CMPUT 404

Web Applications and Architecture

Part 14: TLS

Created by
Abram Hindle (abram.hindle@ualberta.ca)
and Hazel Campbell (hazel.campbell@ualberta.ca).
Copyright 2014-2019.

TLS

Transport Layer Security
- Transport Layer: works on top of TCP/UDP
Security
Formerly called SSL
HTTP inside of TLS is called HTTPS

TLS versions

SSL 2.0: 1995-2011
SSL 3.0: 1996-2015
TLS 1.0: 1999-2020
TLS 1.1: 2006-2020
TLS 1.2: 2008-
TLS 1.3: 2018-

Security

Identity
- Who are you talking to?
Privacy
- Who can hear what you're saying?
Authenticity
- Is what you're hearing really coming from the person you think you're talking to?

Identity

If I go to bank.com how do I know that I'm really talking to bank.com?
Without TLS, I don't: my connection can be intercepted by IP, TCP, and DNS attacks...
Without TLS it's relatively easy to intercept connections meant for a webserver and serve them yourself.

Example: Evil WiFi cafe! I could set up an evil WiFi cafe, where people come, sit down, connect to my "secure" WiFi. Then when someone tries to make a connection to someplace interesting like their bank, my WiFi router can redirect their packets to my own web server! I could make it look just like their bank's website, allowing them to enter their username and password.
Evil WiFi cafe could also simply intercept DNS requests and respond with the address of a fake bank webserver instead.
This is not limited to WiFi: anyone with access to a network where TCP/IP and DNS traffic is going accross, such as one of the many networks that make up the internet, can do the same or similar attacks.
It's not limited to routers either: just sitting next to someone at a wifi cafe would allow you to perform similar attacks

TLS: Identity

With TLS I can ensure the server I'm connected to is the server I meant to connect to.
This is done with certificates.
A certificate works like an ID card.
The server sends a certificate with its domain name (called a "Common Name") and a big number called a public key.
The certificate also includes other details, such as who made it, for example, "Bank Inc."

Okay so the server hands me their certificate, it has a big number and it says it's the bank, but how do I know a fake websitie didn't just make a big number and claim to be the bank?
The certificate also has one crucial thing: a cryptographic signature.
The signature is produced by someone else, and only matches one public key and domain name.

Signed Certificates

The certificate's signature is unique to that certificate.
If you change the public key or the domain name, the signature won't match anymore.
The certificate's signature also comes from a specific entity that made that signature.
It tells us who signed it, and only they could have produced that signature.
No one else can pretend to have signed the certificate, or the signature won't match.

Cryptographic Signatures

I make a private key and a public key as a part of a cryptographic signature algorithm.
I keep my private key secret, known only to me.
I give away my public key to the whole world so everyone knows it's my public key.
I make signatures by combining my private key and some other information I want to sign. In this case its the banks domain name and the bank's different public key.
You can check my signature by using my public key (which I already gave you) and combining it with the signature and information that's been signed to check it.

Certificate Authority (CA)

A certificate authority uses their private key to sign the bank's certificate.
The bank sends their certificate to you so you can check it.
You have the CA's public key.
You combine it with the banks certificate.

Now you know:
- The bank's domain name
- The bank's public key
- That the CA says those are really the bank's
  - But how does the CA know?
The bank sends their certificate when you connect.
The CA's public key is already on your computer. It came with your OS or your browser when you downloaded it.

Identity Problems

You don't have the CA's public key
- There's no way to know if the bank's domain name and public key are correct :(
The CA is evil
- There's no way to know if the bank's domain name and public key are correct :(
The CA's private key got leaked
- There's no way to know if the bank's domain name and public key are correct :(
The bank didn't send a signature from a CA
- There's no way to know if the bank's domain name and public key are correct :(

Identity Problems

Usually these are combatted by the browser makers:
- Only allowing trusted CAs
- Removing bad or leaked CAs
- Not allowing you to view sites without a valid signature from a good CA
- Checking lists of known certificates to make sure someone didn't get a second certificate to pretend to be the bank (Certificate Transparency)
- Checking lists of known bad certificates and certificates that have been "revoked" (CRLs)
- Ensuring the public key is the same as the last time you connected to the site (HPKP)

Certificate and CA in browser

Identity to Privacy

Once the identity of the server is established, and you know the server's public key is really for the server you want to be talking to,
TLS can provide privacy and authenticity.

Privacy

Hiding the content being transmitted back and forth from eavesdropping, snooping, man-in-the-middle, etc.
We want to prevent anyone but us and the server we want to send our information to from being able to read anything exchanged between us and the server

Passwords
Credit card numbers
Search terms
Exact URLs
Cookies
Conversations
Mail
Grades
Interests
Habits
Content
Uploads

Most information that gets transmitted either way

Authenticity

Seperate from privacy
Every message is hashed to ensure it really came from the server (client)
Prevents replay attacks, injection attacks, man-in-the-middle attacks, etc.
Everything encrypted is also authenticated

Handshake

Cloudflare: What happens in a TLS handshake? 2019-04-04

Handshake

TCP Handshake
Client sends TLS "hello"
- TLS version
- Encryption algorithms
- ALPN: Application-Layer Protocol Negotiation
  - e.g., select HTTP/2
- SNI: Server Name Identification
  - Equivalent to Host: header
Server sends TLS "hello"
- Chosen encryption algorithm
- Certificate

Handshake

Client verifies server certificate
- Using CA
Client and server agree on an encryption key
- Either:
Client sends encrypted "ready"
Server sends encrypted "ready"

Perfect Forward Secrecy

Newer versions of TLS
Data remains private even if the server or client later get hacked
Uses Ephemeral Diffie-Hellman Key Exchange

Reducing Latency

Traditional TLS requires at least 2 RTT (round trip time) latency on top of TCP handshake
Newer TLS supports 1 RTT handshake protocol

Cipher Suites

Client and server agree on crypto algorithm to use during handshake
Many different crypto algorithms are available

Cipher Suite Example

TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256 256-bit keys, TLS 1.2

Using TLS 1.2 protocol
Using ECDSA (Elliptic Curve Digital Signature Algorithm)
Using ECDHE (Ephemeral Elliptic Curve Diffie-Hellman Exchange) to agree on an encryption key
Using CHACHA20 encryption algorithm with 256-bit key
Using Poly1305 to check authenticity of most messages
Using SHA-256 to check authenticity of some message

TLS Problems

TLS has had lots of different security problems over the years, typically with fun names!

Renegotiation
FREAK
Logjam
DROWN
BEAST
CRIME
BREACH
padding oracle
Lucky Thirteen
POODLE
Old, insecure crypto algorithms
Truncation
Unholy PAC
Sweet32
Heartbleed
BERserk
Cloudbleed

Avoiding Security Problems

TLS everything
- Make sure all traffic to/from your web app is running over TLS
- Example: it may not help if only login is over TLS because an attacker can replace your TLS-encrypted login page with an unencrypted one by replacing the link on the unencrypted main site

Avoiding TLS Security Problems

Keep software up to date
- No Windows XP
Don't communicate with out-of-date software
Example: if something on your site is meant to be secret, even one bad client can leak the secret!

License

The textual components and original images of this slide deck are placed under the Creative Commons is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Other images used under fair use and copyright their copyright holders.

License

The source code to this slide deck is:


          Copyright (C) 2018 Hakim El Hattab, http://hakim.se, and reveal.js contributors
          Copyright (C) 2019 Hazel Victoria Campbell, Abram Hindle and contributors

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