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Transport Layer Security

Transport Layer Security (TLS) is a cryptographic protocol designed to provide communications security over a computer network. TLS is often used in conjunction with the Secure Sockets Layer (SSL) protocol to secure HTTP communications, but can be used with other protocols as well. TLS is an IETF standard, and is specified in RFC 5246.

Goals

  • Protocol that provides a secure channel between two endpoints;
  • The channel has three properties:
    • Confidentiality - no one besides the endpoints can see the content of the data transmitted;
    • Integrity - no one besides the endpoints can change the content of the data transmitted;
    • Authentication - at least one endpoint of the channel needs to be authenticated so that the other endpoint has guarantees about who it is connected to.

TLS

TLS Layer

  • The TLS layer is a protocol that runs between the application layer and the transport layer;
  • The unencrypted data is provided by the application layer;
  • The TLS encrypts and authenticates the data it sends/receives from the transport layer;
  • The TLS requires a reliable transport layer, such as TCP.

Layers

  • The TLS is divided into two subprotocols:
    • Handshake Protocol - establishes the secure channel;
    • Record Protocol - transmits the encrypted data.

SubProtocols

Record Protocol

  • Fragments, compresses, authenticates (MAC) and encrypts the data;
  • Same TCP connection, with two independent streams of data - client write and server write;
  • Keys, IVs and sequence numbers are different for each stream;
  • Message repetition is detected by the Sequence Number;
  • Message reflection is detected by the MAC, through keys separated for each direction;
  • Keystream reuse: keys and IVs separated for each direction;
  • Traffic analysis through separate encryption keys.

Cryptographic Schemes

  • Depends on the cipher suite agreed;
  • The cipher suite and the compression algorithms are negotiated by the handshake protocol;
  • A cipher suite defines:
    • The hash function used by the HMAC;
    • The symmetric scheme: supports block or stream modes;
    • Key establishment scheme - RSA or Diffie-Hellman.

Handshake Protocol

  • Responsible for:
    • Negotiation of the operating parameters;
    • Authentication of the endpoints;
    • Establishment of a secure key;
  • Endpoint authentication and key establishment:
    • Authentication is optional on both ends;
    • Supports several cryptographic techniques:
      • Key transport - RSA;
      • Key agreement - DH;

RSA Based

RecordRSA

HTTPS

  • HTTP over TLS;
  • Omission port: 443;
  • Check between the URI and the certificate:
    • subjectAltName extension of type dNSName (if exists);
    • The Common Name field in the Subject field.

Message Repetition and Modification

  • Handshake message modification is detected with the Finished message, which guarantees that both endpoints receive the same message;
  • Handshake message repetition of handshake implies that the Finished message is different for each handshake.

Master Secret

  • The change of keys with RSA implies that the browser uses the server's public key to encrypt the pre master secret;
  • The server decrypts the pre master secret using its private key;
  • This process is secure and guarantees the confidentiality of the pre master secret;
  • Perfect forward secrecy is the property of the handshake that guarantees that, if the private key is compromised, it is not possible to decrypt previous master secrets, and, consequently, it is not possible to decrypt record protocol messages.

TLS Sockets in Java

  • SSLSocketFactory and SSLServerSocketFactory:
    • Obtainment of supported cipher suites by default;
    • Creation of socket instances;
  • SSLSocket and SSLServerSocket:
    • Starts the handshake and receives notifications of its completion
    • Defines the enabled protocols (SSL v3.0, TLS v1.0) and enabled cipher suites;
    • Accept/require client authentication;
    • Obtain the negotiated session;
  • SSLSession:
  • Obtain the negotiated cipher suite;
  • Get the authenticated peer identity and certificate chain.