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Computer Systems - Lecuture 17 - TLS - Transport Layer Security

Secure Communication

  • Standards get outdated really fast
  • Initial attempt was Secure Network Programming (SNP)
    • Basically like an API for sockets to retrofit security
  • SSL (Secure Socket Layer)
    • Developed by netscape
    • 1.0 was never released because of security flaws
    • 2.0 was first to be made public
    • 3.0 complete redesign, basis on which TLS was designed

TLS - Transport Layer Security

  • TLS 1.0 1999 - upgrade to SSL 3.0, further improvements to security
  • TLS 1.1 2006 - further upgrades
  • TLS 1.2 2008 - updated primitives
    • You do a "downgrade" attack by pretending you're a older machine that only works with SSL 2.0
    • Added support for AES and various advanced modes of AES
    • 2011 update attempts to fix the downgrade attacks
  • TLS 1.3 2018 draft - significant differences, not backwards compatible, removing weaker crypto primitives, enforcing forward secrecy
  • Problem for older devices, do we let them still try to connect with a crappier protocol or just deny them service

TLS Handshake

  1. TCP connection is established
  2. Client sends ClientHello to server asking for secure connection, listing its support cipher suites
    • RSA, Elliptic curve etc.
  3. Server responds with ClientHello and the cipher suite it supports
    • This also includes the certificate and can request the client send its certificate - used in VPNs and organisational networks
    • Vulnerability to downgrade hack where you decide the cipher suite
  4. Client confirms validity of certificate
  5. Client generates session key
    • Either by picking a random key and encrypting it with the public key of the server or
    • Running the Diffie-Hellman Key Exchange protocol that provides better security
  6. Handshake concludes and both parties share a key that is then used for encrypting/decrypting messages
  • More modern systems will use the Diffie-Hellman key exchange method because of forward secrecy

Diffie-Hellman Key Exchange

  • Provides forward secrecy
    • Means that if the long term key (private key) was compromised, and an attacker had recorded all traffic they would still not be able to decrypt the messages
    • Its protection is still limited to conventional compromises (won't protect against a quantumn computer)
    • In essense the Diffie-Hellman Key exchange works by sending information in a way that both parties can calculate a shared key without ever excplicitly communicating the key
  • Look at Tutorial 11 for more information

Diffie-Hellman Key Exchange Calculation

  • Uses similar techniques to Discrete Log Cryptography Elgamal
  • How it works:
    • Generate some public information like with public keys
    • Large prime number p
    • A generator g
    • Alice will pick a secret value a
      • Computes A = g^a mod p
      • Sends A to Bob
    • Bob will pick a secret value b
      • Computes B = g^b mod p
      • Sends B to Alice
    • Alice calculates the secret s by:
      • s = B^a mod p = g^ba mod p
    • Bob calculates the secret s by:
      • s = A^b mod p = g^ab mod p
    • And it works because they equal each other:
      • g^ba mod p = g^ab mod p

TLS

  • Should always be between the exact process and the exact server
  • It seems to be good
    • It's on the basis that the public are told to check for the padlock to know their communication is secure and protected between their web browser and the server, and is with the genuine server
  • Is our confidence misplaced?
    • Do you trust TLS?

Local TLS Interception

  • Create own local CA and install certificate into root stores
  • Local proxy can intercept TLS connections
    • Acts as a man in the middle
    • Dynamically issues fake certificates using the local CA
    • Virtually invisible to the user
  • Common practice for Anti Virus/Web Protection software
    • Avast, Paspersky, ESET
    • Necessary to monitor and analyse TLS traffic
    • Most attack traffic goes over TLS cause they want to make it harder to detect
  • Not only trusting tools for privacy, also security
  • When acting as Man In The Middle, the local proxy is performing the certificate validation, not the browser
  • Too often this has shown to be flawed:
    • Susceptibility to FREAK
    • Not supporting modern features (OCSP stapling)
    • Downgrade attacks
  • Worst case was superfish/eDellRoot
    • Same root certificate installed on all machines - same private key that was locally stored
    • Once compromised anyone could impersonate websites/code signing
    • Superfish also had faulty certificate checking
    • Proxy was Komodia
      • People don't give a crud about proxy
    • Dad nearly had this

Network Level TLS Interception

  • Many companies intercept TLS connections using the same approach
  • Private network proxy acts as a Man In the Middle and generates fake certificates
  • Domain controllers install root certificate on machine
  • If you don't control the machine, you don't control the trust
    • If you had a machine outside of your control then you can believe that someone has intercepted all of your shit
  • Same problems with poor validation and old cipher suites
    • Hardware routers hardly ever get updated
    • Browsers update like every day for security
    • Routers never really get updated

Server-side problems

  • Often we're not actually connecting directly to the server
  • Web servers increasingly using TLS proxies or Load Balancers
    • Protects against hacking and DDoS
    • Prevalent on cloud services
  • Problem is you don't know what's beyond the TLS proxy
    • Could be shit
  • Proxy has certificates for many different sites
  • Backhaul may not be over a secure connection
  • Must trust proxy for privacy
  • Cannot determine routing of traffic
    • Everything from proxy beyond is hidden
  • Fundamentally breaks concept of end-to-end encryption
  • Proxies analyse and inject content into pages

Cloudbleed

  • Cloudflare - large CDN and TLS proxy service
  • Error in HTML parser led to memory being leaked into returned pages
  • This in turn led to that data being crawled by Google/Bing/etc.
  • Potential breaches of authentication of tokensm passwords, private messages
  • Many big name sites use Cloudflare: Uber, Yelp, OkCupid, Patreon
  • Demonstrates that TLS is being broken

iVote WA - Incapsula

  • An absolute clusterfuck
  • Whilst analysing the WA deployment of iVote we discovered they were running the service through Incapsula
  • They didn't pay for the corporate version and ended up sharing a service with a bunch of shady sites
  • Weird custom encryption
  • For some reason both registration and vote collection were going through the same proxy
  • Their private key turned up everywhere
  • Incapsula injects JavaScript into the page
    • Gathers analytics information
    • Obfuscated to make it difficult to see what is being included
    • Malicious attackers could have injected javascript in string evaluation
  • Incapsula sets persistent cookies
  • Encrypted partial vote is not protected by Public Key Crypto
    • Is only protected by an AES key which is only protected by VoterNo and PIN - one of which the proxy could know
    • Easy to bruteforce
  • They didn't care lol

Have things changed?

  • Nope, we're all screwed cause government services keep screwing up and using TLS proxies

  • New Zealand Online Census

    • Data wasn't even encrypted before sending it
    • NZ falsely claimed that it was an end-to-end connection
    • Stats NZ
      • "You can verify that you are communicating with Stats NZ by selecting the key or the lock"
        • They paid for the extended validation lol
        • Still handed over the private key to Incapsula
      • "All census forms submitted to Stats NZ won't be read by anyone other than the authorised staff at Stats NZ"
        • lol nope Incapsula is reading it???

  • Even with all this technology, it is depending on the person to not be an idiot and deploy it correctly