That’s a huge inefficiency, because that 100 GB of data used as the key is thrown away almost immediately—that doesn’t even account for the time and computational complexities of operating on huge quantities of data. 1. stream ciphers A stream cipher is a single-character-in, single-character-out cipher. significant bit is the output bit. This keystream But this works only when the key doesn’t repeat—the key cannot be shorter than the plaintext message—and is sufficiently random. First, for any key to be truly secure, it should be randomly generated. length of the shift register.


In the case of the OTP, the key is the keystream, but this is not the case in RC4. One LFSR is clocked if the output of a second is a 1, otherwise it repeats its previous output. [i] As it turns out, symmetric algorithms can be further divided into stream ciphers and block ciphers. A stream cipher is an encryption algorithm that encrypts 1 bit or byte of plaintext at a time. In practice, a digit is typically a bit and the combining operation is an exclusive-or (XOR). So the least Many stream ciphers attempt this, but none is as popular as the RC4 cipher. The keystream is pseudorandom, which isn’t as statistically secure as true randomness. Block ciphers must be used in ciphertext stealing or residual block termination mode to avoid padding, while stream ciphers eliminate this issue by naturally operating on the smallest unit that can be transmitted (usually bytes). This has changed since 2011, however, as escalating attacks have targeted vulnerabilities in SSL/TLS, which led the Internet Engineering Task Force to officially ban RC4 in SSL/TLS connections in 2015. bits are used as well.

the XOR of certain bits in the register. That should be true for all keys (there should be no weak keys), even if the attacker can know or choose some plaintext or ciphertext. We demonstrated this using the Caesar Cipher, but we noticed the security of this type of cipher is very weak due to its key size. A long, strong key is worthless without a cryptographically secure cipher, however, and it’s not always feasible to have long keys. Short periods for stream ciphers have been a practical concern.
Such generators include the stop-and-go generator, the alternating step generator and the shrinking generator. feedback polynomials must be primitive modulo 2. A valid_out For a stream cipher implementation to remain secure, its pseudorandom generator should … If digits are added or removed from the message during transmission, synchronisation is lost. synchronize data latching for the stream cipher. Stream Ciphers • Start with a secret key (“seed”) • Generate a keying stream • i-th bit/byte of keying stream is a function of the key and the first i-1 ciphertext bits. SEAL, That generally means a different nonce or key must be supplied to each invocation of the cipher. The primitive polynomials modulo 2 are x11+x2+1 However, this comes at a cost. What is a stream cipher? The stop-and-go generator (Beth and Piper, 1984) consists of two LFSRs. 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. lengths of the constituent LFSRs must be relatively prime, and all of the In the simplest scheme, Bob can encrypt his message “HELLO” using modular addition, similar to the Caesar Cipher. The randomness is what guarantees the statistical impossibility of breaking the encryption. [vi] We repeat this process until we reach the end of the plaintext message. Such schemes are known as self-synchronizing stream ciphers, asynchronous stream ciphers or ciphertext autokey (CTAK). and a3 are the outputs of the three LFSRs, then the output of The elements of stream ciphers are often much simpler to understand than block ciphers and are thus less likely to hide any accidental or malicious weaknesses. SOBER-128, a 64-bit nonce + a 64-bit stream position, Matt J. It’s much easier to settle on one or two keys than to swap keys for every message. primitive modulo 2 polynomial x8+x4+x3+x2+1. shift registers (LFSRs). If one desires an IV, it must be mixed into the key somehow. But how does RC4 actually work? In practice, Alice and Bob agree upon a cipher and a key to use. Please check our privacy policy for more information.

So, we do our best to approximate true randomness by using clever mathematics and programming principles to generate pseudorandom numbers.

Phelix, This can be alleviated by buffering the output. Application designers must also recognize that most stream ciphers provide not authenticity but privacy: encrypted messages may still have been modified in transit. Stream ciphers represent a different approach to symmetric encryption from block ciphers. The real problem here is that we’re humans—we like things to be easy.


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