NetSec Lecture Notes - Lesson 7 - HTTPS

HTTPS

• HTTPS: Hyper Text Protocol Secure = all communication between your browser and a website is encrypted
• Basically HTTP over SSL (Secure Socket Layer)
  • SSL now called TLS (Transport Layer Security)
• As opposed to HTTP, which is a clear text protocol
• Creates a secure channel over an insecure network
• Is reasonable protection against man in them iddle attacks
• Can still provide security even when only one side of the communication is secure

Why is HTTPS not used for all web traffic?

• Crypto slows down web servers (but not by much if done right)
• Some ad networks do not support HTTPS
  • Reduced ad revenue for publishers
  • Stats given are from 2015, probably way out of date

HTTPS Quiz

• Select all of the items that can be encrypted by HTTPS
  • Request URL
    • True
  • Query parameters
    • True
  • Headers
    • True
  • Cookies
    • True
  • Host addresses
  • Port numbers
  • The amount of transferred data
  • Length of the session

HTTPS Threat Model: Network Attacker

• Controls network infrastructure: Routers, DNS
• Eavesdrops, injects, blocks, and modifies packets
• Examples:
  • Wireless network at internet cafe
  • Internet access at hotels (untrusted ISP)

SSL/TLS Overview

• Uses public key crypto

Certificate Authority

• Key challenge is how does Alice obtain Bob’s public key?
  • Standard is to use certificates issued by an authority

Certificates on the Web

* Subject's commonName can be:
	* An explicit name, e.g. cc.gatech.edu
	* A wildcard cert, e.g. \*.gatech.edu
* Matching Rule:
	* \* must occur in leftmost component, does not match "."
	* For example: "\*.a.com" matches "x.a.com" but not "y.x.a.com" * Browsers accept certificates from a large number of CAs
* Top level CAs ~ 60
* Intermediate CAs ~ 1200 * Browser handshake ![img](../assets/content_images/omscs/netsec/L07_img4.png)

HTTPS In the Browser: The Lock Icon

• Intended Goal:
  • Provide the user with identity of page origin
  • Indicate to user that page contents were not viewed or modified by a network attacker
• Indicates that all elements on the page were fetched using HTTPS
• For all elements:
  • HTTPS cert issued bya CA trusted by the browser
  • HTTPS cert is valid (not expired)
  • Domain in URL matches:
    • CommonName or SubjectAlternativeName in cert

HTTPS Disadvantages Quiz

• Which of the following are real disadvantages to using HTTPS
  • Browser caching won’t work properly
  • You need to buy an SSL certificate
    • True
    • Outdated, thanks LetsEncrypt
  • Mixed modes issue - loading insecure content on a secure site
    • True
  • HTTPS uses a lot of serverr esources
  • Proxy caching problems - public caching cannot occur
    • True
  • HTTTPS introduces latencies

Problems with HTTPS and the Lock Icon

• Upgrade from HTTPS to HTTPS
  • SSL stripping attack prevents browser from upgrading, but won’t display any certificate errors
    • aka HTTP downgrading attack
    • Solution is to use HSTS (Strict Transport Security)
      • Policy set for max of 1 year, on server
      • Header tells browser to always connect over HTTPS
      • Subsequent visits must be over HTTPS (self signed certs result in an error)
        • Browser refuses to connect over HTTP or if self-signed cert, requires that entire site be served over HTTPS
      • HSTS flag deleted when users “clears private data”: a tradeoff between security and privacy
• Forged certificates
  • Wrong issuance
  • DigiNotar example
  • Many other examples
  • Breaks the whole cert trust model
  • Functions like a man in the middle attack
  • 

Solutions to Certificate Issues

• Dynamic HTTP public-key pinning
  • Let a site declare CAs that can sign its sert (similar to HSTS)
  • On subsequent HTTPS visits, browser rejects certs issued by other CAs
  • TOFU: Trust On First Use
  • There’s an HPKP extension (HTTP header from server) to facilitate this
• Certificate Transparency
  • Idea: CAs must advertise a public log of all certs they issued
  • Browser will only use a cert if it is published on log server
    • Efficient implementation using Merkle hash trees
  • Companies can scan logs to look for invalid issuance