Internet Computer HTTP Certification

Defining CEL Expressions

CEL (Common Expression Language) is a portable expression language that can be used to enable different applications to more easily interoperate. It can be seen as the computation or expression counterpart to Protocol Buffers.

CEL expressions lie at the heart of the Internet Computer's HTTP certification system. They are used to define the conditions under which a request and response pair should be certified and what should be included from the corresponding request and response objects in the certification.

CEL expressions can be created in two ways, by using the CEL builder, or by directly creating a CEL expression.

Converting CEL expressions to their `String` representation

Note that the CelExpression enum is not a CEL expression itself, but rather a Rust representation of a CEL expression. To convert a CelExpression into its String representation, use CelExpression.to_string or create_cel_expr. This applies to CEL expressions created both by the CEL builder and directly.

use ic_http_certification::cel::CelExpression;

let cel_expr = CelExpression::DefaultCertification(None).to_string();

Alternatively:

use ic_http_certification::cel::{CelExpression, create_cel_expr};

let certification = CelExpression::DefaultCertification(None);
let cel_expr = create_cel_expr(&certification);

Using the CEL builder

The CEL builder interface is provided to ease the creation of CEL expressions through an ergonmic interface. If this interface does not meet your needs, you can also create CEL expressions directly. To define a CEL expression, start with [DefaultCelBuilder]. This struct provides a set of methods that can be used to define how your request and response pair should be certified.

When certifying requests, the request body and method are always certified. To additionally certify request headers and query parameters, use with_request_headers and with_request_query_parameters respectively. Both methods take a [str] slice as an argument.

When certifying a response, the response body and status code are always certified. To additionally certify response headers, use with_response_certification. This method takes the DefaultResponseCertification enum as an argument. To specify header inclusions, use the CertifiedResponseHeaders variant of the DefaultResponseCertification enum. Or to certify all response headers, with some exclusions, use the ResponseHeaderExclusions variant of the DefaultResponseCertification enum. Both variants take a [str] slice as an argument.

Fully certified request / response pair

To define a fully certified request and response pair, including request headers, query parameters, and response headers use DefaultCelBuilder::full_certification. For example:

use ic_http_certification::{DefaultCelBuilder, DefaultResponseCertification};

let cel_expr = DefaultCelBuilder::full_certification()
    .with_request_headers(&["Accept", "Accept-Encoding", "If-Match"])
    .with_request_query_parameters(&["foo", "bar", "baz"])
    .with_response_certification(DefaultResponseCertification::CertifiedResponseHeaders(&[
        "Cache-Control",
        "ETag",
    ]))
    .build();

Partially certified request

Any number of request headers or request query parameters can be certified via with_request_headers and with_request_query_parameters respectively. Both methods will accept empty arrays, which is the same as not calling them at all. If with_request_headers is called with an empty array, or it is not called at all, then no request headers will be certified. Likewise for with_request_query_parameters, if it is called with an empty array, or not called at all, then no request query parameters will be certified. If both are called with an empty array, or neither are called, then only the request body and method will be certified.

For example, to certify only the request body and method:

use ic_http_certification::{DefaultCelBuilder, DefaultResponseCertification};

let cel_expr = DefaultCelBuilder::full_certification()
    .with_response_certification(DefaultResponseCertification::CertifiedResponseHeaders(&[
        "Cache-Control",
        "ETag",
    ]))
    .build();

Alternatively, this can be done more explicitly:

use ic_http_certification::{DefaultCelBuilder, DefaultResponseCertification};

let cel_expr = DefaultCelBuilder::full_certification()
    .with_request_headers(&[])
    .with_request_query_parameters(&[])
    .with_response_certification(DefaultResponseCertification::CertifiedResponseHeaders(&[
        "Cache-Control",
        "ETag",
    ]))
    .build();

Skipping request certification

Request certification can be skipped entirely by using DefaultCelBuilder::response_certification instead of DefaultCelBuilder::full_certification. For example:

use ic_http_certification::{DefaultCelBuilder, DefaultResponseCertification};

let cel_expr = DefaultCelBuilder::response_certification()
    .with_response_certification(DefaultResponseCertification::ResponseHeaderExclusions(&[
        "Date",
        "Cookie",
        "Set-Cookie",
    ]))
    .build();

Partially certified response

Similiarly to request certification, any number of response headers can be provided via the CertifiedResponseHeaders variant of the DefaultResponseCertification enum when calling with_response_certification. The provided array can also be an empty. If the array is empty, or the method is not called, then no response headers will be certified.

