Encryption, Digital Signatures and SSL/HTTPS for Exams

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.

The correct answer is C, MD5. The Message Digest 5 algorithm takes an input of any length and returns a 128-bit digest. It uses no key, and the process cannot be reversed to recover the input, which is exactly why it is a hash and not a cipher; hashes are used to check that a file or password has not been altered. Options A, B and D are all symmetric block ciphers that encrypt and decrypt with a shared key. AES, chosen in 2001 from the Rijndael design, works on 128-bit blocks with keys of 128, 192 or 256 bits. DES, from the 1970s, used a 56-bit key on 64-bit blocks and is now considered breakable. Blowfish is another block cipher of the same family. The other names in the hash family worth remembering are SHA-1, which gives 160 bits, and SHA-256, which gives 256 bits and is used in blockchains.

The correct answer is D, confidentiality of the contents. A digital signature is made by hashing the document and encrypting that digest with the signer's private key. Anyone holding the signer's public key can open the digest, so the document itself travels in the clear and remains readable; confidentiality has to be added separately by encrypting the document with the recipient's public key. Option A is provided, because only the holder of that private key could have produced a digest that the matching public key opens. Option B is provided, because the receiver hashes the document again and compares: any change, even of a single character, gives a different digest. Option C is provided, because the signer cannot later deny having signed, the private key being theirs alone. This distinction between signing and encrypting is the most frequently tested idea in the whole chapter.

The correct answer is C, the receiver's public key. In a key pair, whatever one key locks only the other can unlock. So a message encrypted with the receiver's public key can be opened only by the receiver's private key, which nobody else holds; that is exactly what confidentiality requires. Option A, the sender's private key, is what is used to create a digital signature, which proves who sent the document but hides nothing, since the matching public key is available to all. Option B, the sender's own public key, would produce a message only the sender could read, which defeats the purpose. Option D is impossible in practice, because a private key never leaves its owner and the sender can never have it. The rule to carry into the exam is simple: encrypt with the receiver's public key for secrecy, sign with your own private key for proof.

The correct answer is B, Rijndael. The Advanced Encryption Standard is the Rijndael cipher, designed by the Belgian cryptographers Joan Daemen and Vincent Rijmen, which won an open competition and was adopted in 2001 in place of the ageing Data Encryption Standard. AES is a symmetric block cipher working on 128-bit blocks with key lengths of 128, 192 or 256 bits, and it is used in Wi-Fi security, disk encryption and government communication. Option A, RSA, is asymmetric and was never a candidate to replace DES for bulk encryption. Option C, SHA-1, is a hash function giving a 160-bit digest; it computes no ciphertext and cannot encrypt anything. Option D, RC4, is a stream cipher once common in the older wireless standard WEP and in early SSL, but it was never adopted as the standard and is now discouraged for its weaknesses.

The correct answer is B, the Information Technology Act, 2000. This Act gave electronic records and digital signatures the same legal standing as paper records and handwritten signatures, and its amendment of 2008 broadened the provision to electronic signatures generally. Under the Act a Digital Signature Certificate is issued by a licensed certifying authority, and those authorities are licensed and supervised by the Controller of Certifying Authorities. Option A, the Indian Telegraph Act of 1885, governs telegraph and telecommunication licensing and interception, not electronic signatures. Option C, the Companies Act of 2013, requires directors to use digital signatures for filings but did not create their legal validity. Option D, the Indian Evidence Act of 1872, was itself amended by the 2000 Act so that electronic records could be admitted as evidence, which is the reason it is offered here as a distractor.