arXiv:2610.31178v1 Announce Type: new Abstract: BelT is a block cipher with a 128-bit block size and a 256-bit key, developed in Belarus and adopted as part of the Belarusian national standard STB 34.101.31 for encryption and integrity control. Its specification was issued as a preliminary standard in 2007, becoming a final standard in 2011. BelT employs an eight-round structure combining elements of a Feistel network and the Lai-Massey scheme. This paper presents a complete description of the cipher and explains the principles underlying its design. Software performance and possible optimizations of BelT are discussed in comparison with those of candidates in the Advanced Encryption Standard (AES) competition.