docs: add some proxies docs

docs/user/index.rst

@@ -1,3 +1,41 @@
 Introduction
 ============
 
+Censorship is the suppression of speech, public communication, or other information.
+This may be done on the basis that such material is considered objectionable, harmful, sensitive, or "inconvenient".
+Regardless of the reasons for censorship, the technical measures taken to implement it often look the same.
+The *Bypass Censorship* portal provides a toolkit for circumventing censorship of Internet resources.
+The circumvention methods available will often exploit either collateral freedom, traffic obfuscation, or both in order
+to counter the measures put in place by the censor.
+
+Collateral Freedom
+------------------
+
+*Used by:* :doc:`Web Proxies <proxies>`, :doc:`Tor Bridges <bridges>`
+
+"Collateral freedom" is an anti-censorship strategy that attempts to make it economically prohibitive for censors to
+block an Internet resource.
+The way in which a censor restricts access to resources will require knowing which content to block and which to allow.
+It's incredibly difficult to achieve accuracy with filtering as the Internet is comprised of untagged free-form content
+that must be categorised at speed.
+This results in either over-blocking or under-blocking, and neither of these are desirable properties for the censor.
+
+This can be exploited by circumvention systems by deploying solutions at places that are "too big to block", like cloud
+providers.
+Either encryption or constantly rotating identifiers are then used to prevent censors from identifying requests for
+censored information that is hosted among other content.
+This forces censors to either allow access to the censored information or take down entire services.
+
+Traffic Obfuscation
+-------------------
+
+*Used by:* :doc:`Tor Bridges <bridges>`
+
+"Traffic Obfuscation" is an anti-censorship strategy that attempts to make it difficult to fingerprint traffic.
+This is more commonly used for general censorship circumvention solutions rather than means of accessing specific
+resources.
+There is a long tail of types of traffic on the Internet, including critical infrastructure communications like
+industrial control systems, point-of-sale systems and security systems.
+This can be exploited by circumvention systems by making their traffic look like one of these unclassified systems.
+Not being able to accurately identify the traffic means that the cost of blocking access is unknown, and so it is more
+difficult for a censor to justify the block.