Editing Stream cipher (section)

{{short description|Type of symmetric key cipher}}
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[[Image:A5-1 GSM cipher.svg|280px|thumbnail|
The operation of the [[keystream]] generator in [[A5/1]], an LFSR-based stream cipher used to encrypt mobile phone conversations.]]

<span lang="my" dir="ltr">A</span> '''stream cipher''' is a [[symmetric key algorithm|symmetric key]] [[cipher]] where plaintext digits are combined with a [[pseudorandom]] cipher digit stream ([[keystream]]). In a stream cipher, each [[plaintext]] [[numerical digit|digit]] is encrypted one at a time with the corresponding digit of the keystream, to give a digit of the [[ciphertext]] stream. Since encryption of each digit is dependent on the current state of the cipher, it is also known as '''''state cipher'''''. In practice, a digit is typically a [[bit]] and the combining operation is an [[exclusive-or]] (XOR).

The pseudorandom keystream is typically generated serially from a random [[seed value]] using digital [[shift register]]s. The [[seed value]] serves as the [[cryptographic key]] for decrypting the ciphertext stream. Stream ciphers represent a different approach to symmetric encryption from [[block cipher]]s. Block ciphers operate on large blocks of digits with a fixed, unvarying transformation. This distinction is not always clear-cut: in some [[block cipher mode of operation|modes of operation]], a block cipher primitive is used in such a way that it acts effectively as a stream cipher. Stream ciphers typically execute at a higher speed than block ciphers and have lower hardware complexity. However, stream ciphers can be susceptible to security breaches (see [[stream cipher attack]]s); for example, when the same starting state (seed) is used twice.