In an e-Office system, a confidential file is sent digitally from a section officer to a higher authority. The department wants t…

Question

In an e-Office system, a confidential file is sent digitally from a section officer to a higher authority. The department wants to ensure that only the intended officer can read the file during transmission.

Which of the following ensures confidentiality during transmission?

1. A. Applying a digital signature to the document using a public key infrastructure
2. B. Encrypting the file using the recipient's public key (Correct answer)
3. C. Adding a watermark to the document
4. D. Scanning the file before sending it

Correct Answer

Option B — Encrypting the file using the recipient's public key

Detailed Solution & Explanation

The correct answer is Encrypting the file using the recipient's public key.

Key Points

• Confidentiality means that only the intended recipient can read the content. In public key (asymmetric) cryptography this is achieved by encrypting with the recipient's public key, because only the matching private key — held solely by that recipient — can decrypt the file.
• Even if the transmission is intercepted, the attacker cannot read it without the recipient's private key.

Additional Information

• The two directions of public-key cryptography, and the distinction the question turns on:

• Why option A is wrong: a digital signature is created with the sender's private key and verified with the sender's public key. It establishes authenticity, integrity and non-repudiation — proving who sent the file and that it was not altered — but it does not conceal the contents. A signed document remains fully readable by anyone who intercepts it. This is the central trap in the question.
• Why the other options are wrong: a watermark is a visible or invisible mark for attribution and tamper-evidence, offering no cryptographic protection; scanning a file checks it for malware and has nothing to do with confidentiality in transit.
• The four goals of information security: Confidentiality (encryption), Integrity (hashing), Authentication (digital signatures and certificates) and Non-repudiation (digital signatures).
• Symmetric versus asymmetric encryption: symmetric algorithms such as AES use a single shared key and are fast, but require a secure channel for key exchange; asymmetric algorithms such as RSA and ECC solve the key distribution problem but are slower. In practice, systems use a hybrid approach — asymmetric cryptography to exchange a symmetric session key, then symmetric encryption for the data itself, which is how TLS/HTTPS works.
• In the Indian e-Office context, digital signatures are issued under the Information Technology Act, 2000, by licensed Certifying Authorities, and are commonly held on hardware tokens or as Aadhaar-based eSign credentials.