Harper, a software engineer, is developing an email application. To ensure the confidentiality of email messages. Harper uses a symmetric-key block cipher having a classical 12- or 16-round Feistel ne

CAST-128 (RFC 2144) precisely matches all described characteristics: a 12- or 16-round Feistel network, a 64-bit block size, and four large 8x32-bit S-boxes derived from bent functions. Its defining per-round use of a masking key (Km) and a rotation key (Kr) combined with modular addition, subtraction, and XOR operations is unique to CAST-128 among common ciphers.

AES uses a 128-bit block size and a substitution-permutation network rather than a Feistel structure, and it does not use Km or Kr key pairs.

The GOST block cipher uses a 64-bit block and Feistel structure but employs 32 rounds with small 4-bit S-boxes, not the large 8x32-bit S-boxes described.

DES uses a 64-bit block and a 16-round Feistel network but relies on 6-to-4-bit S-boxes and does not incorporate masking keys or rotation keys as described.