Cryptographic Access Control in a Distributed File System

Cryptographic Access Control in a Distributed File System
Anthony Harrington
Christian D. Jensen
Department of Computer Science
Trinity College Dublin
Anthony.Harrington,Christian.Jensen @cs.tcd.ie
ABSTRACT
Traditional access control mechanisms rely on a reference monitor
to mediate access to protected resources. Reference monitors are
inherently centralized and existing attempts to distribute the functionality of the reference monitor suffer from problems of scalability.
Cryptographic access control is a new distributed access control
paradigm designed for a global federation of information systems.
It defines an implicit access control mechanism, which relies exclusively on cryptography to provide confidentiality and integrity
of data managed by the system. It is particularly designed to operate in untrusted environments where the lack of global knowledge
and control are defining characteristics.
The proposed mechanism has been implemented in a distributed
file system, which is presented in this paper along with a preliminary evaluation of the proposed mechanism.
1.
INTRODUCTION
The widespread availability of networks such as the Internet as a
medium for communication has prompted a proliferation of both
stationary and mobile devices capable of sharing and accessing data
across networks spanning multiple administrative domains. Such
collaboration increases user dependence on remote elements of the
system which are often outside their control and heightens their
need for effective security systems. The shift toward the networking paradigm has resulted in a fundamental re-evaluation of computer security and its place in system design.
Current access control mechanisms invoke a reference monitor [1]
to verify a principal’s right to access a protected object every time
the object is referenced. The access rights of all principals in the
system can be viewed as an access control matrix [14], which is
stored either in the form of an access control list (ACL) for each
object in the system or in the form of a capability list for every
principal in the system. ACLs require the authenticated identity of
the requesting principal to be known before access can be granted,
which reduces the ability to define dynamic access control poli-
cies. Moreover, ACLs do not scale well, because it is difficult to
delegate the right to delegate, i.e. modify the ACL. This means that
the ACLs have to be organised into a hierarchy where the right to
delegate access rights corresponds to the right to modify the ACL
in a specific branch of the hierarchy, Thus, the ACL hierarchy must
reflect the structure of the entire organisation.1
On the other hand, capabilities provide the necessary flexibility to
grant and delegate access rights in large scale open networks, but
they only control access to the object on the managing server. The
capability does not control access to the object while it is transferred across a potentially hostile network or while it is temporarily
cached on an intermediate server.
Reference monitors and access control matrices are fundamental
concepts in access control; originally developed in the context of
centralized systems. Different extensions have been introduced
to extend their use to distributed systems. Strong authentication
mechanisms, such as Kerberos [12], X.509 [19] and BAN logic [15]
(an extension of BAN logic is used in the Taos operating system [13,
34]), allow ACLs to be used in a networked environment and capabilities can be partly encrypted to ensure the integrity and authenticity of the capability in a distributed system, e.g., this method is
used in Amoeba [32]. However, the reference monitor is inherently
centralized, as the interpretation of access control state, i.e., the encoded access control matrix, is limited to the server that manages
the particular object.
A distributed reference monitor would need a consistent view of access control state, which is theoretically impossible, if failures may
occur in the system [8]. Moreover maintaining consistency limits
the scalability of the access control mechanism. In addition, a distributed reference monitor enforces a homogeneous security policy
on a heterogeneous environment and requires global knowledge on
the part of the entity that defines security policy. Finally, the reference monitor only controls access to the protected object on the
server that stores the object, additional mechanisms are needed to
ensure confidentiality and integrity of data while they transferred
across the network or temporarily cached on an intermediate server.
We therefore need to rethink the notion of access control in largescale open systems, such as the Internet, in a way that separates
enforcement of access control policies from the storage of objects
in the system and, at the same time, ensures the confidentiality and
integrity of data stored and transferred in the system.
In this paper we propose Cryptographic Access Control, a novel
1
This is successfully demonstrated in section 4.3 of RFC 2693 [7].
access control mechanism that eliminates the need for a reference
monitor and relies exclusively on cryptography to ensure confidentiality and integrity of data stored in the system. Data are encrypted
as the applications store them on a server, which means that the
storage system only manages encrypted data. Read access to the
physical storage device is granted to all principals (only those who
know the key are able to decrypt the data) and write access can
be granted to everybody, providing that the modifications do not
overwrite existing data, e.g. by using a journaling mechanism that
tracks modifications of objects instead of applying them directly to
the stored version of objects.