For example, to certify only the response body and status code:

use ic_http_certification::DefaultCelBuilder;

let cel_expr = DefaultCelBuilder::response_certification().build();

This can also be done more explicitly:

use ic_http_certification::{DefaultCelBuilder, DefaultResponseCertification};

let cel_expr = DefaultCelBuilder::response_certification()
    .with_response_certification(DefaultResponseCertification::CertifiedResponseHeaders(&[]))
    .build();

The same applies when both when using DefaultCelBuilder::response_certification and DefaultCelBuilder::full_certification.

use ic_http_certification::DefaultCelBuilder;

let cel_expr = DefaultCelBuilder::full_certification()
    .with_request_headers(&["Accept", "Accept-Encoding", "If-Match"])
    .with_request_query_parameters(&["foo", "bar", "baz"])
    .build();

To skip response certification completely, then certification overall must be skipped completely. It wouldn't be useful to certify a request without certifying a response. So if anything is certified, then it must at least include the response. See the next section for more details on skipping certification entirely.

Skipping certification

To skip certification entirely, use skip_certification, for example:

use ic_http_certification::DefaultCelBuilder;

let cel_expr = DefaultCelBuilder::skip_certification();

Skipping certification may seem counter-intuitive at first, but it is not always possible to certify a request and response pair. For example, a canister method that will return different data for every user cannot be easily certified.

Typically these requests have been routed through raw Internet Computer URLs in the past, but this is dangerous because raw URLs allow any responding replica to decide whether or not certification is required. In contrast, by skipping certification using the above method with a non-raw URL, a replica will no longer be able to decide whether or not certification is required and instead this decision will be made by the canister itself and the result will go through consensus.

Directly creating a CEL expression

To define a CEL expression, start with the CelExpression enum. This enum provides a set of variants that can be used to define different types of CEL expressions supported by Internet Computer HTTP Gateways. Currently only one variant is supported, known as the "default" certification expression, but more may be added in the future as HTTP certification evolves over time.

When certifying requests, the request body and method are always certified. To additionally certify request headers and query parameters, use the headers and query_paramters of DefaultRequestCertification struct. Both properties take a str slice as an argument.

When certifying a response, the response body and status code are always certified. To additionally certify response headers, use the CertifiedResponseHeaders variant of the DefaultResponseCertification enum. Or to certify all response headers, with some exclusions, use the ResponseHeaderExclusions variant of the DefaultResponseCertification enum. Both variants take a str slice as an argument.

Note that the example CEL expressions provided below are formatted for readability. The actual CEL expressions produced by CelExpression::to_string and create_cel_expr are minified. The minified CEL expression is preferred because it is more compact, resulting in a smaller payload and a faster evaluation time for the HTTP Gateway that is verifying the certification, but the formatted versions are also accepted.

Fully certified request / response pair

To define a fully certified request and response pair, including request headers, query parameters, and response headers:

use ic_http_certification::cel::{CelExpression, DefaultCertification, DefaultRequestCertification, DefaultResponseCertification};

let cel_expr = CelExpression::DefaultCertification(Some(DefaultCertification {
  request_certification: Some(DefaultRequestCertification {
    headers: &["Accept", "Accept-Encoding", "If-Match"],
    query_parameters: &["foo", "bar", "baz"],
  }),
  response_certification: DefaultResponseCertification::CertifiedResponseHeaders(&[
    "ETag",
    "Cache-Control",
  ]),
}));