Why is the choice of key and encryption algorithm important?

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The choice of algorithm only affects the speed of encryption, not its security.

Any key will work as long as it is kept secret, and the algorithm choice doesn$$t impact security.

The key and algorithm together determine how secure the encryption is$,$ as a weak key or algorithm can make the data vulnerable to attacks.

The algorithm determines how large the encrypted data will be$,$ while the key defines the format of the ciphertext.