https://blog.amuralmanji.com/categories/cryptography/Recent content in Cryptography onHugoenWed, 15 Oct 2025 19:30:00 +0400https://blog.amuralmanji.com/posts/ae_clefia_aes/Wed, 15 Oct 2025 19:30:00 +0400https://blog.amuralmanji.com/posts/ae_clefia_aes/<p>An interesting Link shows that the <strong>CLEFIA S-box</strong> and the <strong>Rijndael (AES) S-box</strong> are <strong>affine equivalent</strong>—meaning one can be transformed into the other through invertible affine transformations over the finite field $\mathbb{F}_2^{8}$.</p> <p>Formally, two functions ( f ) and ( g ) are affine equivalent if there exist invertible affine mappings ( A ), ( B ), and constants ( a ), ( b ) such that:</p> <p>$$ f(x) = (B \circ g \circ A)(x \oplus a) \oplus b $$</p>https://blog.amuralmanji.com/posts/clefia_sbox_normal_basis/Fri, 18 Apr 2025 14:00:00 +0400https://blog.amuralmanji.com/posts/clefia_sbox_normal_basis/<p>A few years ago, I explored one of my favorite cryptographic building blocks: the substitution box (S-box). Back then, I came across the elegant paper <em>A Very Compact Rijndael S-box</em>, which showed how to eliminate lookup tables by leveraging <strong>normal basis representation</strong> and logical operations in $F_{2^{8}}$. That approach deeply resonated with me—compact, efficient, and mathematically sound.</p> <p>Recently, I decided to revisit that work—but this time with a new challenge: re-implementing the <strong>CLEFIA S-box</strong> using the same principles. CLEFIA, Sony’s lightweight block cipher, uses two $8 \times 8$ S-boxes ($S_0$ and $S_1$), and I focused on reconstructing <strong>S₁</strong> using field inversion and affine transformations—all without tables.</p>