In order to separate read and write access, the cryptographic storage system must implement an asymmetric encryption algorithm.
Data are encrypted with the private key, also called the encryption
key and decrypted with the public key, also called the decryption
key. Data that are not encrypted with the encryption key cannot be
decrypted with the decryption key, so an earlier version must be
read from the log. This allow us to selectively grant read-access
and write-access to the data, by respectively giving the decryption
key, the encryption key or both to another principal.
The advantage of this approach is that it is inherently distributed,
i.e., the tokens needed to access data only have meaning where data
is used, so there is no need for coordination of access control state
among clients and servers in the system. Moreover, no additional
mechanism is needed to protect data in transit between users and
the servers that stores data, since cryptographically stored data can
be transferred across the network “in the clear” because the contents are already encrypted.
By employing security protocols at the application level, the cryptographic access control model respects the End-To-End argument
[23]. The targeted operational domain for this access control method,
i.e. large-scale open systems characterised by a lack of central authority, is such that the cryptography overhead which must be borne
by all applications is justified.
We have implemented cryptographic access control in a file system
that allows users to securely access and store files on a server from
any machine in the network. This file server implements no access
control mechanism, so users can access their files from any user
account and any machine connected to the network. The design
and implementation of this prototype is outlined in this paper along
with preliminary performance measurements.
place complete trust in their own machine. The cryptographic access control mechanism seeks to reposition the burden of access
control from the server to the client.
2.1 Basic Mechanism
The basic cryptographic access control mechanism is based on asymmetric cryptography, where objects are encrypted with the private
key, also called the encryption key, and can only be decrypted by
someone who knows the public key, also called the decryption key.
A separate key-pair may be associated with every file and should
only be distributed to principals who are allowed to access the file’s
data. Restricting the distribution of keys allows us to export the actual file to everybody. The two keys effectively act as capabilities,
where the decryption key corresponds to a read-capability and the
encryption key corresponds to a write-capability. However, capabilities are part of an access control mechanism and the semantics
can only be interpreted by the object server. The semantics of the
encryption/decryption key-pair is universally valid.
Asymmetric cryptography ensures secrecy and authenticity of the
stored data, but neither integrity nor availability are guaranteed.
Everybody can write the file, so integrity is easily compromised
by overwriting the file with garbage. This also denies legitimate
users access to the stored data, thus compromising availability.
Overwriting data without knowing the proper encryption key is easily detected because the result of decryption is unlikely to respect
the format of the file.2 However, detection is deferred to runtime
and the process may already have modified other data that are now
in an inconsistent state. Detection of corrupted data may happen
until the last read from disk is completed, so we either have to execute all programs within transactions that can be restarted if corrupt
data are detected, or we need to prevent users from overwriting data
arbitrarily. Executing long running programs as transactions isn’t
practical, so we decide to prevent users arbitrary write access to
data.
Integrity and availability can be ensured by using a log-structured
file system [22, 30, 25, 6], where modifications are written to a
log instead of overwriting disk blocks. An attacker may write false
log entries, but this is easily detected and corrupt disk blocks can
be ignored, i.e. by reading the latest entry written to the log that
decrypts correctly.
2.2 The Problem of Log Compression
The rest of this paper is organized as follows. we define Cryptographic Access Control in Section 2. The design and implementation of our prototype file system is presented in Section 3. We
present the evaluation of our prototype in Section 4. Related work
is discussed in Section 5 and our conclusions are presented in Section 6.
2.
CRYPTOGRAPHIC ACCESS CONTROL
The cryptographic access control mechanism is designed to operate in an open network where establishing the identity of the client
conveys no information about the likely behaviour of the client and
thus, is irrelevant to the secure operation of the server. It is flexible enough to operate across multiple administrative domains with
varying security requirements and offers a high degree of resource
protection in these untrusted environments. It makes no assumptions about the security of remote elements in the system such as
the transport mechanism or file server and only requires clients to
In order to prevent the log from growing indefinitely and limit
fragmentation of user data on the file server, most log-structured
file systems implement a mechanism to compress logs and reclaim
space allocated to entries that have become obsolete. This means
that the server needs some way to distinguish valid log-entries from
invalid log-entries created by an attacker. A simple approach is to
provide the server with the decryption key, which allows it to identify incorrect log-entries and remove them from the log. However,
the decryption key alone does not allow the server to detect invalid
log-entries in unstructured binary data, so we require that the client
signs log-entries, with the encryption, key before they are sent to
the server.
