The Top 30 OpenSSL Interview Questions and Answers for 2023

As technology continues to evolve security has become more important than ever before. OpenSSL is an open source toolkit that implements Transport Layer Security (TLS) and Secure Sockets Layer (SSL) protocols. It is used to provide communication security and data integrity for applications communicating over networks.

With the prevalence of OpenSSL, knowledge of it is becoming increasingly valuable for IT professionals Whether you’re prepping for an interview or just want to expand your skillset, familiarizing yourself with some common OpenSSL interview questions is a great idea

In this article, I’ll provide 30 of the top OpenSSL interview questions along with detailed answers. Read on to learn key concepts, practical applications, troubleshooting techniques and more related to OpenSSL. This comprehensive guide will help you ace your next tech interview!

1. What is OpenSSL and what is it used for?

OpenSSL is an open source project that provides a robust commercial-grade toolkit for Transport Layer Security (TLS) and Secure Sockets Layer (SSL) protocols. It is used to enable secure communications between a client and a server providing encryption of traffic as it flows across a network.

Some of the core uses of OpenSSL include:

• Implementing HTTPS, FTPS, SMTPS and other secure communication protocols.

• Encrypting sensitive data transmission over networks to prevent eavesdropping and data theft.

• Providing certificate and public key infrastructure management for authentication and data integrity.

• Creating and managing digital certificates, public/private key pairs, and certificate signing requests.

• Supporting a wide range of encryption algorithms and cryptographic functions.

2. Explain asymmetric vs symmetric encryption and how OpenSSL uses both.

Symmetric encryption uses a single shared key for both encryption and decryption. It is faster but the key distribution is problematic.

Asymmetric encryption uses a public-private key pair – the public key encrypts data while only the private key can decrypt it. It enhances security but is slower.

OpenSSL utilizes both symmetric and asymmetric encryption:

• For asymmetric encryption, OpenSSL provides algorithms like RSA, DSA, ECC for public-private key pairs.

• For symmetric encryption, it supports AES, DES, RC4, etc. and handles key exchange securely.

• During an SSL handshake, the client and server agree on an asymmetric cipher to exchange keys securely.

• These keys are then used to derive symmetric session keys which encrypt the actual data transmission due to higher speed.

3. How does OpenSSL provide certificate management capabilities?

OpenSSL provides robust certificate management capabilities through tools like:

• openssl req: Generates certificate signing requests (CSRs) and self-signed certificates.

• openssl ca: Implements a Certification Authority (CA) to issue and revoke certificates.

• openssl x509: Examines and generates x.509 certificates. Handles tasks like signing certs, converting formats, printing contents.

• openssl crl: Manages certificate revocation lists (CRLs) to revoke compromised or invalid certificates.

It also provides an API for software developers to integrate certificate management functions into their own applications. Overall, these capabilities allow for creation, validation, revocation and overall lifecycle management of digital certificates within the OpenSSL ecosystem.

4. How does OpenSSL handle encryption and decryption?

OpenSSL handles encryption and decryption in several ways:

• It implements various symmetric encryption algorithms like AES, Blowfish, DES, IDEA etc. Symmetric algorithms use the same key to encrypt and decrypt data.

• For asymmetric encryption, OpenSSL provides support for algorithms like RSA, DSA, and ECC using public-private key pairs.

• OpenSSL stores keys and certificates securely in a keystore. Developers can use the API to encrypt/decrypt data using stored keys.

• During an SSL handshake, the client and server agree on encryption ciphers and exchange keys to establish a secure SSL session.

• OpenSSL APIs allow passing plaintext and obtaining encrypted ciphertext and vice versa.

• Cipher suites handle tasks like checksum validation, compression, and encryption padding to securely encrypt application data.

5. Explain how OpenSSL verifies the identity of certificates.

OpenSSL certificate verification involves the following steps:

• It checks if the certificate is issued and digitally signed by a trusted Certificate Authority (CA).

• It verifies the validity period of the certificate and checks if it is expired.

• It looks for certificate revocation by checking against CRLs.

