Which cipher is used only one time and then discarded?

Which cipher is used only one time and then discarded?

• A. One Time Pad
• B. Caesar Cipher
• C. Symmetric Encryption
• D. Asymmetric encryption

Answer: (A)

The concept being tested is using a cipher only once and then discarding the key. The one-time pad is the only method designed for that: it uses a truly random key as long as the message, and that key is used exactly once and then destroyed. When the key is never reused, the ciphertext provides no information about the plaintext without the key, which is why the method is said to offer perfect secrecy if used correctly. The other options don’t fit this requirement. The Caesar cipher is a simple shift that leaves recognizable patterns and can be cracked with basic frequency analysis, and nothing in its design requires discarding the key after one use. Symmetric encryption covers a broad family of algorithms that typically reuse the same secret key for many messages, so discarding after a single use is not inherent. Asymmetric encryption uses a public/private key pair and is also intended for multiple messages over time rather than a single-use discard model. So, the one-time pad is the best answer because its defining property is one-time use and immediate discard of the key.

The concept being tested is using a cipher only once and then discarding the key. The one-time pad is the only method designed for that: it uses a truly random key as long as the message, and that key is used exactly once and then destroyed. When the key is never reused, the ciphertext provides no information about the plaintext without the key, which is why the method is said to offer perfect secrecy if used correctly.

The other options don’t fit this requirement. The Caesar cipher is a simple shift that leaves recognizable patterns and can be cracked with basic frequency analysis, and nothing in its design requires discarding the key after one use. Symmetric encryption covers a broad family of algorithms that typically reuse the same secret key for many messages, so discarding after a single use is not inherent. Asymmetric encryption uses a public/private key pair and is also intended for multiple messages over time rather than a single-use discard model.

So, the one-time pad is the best answer because its defining property is one-time use and immediate discard of the key.