Providing the server with the decryption key may compromise con2
Detection of corruption is difficult in unstructured binary
data, such as image bitmaps, but the modifications proposed
in 2.2 and 2.3 solve this problem.
fidentiality on compromised servers or servers that are simply not
trustw
orthy. A solution to this problem is presented in Section 2.3.
2.3 A Necessary Optimisation
So far, we have assumed that asymmetric cryptography is sufficiently fast to encrypt/decrypt all data to be stored in the system.
In reality, asymmetric cryptography is too slow – several orders
of magnitude slower than symmetric cryptography. We therefore
propose an extension that uses symmetric cryptography to encrypt
and decrypt data, while asymmetric cryptography is reserved for
generation and verification of digital signatures.
stored on the machines of clients that are authorized to read the
file. The client will possess this key and can thus decrypt the data
however it must also ensure that the data has not been tampered
with on the server side or in transit. This is accomplished through
the use of message digests. When servicing the clients read request
the server also returns the hash of the file which was signed with
the encryption key associated with this file. The client then creates
his own hash of the data, decrypts the hash supplied by the server
and compares the two digests. If they match, the authenticity and
integrity of the data is established.
2.4 Discussion
Data stored on the server are encrypted with a symmetric algorithm
and signed with the encryption key as before. The digital signature
is used by clients to guarantee integrity of the data and by the server
to provide a means for log compression as described in Section 2.2.
The addition of symmetric cryptography to the system means that
we now have two cryptographic algorithms and three cryptographic
keys that are used in the following way:
read clients that are authorized to read the file need the symmetric
key to decrypt data received from the server and the decryption key to verify the signature that proves the integrity of the
data.
write clients that are authorized to write the file need the symmetric key to encrypt the data before sending them to the server
and the encryption key to generate the signature that proves
integrity of the data.
log compression the server only needs the decryption key to
verify the signature of log-entries in order to weed out corrupt entries and merge correct log-entries with the original
file data. It is important to note that the addition of symmetric
cryptography solves the confidentiality problem introduced
by giving the decryption key to the server.
The modification introduced above means that when a user wishes
to store a new file on the server he must first encrypt the data using symmetric key cryptography. Next he must produce a hash or
message digest of the encrypted file contents. The message digest
is then signed using the encryption key known only to clients with
write access to the file. This message digest and the decryption
key used to verify the signature are then sent to the server along
with the data to be stored. Upon receipt of the data and digest the
server will generate its own digest from the encrypted data using
the same algorithm. It then decrypts the client-generated digest using the file’s decryption key and compares the two digests. If they
match, the write request is considered valid. The server stores the
decryption key associated with the file and uses this stored key for
validating future write requests from the user. If the data originates
from a malicious client then the message digest cannot be signed
using the valid encryption key and the write operation is considered
invalid. When a user wishes to update the contents of a file which
already exists the server, he encrypts the data and signs it as before.
When the server receives the data from the user, it can retrieve the
decryption key associated with the file and validate the data immediately. This means that we no longer need a log-structured file
system to ensure integrity, but we may still decide to use one in
order to improve performance.
When the client issues a read request the data returned is encrypted
with the symmetric key associated with that file. This key is only
It is a fundamental requirement of our system that the user have
complete trust in the client machine. All data stored locally are
unencrypted and if a malicious agent succeeds in compromising
the client machine then data confidentiality is destroyed. The keys
necessary to read and update the users files are also stored on the
local machine. If an attacker obtains these keys then the system has
been fully compromised.
The system is designed to operate over a public unsecured network.
All data are encrypted by the client before the remote storage procedures are invoked. This ensures that all data is encrypted prior
to being transmitted across the network. All data sent from the
server to the client is also encrypted. The use of digital signatures
protects against deliberate or accidental changes to the payload of
the network packets. This effectively means that data integrity and
confidentiality are guaranteed and the user need not place any trust
in the network.
