Encryption: symmetric and asymmetric methods

Fundamentals

• Encryption converts readable plaintext into unreadable ciphertext using an algorithm (cipher) and a key. Decryption reverses it.
• It is needed because data sent over networks can be intercepted; encryption means intercepted data is useless without the key. It also protects stored data, for example on a stolen laptop.
• Benefits: confidentiality of messages, passwords, payments. Drawbacks: processing time, lost keys mean lost data, it does not stop data being intercepted or deleted, and weak keys or algorithms can be broken. The security depends on keeping the (secret) key secret, not on hiding the algorithm.

Symmetric encryption

In symmetric encryption the same key is used to encrypt and decrypt. The sender and receiver must both know this shared key.

• Benefits: fast, suitable for large amounts of data.
• Drawback, the key exchange problem: the key must be shared before secure communication starts. If it is sent over the network it could be intercepted, and anyone with the key can read every message. Each pair of people needs its own key.

Asymmetric encryption

In asymmetric (public key) encryption each person has a key pair: a public key that anyone may know, and a private key that is kept secret. The keys are mathematically linked.

• To send a message to B, A encrypts it with B's public key. Only B's private key can decrypt it.
• Benefits: no need to share a secret key in advance, so the key exchange problem is solved; also allows digital signatures (encrypt with your private key, anyone can check with your public key).
• Drawbacks: much slower than symmetric encryption. In practice asymmetric encryption is used to exchange a symmetric key, which then encrypts the rest of the session (as in HTTPS).

Worked example

Amira wants to send Ben a confidential file using asymmetric encryption. Describe the process.

1. Ben generates a key pair and publishes his public key; his private key stays secret.
2. Amira encrypts the file using Ben's public key.
3. The ciphertext is sent; anyone intercepting it cannot decrypt it with the public key.
4. Ben decrypts it using his private key.

Answer: Encrypt with the receiver's public key; decrypt with the receiver's private key.

Check yourself

1. What is a feature of symmetric encryption?
   • Each user has a public and a private key
   • The same key is used to encrypt and decrypt
   • Messages are encrypted with the receiver's public key
   • No key is used
   Show answer

   The same key is used to encrypt and decrypt. Symmetric means the same key both ways.

2. In asymmetric encryption, which key does the sender use to encrypt a confidential message to Ben?
   • Ben's private key
   • The sender's private key
   • A key chosen at random and never shared
   • Ben's public key
   Show answer

   Ben's public key. Only Ben's private key can then decrypt it.

3. Which key decrypts a message that was encrypted with Ben's public key?
   • Ben's public key
   • Ben's private key
   • The sender's public key
   • Any public key
   Show answer

   Ben's private key. The keys work as a pair.

4. Describe the key exchange problem in symmetric encryption.
   Show answer

   Both parties need the same secret key before they can communicate securely, but sending the key over the network risks it being intercepted, and then all messages could be decrypted.. Explain why sharing the key is risky.

5. Why is symmetric encryption often used for the bulk of data in a secure session?
   • It is much faster than asymmetric encryption
   • It needs no key
   • It is the only method that can be decrypted
   • It uses a public key
   Show answer

   It is much faster than asymmetric encryption. Asymmetric is slow, so it is used just to exchange a symmetric session key.

6. What is ciphertext?
   • The original readable message
   • The algorithm used to encrypt
   • Encrypted data that cannot be understood without the key
   • A list of public keys
   Show answer

   Encrypted data that cannot be understood without the key. Plaintext goes in, ciphertext comes out.

7. Give one limitation of encryption.
   Show answer

   It does not stop data being intercepted or deleted; it only stops it being understood. Also, if the key is lost the data cannot be recovered.. Encryption protects confidentiality only.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Computer Science specification (2026)

Last updated 8 November 2026. Spotted a mistake? Tell us.