had access to the shared secret, which is, in this case, the symmetric key. Digital signatures can also be incorporated into Cryptfs. Executables downloaded over the Internet can often be of questionable origin or integrity. If you trust the person who signed the executable, then you can have a higher degree of certainty that the executable is safe to run if the digital signature is verifiable. The verification of the digital signature can be dynamically performed at the time of execution. As previously mentioned, in addition to the extensions to the Cryptfs stackable layer, this effort is requiring the development of a cryptfs library, a set of PAM modules, hooks into GNOME and KDE, and some utilities for managing file encryption. Applications that copy files with Extended Attributes must take steps to make sure that they preserve the Extended Attributes. 7 7 Conclusion Linux currently has a comprehensive framework for managing filesystem security. Standard file security attributes, process credentials, ACL, PAM, LSM, Device Mapping (DM) Crypt, and other features together provide good security in a contained environment. To extend access control enforcement over individual files beyond the local environment, you must use encryption in a way that can be easily 7 See http://www.suse.de/~agruen/ ea-acl-copy/
Linux Symposium 2004 • Volume One • 285 applied to individual files. The currently employed processes of encrypting and decrypting files, however, is inconvenient and often obstructive. By integrating the encryption and the decryption of the individual files into the filesystem itself, associating encryption metadata with the individual files, we can extend Linux security to provide seamless encryption-enforced access control and integrity auditing. 8 Recognitions We would like to express our appreciation for the contributions and input on the part of all those who have laid the groundwork for an effort toward transparent filesystem encryption. This includes contributors to FiST and Cryptfs, GnuPG, PAM, and many others from which we are basing our development efforts, as well as several members of the kernel development community. 9 Legal Statement This work represents the view of the author and does not necessarily represent the view of IBM. IBM and Lotus Notes are registered trademarks of International Business Machines Corporation in the United States, other countries, or both. Other company, product, and service names may be trademarks or service marks of others. References [1] E. Zadok, L. Badulescu, and A. Shender. Cryptfs: A stackable vnode level encryption file system. Technical Report CUCS-021-98, Computer Science Department, Columbia University, 1998. [2] J.S. Heidemann and G.J. Popek. File system development with stackable layers. ACM Transactions on Computer Systems, 12(1):58–89, February 1994. [3] E. Zadok and J. Nieh. FiST: A Language for Stackable File Systems. Proceedings of the Annual USENIX Technical Conference, pp. 55–70, San Diego, June 2000. [4] S.C. Kothari, Generalized Linear Threshold Scheme, Advances in Cryptology: Proceedings of CRYPTO 84, Springer-Verlag, 1985, pp. 231–241. [5] Matt Blaze. “Key Management in an Encrypting File System,” Proc. Summer ’94 USENIX Tech. Conference, Boston, MA, June 1994. [6] For more information on Extended Attributes (EA’s) and Access Control Lists (ACL’s), see http://acl.bestbits.at/ or http://www.suse.de/~agruen/ acl/chapter/fs_acl-en.pdf [7] For more information on GnuPG, see http://www.gnupg.org/ [8] For more information on OpenSSL, see http://www.openssl.org/ [9] For more information on IBM Lotus Notes, see http://www-306.ibm. com/software/lotus/. Information on Notes security can be obtained from http://www-10.lotus.com/ldd/ today.nsf/f01245ebfc115aaf 8525661a006b86b9/ 232e604b847d2cad8 8256ab90074e298?OpenDocument [10] For more information on Pluggable Authentication Modules (PAM), see http://www.kernel.org/pub/ linux/libs/pam/
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Proceedings of the Linux Symposium
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The State of ACPI in the Linux Kern
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Conference Organizers Andrew J. Hut
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TCP Connection Passing Werner Almes
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Linux Symposium 2004 • Volume One
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Cooperative Linux Dan Aloni da-x@co
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