In the envisaged operational environment for cryptographic access
control the server may be located in an administrative domain outside the control of the client. With this in mind the extent of the
trust that the user must place in the server is a critical factor in
determining the strength of the system. We have three principal
requirements to meet before the storage of data on the server can
be considered secure. We must ensure the confidentiality, integrity
and availability of the data.
Confidentiality Data is stored in encrypted format and the server
does not possess the keys necessary to decrypt the data. An
assailant who compromises the server but who does not possess the necessary keys cannot access the data in a meaningful manner. All data stored on the server can thus be considered confidential.
Integrity When a client sends data to the server it also sends the
file’s public key and a message digest of the data signed with
the file’s private key. This digital signature is stored with the
file data on the server. Future read requests from the client
result in the file data and digital signature being sent to the
client. If the data has been altered on the server the signed
digest will be invalid and the user will be aware that the integrity of the data has been compromised.
Availability The server performs three tasks: it stores data on disk,
it provides data to any client requesting a particular object
and it updates data modified by authorized clients. There is
no direct way for the client to verify that the server actually
performs these tasks. A server may receive an update from
an authorized client, but decide not to commit it to disk. It
may also segregate two collaborating principals by keeping
two copies of a particular file and serve each client from a
Server
Client
logon()
read()
read()
logon
read
Server
data
Network
(a) Centralised system
(b) LAN file server
re
ad
User Space
Client
Client
read()
authentication
read
Server
Server
data
E K(read )
E K(data )
TCB
Network
Network
(c) Distributed file systems
(d) Cryptographic Access control
Figure 1: Trusted Computing Base in different types of file systems
separate copy of the file. These problems are common to all
distributed file systems, which is why clients are required to
interact with a pre-specified set of trusted servers. However,
we believe that these problems can be solved, either through
a system of server issued receipts [18], or though a system of
client maintained version numbers. Otherwise, clients simply have to select trustworthy servers in the same way that
they would with a traditional distributed filesystem, except
that they only need to require availability from the server,
confidentiality and integrity are guaranteed by the cryptographic access control mechanism, i.e., clients are more free
to select servers.
Contrary to traditional access control mechanisms, cryptographic
access control implements a client centric view of the trusted computing base (TCB). The trusted computing base of a number of
popular filesystem architectures is illustrated in gray in Figure 1.
Figure 1 (a) shows a traditional centralized file system, where all
file system components belong to the trusted computing base of the
local operating system. Figure 1 (b) shows a traditional file server,
e.g., NFS [24, 31], where both the file server and the intermediate
network are considered to be secure, but the client must be authenticated, e.g., through a logon procedure. Figure 1 (c) shows an
example of a global file system, where the network is considered
insecure, while the server is considered secure and generally stores
data unencrypted on disk. Consequently, the clients and servers
have to authenticate each other. Moreover, data has to be encrypted
on the server before it is sent to the client, which decrypts the data,
performs the required operations on the data before it re-encrypts
them and sends them back to the server. The server then has to
decrypt the data before they can be written to disk. Finally, Figure 1 (d) shows the trusted computing base in a system with cryptographic access control, where only the client is trusted. Data can
be sent from the server without prior authentication and the client
must decrypt the data before it can perform any operations on them.
The client then re-encrypts the data before sending them back to the
server, which stores the encrypted data back on disk.
We believe that cryptographic access control will improve the performance of file system operations across untrusted networks, because we only need one decryption and signature validation for a
read operation and one encryption and signature creation for a write
operation as opposed to one authentication (which may include signature validation) one encryption and one decryption for every read
operation and one authentication, one encryption and one decryp-
tion for every write operation.
3. SYSTEM DESIGN AND IMPLEMENTATION
In the following we present the design and implementation of CNFS [10],
a network file system that implements cryptographic access control. A key design goal for CNFS is to minimize the number of
trusted components in the system. The principal system components are the client module, the key distribution mechanism, the
network connecting the file server and client and the file server itself. Use of the cryptographic access control mechanism requires
that the user need only completely trust the client machine and the
key distribution mechanism, while only minimal trust is required in
the file server.
3.1 System Architecture
The architecture of the CNFS System as illustrated in Figure 2 is
based on the traditional client/server model. The server and client
have been implemented in user level space. The overhead introduced by the additional context switches was offset by the flexibility offered by a user level solution which does not require root
privileges or kernel recompilation to install. The ease of debugging
user level applications coupled with the widespread availability of
cryptography libraries enabled more rapid development of the prototype.
