Implementation of Authentication Mechanism for a Virtual File System

Transcription

1 Implementatin f Authenticatin Mechanism fr a Virtual File System Prject fr Operating Systems Curse (CS 5204) Implemented by- Vinth Jagannathan Abhishek Ram Under the guidance f Dr Dennis Kafura

2 Abstract The aim f the prject is the implementatin f an authenticatin mechanism t cntrl access t a virtual file system Our authenticatin mechanism is mdeled n Kerbers The PGP algrithm is used fr encryptin and decryptin The virtual file system emulates the VFAT file structure used in Unix The disk layut f the files stred in the VFS is placed in the index blcks f VFS that are similar t the inde structure in Unix file system Access cntrl lists are implemented fr each file in the file system t cntrl access The purpse f ding the prject was t gain an insight int the wrking f security mechanisms and hw t apply a security layer t a file system in a distributed envirnment

3 Prject Design Kerbers Mechanism Kerbers Server (Ticket Granting Server Authenticatin Server) Key distributin service Wrkstatin User File Server The design f the Kerbers mechanism can be divided int three basic mdules 1 Wrkstatin This is the mdule that interacts with the user It allws the user t lgin and participates, alng with the Kerbers server, in the verificatin f the user s identity Each time the user subsequently requests a service ver the netwrk, it perfrms the validatin f the user in a way transparent t the user 2 Kerbers Server This mdule is respnsible fr verifying the user s identity and granting tickets t the user whenever the user needs sme service ver the netwrk The tickets are used in authenticatin f the user 3 File Server This mdule takes care f the maintenance f the virtual file system It receives the users requests fr peratins n files and checks whether they have the permissins fr executing thse Permissins are checked by referring t the access cntrl matrix that is maintained fr each file in the virtual file system Kerbers authenticatin mechanism When the user lgs in User name is sent by wrkstatin t the Kerbers server The Kerbers server btains frm the database an encryptin key fr the user and fr the service name

4 The Kerbers server frms a ticket that is in the frmat Ticket=(lgin-name, net address f wrkstatin, TGS sessin key) The TGS sessin key is a randm number generated fr that sessin The ticket is encrypted using the service key A message is then frmed as fllws Message=(TGS sessin key, sealed-ticket) The message is encrypted using the user encryptin key and sent t the wrkstatin The wrkstatin then prmpts the user fr his passwrd, which wuld be the user key The passwrd is used t decrypt the message and then deleted The TGS sessin key and the sealed ticket are extracted frm the message and then stred When the user requests sme netwrk service The wrkstatin builds the fllwing message t be sent t the Kerbers server Message=(sealed-ticket,sealed-authenticatr) The authenticatr, which is sealed using the TGS sessin key, has the fllwing frmat Authenticatr=(lgin-name, wrkstatin net address) The Kerbers server decrypts the sealed ticket using the servic key and extracts the sessin key frm the sealed ticket It then decrypts the sealed authenticatr using the sessin key and cmpares the lgin name and wrkstatin address in the ticket and the authenticatr t validate the user If the user is validated a new sessin key is generated randmly and a new ticket is built- Ticket=(lgin-name, net address f wrkstatin, new sessin key) The ticket is encrypted using the encryptin key fr the file server

5 A new message is frmed- Message=(new sessin key, sealed-ticket) The message is encrypted using the TGS sessin key and sent t the wrkstatin The wrkstatin decrypts the message using the TGS sessin key and extracts the new key It builds an authenticatr as fllws Authenticatr=(lgin-name, wrkstatin net address) The authenticatr is encrypted using the new sessin key The wrkstatin builds the fllwing message t be sent t the file server Message=(sealed-ticket,sealed-authenticatr) The file server, n receiving the message, decrypts the sealed ticket using its key and extracts the new sessin key Using this it decrypts the authenticatr It then vaildates the user by cmparing the lgin name and the wrkstatin address in the ticket and the authenticatr Encryptin and Decryptin Functins int encrypt(char * data, char *pwd) The functin encrypts data using the given key It takes a pinter t the data and a pinter t the key as parameters and returns the length f the encrypted data int decrypt(char *data, char *pwd, int len) The functin decrypts data using the given key It takes a pinter t the data, the passwrd, and the length f the encrypted data as parameters Returns length f decrypted data

