Tools / SSL Certificate Interview Questions

An SSL certificate is a digital certificate that authenticates a website's identity and enables an encrypted connection. SSL stands for Secure Sockets Layer, a security protocol that creates an encrypted link between a web server and a web browser.

Companies and organizations need to add SSL certificates to their websites to secure online transactions and keep customer information private and secure.

In short: SSL keeps internet connections secure and prevents criminals from reading or modifying information transferred between two systems. When you see a padlock icon next to the URL in the address bar, that means SSL protects the website you are visiting.

To generate an SSL certificate, start by creating a private key using OpenSSL. Next, create a Certificate Signing Request (CSR) which includes details like domain name and organization. Submit the CSR to a Certificate Authority (CA). The CA will validate your identity and issue the SSL certificate. Install this certificate on your server.

A Certificate Authority (CA) in SSL plays a crucial role in the security of web transactions. It issues digital certificates to entities or individuals after verifying their identity. These certificates serve as proof that the entity is who it claims to be, thus fostering trust among users. The CA also maintains a list of issued certificates and their status – valid, expired, or revoked. In case of any discrepancies, browsers can cross-verify with this list.

Websites need SSL certificates to keep user data secure, verify ownership of the website, prevent attackers from creating a fake version of the site, and convey trust to users.

A cipher suite is a set of algorithms for use in establishing a secure communications connection. There are a number of cipher suites in wide use, and an essential part of the TLS handshake is agreeing upon which cipher suite will be used for that handshake.

The SSL/TLS handshake is a series of steps that allows two parties, typically a client and a server, to authenticate each other, agree on encryption standards, and establish a secure channel for transferring data.

1. Introduction (ClientHello): Your browser sends a "ClientHello" message to the server when you request a secure website. This message contains essential information, including the SSL/TLS versions it supports and the cipher suites it can use.

2. Server's Response (ServerHello): The server replies with a "ServerHello" message, including the highest SSL/TLS version and cipher suite both parties support.

3. Server's Credentials: The server presents its digital certificate, verified by a Certificate Authority (CA) such as www.SSL.com, like an ID card providing its authenticity.

4. Client's Verification and Key Generation: Your browser validates the server's certificate. Once verified, it uses the server's public key to encrypt a "premaster secret", a unique session key, and sends it back to the server.

5. Establishing a Secure Connection: The server decrypts the premaster secret with its private key. The server and client then compute the session key, which will be used for symmetric encryption of all communication.

In an SSL/TLS connection, the public and private keys play crucial role in establishing a secure communication channel. The public key is used during the handshake process to encrypt data sent from the client to the server. This encrypted data can only be decrypted using the corresponding private key held by the recipient server. Thus, even if intercepted, the data remains unreadable without the private key. Conversely, the private key is also used for creating digital signatures which are verified using the public key, ensuring authenticity of the sender.

SSL certificates are validated through a process called SSL handshake. When a client connects to an SSL-secured server, it requests the server's public key in its certificate. The client checks if the certificate was issued by a trusted party (known as a Certificate Authority), is not expired, and has not been revoked. It also verifies that the certificate matches the site it's connecting to. If these checks pass, the client uses the public key to encrypt a random symmetric encryption key and sends it to the server with the encrypted URL required and other encrypted HTTP data. The server decrypts the symmetric encryption key using its private key and uses the symmetric key to decrypt the URL and HTTP data.

Certificate Signing Request or CSR is encoded information that contains the applicant's information such as a common name, the name of an organization, email address, city, state, and country. This encoded information is used by the certifying authority (CA) to issue an SSL certificate to the applicant.

CSR is base 64 encoded text to start with "--BEGIN CERTIFICATE REQUEST--" and end with "--END CERTIFICATE REQUEST--" lines.