Round (cryptography)

In <a href="/facts/Cryptography/e94PEVau">cryptography</a>, a round or round function is a basic transformation that is repeated (<a href="/facts/Iteration/AMc1p7BL">iterated</a>) multiple times inside the algorithm. Splitting a large algorithmic function into rounds simplifies both implementation and <a href="/facts/Cryptanalysis/7CPEQlJI">cryptanalysis</a>.
For example, encryption using an oversimplified three-round cipher can be written as 
 
 
 
 C
 =
 
 R
 
 3
 
 
 (
 
 R
 
 2
 
 
 (
 
 R
 
 1
 
 
 (
 P
 )
 )
 )
 
 
 {\displaystyle C=R_{3}(R_{2}(R_{1}(P)))}
 
, where C is the <a href="/facts/Ciphertext/luNTs4sY">ciphertext</a> and P is the <a href="/facts/Plaintext/tWnBDCgE">plaintext</a>. Typically, rounds 
 
 
 
 
 R
 
 1
 
 
 ,
 
 R
 
 2
 
 
 ,
 .
 .
 .
 
 
 {\displaystyle R_{1},R_{2},...}
 
 are implemented using the same function, parameterized by the round constant and, for <a href="/facts/Block_cipher/MJywAyfQ">block ciphers</a>, the round key from the <a href="/facts/Key_schedule/H2klHRyU">key schedule</a>. Parameterization is essential to reduce the self-similarity of the cipher, which could lead to <a href="/facts/Slide_attack/nHYbtVPn">slide attacks</a>.
Increasing the number of rounds "almost always" protects against <a href="/facts/Differential_cryptanalysis/84yp0SDC">differential</a> and <a href="/facts/Linear_cryptanalysis/Hvg2bJpC">linear cryptanalysis</a>, as for these tools the effort grows exponentially with the number of rounds. However, increasing the number of rounds does not always make weak ciphers into strong ones, as some attacks do not depend on the number of rounds.
The idea of an <a href="/facts/Iterative_cipher/qHTBoCnh">iterative cipher</a> using repeated application of simple <a href="/facts/Commutative_property/WaYxJLxd">non-commutating</a> operations producing <a href="/facts/Diffusion_and_confusion/PKqvzyKY">diffusion and confusion</a> goes as far back as 1945, to the then-secret version of <a href="/facts/C._E._Shannon/EremsCws">C. E. Shannon</a>'s work "<a href="/facts/Communication_Theory_of_Secrecy_Systems/wXHm9G3h">Communication Theory of Secrecy Systems</a>"; Shannon was inspired by mixing transformations used in the field of <a href="/facts/Dynamical_systems_theory/LM6aDhV0">dynamical systems theory</a> (cf. <a href="/facts/Horseshoe_map/hh7Ipz1v">horseshoe map</a>). Most of the modern ciphers use <a href="/facts/Iterative_design/4GPzXnGT">iterative design</a> with number of rounds usually chosen between 8 and 32 (with 64 and even 80 used in <a href="/facts/Cryptographic_hash/cuxkgbST">cryptographic hashes</a>).
For some <a href="/facts/Feistel_cipher/CPtzaEET">Feistel-like cipher</a> descriptions, notably that of the <a href="/facts/RC5/RjIGUnNk">RC5</a>, a term "half-round" is used to define the transformation of part of the data (a distinguishing feature of the Feistel design). This operation corresponds to a full round in traditional descriptions of Feistel ciphers (like <a href="/facts/Data_encryption_standard/STfBjTGb">DES</a>).

Round (cryptography) open-in-new

Round (cryptography)