Which algorithm was adopted in 2001 as the Advanced Encryption Standard, replacing DES?

The correct answer is C, conventional encryption. Symmetric encryption uses one shared secret key for both locking and unlocking data, and because it is the older and simpler of the two families it is variously called conventional, secret key or private key encryption. Option A, public-key encryption, is the opposite arrangement: it uses a pair of keys, a public one that may be shared freely and a private one that is never shared. Option B, asymmetric encryption, is just another name for that same public-key family, so it too is the opposite of what the question asks. Option D, hybrid encryption, describes the practical combination used on the internet, where an asymmetric exchange settles a symmetric session key that then carries the data; it is not a synonym for symmetric encryption. DES, Triple DES and AES are the standard symmetric examples, while RSA and Diffie-Hellman are asymmetric.

The correct answer is B, one key. The defining feature of symmetric encryption is that a single shared secret key performs both operations: the sender encrypts the plaintext with it and the receiver decrypts the ciphertext with the very same key. That is also its practical problem, because the key must be delivered to the other party over some safe channel before any message can be exchanged. Option A, two keys, describes asymmetric or public key encryption, where a public key and a private key form a linked pair. Option C, no key, describes a hash function such as MD5 or SHA-256, which produces a fixed-length digest without any key and cannot be reversed; hashing is therefore not encryption at all. Option D, three keys, is a distractor built on the name Triple DES, which in fact applies the DES algorithm three times and is still symmetric.

The correct answer is B, Secure Sockets Layer. SSL was developed by Netscape in the mid-1990s to encrypt the traffic between a web browser and a web server, and its successor, Transport Layer Security or TLS, standardised from 1999, is what actually protects websites now; the older name survives in common speech and in the phrase SSL certificate. Options A, C and D are invented expansions of the kind objective papers use to test whether a candidate has memorised the exact words. A socket in networking is the combination of an IP address and a port number, which is why the protocol that sits above the transport layer and below the application layer was named for sockets. Remember the chain: SSL became TLS, and HTTP protected by either of them is called HTTPS, which runs on port 443.

The correct answer is D, 443. HTTPS is HTTP carried inside an SSL or TLS tunnel, and the port reserved for it is 443, which is why a padlock appears in the address bar when a site is reached on that port with a valid certificate. Option C, port 80, is the default for ordinary unencrypted HTTP, and it is the pair 80 and 443 that examiners test most often. Option A, port 21, belongs to the File Transfer Protocol, which uses 21 for control and 20 for data. Option B, port 25, is the Simple Mail Transfer Protocol used to send electronic mail, with 110 for POP3 and 143 for IMAP on the receiving side. Other numbers worth memorising are 22 for SSH, 23 for Telnet and 53 for DNS. These port numbers are fixed by convention and appear in SSC, Railway and banking papers almost every year.

The correct answer is B, public key cryptography. RSA, named after Rivest, Shamir and Adleman, works with a linked pair of keys and takes its security from the difficulty of factorising the product of two large prime numbers; it is used both to exchange keys and to create digital signatures. Option A is wrong because symmetric cryptography uses one shared key, and its standard examples are DES, Triple DES and AES. Option C, hashing, is not encryption at all: a hash function such as MD5 or SHA-256 uses no key, produces a fixed-length digest and cannot be reversed, so nothing encrypted by it can be recovered. Option D, steganography, hides the very existence of a message by concealing it inside an image, audio file or document, instead of scrambling its content. A reliable memory aid is that RSA and Diffie-Hellman are asymmetric, while DES and AES are symmetric.