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280 • Linux Symposium 2004 • Volume One 5.6 Reiser4 ReiserFS version 4 (Reiser4)[29], while still in the development stage, features pluggable security modules. There are currently proposed modules for Reiser4 that will perform encryption and auditing. 5.7 Network Filesystem Security Much research has taken place in the domain of networking filesystem security. CIFS, NFSv4, and other networking filesystems face special challenges in relation to user identification, access control, and data secrecy. The NFSv4 protocol definition in RFC 3010 contains descriptions of security mechanisms in section 3[30]. 6 Proposed Extensions to Cryptfs Our proposal is to place file encryption metadata into the Extended Attributes (EA’s) of the file itself. Extended Attributes are a generic interface for attaching metadata to files. The Cryptfs layer will be extended to extract that information and to use the information to direct the encrypting and decrypting of the contents of the file. In the event that the filesystem does not support Extended Attributes, another filesystem layer can provide that functionality. The stackable framework effectively allows Cryptfs to operate on top of any filesystem. The encryption process is very similar to that of GnuPG and other public key cryptography programs that use a hybrid approach to encrypting data. By integrating the process into the filesystem, we can achieve a greater degree of transparency, without requiring any changes to userspace applications or libraries. Under our proposed design, when a new file is created as an encrypted file, the Cryptfs layer generates a new symmetric key K s for the encryption of the data that will be written. File creation policy enacted by Cryptfs can be dictated by directory attributes or globally defined behavior. The owner of the file is automatically authorized to access the file, and so the symmetric key is encrypted with the public key of the owner of the file K u , which was passed into the Cryptfs layer at the time that the user logged in by a Pluggable Authentication Module linked against libcryptfs. The encrypted symmetric key is then added to the Extended Attribute set of the file: {K s }K u Suppose that the user at this point wants to grant Alice access to the file. Alice’s public key, K a , is in the user’s GnuPG keyring. He can run a utility that selects Alice’s key, extracts it from the GnuPG keyring, and passes it to the Cryptfs layer, with instructions to add Alice as an authorized user for the file. The new key list in the Extended Attribute set for the file then contains two copies of the symmetric key, encrypted with different public keys: {K s }K u {K s }K a Note that this is not an access control directive; it is rather a confidentiality enforcement mechanism that extends beyond the local machine’s access control. Without either the user’s or Alice’s private key, no entity will be able to access the decrypted contents of the file. The machine that harbors such keys will enact its own access control over the decrypted file, based on standard UNIX file permissions and/or ACL’s. When that file is copied to a removable media or attached to an email, as long as the Extended Attributes are preserved, Alice will have all the information that she needs in order to retrieve the symmetric key for the file and de-
Linux Symposium 2004 • Volume One • 281 Kernel User Kernel crypto API cryptfs library Change file encryption attributes VFS syscall Login/GNOME/KDE/... Cryptfs layer Keystore Key retrieval PAM Module Prompt for user authentication action (conversation) File Structure Security Attributes Additional layers (optional) USB Keychain device, Smartcard, TPM, GnuPG Keyring, etc... Authentication PAM Module Filesystem PAM Figure 1: Overview of proposed extended Cryptfs architecture crypt it. If Alice is also running Cryptfs, when she launches an application that accesses the file, the decryption process is entirely transparent to her, since her Cryptfs layer received her private key from PAM at the time that she logged in. If the user requires the ability to encrypt a file for access by a group of users, then the user can associate sets of public keys with groups and refer to the groups when granting access. The userspace application that links against libcryptfs can then pass in the public keys to Cryptfs for each member of the group and instruct Cryptfs to add the associated key record to the Extended Attributes. Thus no special support for groups is needed within the Cryptfs layer itself. 6.1 Kernel-level Changes No modifications to the 2.6 kernel itself are necessary to support the stackable Cryptfs layer. The Cryptfs module’s logical divisions include a sysfs interface, a keystore, and the VFS operation routines that perform the encryption and the decryption on reads and writes. By working with a userspace daemon, it would be possible for Cryptfs to export public key cryptographic operations to userspace. In order to avoid the need for such a daemon while using public key cryptography, the kernel cryptographic API must be extended to support it. 6.2 PAM At login, the user’s public and private keys need to find their way into the kernel Cryptfs layer. This can be accomplished by writing a Pluggable Authentication Module, pam cryptfs.so. This module will link against libcryptfs and will extract keys from the user’s GnuPG keystore. The libcryptfs library will use the sysfs interface to pass the user’s keys into the Cryptfs layer. 6.3 libcryptfs The libcryptfs library works with the Cryptfs’s sysfs interface. Userspace utilities, such as pam cryptfs.so, GNOME/KDE, or stand-alone utilities, will link against this library and use it
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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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One - The Linux Kernel Archives

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