CA Signing vs Certificate Public Key

TL;DR

A CA (Certificate Authority) uses a signing algorithm to create digital signatures for certificates, verifying their authenticity. The certificate itself contains a public key algorithm which is used for encryption/decryption and secure communication. They are different but related – the CA’s signature proves the public key within the certificate is trustworthy.

Understanding the Difference

1. CA Signing Algorithm: This is how the CA signs the certificate. Think of it like a notary stamping a document. The CA uses its private key and a signing algorithm (e.g., SHA256withRSA, ECDSA with SHA-256) to create a digital signature.
   • Purpose: To prove that the certificate is genuine and hasn’t been tampered with.
   • Example Algorithms: SHA256withRSA, SHA384withRSA, ECDSA with SHA-256, ECDSA with SHA-384.
2. Certificate Public Key Algorithm: This defines the type of encryption used by the certificate.
   • Purpose: To encrypt data, verify digital signatures (using the corresponding private key), and establish secure connections.
   • Example Algorithms: RSA, ECDSA, EdDSA. These determine how strong the encryption is and what protocols can be used.

How They Work Together

1. Certificate Creation: When a certificate is created, it includes information like the website’s domain name, the public key, and details about the issuing CA.
2. CA Signing Process: The CA takes all this information, hashes it (creates a unique fingerprint), and then encrypts that hash using its private key and the chosen signing algorithm. This encrypted hash is the digital signature.

   openssl x509 -in certificate.pem -text -noout | grep Signature Algorithm

3. Verification: When your browser connects to a website, it receives the certificate and verifies the CA’s signature using the CA’s public key (which is pre-trusted in your browser). If the signature is valid, it confirms that the certificate hasn’t been altered.
   • If the verification fails, your browser will show a security warning.

Practical Example

Imagine you have an RSA certificate with a 2048-bit key.

• Public Key Algorithm: RSA (2048-bit) – This is the encryption method used within the certificate.
• CA Signing Algorithm: SHA256withRSA – The CA uses this to sign the certificate, ensuring its authenticity.

Checking Certificate Details

1. Using a Web Browser: Most browsers allow you to view certificate details by clicking on the padlock icon in the address bar.
   • Look for sections like “Signature Algorithm” and “Public key algorithm”.
2. Using OpenSSL (Command Line): You can use OpenSSL to inspect a certificate’s details.

   openssl x509 -in your_certificate.pem -text -noout

   This command will display the certificate’s information, including both the signing algorithm and the public key algorithm.

Key Takeaways

• The CA’s signing algorithm proves the certificate is valid.
• The certificate’s public key algorithm defines how secure communication happens.
• They work together to establish trust and enable encrypted connections.