Advanced Cryptography Interview Questions

Cryptography plays a critical role in securing assets in digital space. Cryptography plays a vital role in securing networking by providing confidentiality, integrity, and authentication. It prevents unauthorized access and tampering of data transmitted across networks. It ensures secure communication in various applications. These include SSL/TLS for web traffic, VPNs for private connections, email encryption, and Wi-Fi security protocols like WPA. Cryptography uses techniques such as encryption, hashing, and digital signatures. These methods help maintain privacy, verify identities, and safeguard sensitive information in today’s interconnected world.

This blog provided Interview Questions and Answers on Advanced Cryptography. These may be asked in a job interview for a cyber security role.

Q1. What is Blowfish in the field of cryptography?

Ans: Blowfish is a symmetric key block cipher. It has a 64-bit block size. The key length varies from 32 bits to 448 bits. This algorithm is developed in 1993 by Bruce Schneier and is able to replace the DES algorithm. As of now, blowfish is secure against any known vulnerabilities.

Q2. What is Skipjack in the field of cryptography?

Ans: Skipjack is an encryption algorithm that uses an 80-bit key to encrypt or decrypt 64-bit block data.

Q3. What is Twofish in the field of cryptography?

Ans: Blowfish is a symmetric key block cipher of 128-bit size and key length varies up to 256 bits. Twofish is an improved version of the Blowfish algorithm.

Q4. What is the Diffie-Hellman Algorithm?

Ans: Diffie-Hellman algorithm used to secure communication on a public channel. This algorithm is based on Elliptic Curve Cryptography (ECC) and uses the concept of elliptic curves over finite fields.

It allows two parties to securely exchange cryptographic keys over a public channel. By agreeing on public parameters and generating private keys, they compute public keys and exchange them. Each party then derives the same shared secret, which can be used for secure communication, ensuring confidentiality.

Q5. What are Public and Private keys in the field of Cryptography?

Ans: Public key and Private key both are used as a key pair generated by an asymmetric algorithm. The public key is shared openly for encryption or verification. The private key stays secret and is used for decryption or signing. This pair ensures secure communication, data integrity, and authentication.

Q6. What is the importance of the Key Length of Encryption Algorithm?

Ans: Key length is a critical aspect to determine the strength of the encryption algorithm. A longer key length increases the number of possible combinations. This makes it more difficult for attackers to break the encryption through brute force attacks. Shorter key lengths are more vulnerable to such attacks, while longer keys provide stronger protection against modern computational capabilities.

Q7. What is the maximum Key Length of RSA, DSA, and Elliptic Curve Cryptography?

Ans:

Algorithm	Maximum Key Length
RSA	4096 bits
DSA	2048 bits
Elliptical Curve Cryptography	256 bits

Q8. Explain the concept of Hash Functions.

Ans: A hash function converts arbitrary data to fixed-size values. These are also called hash values or digest. These functions are used to check the integrity of data.

Q9. What is Public Key Infrastructure (PKI)?

Ans: A Public Key Infrastructure (PKI) is a system for the generation, distribution, and revoke of Digital Signature Certificates (DSC).

Q10. List out some Cryptographic Attacks.

Ans: Below are some cryptographic attacks:

• Birthday Attack: A cryptographic attack exploits the birthday paradox. It aims to find two different inputs that produce the same hash value, known as a collision. This weakens hash function security by making it easier to find collisions faster than brute force.
• Hash Function Security Summary: Secure hash functions are resistant to collisions, preimage, and second preimage attacks. They provide unique fixed-size outputs for distinct inputs, ensuring data integrity and authenticity. However, weaknesses in older hash algorithms (e.g., MD5, SHA-1) can compromise security.
• Rainbow Table: A precomputed table used for reverse-engineering hashed passwords by comparing hashes to known plaintext values. It speeds up brute force attacks by saving computation time. However, salts or more complex hash functions can thwart it.
• Side Channel Attacks: These attacks exploit physical aspects of cryptographic systems. They use timing information, power consumption, or electromagnetic leaks. Their goal is to gain insights into secret keys or data. These attacks bypass algorithmic security by targeting implementation flaws.

