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The following sections describe the concepts you need to understand to use the access policy language. They're presented in a logical order, with the first terms you need to know at the top of the list.
A permission is the concept of allowing or disallowing some kind of access to a particular resource. Permissions essentially follow this form: "A is/isn't allowed to do B to C where D applies." For example, Jane (A) has permission to receive messages (B) from John's Amazon SQS queue (C), as long as she asks to receive them before midnight on May 30, 2009 (D). Whenever Jane sends a request to Amazon SQS to use John's queue, the service checks to see if she has permission and if the request satisfies the conditions John set forth in the permission.
A statement is the formal description of a single permission, written in the access policy language. You always write a statement as part of a broader container document known as a policy (see the next concept).
A policy is a document (written in the access policy language) that acts as a container for one or more statements. For example, a policy could have two statements in it: one that states that Jane can use John's queue, and another that states that Bob cannot use John's queue. As shown in the following figure, an equivalent scenario would be to have two policies, one containing the statement that Jane can use John's queue, and another containing the statement that Bob cannot use John's queue.
The AWS service implementing access control (e.g., Amazon SQS) uses the information in the statements (whether they're contained in a single policy or multiple) to determine if someone requesting access to a resource should be granted that access. We often use the term policy interchangeably with statement, as they generally represent the same concept (an entity that represents a permission).
The issuer is the person who writes a policy to grant permissions for a resource. The issuer (by definition) is always the resource owner. AWS does not permit AWS service users to create policies for resources they don't own. If John is the resource owner, AWS authenticates John's identity when he submits the policy he's written to grant permissions for that resource.
The principal is the person or persons who receive the permission in the policy. The principal is A in the statement "A has permission to do B to C where D applies." In a policy, you can set the principal to "anyone" (i.e., you can specify a wildcard to represent all people). You might do this, for example, if you don't want to restrict access based on the actual identity of the requester, but instead on some other identifying characteristic such as the requester's IP address.
The action is the activity the principal has permission to
perform. The action is B in the statement "A has permission to do B to C where D
applies." Typically, the action is just the operation in the request to AWS. For example, Jane sends a request to Amazon SQS
=ReceiveMessage. You can
specify one or multiple actions in a policy.
The resource is the object the principal is requesting access to. The resource is C in the statement "A has permission to do B to C where D applies."
The conditions are any restrictions or details about the permission. The condition is D in the statement "A has permission to do B to C where D applies." The part of the policy that specifies the conditions can be the most detailed and complex of all the parts. Typical conditions are related to:
Date and time (e.g., the request must arrive before a specific day)
IP address (e.g., the requester's IP address must be part of a particular CIDR range)
A key is the specific characteristic that is the basis for access restriction. For example, the date and time of request.
You use both conditions and keys
together to express the restriction. The easiest way to understand how you actually
implement a restriction is with an example: If you want to restrict access to before
May 30, 2010, you use the condition called
DateLessThan. You use the
and set it to the value
2010-05-30T00:00:00Z. AWS defines the
conditions and keys you can use. The AWS
service itself (e.g., Amazon SQS or Amazon SNS) might also define service-specific keys. For more information
about conditions, see Condition. For
more information about the available keys, see Available Keys.
The requester is the person who sends a request to an AWS service and asks for access to a particular resource. The requester sends a request to AWS that essentially says: "Will you allow me to do B to C where D applies?"
Evaluation is the process the AWS service uses to determine if an incoming request should be denied or allowed based on the applicable policies. For information about the evaluation logic, see Evaluation Logic.
The effect is the result that you want a policy statement to return at evaluation time. You specify this value when you write the statements in a policy, and the possible values are deny and allow.
For example, you could write a policy that has a statement that denies all requests that come from Antarctica (effect=deny given that the request uses an IP address allocated to Antarctica). Alternately, you could write a policy that has a statement that allows all requests that don't come from Antarctica (effect=allow, given that the request doesn't come from Antarctica). Although the two statements sound like they do the same thing, in the access policy language logic, they are different. For more information, see Evaluation Logic.
Although there are only two possible values you can specify for the effect (allow or deny), there can be three different results at policy evaluation time: default deny, allow, or explicit deny. For more information, see the following concepts and Evaluation Logic.
A default deny is the default result from a policy in the absence of an allow or explicit deny.
An allow results from a statement that has effect=allow, assuming any stated conditions are met. Example: Allow requests if they are received before 1:00 p.m. on April 30, 2010. An allow overrides all default denies, but never an explicit deny.
An explicit deny results from a statement that has effect=deny, assuming any stated conditions are met. Example: Deny all requests if they are from Antarctica. Any request that comes from Antarctica will always be denied no matter what any other policies might allow.