Final answer:
If the key generation is improper and the same key is used multiple times in stream ciphers, the encrypted data will be the same for each encryption, which can lead to significant security issues.
Step-by-step explanation:
If the key generation and handling is not done properly in stream ciphers, and the same key is used to encrypt data two or more times, the most likely consequence is that the encrypted data will be the same for each encryption. This occurs because stream ciphers operate by combining the key with a nonce (number used once) to produce a keystream, which is then XORed with the plaintext to create the ciphertext. When the same key and nonce are used multiple times, the same keystream is generated, resulting in identical ciphertexts. This can lead to significant security vulnerabilities, as an attacker can use this fact to perform ciphertext-only attacks and potentially retrieve the plaintext without needing the key.