Client Architecture
All data to be stored on the server is encrypted on the client side
prior to transmission across the network and all data to be read
from the server is only decrypted on the client side. The full responsibility for ensuring the authenticity and integrity of file data
rests with the client. Thus the client must also produce and validate
hashes of all data stored on the server.
Server Architecture
The CNFS server is designed to be as simple as possible and the
system intelligence should in the main reside on the client side.
Ideally the file server would simply serve disk blocks on request
from the client. Although the CNFS server codebase is taken from
the Linux NFS server there are significant differences in design.
The NFS failure model assumes a stateless service and all file operations are designed to be idempotent. These file operations are
carried out on data units which match the underlying NFS block
size and the NFS server has no concept of the file as a logical unit
in itself.
CNFS Server
CNFS Client
Application program
CNFS Server
CNFS Libraruy
Kernel
Kernel
Virtual File System
Unix
File
System
Virtual File System
NFS
Client
Unix File System
Figure 2: CNFS architecture
However the write validation routines implemented by the cryptographic access control mechanism require the CNFS server to
perform digital signature verification on the entire file before the
data can be written to disk. Fundamental changes to the NFS write
procedure were required to accommodate this functionality.
When dealing with updates to a file which is a multiple of the block
size the CNFS server must maintain state information in relation to
which clients were updating open files. It is also necessary to buffer
data written from the client until the entire file is transferred across
the network. The file name and client identity are established from
information included in each RPC request header. This information
is maintained in memory while a client has a server resident file
opened in write mode on his machine.
When the client writes the first block to the CNFS server, this block
is validated by the cryptographic access control mechanism. If
this operation is successful, state information in relation to file and
client identity is maintained. Each subsequent data block can then
be buffered until all data is received from the client. The buffer is
passed to the validation routines and ultimately written to disk or
flushed from memory.
This also has the effect of delaying the write to disk operation and
has important consequences for the NFS failure model. Buffering
data increases the likelihood of cache inconsistency faults. The
window of vulnerability associated with lost updates due to server
crashes is also widened.
Use of Cryptography in CNFS
Employing cryptography at the level of the file system itself enables
applications and users to access protected data in a transparent fashion. Encryption in the CNFS System is employed at the granularity
of the file level. This necessitates a high overhead for file generation operations on the client as a symmetric key and an asymmetric
key-pair must be generated for each new file created. However the
advantages offered by this approach, namely increased flexibility
and security outweigh the disadvantages.
The symmetric and encryption keys for files are never stored on
the file server. This is a fundamental requirement in ensuring that
compromising the file server will not compromise the data stored
there. These keys are to be stored on the client which owns or
generated the files originally. Only the decryption key associated
with a file is stored on the server.
In the case of CNFS sharing files is inextricably intertwined with
key distribution. As files are meaningless without the appropriate
keys the file owner must make the key available to whomever (s)he
wishes to share his files with. Studies of file system usage properties have noted that file sharing is unusual[26]. It seems logical
that sharing protected files among large numbers of users would be
rarer still. As the symmetric key and private asymmetric key are
created and stored on the client machine, key distribution is at the
file owners discretion.
3.2 CNFS Implementation
In this section we present the implementation of cryptographic access control in the CNFS system. The cryptographic access control
mechanism was originally designed to operate on a log-structured
file system. However, the use of symmetric cryptography as a necessary optimization allows us sufficient flexibility to use the standard Linux NFS codebase.
CNFS Ciphers As stated earlier only the decryption key is stored
on the server. The absolute file path is hashed and normalized to
produce a reference value for the relevant key. The decryption keys
associated with all files on the server are maintained in a binary tree
which is sorted by the hash value. When the server wishes to find a
file’s public key it reconstructs the file path from the filehandle and
searches for a match. A lookup operation on this tree is an O(log
n) operation. As each new key is registered with the server an extra
node is added to the tree, this information is also written to a file
stored on the server so that they keys can be restored if the server
fails. Each time the server is started it must load all public keys
from this file into memory.
