Encryption, Encryption Protocols and Digital Certificates

Computer Science A2 · Security. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Encryption turns readable plain text into unreadable cipher text with an algorithm and a key. Only someone with the right key can turn it back.

In symmetric cryptography the same secret key encrypts and decrypts, so the key must reach the other person safely. In asymmetric cryptography each person has a matching pair of keys: a public key that anyone may know and a private key that is never shared. What one key encrypts, only the other key of the pair can decrypt.

The same pair of keys also proves who sent a message. This is the basis of digital signatures, digital certificates and SSL/TLS, the protocol that protects data between a browser and a web server.

Rules to remember

Private message to a person: the sender encrypts with the receiver's public key; only the receiver's private key can decrypt it.
Verified message to the public: the sender encrypts with their own private key; anyone can decrypt it with the sender's public key, which proves who sent it.
Digital signature: the message is hashed to give a digest, and the digest is encrypted with the sender's private key. Matching digests show the message is authentic and has not been changed.
Digital certificate: issued by a Certificate Authority (CA) after it checks the applicant; it holds the owner's name, the owner's public key, the issuer, a serial number, valid dates and the CA's digital signature.
SSL/TLS: the server sends its certificate, the browser checks it, a session key is agreed with asymmetric encryption, then the data is sent with faster symmetric encryption.
Each extra bit in a key doubles the number of possible keys: n bits give 2ⁿ keys.
Quantum cryptography sends the key as photons. Benefit: any eavesdropping changes the photons and is detected. Drawbacks: high cost, special equipment (fibre optic links) and limited distance.

Common mistake

Students mix up which key is used. To keep a message secret, use the receiver's public key. To prove who sent it, use the sender's private key. A private key is never sent to anyone.

Worked example

Sana must send Omar a report that only Omar can read, and Omar must be sure it came from Sana. Which keys does Sana use?

Secrecy: only Omar may read it, so it must need a key that only Omar has, his private key. Sana therefore encrypts the report with Omar's public key.
Proof of sender: the signature must need a key that only Sana has. Sana hashes the report and encrypts the digest with her own private key.
Omar decrypts the report with his private key, decrypts the signature with Sana's public key, and compares that digest with his own hash of the report.

Answer: Omar's public key to encrypt the report, and Sana's private key to make the digital signature.

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