• It performs hostname verification by matching the domain name against certificate details.

• It checks the chain of trust by walking up the chain of issued certificates up to a trusted root CA.

• It validates certificate integrity by checking the signature against the CA’s public key.

• It performs cryptographic validation of the certificate’s public key using algorithms like RSA, DSA, or ECC.

This multifactor verification process ensures that the certificate is valid, current, trusted, and authentic.

6. How can you generate a self-signed certificate using OpenSSL?

To generate a self-signed certificate using OpenSSL:

1. Generate a 2048-bit private key:

openssl genrsa -out selfsigned.key 2048

1. Create a self-signed certificate valid for 365 days:

openssl req -new -x509 -key selfsigned.key -out selfsigned.crt -days 365

1. Enter details like organization name, location, etc. when prompted.

2. Install the key and certificate on your server.

3. Add the certificate to your trusted root store to avoid trust warnings.

Self-signed certificates are not genuinely trusted but can be useful for testing purposes. For production use, OPT for a CA-signed certificate.

7. How does OpenSSL handle encryption key management?

OpenSSL handles encryption key management through features like:

• Asymmetric key generation – APIs to generate public-private key pairs.

• Secure key storage – Keys stored encrypted in PEM/DER formats with password protection.

• Key distribution – Keys exchanged securely during SSL handshake using asymmetric encryption.

• Key derivation – Master secrets derived from session keys to encrypt application data.

• Key revocation – Lost or compromised keys can be invalidated.

• Key reuse – Keys cached for reuse in new sessions with the same client.

• Perfect Forward Secrecy – Unique keys per session prevent retrospective decryption.

• Key logging – Optional logging of session keys for debugging purposes.

Together these capabilities allow proper lifecycle management of cryptographic keys within OpenSSL.

8. How can you inspect the contents of an x509 certificate on OpenSSL?

To inspect the contents of an X509 certificate using OpenSSL:

1. Obtain the certificate file (CERT.pem).

2. Run the following command:

openssl x509 -in CERT.pem -text -noout

1. This will output the contents of the certificate in a human-readable text format including the issuer, subject, public key, signature algorithm, validity period etc.

2. Alternatively, you can check selective certificate fields by passing parameters like -issuer, -subject, -dates etc.

3. To view the certificate in its original encoded form, remove the -text option.

This allows thorough inspection of the certificate without needing to decode it manually.

9. How does OpenSSL perform certificate revocation checking?

OpenSSL performs certificate revocation checking in two ways:

1. Certificate Revocation Lists (CRLs): CRLs contain lists of revoked certificates published periodically by the issuing Certificate Authority (CA). OpenSSL allows checking certificates against these CRLs.

2. Online Certificate Status Protocol (OCSP): This is a more timely method where the client queries the CA’s OCSP responder to check a certificate’s current status in real-time.

During verification, OpenSSL first checks locally cached CRLs and then makes an OCSP query if needed. OCSP offers better security but may have connectivity issues. CRLs are updated periodically but avoid downtime.

10. How can you debug SSL connection issues with OpenSSL?

Some ways to debug SSL connection issues with OpenSSL:

• Enable SSL debugging by setting SSL_DEBUG=1 as an environment variable.

• Use openssl s_client -connect host:port to test connecting to a server.

• Check encryption cipher issues with openssl ciphers.

• Verify certificate issues using openssl verify cert.pem.

• Confirm handshake details with openssl s_client -tls1 -debug -connect host:443.

• Use Wireshark to inspect traffic between client and server.

• Analyze server logs for SSL-related error messages.

• Check for DNS failures, incorrect SSL ports, firewall blocks, unsupported protocols etc.

• Confirm SSL module is properly installed on the server.

11. Explain SSL/TLS handshake process using OpenSSL.

The SSL/TLS handshake process using OpenSSL involves the following main steps:

1. Client sends “ClientHello” message with supported SSL versions, ciphers, and other data.

2. Server responds with “ServerHello” message selecting appropriate cipher suite.

3. Server sends its public key