Revised Version of Block Cipher CHAM

CHAM is a family of lightweight block ciphers published in 2017 [22]. The CHAM family consists of three ciphers, CHAM-64/128, CHAM-128/128, and CHAM-128/256. CHAM can be implemented with a remarkably low area in hardware compared to other lightweight block ciphers, and it also performs well on software. We found new (related-key) differential characteristics and differentials of CHAM using a SAT solver. Although attacks using the new characteristics are limited to the reduced rounds of CHAM, it is preferable to increase the number of rounds to ensure a sufficient security margin. The numbers of rounds of CHAM-64/128, CHAM-128/128, and CHAM-128/256 are increased from 80 to 88, 80 to 112, and 96 to 120, respectively. We provide strong evidence that CHAM with these new numbers of rounds is secure enough against (related-key) differential cryptanalysis. Because increasing the number of rounds does not affect the area in low-area hardware implementations, the revised CHAM is still excellent in lightweight hardware implementations. In software, the revised CHAM is still comparable to SPECK, one of the top-ranked algorithms in software.