Description
Introduction
Access control mechanisms in operating systems have evolved over the years to include access control
lists as well as a variety of mandatory access control (MAC) mechanisms, including multi-level security,
integrity levels, type enforcement, and limited forms of role-based access control.
An operating system, however, can only deal with the users and resources it knows about. It manages
access between subjects (users) and objects (resources provided by the system, such as files and
devices). There’s an underlying assumption that these subjects (users) have accounts on the system
and the objects are known to and managed by the system (e.g., files in the file system).
For many applications, however, this is not the case. Applications may run as services that are launched
by a specific user. These services, in turn, interact with users who may not have accounts on the
system. For instance, you can log onto eBay and interact with it but you don’t have an account on any of
the systems that provide the eBay’s service. Similarly, objects may be entities that are also unknown to
the operating system, such as fields or tables in a database or media streams.
This is a problem that affects many services. Services often have to put together their own solutions to
manage their user accounts and access permissions (authorizations). To address this problem, Google
built Zanzibar: Google’s Consistent, Global Authorization System
(https://www.usenix.org/system/files/atc19-pang.pdf) . This provides a consistent, large-scale service for
managing access control policies that any application can use. Google uses this for services that include
Calendar, Cloud, Drive, Maps, Photos, and YouTube.
Your Assignment
Your assignment is to design and implement an authentication and access control (authorization) library
that can be used by services that need to rely on their own set of users rather than those who have
accounts on the computer and manage their own sets of objects and operations on those objects.
The access control system that you design will support:
Users
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A collection of users and passwords that is used for authenticating users and for tracking members of
domains (user groups). For example
{ “anika”, “password” }, { “fang”, “123456”}, { “liam”, “abc123” }, …
Domains
A named collection of zero or more users. For example,
admins = { “anika”, “arun”, “wei”, “yash” }
premium_subscribers = { “fang”, “noah”, “riya” }
normal_subscribers = { “liam”, “ravi”, “olivia” }
Types
A named collection of zero or more objects. Objects are any strings that will have meaning to the
application. For example, they might be file names, directory names, subscribed features, media
streams, etc. For example,
premium_content = { “hbo”, “showtime”, “disney” }
normal_content = { “cbs”, “nbc”, “fox”, “abc”, “wor”, “pix”, “pbs” }
Access permissions
A set of access rights that defines operations that domains can perform on types. For example,
“view”: (premium_user, premium_content)
“delete”: (“admins”, normal_content)
“delete”: (“admins”, premium_content)
The access rights are meaningful only for the application that is making use of this service. That is,
view and delete in these examples are arbitrary strings. Note that the delete operation does not specify
any objects but rather just an operation that those in the user group domains may perform.
This system is similar to SELinux’s (Security Enhanced Linux) type enforcement system.
The above examples are conceptual. It is up to you to decide what storage structures are the most
convenient to implement.
Groups
This is an individual assignment. You may not collaborate in your development of this project.
Languages
You may write this assignment in C, C++, Go, Java, or Python.
Environment
Your submissions will be tested on Rutgers iLab Linux systems. You can develop this on any other
system but you are responsible for making sure that it will work on the iLab systems.
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Specifications
Your implementation will be one that runs locally rather than as a network service and can be
incorporated within any application. For this assignment, you do not need to handle any concurrent
operations, so you need not implement locking.
Applications that use this service can be assumed to be trusted and trustworthy: they will not try to use
the interfaces incorrectly or subvert the system in any way. You should check for basic usage (correct
number of arguments or empty strings) but you can assume that anyone using your code for managing
access control will not
You will write a single driver program named auth.
This program will take commands as parameters, each of which implements a specific function of the
interface. The program should be suitable for use and testing by shell scripts. This means should follow
the defined interface and not generate extraneous output.
The program gets all the parameters it needs from the command line and exits after running the
requested command. It will never prompt for user input or read data from the standard input. User
accounts and access permissions must be stored persistently in one or more files so that they can be
accessed again when the application (or another program using the same authorization service) is run
again. If you choose to use a structure like a Python dictionary, you are responsible for storing it into a
file before the program exits and reading it in when the program runs.
If the number of arguments is incorrect or if the command parameter is invalid, you should print a
descriptive error message and exit. Error messages start with the string Error: followed by a space and
a one-line error message.
API
This section describes the operations that you need to implement. It is up to you to define the
appropriate return values, exceptions, and other details that you feel are needed for your design.
Each of these functions and their parameters will be invoked from the main program and each of the
parameters will be a single argument on the command line. For example,
auth AddUser paul “monkey brains”
is the command to add a new user named paul with the password monkey brains .
Responses are either:
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A single line with the text Success to indicate a successful operation if there is no other output (i.e., a
list of items) that needs to be presented.
A single line starting with Error: followed by an optional message separated from the Error: string
with a space to indicate what went wrong.
A list of items, one item per line.
AddUser(“user”, “password”)
Define a new user for the system along with the user’s password, both strings.
The username cannot be an empty string. The password may be an empty string.
Test program:
auth AddUser myname mypassword
The program should return one of:
Success
Error: user exists – if the user already exists.
Error: username missing – if the username is an empty string.
Authenticate(“user”, “password”)
Validate a user’s password by passing the username and password, both strings.
Test program:
auth Authenticate myname mypassword
The program should clearly report
Success
Error: no such user
Error: bad password
SetDomain(“user”, “domain”)
Assign a user to a domain. Think of it as adding a user to a group.
If the domain name does not exist, it is created. If a user does not exist, the function should return an
error. If the user already exists in the domain, the command will succeed but take no action. You should
never have duplicate users in a domain.
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A user may belong to multiple domains.
The domain name must be a non-empty string.