The Twofish cipher was chosen as the symmetric cipher [29]. To
date no significant success have been reported in crypt-analysing
this cipher. We obtained the reference implementation which was
submitted to the AES panel. The key size used in our prototype
is 128 bits. This is deemed sufficient for security in the envisaged
operational domain.
In the CNFS system asymmetric cryptography is only used to sign
and verify the message digests generated from the contents of the
encrypted files.
The RSAEuro library we used provides a mature and extensive API
with support for RSA encryption, decryption, key generation, and
the generation and verification of Message Digests using MD2,
MD4
and MD5. We use the MD5 hashing algorithm to generate
a 128 bit digests of the file data which are signed with the private
asymmetric key. The asymmetric key length used in the prototype
implementation is 1024 bits.
4.
EVALUATION
In order to evaluate the prototype we have performed an initial performance evaluation and an analysis of the security properties of
the proposed mechanism.
4.1 CNFS Server Performance
We have compared the performance of the CNFS server with the
performance of the unmodified user level NFS server. The lack
of cryptography in the standard NFS server mitigates against direct comparison with the CNFS server. This evaluation is primarily
intended to determine whether the overhead introduced by cryptographic access control is acceptable.
The execution time of the read() and write() procedures on the
server have been measured. TCP stack and XDR operation times
are not included. Two file sizes were chosen for the tests. The first
size is 8K, the underlying NFS V2 and CNFS data block size. The
second file size of 1MB was chosen as an arbitrary large file size.
The tests were carried out on a lightly loaded Pentium Pro 233
MHz machine with 128 MB SDRAM. The user level CNFS server
and client were run on the same machine. A local directory was
mounted and requests for file operations in this directory were handled by the CNFS server. For the purposes of our tests caching was
disabled on the client and server side. All experiments were run 20
times and the arithmetic mean and standard deviation of the results
of these experiments are presented in the tables below.
Read Performance
The absence of centralized access control allows the CNFS server
to process read requests without validation. There is an obvious
performance benefit and the CNFS read operation on the K file is
faster than the read operation on the unmodified NFS server.
Access control and request caching functionality were closely integrated in the NFS codebase. The modifications required to the
read() operation on the CNFS server involved removing the ACL
lookup operation. Some weakening of the caching strategy was
unavoidable. Compromising the caching
strategy results in a read
slower on the CNFS server.
operation on the 1MB file which is The results of these operations are displayed in Table 1.
Write performance
The write operation for a file of size K on the CNFS server is two
orders of magnitude slower than the comparable operation on the
unmodified server. This difference is accounted for by the overhead
introduced by the digital signature validation. A similar overhead
would be introduced by any mechanism that ensures the integrity
of data transferred over a network. The buffering mechanism used
in CNFS for larger files accounts for the relatively small difference
in performance between the two servers for larger file sizes.
4.2 Security Analysis
The design of the CNFS system makes it very difficult for a malicious agent to mount a man in the middle attack. Minimal trust is
placed in the server and any interceptor of messages can only inflict as much damage as a malicious server. The strong encryption
safeguards data confidentiality and the signature validation routines
protect data integrity.
Replay attacks are a more plausible threat. However these can be
countered by the use of version numbers or timestamps.
A denial of service attack could be launched by attempting to fill
the server with garbage data, however the signature validation routines will counter this attack. The current implementation of the
CNFS server validates the first 8K data block of a file and buffers
subsequent blocks until all data have been received. A more sophisticated denial of service attack would be to allow the first 8K
of a file to be validated and then substitute bogus data for the remainder of the file. In such a scenario the CNFS client will detect
the write failure and may then decide to reissue the write operation
and sign the first 8K and every subsequent 48K of data, thus reducing the effectiveness of the attack. A sliding scale could be used to
determine the optimum size data block to sign.
5. RELATED WORK
The idea of securing stored data through encryption is already well
established. It was originally proposed in the context of the cryptographic file system (CFS) [4], which is implemented by a user level
NFS server that runs on the user’s own machine. The CFS server
can use any underlying file system, including NFS, to store data
on disk. The Transparent Cryptographic File System (TCFS) [5]
provides similar functionality but has migrated the cryptography
to the kernel level thus increasing the transparency to the user.
CryptFS [35] provides an interesting departure in design in that
it has been implemented as a stackable v-node interface and can
be used on top of any underlying file system without modification
to that system. Similar to cryptographic access control, the use of
cryptography in these systems limit the trust required in the server.
