TLS - Transport-Layer Security

Asymmetric Key Authentication:
     - Relies on two keys: Public key, Private key
     - Both keys are related, but impossible (computationally infeasable) to
       identify the private key based on the public key [1][2]
     - The public key can be distributed publicly
          - Used to encrypt message to the owner of the private paired key
          - Used to verify signatures from the private key
     - The private key is kept secret
          - Used to decrypt message from the public paired key
          - Used to as a digital signature

Basics of an Asymmetric Key handshake:
     1. Client reaches out to server, requesting a secure connection
     2. Server acknowledges request, sends back it's public key
          - This is commonly known as a certificate. Often signed by a
            third-party to ensure it is what it's supposed to be.
     3. Client uses this public key to encrypt a secret, and sends the package
     back to the server.
     4. The server then uses it's private key to decrypt the public-key
     encrypted secret, and uses that secret hence forth to encrypt all traffic.
     5. A private connection is now established.

Basics of Certificates
     1. A certificate is a vessel for a server to provide authentication
     information.
     2. Typically a certificate will contain the following information:
          - A UUID of the certificate itself
          - The subject of the certificate
          - The signature, and signature algorithm used
          - The issuer of the certificate, as well as dates when it is valid
          - The purpose of the key
          - The thumbprint, and algorithm, used to hash the key
          - The public key itself
     3. Certificate Authorities act as a third part to verify the integrity of
     public keys. 

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TLS: An examination into the Security of the Internet, Part 1


TLS, more often referred to as SSL, is the means by which a secure connection is
established over a computer network. Most often these connections are
established over the Internet, between a client (ex., web browser) and a server
(ex., a web site). In the specific case of web sites, HTTP is layered on TLS/SSL
to ensure a secure and private connection; HTTPS is not a separate protocol,
rather a combination or protocols. Establishing a secure connection thus
requires a few steps:

Step 1: Establish an TCP connection
     TCP is a transport-layer protocol that establishes a connection which is
     reliable and fault-tolerant. As opposed to UDP, TCP will seek to verify that
     information is transferred successfully and as intended. This is important
     for the next step. The connection is established as follows:

          1. A client will contact a server and announce it wishes to establish
          a connection. (Called a SYN, short for synchronize). This SYN is a
          number, stored as part of the TCP header; we'll call it A.
     
          2. The server will respond to the client announcing it has received
          the client's wish, and also state that it wishes to establish a
          connection. (Called a SYN-ACK, short for synchronize-acknowledge).
          Ths SYN-ACK is actually two values: One is the ACK value, A+1. The 
          second is the server's SYN value, which we'll call B.
          
          3. The client then acknowledged the server's wish, thus establishing a
          connection (Called simply an ACK). Because it was expecting an ACK
          value in step 2, and expecting that value to be A+1, it can verify
          that this connection is the same as the one it started.  Additionally
          and similarly, the ACK which gets returned to the server as B+1.

          4. At this point, the connection is established. Both client and
          server have assured themselves of a proper connection thanks to the
          three-way handshake described above. From this point forward, the
          server has bound a specific port to listen for any further
          communications with the client. 

Step 2: Establish an SSL/TLS Connection
     TLS, Transport Layer Security, is appropriately on the same layer as TCP,
     the transport layer. TLS relies on public key authentication to establish a
     secure connection between the aforementioned client and server. The
     connection is established as follows:

          1. A client will announce to the server it wishes to establish a
          TLS/SSL connection. It will include information such as it's TLS/SSL
          version, the ciphersuites it wishes to use, and which compression
          methods it wishes to use.

          2. The server then uses the highest possible TLS/SSL version, chooses
          one of the ciphersuites available to the client, chooses one of the
          compression methods available to the client, and sends it's
          certificate. A certificate is basically a container for a server's
          public key, but with many additional details, and often signed by a
          certificate authority, to further verify the certificate contains the
          proper key. More on certificates in part 2.

          3. The client then uses the server's public key to encrypt a secret.
          This secret is then sent back to the server.

          4. The server decrypts the secret with it's private key. This secret
          is now shared by only the client and the server, and from this point
          on is used for symmetric encryption. 

     From this point forward, the connection is encrypted and secure from
     external threats. Of course, this all depends on a trusted certificate and
     proper encryption algorithms.

Step 3: Establish an HTTP connection
     HTTP is an application-layer protocol, and is responsible for translating
     the information from the transport layer into information used by an
     application. Your web browser, for example, will utilize HTTP to translate
     a bunch of hexidecimal information into alpha-numeric information, which is
     then formatted and presented to you as a web page. Security is previously
     established thanks to TLS/SSL, and reliability is previously established
     thanks to TCP.

Of course this is an over-simplification of the whole stream. TCP itself is
worthy of many research papers. However, many sources on TCP already do an apt
job of explaining it in understandable ways. I felt that SSL was less-so
explained, so I wanted to dive a bit more into that. In Part 2, I'll dive more
into the TLS handshake, what certificates are and how they play a role, and the
importance of secure ciphersuites for keys.

          
============================
Sources
     [1] https://en.wikipedia.org/wiki/Public-key_cryptography
     [2]
     https://security.stackexchange.com/questions/6290/how-is-it-possible-that-people-observing-an-https-connection-being-established-w
     [3]
     https://security.stackexchange.com/questions/20803/how-does-ssl-tls-work