6 Virtual File System The Virtual File System (VFS) is used t stre the files It is a simple emulatin f the VFAT structure in Unix file system The file server perates n the VFS t cntrl the access mechanism The structure f VFS is depicted in the figure shwn belw Super blck Index blck Index blck Index blck Index blck Data blck Data blck Data blck Data blck Structure f VFS

7 The Virtual File System (VFS) cntains ne super blck, 100 index blcks and 1000 data blcks Structure f Super blck: Number f files Free space available Starting address f free space Access Matrix Number f files: This cunt is used t enhance searching thrugh the VFS This cunt is updated whenever a file is written in t the VFS r deleted frm the VFS (this is dne by the update functin) Free space available: This is used t check whether the incming file can be written in t the VFS r nt If the file s size is greater than the ttal free space available, then the file cannt be written int the VFS Starting address f free space: This specifies the starting lcatin f the free pl When a file is deleted frm VFS, the memry allcated t that file is attached t the beginning f the free pl and the starting address f free pl is mdified accrdingly Access Matrix: This matrix cntains the access permissin values fr (file, user) pairs This is implemented as a tw dimensinal array Structure f Index blck: Name f the file Size f the file Starting blck f the file

8 The index blck cntains the disk layut f the files stred in the VFS This is similar t the inde in the Unix file system Size f the file: The size f the file (in bytes) is stred in this variable When a file is deleted frm VFS, the free space variable is incremented by the size f the file Structure f Data blck: Data (100 Characters) Link t next data blck As the name says, the data blck is used t hld the cntents f the files The data blcks are implemented using the linked list mechanism Functins used t access the VFS READ: int read (char *fname, char *uname, char *buffer) The access permissins are checked using the fname and uname, and if the user (uname) has READ_ONLY permissin t the file (fname), the cntent f the file is returned in the buffer WRITE: int write (char *fname, char *uname, char *buffer) This functin is used t create a new file and stre it in the VFS When a file is created, the wner is autmatically assigned with FULL_CONTROL and all ther users are granted with READ_ONLY access S when a file is created, nly the wner f the file can change the access cntrl settings SET_ACCESS: int set_access (char *wner, char *fname, char *uname, int access) The access cntrl settings fr the file (fname) can be changed nly if wner has FULL_CONTROL access t the file

9 Access Cntrl Mechanism A simple access cntrl mechanism is implemented in the file server The access mechanism has three levels f access, i N access ii iii Read nly Full Cntrl (Read and set access permissins t ther users) The access mechanism is implemented as an access cntrl matrix that is f the frm (file_name, user_name) If a user X has READ_ONLY access t a file A, then in the access matrix the psitin (A, X) will have the value set fr READ_ONLY (ie 1 Fr NO_ACCESS it is 0 and fr FULL_CONTROL it is 2) If the user X wants t change the access cntrl settings f the file A, as he des nt have the permissin t change access cntrl settings, he will nt be allwed t perfrm the task Pretty Gd Privacy PGP r Pretty Gd Privacy is a pwerful cryptgraphic prduct family that enables peple t securely exchange messages, and t secure files, disk vlumes and netwrk cnnectins with bth privacy and strng authenticatin Privacy means that nly the intended recipient f a message can read it By prviding the ability t encrypt messages, PGP prvides prtectin against anyne eavesdrpping n the netwrk Even if the infrmatin is intercepted, it is cmpletely unreadable t the snper Authenticatin identifies the rigin f the infrmatin, certainty that it is authentic, and that it has nt been altered Authenticatin als prvides an extremely valuable tl in netwrk security: verificatin f the identity f an individual

10 Summary The prject has enabled us t understand the key issues that are invlved in the develpment f an authenticatin mechanism The wrking f the Kerbers and the PGP encryptin algrithm has been understd We gt an insight abut the Unix file system architecture and strage mechanism by implementing the Virtual File System Thugh the VFS was a simple and small file system, it gave a cmplete verview abut mst f the features f the actual file system, such as memry allcatin, free pl management, etc The implementatin f the access cntrl lists fr files expsed us t the file prtectin cncept as well Sme extensins f the prject may be t implement the same fr a distributed file systems Maintaining capability lists fr the users f the systems is anther mdificatin pssible