3.1. Definitions

encryption algorithm:

process that transforms plaintext into ciphertext (Clause 2.19 of [ISO-IEC18033-1])¶

decryption algorithm:

process that transforms ciphertext into plaintext (Clause 2.14 of [ISO-IEC18033-1])¶

basic block cipher:

block cipher that, for a given key, provides a single invertible mapping of the set of fixed-length plaintext blocks into ciphertext blocks of the same length¶

block:

string of bits of a defined length (Clause 2.6 of [ISO-IEC18033-1])¶

block cipher:

symmetric encipherment system with the property that the encryption algorithm operates on a block of plaintext -- i.e., a string of bits of a defined length -- to yield a block of ciphertext (Clause 2.7 of [ISO-IEC18033-1])¶

Note: In GOST R 34.12-2015, it is established that the terms "block cipher" and "block encryption algorithm" are synonyms.¶

encryption:

reversible transformation of data by a cryptographic algorithm to produce ciphertext -- i.e., to hide the information content of the data (Clause 2.18 of [ISO-IEC18033-1])¶

round key:

sequence of symbols that is calculated from the key and controls a transformation for one round of a block cipher¶

key:

sequence of symbols that controls the operation of a cryptographic transformation (e.g., encipherment, decipherment) (Clause 2.21 of [ISO-IEC18033-1])¶

Note: In GOST R 34.12-2015, the key must be a binary sequence.¶

plaintext:

unencrypted information (Clause 3.11 of [ISO-IEC10116])¶

key schedule:

calculation of round keys from the key,¶

decryption:

reversal of a corresponding encipherment (Clause 2.13 of [ISO-IEC18033-1])¶

symmetric cryptographic technique:

cryptographic technique that uses the same secret key for both the originator's and the recipient's transformation (Clause 2.32 of [ISO-IEC18033-1])¶

cipher:

alternative term for encipherment system (Clause 2.20 of [ISO-IEC18033-1])¶

ciphertext:

data that has been transformed to hide its information content (Clause 3.3 of [ISO-IEC10116])¶

3.2. Notation

The following notation is used in the specification:¶

V*

the set of all binary vector strings of a finite length (hereinafter referred to as the strings), including the empty string¶

V_s

the set of all binary strings of length s, where s is a nonnegative integer; substrings and string components are enumerated from right to left, starting from zero¶

U[*]W

direct (Cartesian) product of two sets U and W¶

|A|

the number of components (the length) of a string A belonging to V* (if A is an empty string, then |A| = 0)¶

A||B

concatenation of strings A and B both belonging to V* -- i.e., a string from V_(|A|+|B|), where the left substring from V_|A| is equal to A and the right substring from V_|B| is equal to B¶

A<<<_11

cyclic rotation of string A belonging to V_32 by 11 components in the direction of components having greater indices¶

Z_(2^n)

ring of residues modulo 2^n¶

(xor)

exclusive-or of two binary strings of the same length¶

[+]

addition in the ring Z_(2^32)¶

Vec_s: Z_(2^s) -> V_s

bijective mapping that maps an element from ring Z_(2^s) into its binary representation; i.e., for an element z of the ring Z_(2^s), represented by the residue z_0 + (2*z_1) + ... + (2^(s-1)*z_(s-1)), where z_i in {0, 1}, i = 0, ..., n-1, the equality Vec_s(z) = z_(s-1)||...||z_1||z_0 holds¶

Int_s: V_s -> Z_(2^s)

the mapping inverse to the mapping Vec_s, i.e., Int_s = Vec_s^(-1)¶

PS

composition of mappings, where the mapping S applies first¶

P^s

composition of mappings P^(s-1) and P, where P^1=P¶