The exclusive use of symmetric cryptography means that these systems must rely on the access control mechanism of the underlying
file system to allow users to separate read and write access to the
files.
Many distributed file systems seek to support flexible user defined
access control across different administrative domains. AFS [11,
27, 28] is probably the most widely used wide-area file system.
It uses Kerberos [12] to authenticate users, which requires coordination of users and access rights among systems in the different
administrative domains. Cryptographic access control does not require users to be known to the server that stores data, which means
that data can be freely shared among different administrative domains. SFS [9, 17, 16] is a global decentralized file system, which
is similar to AFS. However, it does not rely on a single authentication service, but instead, public-key cryptography is used to
authenticate all entities in the system. The identity of the trusted
third party, which is required to authenticate the user and the client
machine is encoded in the file name. SFS is more flexible than
AFS, but essentially suffers from the same disadvantages. Other
distributed file systems, such as WebFS [33, 2] and Truffles [21],
rely on a trusted third party to authenticate users to servers. This
means that user can only collaborate if the system administrators
have agreed to coordinate the security policies of their respective
domains. Moreover, they generally rely on some trusted infrastructure, e.g., authentication in WebFS is based on X.509 certificates [3]. Coordination among different administrative domains
requires some degree of global knowledge and may prove an impediment to the scalability of the access control mechanism.
Operation
8K Read
8K Write
1MB Read
1MB Write
NFS
Mean
SD
0.79 ms
0.09
0.89 ms
0.11
792.84 ms
83.11
1187.89 ms 156.30
CNFS
Mean
SD
0.63 ms
0.02
23.062 ms 1480.16
874.168 ms
150.85
1480.16 ms
222.16
Overhead
Table 1: NFS and CNFS Server Routines Performance
Capability File Names [20] is a mechanism, that facilitates collaboration among users, without requiring additional services for identification and authentication. However, it still requires the server
to invoke a reference monitor to authorize clients. Moreover, security of data in transit is not addressed by the mechanism itself,
so it requires an additional mechanism to provide a secure channel
between client and server.
has created a series of new challenges for security systems. Existing access control mechanisms need to be adapted to reflect the
changing operational environment. We have described these challenges and have offered a critique of the suitability of the cryptographic access control mechanism for use in modern distributed
systems. We have found the cryptographic access control mechanism to be flexible and scalable yet powerful enough to effectively
protect resources in insecure environments.
6.
7. REFERENCES
CONCLUSIONS & FUTURE WORK
In this paper we addressed the problems of scalable access control
in large open systems, where the authenticated identity of a principal conveys no a priori information about the likely behaviour of
that principal.
We proposed a novel access control mechanism, called cryptographic
access control, that relies exclusively on cryptography to guarantee
confidentiality and integrity of objects (files) stored on potentially
untrusted servers in the system. This mechanism eliminates the
need to invoke a reference monitor (a potential bottleneck) on the
server. Instead, the server delivers data to any requesting client. Access control is performed implicitly on the client machine through
the principal’s ability to encrypt, decrypt and sign data stored in
the system. This means that access control can be performed in a
decentralized way on the client, without relying on trusted code or
tamper proof devices.
We presented the design and implementation of cryptographic access control in a distributed file system called CNFS, which is based
on a standard NFS server. We evaluated the performance of read
and write operations on the server, which showed that reading a
small file was slightly faster than a standard NFS server, while writing a small file was substantially more expensive because the CNFS
server has to verify the signature of data before they are written to
disk. The cost of signature verification is amortized when larger
files are written to disk, so both read and write operations of larger
files are comparable to a standard NFS server.
Security policies are defined by the users and enforced by the way
that they distribute keys. The current prototype lacks a secure key
distribution mechanism, which would facilitate file sharing.
The current prototype is implemented as a user level library and a
modified NFS server. The user level library makes the implementation portable at the cost of transparency and performance, but we
would like to integrate the client code into the operating system
in order to improve performance. Re-implementing the server, using a log-structured file system, would allow us to defer signature
validation until the log is compressed, which should significantly
improve the performance of the write operation.
The rapid growth of mobile computing and the deployment of large
distributed systems which span multiple administrative domains
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