cryptologist Interview Questions and Answers

1. What is cryptography?

   • Answer: Cryptography is the practice and study of techniques for secure communication in the presence of adversarial behavior. It involves transforming readable information (plaintext) into an unreadable format (ciphertext) and back again using encryption and decryption algorithms, respectively. It also encompasses the study of authentication, digital signatures, and other security primitives.

2. Explain symmetric-key cryptography.

   • Answer: Symmetric-key cryptography uses the same secret key for both encryption and decryption. Examples include AES (Advanced Encryption Standard) and DES (Data Encryption Standard). It's faster than asymmetric cryptography but requires secure key exchange.

3. Explain asymmetric-key cryptography.

   • Answer: Asymmetric-key cryptography uses a pair of keys: a public key for encryption and a private key for decryption. The public key can be widely distributed, while the private key must be kept secret. RSA and ECC (Elliptic Curve Cryptography) are common examples. It's slower than symmetric but solves the key exchange problem.

4. What is a hash function?

   • Answer: A hash function is a one-way function that takes an input of arbitrary size and produces a fixed-size output (hash value or digest). It's crucial for data integrity verification and digital signatures. Characteristics include collision resistance, pre-image resistance, and second pre-image resistance.

5. What is a digital signature?

   • Answer: A digital signature is a cryptographic technique used to verify the authenticity and integrity of data. It uses a private key to create a signature that can be verified using the corresponding public key. This ensures non-repudiation, authentication, and data integrity.

6. Explain the concept of a certificate authority (CA).

   • Answer: A Certificate Authority is a trusted third party that issues and manages digital certificates. These certificates bind a public key to an identity, enabling secure communication and verification of identities online. CAs play a crucial role in public key infrastructure (PKI).

7. What is a block cipher?

   • Answer: A block cipher operates on fixed-size blocks of plaintext, encrypting them one block at a time. AES is a widely used example. Modes of operation, like CBC (Cipher Block Chaining), are used to handle data longer than a single block.

8. What is a stream cipher?

   • Answer: A stream cipher encrypts data bit by bit or byte by byte using a keystream generated from a secret key. RC4 is a well-known example (though now considered insecure). They are often faster than block ciphers for continuous data streams.

9. What are the different modes of operation for block ciphers?

   • Answer: Common modes of operation include ECB (Electronic Codebook), CBC (Cipher Block Chaining), CTR (Counter), GCM (Galois/Counter Mode), and CFB (Cipher Feedback). Each mode has different properties regarding efficiency, security, and error propagation.

10. Explain the difference between confidentiality, integrity, and availability.

    • Answer: Confidentiality ensures that only authorized parties can access data. Integrity ensures that data has not been tampered with. Availability ensures that data and resources are accessible to authorized users when needed. These are the three core principles of information security (CIA triad).

11. What is a known-plaintext attack?

    • Answer: A known-plaintext attack is a cryptanalytic attack where the attacker has access to both the plaintext and the corresponding ciphertext. This information can be used to deduce the encryption key or other secrets.

12. What is a chosen-plaintext attack?

    • Answer: In a chosen-plaintext attack, the attacker can choose the plaintext to be encrypted and obtain the corresponding ciphertext. This allows for a more powerful attack compared to a known-plaintext attack.

13. What is a chosen-ciphertext attack?

    • Answer: A chosen-ciphertext attack gives the attacker the ability to choose ciphertexts and obtain their corresponding plaintexts. This is a more powerful attack than known- or chosen-plaintext attacks.

14. What is differential cryptanalysis?

    • Answer: Differential cryptanalysis is a cryptanalytic technique that studies the propagation of differences through a cryptographic algorithm. It analyzes how differences in the input affect differences in the output to find weaknesses.

15. What is linear cryptanalysis?

    • Answer: Linear cryptanalysis is a known-plaintext attack that uses linear approximations to describe the behavior of a block cipher. It tries to find linear relationships between the plaintext, ciphertext, and key bits.

16. Explain the concept of perfect secrecy.

    • Answer: Perfect secrecy, as defined by Claude Shannon, means that the ciphertext reveals no information about the plaintext beyond the length of the message. This requires a key as long as or longer than the message itself.

17. What is a key exchange protocol? Give an example.

    • Answer: A key exchange protocol is a method for two or more parties to securely agree on a shared secret key over an insecure channel. Diffie-Hellman is a classic example.

18. What is a public key infrastructure (PKI)?

    • Answer: PKI is a system that provides the infrastructure for creating, managing, distributing, using, storing, and revoking digital certificates and managing public-key cryptography.

19. What is elliptic curve cryptography (ECC)?

    • Answer: ECC is an approach to public-key cryptography based on the algebraic structure of elliptic curves over finite fields. It offers comparable security to RSA with smaller key sizes, making it suitable for resource-constrained devices.

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