Encryption

Encryption keys, plaintext and ciphertext

• The original data being sent is known as plaintext.
• Once it has gone through an encryption algorithm, it produces ciphertext.
• Note that, when encrypting text, block cipher is usually used.
• Here, the encryption algorithm is applied to a group of contiguous bits (for example, 128bits) rather than one bit at a time (which is known as stream cipher).
• With block cipher, each plaintext block is XORed with the previous ciphertext block and then encrypted – this is known as block chaining.
• This prevents identical blocks of plaintext producing the same ciphertext each time they are encrypted.

Symmetric encryption

• Symmetric encryption uses a secret key; the same key is used to encrypt and decrypt the encoded message.
• Consider a simple system which uses 10-denary-digit encryption (which gives about 10 billion possibilities).
• Suppose our secret key is 4 2 9 1 3 6 2 8 5 6, which means each letter in a word is shifted across the alphabet +4, +2, +9, and so on, places.

• One issue with symmetric encryption is that both sender and recipient need to use the same secret key.
• This is a security risk here, since the sender has to supply the key to the recipient.
• This key could be intercepted.
• This is referred to as the key distribution problem.

Asymmetric encryption

• Asymmetric encryption uses two keys – a public key, available to all users, and a private key, known to a specific person or computer.
• Suppose Tom and Meera work for the same company.
• Jane wishes to send a confidential document to Tom.
• Here’s how he could do it.