Q11. What are the common applications of cryptography to secure networking?

Ans:

• SSL/TLS (Secure Sockets Layer / Transport Layer Security): Secures communication over the internet by encrypting data. It is transmitted between clients and servers. This ensures confidentiality, integrity, and authentication.
• VPN (Virtual Private Network): Uses encryption protocols, like IPsec, to create secure tunnels over untrusted networks. This provides private communication over public internet connections.
• SSH (Secure Shell): It secures remote administration of systems by encrypting the data transmitted between client and server. This protection helps against eavesdropping and man-in-the-middle attacks.
• IPsec (Internet Protocol Security): Secures IP communications by authenticating and encrypting each IP packet. This method is commonly used in VPNs for secure site-to-site and client-to-site connections.
• Email Encryption (e.g., PGP, S/MIME): Protects email privacy by encrypting message content, ensuring that only the intended recipient can read the email.
• HTTPS (HyperText Transfer Protocol Secure): This is a protocol for secure communication over the internet. It utilizes SSL/TLS to encrypt HTTP traffic. This ensures data confidentiality and integrity for websites and online transactions.
• Digital Signatures: These are used in various networking protocols. They ensure the authenticity and integrity of data. They are often used in software distribution, email, and secure transactions.
• Wi-Fi Security (WPA2/WPA3): Wi-Fi Protected Access (WPA) protocols use cryptography to protect wireless networks from unauthorized access and eavesdropping.

Q12. What is WPA encryption?

Ans: Wi-Fi Protected Access (WPA) is a security standard introduced in 2003 to secure wireless network systems. WPA replaced Wired Equivalent Privacy (WEP) as this protocol offers more security in user authorization and managing security keys. WPA uses Temporal Key Integrity Protocol (TKIP) to secure wireless traffic. WPA is now obsolete as WPA2 provides stronger encryption.

Q13. How are digital signatures generated and verified?

Ans:

• Generation: A digital signature is created using the sender's private key. The sender first generates a hash of the document or message and then encrypts the hash with their private key. The resulting encrypted hash is the digital signature.
• Verification: The recipient uses the sender's public key to decrypt the digital signature, revealing the hash value. They then compute the hash of the received document and compare it to the decrypted hash. If both hashes match, the signature is valid, confirming both the document's integrity and the sender's authenticity.

Q14. What is Safer in the field of cryptography?

Ans: "Safer" is a reference to a family of cryptographic algorithms, notably the Safer K-64 and Safer K-128 block ciphers. These algorithms were designed as alternatives to older encryption schemes like DES. Safer algorithms provide a secure method for encrypting data and have a higher resistance to certain attacks.

Q15. What is a One-time Pad?

Ans: A One-time Pad (OTP) is a theoretically unbreakable encryption scheme. It requires a key that is as long as the message, is used only once, and is completely random. When the key is XORed with the plaintext message, it produces the ciphertext. The key must remain secret, and it must be securely shared and never reused.

Q16. What is a Birthday Attack?

Ans: A Birthday Attack is a cryptographic attack that exploits the mathematics behind the birthday paradox. It targets hash functions by finding two different inputs that produce the same hash (collision). As the number of possible inputs grows, the probability of finding a collision increases rapidly. This makes it easier to break some cryptographic systems.

Q17. What is the Secure Hash Algorithm?

Ans: The Secure Hash Algorithm (SHA) is a family of cryptographic hash functions. They are designed to produce a fixed-size output (hash value) from any input data. SHA-256 is a widely used member of this family. It produces a 256-bit hash. It is commonly used in blockchain technologies and digital certificates.

Q18. What are Message Authentication Codes (MACs)?

Ans: A Message Authentication Code (MAC) is a short piece of information. It is used to authenticate a message. It also verifies its integrity. It is typically created by combining the message with a secret key. This is done using a cryptographic hash function or a symmetric encryption algorithm. The MAC ensures that the message has not been altered and that it comes from a trusted source.

