Welcome to the next step in our journey to secure Node.js applications. In this lesson, we will delve into the concept of the Chain of Trust in TLS. Previously, we were introduced to digital certificates, focusing on their necessity for TLS. Now, we will build on that foundation to understand how the chain of trust is used to secure communications over HTTPS. By the end of this lesson, you will be able to establish a certificate chain and verify certificates, reinforcing the security of your applications.

The chain of trust is a critical concept in TLS that ensures the authenticity and integrity of certificates used in secure communications. It involves a hierarchy of certificates, starting from a trusted root CA down to the server certificate. Each certificate in the chain is signed by the one above it, creating a chain of trust. This structure allows clients to verify the legitimacy of a server's certificate, ensuring that they are communicating with the intended party. The chain consists of the following components:

1. Root CA: The root CA is the top-most certificate in the chain and is self-signed. It is inherently trusted by clients and browsers, forming the foundation of the chain of trust. The root CA's public key is widely distributed and used to verify the signatures of intermediate CAs.

2. Intermediate CAs: These are optional but commonly used to create a more manageable and secure hierarchy. Intermediate CAs are signed by the root CA or another intermediate CA, and they sign the server certificates. This delegation allows the root CA to remain offline and secure, reducing the risk of compromise.

3. Server Certificate: This is the certificate presented by the server during a TLS handshake. It is signed by an intermediate CA and contains information about the server's identity, such as its domain name. The server certificate is what clients verify to ensure they are communicating with the correct server.

When a client connects to a server over HTTPS, the server presents its certificate chain. The client then performs the following steps to verify the chain of trust:

1. Certificate Path Validation: The client checks each certificate in the chain, starting from the server certificate up to the root CA. It ensures that each certificate is signed by the one above it and that the signatures are valid.

2. Trust Anchor Verification: The client verifies that the root CA is in its list of trusted root certificates. If the root CA is trusted, the entire chain is considered valid.

3. Certificate Revocation Check: The client checks if any certificate in the chain has been revoked. This is done using Certificate Revocation Lists (CRLs) or the Online Certificate Status Protocol (OCSP).

4. Expiration and Validity Check: The client ensures that each certificate in the chain is within its validity period and has not expired.

By successfully completing these steps, the client can trust the server's identity and establish a secure connection. Understanding and implementing the chain of trust is essential for ensuring secure communications and protecting sensitive data from unauthorized access.

A Certificate Signing Request (CSR) is a crucial component in the process of obtaining a digital certificate. It is a message sent from an applicant to a CA to apply for a digital identity certificate. The CSR contains information about the entity requesting the certificate and the public key that will be included in the certificate. Here’s a breakdown of the key elements involved in a CSR:

1. Public Key: The CSR includes the public key that will be part of the certificate. This key is used by others to encrypt data that only the certificate holder can decrypt with their private key.

2. Distinguished Name (DN): This is a set of attributes that uniquely identify the entity requesting the certificate. It typically includes the Common Name (CN), Organization (O), Organizational Unit (OU), Country (C), and other relevant details.

3. Signature: The CSR is digitally signed by the applicant using their private key. This signature ensures the integrity of the CSR and verifies that the applicant possesses the corresponding private key.

4. Optional Extensions: CSRs can also include optional extensions, such as specifying the intended use of the certificate or additional attributes.

To create a CSR, you typically use a tool like OpenSSL. Here’s a basic command to generate a CSR:

In this command, server.key is the private key file, and server.csr is the output CSR file. The -subj option specifies the Distinguished Name attributes.

As you might have noticed, the commands for generating a self-signed certificate and for generating a CSR differ only by the parameter -x509.

Once the CSR is generated, it is submitted to a CA, which verifies the information and issues a digital certificate if the request is approved. Understanding CSRs is essential for managing digital certificates and ensuring secure communications in your applications.

Creating a self-signed chain of trust involves generating a root certificate, an intermediate certificate, and a server certificate using Certificate Signing Requests (CSRs). This process is useful for testing and development environments where a trusted third-party CA is not necessary. Here's how you can create a self-signed chain of trust using OpenSSL:

1. Generate a Root Certificate:

   • Create a private key and a self-signed root certificate:

2. Generate an Intermediate Certificate:

   • Create a private key for the intermediate CA:
   • Create a CSR for the intermediate CA:
   • Create an extension file that defines the non-self-signed intermediate cert as a CA:
   • Sign the intermediate CSR with the root certificate to create the intermediate certificate:

3. Generate a Server Certificate:

   • Create a private key for the server:
   • Create a CSR for the server:
   • Sign the server CSR with the intermediate certificate to create the server certificate:

4. Verify the Server Certificate:

   • Verify the server certificate by giving the root CA, as well as the other untrusted certificates.

By following these steps, you create a self-signed chain of trust that includes a root CA, an intermediate CA, and a server certificate. This setup allows you to simulate a real-world certificate hierarchy in a controlled environment, facilitating the testing and development of secure applications.

The chain of trust is fundamental to secure communications over the internet. By understanding and implementing it, you ensure that your applications can verify the identity of servers and clients, preventing unauthorized access and data breaches. As you proceed to the practice section, remember that mastering the chain of trust is a crucial step in building secure and reliable applications. Let's get started!