What is a cipher?
Answer
A set of rules or an algorithm for encryption and decryption
Explanation
### Why this is right
A cipher specifies the rules that transform data during encryption and reverse that transformation during decryption. A key supplies the particular value or method used with those rules.
### The concept
Encryption combines an algorithm with a key to transform plaintext into ciphertext. The cipher is the algorithm, not the key or the resulting ciphertext. A hash is different because it is designed as a one-way transformation rather than a reversible encryption operation.
### The other options
- **B. A secret shared only by two users**: this describes a possible symmetric key, not the algorithm that processes data.
- **C. A certificate that identifies a sender**: a certificate associates identity information with a public key.
- **D. A one-way checksum that cannot be reversed**: this describes hashing, not a reversible cipher.
### Terms
- **Plaintext**: readable data before encryption.
- **Ciphertext**: transformed data produced by encryption.
### Remember
The cipher is the rule; the key is the value or method used with the rule.
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