Q19. How do digital timestamps support Digital Signatures?

Ans: Digital timestamps provide a way to prove the exact time a digital signature was created. When a digital signature is time-stamped, it helps verify that the signature was applied to a document before a certain event (e.g., the document’s expiration or a specific change). This ensures non-repudiation and supports legal and regulatory compliance.

Q20. Is private key encryption to verify identity a weakness?

Ans: Private key encryption to verify identity is generally considered secure if managed correctly. However, weaknesses can arise if the private key is compromised or if poor key management practices are followed. Additionally, misuse or loss of the private key can lead to identity fraud.

Q21. Can Hash Length Extension attacks be avoided by changing the data structure?

Ans: Yes, Hash Length Extension attacks can be mitigated by using hash functions resistant to such attacks. Examples include SHA-3 or HMAC (Hash-based Message Authentication Code). Changing the data structure alone may not prevent these attacks if the underlying hash function is vulnerable.

Q22. Is it possible to send encrypted data over an unencrypted network while hiding the fact that it is encrypted?

Ans: Yes, using Steganography or Obfuscation techniques, you can hide the fact that data is encrypted. Steganography hides the encrypted data inside other types of files (e.g., images, audio), making it less detectable. However, this does not guarantee security, and proper cryptographic protection is still necessary.

Q23. How password-based encryption works?

Ans: Password-based encryption (PBE) involves using a password to generate a cryptographic key. This key is then used to encrypt the data. Typically, the password undergoes key derivation functions (e.g., PBKDF2, bcrypt) to generate a stronger key before encryption. This ensures that even weak passwords produce strong encryption.

Q24. Is SHA-256 + Salt still safe for password storage?

Ans: Yes, SHA-256 with salt is still safe for password storage. However, it is not the best practice on its own. While adding a salt makes it harder to use recomputed hash tables (rainbow tables) for attacks. It is recommended to use more advanced techniques like bcrypt, Argon2, or PBKDF2 for better security against modern attack methods (e.g. GPU-based brute force attacks).

Q25. What are the privacy advantages of a DNS encryption service such as DNScrypt?

Ans: DNS encryption services like DNScrypt protect users’ privacy by encrypting DNS queries between the client and the DNS resolver. This stops third parties, including ISPs and malicious actors, from intercepting DNS traffic. It also prevents tampering. This provides protection against surveillance and man-in-the-middle attacks.

Q26. Why can't hashes be reversed?

Ans: Hash functions are designed to be one-way functions. They produce a fixed-length output (hash) from variable-length input data. There is no feasible way to reverse the hash to retrieve the original input. This happens because information is lost during the hashing process. Multiple inputs may map to the same hash, which are known as collisions.

Q27. How can you encrypt email messages?

Ans: Email messages can be encrypted using protocols like PGP (Pretty Good Privacy) or S/MIME (Secure/Multipurpose Internet Mail Extensions). These protocols use asymmetric encryption. The sender encrypts the message with the recipient’s public key. The recipient decrypts it using their private key.

Q28. Can I slow down a brute force attack by encoding password input data?

Ans: While encoding password input data (e.g., using Base64) can obscure the data during transmission, it does not prevent a brute force attack on the encoded data. To slow down brute force attacks, use more effective methods. These include using slow hash algorithms such as bcrypt or PBKDF2. Additionally, adding a salt and implementing account lockouts or delays can be beneficial.

Q29. How does a Digital Signature Certificate (DSC) work?

Ans: A Digital Signature Certificate (DSC) is an electronic document. A Certification Authority (CA) issues it. It confirms the identity of the certificate holder. It contains the public key, the identity of the certificate holder, and the signature of the CA. DSCs are used to authenticate users, sign documents, and ensure data integrity.

Q30. What are the different classes of Digital Signature Certificates?

Ans: Digital Signature Certificates are typically classified into three classes:

• Class 1: For individual use, to validate the email address.
• Class 2: For individuals and organizations, to validate identities for e-filing and other non-critical purposes.
• Class 3: For high-security purposes, such as online transactions, ensuring stronger authentication.