Test program:
auth SetDomain user domain_name
The program should report
Success
Error: no such user – if the user does not exist
Error: missing domain – if the domain name is an empty string. Your program should check that the
domain is a valid name before checking that the user exists. That avoids having to check for a user
only to find out that the domain name is an empty string.
DomainInfo(“domain”)
List all the users in a domain.
The group name must be a non-empty string.
Test program:
auth DomainInfo domain_name
The program should output:
A list of all the users in that domain, one per line
Nothing if the domain does not exist or there are no users in a domain (empty list). A non-existent
domain name is treated the same as one with no users in it.
Error: missing domain – if the domain name is an empty string.
Sample output:
alice
bob
charles
david
ellen
with no leading tabs or spaces. This output format fits into the Unix tools philosophy, which makes the
output suitable for input in a pipeline of commands.
SetType(“objectname”, “type”)
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Assign a type to an object. You can think of this as adding an object to a group of objects of the same
type.
If the type name does not exist, it is created.
The object can be any non-null string.
Test program:
auth SetType object type_name
The program should report
Success
Error: Failure if the object or the type names are empty strings.
TypeInfo(“type”)
List all the objects that have a specific type, one per line.
The type name must be a non-empty string.
Test program:
auth TypeInfo type_name
The program should output
A list of all the objects that have been assigned type_name with SetType .
Nothing if the type does not exist or there are no objects associated with that type (empty list).
Error: if the type name is an empty string.
Sample output:
alice_file
bob_file
charles_file
david_file
ellen_file
with no leading tabs or spaces. This output format fits into the Unix tools philosophy, which makes the
output suitable for input in a pipeline of commands.
AddAccess(“operation”, “domain_name”, “type_name”)
Define an access right: a string that defines an access permission of a domain to an object. The access
permission can be any arbitrary string that makes sense to the service.
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The domain name and type name must be non-empty strings.
If the domain name or type names do not exist then they will be created. If the operation already exists
for that domain and type, it will not be added a second time but it will not be treated as an error.
This is the key function that builds the access control matrix of domains and types.
Test program:
auth AddAccess operation domain_name type_name
The program will accept three strings and report
Success – if the operation was added to the access control matrix
Error: missing operation – if the operation is null
Error: missing domain – if the domain is null
Error: missing type – if the type is null
CanAccess(“operation”, “user”, “object”)
Test whether a user can perform a specified operation on an object.
The logic in this function in pseudocode is:
for d in domains(user)
for t in types(object)
if access[d][t] contains operation
return true
return false
That is, find all domains for the user and find all types for the object. Then see if any of those domaintype combinations have the desired operation set.
Test program:
auth CanAccess operation user object
The program will check whether the user is allowed to perform the specified operation on the object.
That means that there exists a valid access right for an operation in some (domain, type) where the user
is in domain and the object is in the corresponding type.
As with AddAccess, the program will accept three strings.
Note that the parameters here are user names and object names, not user domains and object types.
The program will return:
Success – if the access is permitted
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Error: access denied – for all other cases. This means the operation does not exist for that any
domains for that user-object or the user does not exist. CanAccess provides the answer to the
question, can user A access object B? If the operation does not exist, the answer is no. You don’t
need to provide an explanation of the error.
Command syntax error handling
Your program should print an error if an invalid command is entered or an incorrect number of
arguments is specified. For example:
auth Add myname mypassword
Error: invalid command Add
auth Authenticate myname mypassword mypassword2
Error: too many arguments for Authenticate
Notes and assumptions
Note that this is not a complete interface. It is, for example, notably missing are operations to delete or
change users, user groups, object groups, and permissions. It also has no support for wildcards or
regular expression patterns.
Design your program so that a user may be a member of multiple domains and an object may have
multiple types associated with it. This means that you will need to check all appropriate access rights to
see if a user is allowed access to an object.
Because this is implemented as a program that accepts a single command at a time, you will need to
save your results in one or more files after running each command. Saving and reading your lists and
tables may be the most time-consuming part of designing your program. Think about good ways to
implement this so your code can remain simple.
Hard-coded paths
Because the program will be run from other accounts, it is imperative that you do not include hardcoded full pathnames in your program (e.g., do not use a rooted pathname like
“ /ilab/users/pxk/src/access/tables ”). You may store your file in the current directory or, if using multiple
files, in a subdirectory (e.g., “ ./tables/ ”) so that cleanup will be easy. Create the directory if it’s missing.
Don’t assume that anything is set up before we run the program.
What to submit
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Your submission will generate one program, named auth, that takes the following commands: AddUser,
Authenticate, SetDomain, DomainInfo, SetType, TypeInfo, AddAccess, and CanAccess.
Documentation and components
Clear and simple documentation is crucial so that we don’t waste time trying to figure out how to compile
and run your program. At a minimum, specify clearly:
1. How to compile your programs (a build script or Makefile would be useful). There should be
NOreliance on any IDE (e.g., eclipse) or third-party libraries (with the exception listed in the next
section). Instructions should specify command-line commands only.
2. Any setup that is needed, such as creating a subdirectory to hold your access permission files.
3. Example test scripts that you used to validate the program, testing error cases as well as success
cases.
Code
Submit only your documentation and source files (and Makefile, if you have one). DO NOT submit any
compiled files (e.g., object files, Java .class files or executables).
If you feel a need to do so, you may include third-party support only for serializing or parsing stored
data. If the packages are not installed on Rutgers systems, you will need to submit the source
components and make sure they are integrated into your build script.
Make sure the instructions you submit to compile and run the programs makes sense. Try to follow them
based on a clean download of what you submitted. Better yet, have a friend try to follow them.
Note that there are 150 students in this class. With a smaller class, we would be able to devote a several
minutes to figuring out how to compile and run your program. With a class this size, you will need to
make sure it is trivial to build your program and that it conforms to the interface.
