Transport Layer Security (TLS)

What is SSL?

• Protocol by netscape to enable ecomerce transaction
• Implemented on top of TCP, while alloweing protocols above it to operate unchanged
• End result of correct use: 3rd party observer can only infer
  • Connection end-points
  • Type of encryption
  • Frequency
  • Approximate amount of data sent
  • No modifying / reading actual data

What is TLS? Renaming of SSL (basically, SSL 3.0)

Encryption, Authentication, and Integrity

TLS goal: provide 3 services to apps running above it:

• Encryption: obfuscate what’s sent
• Authenticatoin: verify validity of provided id material
• Data Integrity: detect message tampering

Requirements to establish secure data channel? (encryption)

• Agreement from connection peers on
  • Ciphersuites
  • Keys to encrypt the data
• This is established by the TLS handshake

TLS Handshake: allows peers to negotiate shared secret key over unencrypted channel

• Also lets clients verify server identity (authentication)

Also, have MAC (message authentication code)

• Sign each message with it- 1-way cryptographic hash function
• Use to ensure data integrity

TLS Handshake

In order for client / server to excahnge data oer TLS, must negotiate the encrypted tunnel

• TLS protocol version
• Ciphersuite
• Verify certifacites

The process (in order)

• Runs over TCP- so need to do the TCP 3 way handshake
• Client
  • Sends: specifications in plain text (ie TLS protocol version, supported cipher-suites, other TLS options)
• Server
  • Decides: TLS protocol, ciphersuite- attaches certificate
  • Sends: response
• Client (if happy with certificate)
  • Generates: new symmetric key, encrypts with server’s public key-
  • Sends: message to server, saying to switch to encrypted communication going forward
• Server
  • Decrypts: symmetric key sent by client, checks message integrity by verifying MAC-
  • Sends: encrypted “Finished” message
• Client
  • Decrypts: message with symmetric key, verifies MAC
  • If all is well, tunnel is now established

Takeaways

• Complicated
• Requires TCP handshake + 2 extra round trips

Most of this process is, luckily, handled by the server and browser. We just neeed to provide / configure certificates

ALPN (Application Layer Protocol Negotiation)

How do you use a custom application protocols for network peer communication?

Slow/impractical solution: assign a well-known port to predetermined protocol and configure clients/servers as such.

Instead, we reuse port 443 (which is reserved for secure HTTPS sessions running over TLS)

• TLS gives reliability- end-to-end encrypted tunnel
• But what about how we negotiate the protocol?
  • Could use HTTP upgrade mechanism
  • Better to use ALPN- avoid extra roundtrip

ALPN: TLS extension which introduces support for application protocol negotiation into the TLS handshake. How?

• Client adds ProtocolNameList field (list of support app protocols) into ClientHello message
• Server inspects the ProtocolNameList field and returns ProtocolName field indicated selected protocol as part of the ServerHello message

Server Name Indication (SNI)