Enable HTTP Compression Facilities

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Compression feature adds the ability to compress outgoing content using gzip, deflate or custom encoder and thus reduce the size of the response.

This feature is defined in the class io.ktor.features.Compression and no additional artifacts are required.


When the configuration block is omitted, the default configuration is used. It includes the following encoders:

  • gzip
  • deflate
  • identity

If you want to select specific encoders you need to provide a configuration block:

install(Compression) {

Each encoder can be configured with a priority and some conditions:

install(Compression) {
    gzip {
        priority = 1.0
    deflate {
        priority = 10.0 
        minimumSize(1024) // condition

Encoders are sorted by specified quality in an Accept-Encoding header in the HTTP request, and then by specified priority. First encoder that satisfies all conditions wins.

In the example above when Accept-Encoding doesn’t specify quality, gzip will be selected for all contents less than 1K in size, and all the rest will be encoded with deflate encoder.

Some typical conditions are readily available:

  • minimumSize – minimum size of the response to compress
  • matchContentType – one or more content types that should be compressed
  • excludeContentType – do not compress these content types

You can also use a custom condition by providing a predicate:

gzip {
    condition {
        parameters["e"] == "1"

Security with HTTPS

HTTPS with any kind of compression is vulnerable to the BREACH attack. This kind of attack allows a malicious attacker to guess a secret (like a session, an auth token, a password, or a credit card) from an encrypted HTTPS page in less than a minute.

You can mitigate this attack by:

  • Completely turn off HTTP compression (which might affect performance).
  • Not placing user input (GET, POST or Header/Cookies parameters) as part of the response (either Headers or Bodies) mixed with secrets (including a Set-Cookie with a session_id).
  • Add a random amount of bytes to the output for example in an html page, you can just add <!-- 100~500 random_bytes !--> making it much harder to guess the secret for an attacker in a reasonable time.
  • Ensure that your website is completely HTTPS and has HSTS enabled, and adding a conditional header checking the Referrer page. (If you have a single page without HTTPS, the malicious attacker can use that page to inject code using the same domain as Referrer).
  • Adding CSRF protection to your pages.
application.install(Compression) {
    gzip {
        condition {
            // @TODO: Check: this is only effective if your website is completely HTTPS and has HSTS enabled. 
            request.headers[HttpHeaders.Referrer]?.startsWith("https://my.domain/") == true

TL;DR; Even when HTTPS prevents an eavesdropper to know the content of a request, it does not hide the response length. So one of your users could be connecting to an evil access point, for example by connecting to a public network or one with a well-known password, or a private network with an Evil Twin. That access point can intercept all the encrypted messages and measure the length. Then can modify any non-https connection (or social engineer the user to access an https page controlled by the attacker) to inject a javascript or place images pointing to the vulnerable page mutating an input (get, post or header parameters) that are reflected in either the headers or the response body, then the access point can measure the length of the responses to guess a secret with as little as 100 to 10000 requests that are forced to be done by your browser with either the javascript or image requests without the user ever noticing.


You can provide your own encoder by implementing the CompressionEncoder interface and providing a configuration function. Since content can be provided as a ReadChannel or WriteChannel, it should be able to compress in both ways. See GzipEncoder as an example of an encoder.