What is HTTPS, SSL/TLS?


What is HTTPS?

Hypertext transfer protocol secure (HTTPS) is the secure version of HTTP, which is the primary protocol used to send data between a web browser and a website. HTTPS is encrypted in order to increase security of data transfer. This is particularly important when users transmit sensitive data, such as by logging into a bank account, email service, or health insurance provider.

How does HTTPS work?

HTTPS uses an encryption protocol to encrypt communications. The protocol is called Transport Layer Security (TLS), although formerly it was known as Secure Sockets Layer (SSL). This protocol secures communications by using what’s known as an asymmetric public key infrastructure. This type of security system uses two different keys to encrypt communications between two parties:

  1. The private key - this key is controlled by the owner of a website and it’s kept, as the reader may have speculated, private. This key lives on a web server and is used to decrypt information encrypted by the public key.
  2. The public key - this key is available to everyone who wants to interact with the server in a way that’s secure. Information that’s encrypted by the public key can only be decrypted by the private key.

What is SSL/TLS?

The SSL and TLS protocols provide communications security over the internet and allow client/server applications to communicate in a way that is confidential and reliable. The protocols have two layers: a Record Protocol and a Handshake Protocol, and these are layered above a transport protocol such as TCP/IP. They both use asymmetric and symmetric cryptography techniques.

An SSL or TLS connection is initiated by an application, which becomes the SSL or TLS client. The application which receives the connection becomes the SSL or TLS server. Every new session begins with a handshake, as defined by the SSL or TLS protocols.

SSL/TLS handshake

The SSL or TLS handshake enables the SSL or TLS client and server to establish the secret keys with which they communicate.

Following are the high-level tasks achieved by SSL/TLS handshake:

  • Agree on the version of the protocol to use.
  • Select cryptographic algorithms.
  • Authenticate each other by exchanging and validating digital certificates.
  • Use asymmetric encryption techniques to generate a shared secret key, which avoids the key distribution problem. SSL or TLS then uses the shared key for the symmetric encryption of messages, which is faster than asymmetric encryption.

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In overview, the steps involved in the SSL handshake are as follows:

  1. The SSL or TLS client sends a “client hello” message that lists cryptographic information such as the SSL or TLS version and, in the client's order of preference, the CipherSuites supported by the client. The message also contains a random byte string that is used in subsequent computations. The protocol allows for the “client hello” to include the data compression methods supported by the client.
  2. The SSL or TLS server responds with a “server hello” message that contains the CipherSuite chosen by the server from the list provided by the client, the session ID, and another random byte string. The server also sends its digital certificate. If the server requires a digital certificate for client authentication, the server sends a “client certificate request” that includes a list of the types of certificates supported and the Distinguished Names of acceptable Certification Authorities (CAs).
  3. The SSL or TLS client verifies the server's digital certificate.
  4. The SSL or TLS client sends the random byte string that enables both the client and the server to compute the secret key to be used for encrypting subsequent message data. The random byte string itself is encrypted with the server's public key.
  5. If the SSL or TLS server sent a “client certificate request”, the client sends a random byte string encrypted with the client's private key, together with the client's digital certificate, or a “no digital certificate alert”. This alert is only a warning, but with some implementations, the handshake fails if client authentication is mandatory.
  6. The SSL or TLS server verifies the client's certificate.
  7. The SSL or TLS client sends the server a “finished” message, which is encrypted with the secret key, indicating that the client part of the handshake is complete.
  8. The SSL or TLS server sends the client a “finished” message, which is encrypted with the secret key, indicating that the server part of the handshake is complete.
  9. For the duration of the SSL or TLS session, the server and client can now exchange messages that are symmetrically encrypted with the shared secret key.

References

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