Security
- Main article: Stream cipher attack
For a stream cipher to be secure, its keystream must have a large period and it must be impossible to recover the cipher's key or internal state from the keystream. Cryptographers also demand that the keystream be free of even subtle biases that would let attackers distinguish a stream from random noise, and free of detectable relationships between keystreams that correspond to related keys or related cryptographic nonces. This should be true for all keys (there should be no weak keys), and true even if the attacker can know or choose some plaintext or ciphertext.
As with other attacks in cryptography, stream cipher attacks can be certificational, meaning they are not necessarily practical ways to break the cipher but indicate that the cipher might have other weaknesses.
Securely using a secure synchronous stream cipher requires that one never reuse the same keystream twice; that generally means a different nonce or key must be supplied to each invocation of the cipher. Application designers must also recognize that most stream ciphers don't provide authenticity, only privacy: encrypted messages may still have been modified in transit.
Short periods for stream ciphers have been a practical concern. For example, 64-bit block ciphers like DES can be used to generate a keystream in output feedback (OFB) mode. However, when not using full feedback, the resulting stream has a period of around 232 blocks on average; for many applications, this period is far too low. For example, if encryption is being performed at a rate of 8 megabytes per second, a stream of period 232 blocks will repeat after about a half an hour.
Some applications using the stream cipher RC4 are attackable because of weaknesses in RC4's key setup routine; new applications should either avoid RC4 or make sure all keys are unique and ideally unrelated (such as generated by a well-seeded CSPRNG or a cryptographic hash function) and that the first bytes of the keystream are discarded.
Read more about this topic: Stream Cipher
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