Cryptography

Correct Answer: SHA-1 is vulnerable to collision attacks, allowing attackers to forge certificates

The correct answer is that SHA-1 is vulnerable to collision attacks, allowing attackers to forge certificates. SHA-1 (Secure Hash Algorithm 1) was once widely used for digital signatures and certificate validation, but it has been proven to be cryptographically weak. In 2017, researchers demonstrated the first practical collision attack against SHA-1, meaning they could create two different inputs that produce the same hash output. For digital certificates, this vulnerability means an attacker could potentially forge a certificate that has the same hash value as a legitimate certificate, enabling various attacks including man-in-the-middle attacks. As for the other options: SHA-1 creating certificates that expire too quickly is incorrect. The expiration of certificates is determined by the certificate authority during issuance, not by the hashing algorithm used. SHA-1 requiring more computational resources than alternatives is also incorrect. In fact, SHA-1 is computationally less intensive than newer algorithms like SHA-256, which is one reason organizations might have been reluctant to upgrade (despite the security benefits). SHA-1 producing longer key lengths incompatible with modern browsers is incorrect. SHA-1 doesn't determine key length; it produces a 160-bit hash value regardless of input size. Also, the compatibility issue with browsers is related to browsers actively rejecting SHA-1 certificates for security reasons, not due to key length incompatibility.

Correct Answer: Caesar cipher with shift value of 3

To identify the encryption method, I need to analyze the transformation pattern from plaintext to ciphertext. Examining 'Hello' → 'Khoor': H → K (shift 3 letters forward) E → H (shift 3 letters forward) L → O (shift 3 letters forward) L → O (shift 3 letters forward) O → R (shift 3 letters forward) And 'World' → 'Zruog': W → Z (shift 3 letters forward) O → R (shift 3 letters forward) R → U (shift 3 letters forward) L → O (shift 3 letters forward) D → G (shift 3 letters forward) This consistent pattern of shifting each letter 3 positions forward in the alphabet is a characteristic of a Caesar cipher with a shift value of 3. The Vigenère cipher would use a repeating key to shift letters by varying amounts, not the consistent shift seen here. ROT13 is a specific Caesar cipher that shifts 13 places, not 3. "Double rotation" would imply 26 places (full circle back to original). XOR encryption with a single-byte key would produce different patterns than the consistent alphabetic shift observed here.

Correct Answer: AES (Advanced Encryption Standard)

AES (Advanced Encryption Standard) is the correct answer because it is a symmetric block cipher that uses a substitution-permutation network architecture with exactly the specifications described in the question: a fixed block size of 128 bits and support for key sizes of 128, 192, or 256 bits. SHA-256 is incorrect because it is a cryptographic hash function, not an encryption algorithm. It produces a digest (hash) of fixed length and does not use keys for encryption/decryption operations. RSA is incorrect because it is an asymmetric (public-key) encryption algorithm based on the mathematical difficulty of factoring large integers. RSA does not use a substitution-permutation network and does not have fixed block sizes of 128 bits. Triple DES is incorrect because it is based on the DES algorithm which uses a Feistel network (not a substitution-permutation network). Triple DES typically uses a block size of 64 bits, not 128 bits, and has different key length specifications than those mentioned in the question.