Encryption, Encryption Protocols and Digital certificates

Encryption is the process of scrambling readable data (plaintext) into an unreadable form (ciphertext) using a key, so that it cannot be understood by anyone who intercepts it.

It is used because data travelling over the internet can be intercepted by unauthorised people. If the data is encrypted, an interceptor sees only ciphertext, which is meaningless without the correct key — so confidential data (passwords, bank details) stays secure even if intercepted.

Public key: This key is made freely available to anyone. It is published/shared openly. Data encrypted with someone's public key can only be decrypted by the matching private key, so it is used to send confidential data TO the owner of the key pair.

Private key: This key is kept secret by its owner and never shared. It is the only key that can decrypt data encrypted with the matching public key, and it is also used to create digital signatures (by encrypting a message digest) that the public can verify with the public key.

The two keys are a mathematically linked pair: what one key encrypts, only the other can decrypt.

(a) Confidential email to Raj — encrypt with RAJ'S PUBLIC key. Maya uses Raj's public key to encrypt the email. Because the keys are a pair, only the matching private key can decrypt it, and only Raj holds his private key. Therefore only Raj can read the email, giving confidentiality. (Using Maya's own keys here would not restrict who can read it.)

(b) Verified press release to the public — encrypt the digest with the DEPARTMENT'S OWN PRIVATE key. The department creates a digital signature by hashing the release and encrypting that digest with its private key. Any member of the public can decrypt the signature with the department's public key and compare digests. A successful verification proves the release came from the department (authentication) and was not altered (integrity). The message itself is not secret — the goal is verification, not confidentiality.

(a) Purpose of SSL/TLS: SSL/TLS are protocols that create a secure, encrypted connection between a client (e.g. a web browser) and a server. They authenticate the server using its digital certificate so the client knows it is talking to the genuine server, agree a shared symmetric session key during a handshake, and then encrypt all data exchanged so it cannot be read or altered in transit. This is what turns HTTP into the secure HTTPS.

(b) Appropriate situations:

• Online banking / payments: highly sensitive financial data (card numbers, passwords) is transmitted, so encryption is essential to prevent interception and the certificate reassures the user the site is genuine (not a fake/phishing site).
• Logging in to any account (e.g. email or e-commerce): usernames and passwords are sent over the network, so SSL/TLS prevents them being captured by an eavesdropper on the connection.

Acquiring the certificate:

1. The organisation generates a key pair — a public key and a private key (kept secret).
2. It produces a Certificate Signing Request (CSR) containing its identity details and its public key, and submits it to a Certificate Authority (CA), a trusted third party.
3. The CA verifies the organisation's identity. If satisfied, it issues a digital certificate that binds the organisation's identity to its public key.
4. The CA signs the certificate using the CA's own private key, so its authenticity can later be checked by anyone holding the CA's public key.

Producing a digital signature (using the key pair): 5. The sender hashes the message to produce a message digest. 6. The sender encrypts the digest with its own private key — this encrypted digest is the digital signature, sent with the message. The recipient can obtain the sender's verified public key from the certificate.

Verifying the signature: 7. The recipient first checks the certificate is valid by verifying the CA's signature on it (using the CA's public key), confirming the public key inside really belongs to the sender. 8. The recipient then decrypts the signature with the sender's public key to recover the digest, independently hashes the received message, and compares the two digests. A match confirms authentication (it came from the sender) and integrity (it was not altered).

What it is: Quantum cryptography uses the principles of quantum mechanics (the behaviour of particles such as photons) to secure communication. Its best-known use is quantum key distribution (QKD), where a secret key is shared by transmitting photons. A fundamental property of quantum mechanics is that measuring/observing a quantum state changes it, so any attempt to intercept the key alters the photons.

Benefit — very high security / eavesdropping is detectable: Because intercepting the photons disturbs them, the two communicating parties can detect that an eavesdropper is present (the error rate rises) and can abandon the compromised key. This makes the key exchange far more secure than classical methods, where interception can go unnoticed. It is also resistant to future attacks because its security rests on physics rather than on a hard maths problem that a powerful (quantum) computer might one day solve.

Drawback — cost and practical limits: Quantum cryptography is expensive and relies on specialised hardware (e.g. photon sources/detectors and, often, dedicated fibre links). It currently works only over limited distances and is still an emerging/developing technology, so it is not yet practical for everyday widespread use.

Conclusion: Quantum cryptography offers exceptional, eavesdrop-detecting security, but high cost, hardware requirements and distance limitations mean it is presently restricted to specialised high-security applications.