curve25519.h

// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * Copyright 2024- IBM Corp.
 *
 * X25519 scalar multiplication with 51 bits limbs for PPC64le.
 *   Based on RFC7748 and AArch64 optimized implementation for X25519
 *     - Algorithm 1 Scalar multiplication of a variable point
 */

#include <linux/types.h>
#include <linux/jump_label.h>
#include <linux/kernel.h>

#include <linux/cpufeature.h>
#include <linux/processor.h>

typedef uint64_t fe51[5];

asmlinkage void x25519_fe51_mul(fe51 h, const fe51 f, const fe51 g);
asmlinkage void x25519_fe51_sqr(fe51 h, const fe51 f);
asmlinkage void x25519_fe51_mul121666(fe51 h, fe51 f);
asmlinkage void x25519_fe51_sqr_times(fe51 h, const fe51 f, int n);
asmlinkage void x25519_fe51_frombytes(fe51 h, const uint8_t *s);
asmlinkage void x25519_fe51_tobytes(uint8_t *s, const fe51 h);
asmlinkage void x25519_cswap(fe51 p, fe51 q, unsigned int bit);

#define fmul x25519_fe51_mul
#define fsqr x25519_fe51_sqr
#define fmul121666 x25519_fe51_mul121666
#define fe51_tobytes x25519_fe51_tobytes

static void fadd(fe51 h, const fe51 f, const fe51 g)
{
	h[0] = f[0] + g[0];
	h[1] = f[1] + g[1];
	h[2] = f[2] + g[2];
	h[3] = f[3] + g[3];
	h[4] = f[4] + g[4];
}

/*
 * Prime = 2 ** 255 - 19, 255 bits
 *    (0x7fffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffed)
 *
 * Prime in 5 51-bit limbs
 */
static fe51 prime51 = { 0x7ffffffffffed, 0x7ffffffffffff, 0x7ffffffffffff, 0x7ffffffffffff, 0x7ffffffffffff};

static void fsub(fe51 h, const fe51 f, const fe51 g)
{
	h[0] = (f[0] + ((prime51[0] * 2))) - g[0];
	h[1] = (f[1] + ((prime51[1] * 2))) - g[1];
	h[2] = (f[2] + ((prime51[2] * 2))) - g[2];
	h[3] = (f[3] + ((prime51[3] * 2))) - g[3];
	h[4] = (f[4] + ((prime51[4] * 2))) - g[4];
}

static void fe51_frombytes(fe51 h, const uint8_t *s)
{
	/*
	 * Make sure 64-bit aligned.
	 */
	unsigned char sbuf[32+8];
	unsigned char *sb = PTR_ALIGN((void *)sbuf, 8);

	memcpy(sb, s, 32);
	x25519_fe51_frombytes(h, sb);
}

static void finv(fe51 o, const fe51 i)
{
	fe51 a0, b, c, t00;

	fsqr(a0, i);
	x25519_fe51_sqr_times(t00, a0, 2);

	fmul(b, t00, i);
	fmul(a0, b, a0);

	fsqr(t00, a0);

	fmul(b, t00, b);
	x25519_fe51_sqr_times(t00, b, 5);

	fmul(b, t00, b);
	x25519_fe51_sqr_times(t00, b, 10);

	fmul(c, t00, b);
	x25519_fe51_sqr_times(t00, c, 20);

	fmul(t00, t00, c);
	x25519_fe51_sqr_times(t00, t00, 10);

	fmul(b, t00, b);
	x25519_fe51_sqr_times(t00, b, 50);

	fmul(c, t00, b);
	x25519_fe51_sqr_times(t00, c, 100);

	fmul(t00, t00, c);
	x25519_fe51_sqr_times(t00, t00, 50);

	fmul(t00, t00, b);
	x25519_fe51_sqr_times(t00, t00, 5);

	fmul(o, t00, a0);
}

static void curve25519_fe51(uint8_t out[32], const uint8_t scalar[32],
			    const uint8_t point[32])
{
	fe51 x1, x2, z2, x3, z3;
	uint8_t s[32];
	unsigned int swap = 0;
	int i;

	memcpy(s, scalar, 32);
	s[0]  &= 0xf8;
	s[31] &= 0x7f;
	s[31] |= 0x40;
	fe51_frombytes(x1, point);

	z2[0] = z2[1] = z2[2] = z2[3] = z2[4] = 0;
	x3[0] = x1[0];
	x3[1] = x1[1];
	x3[2] = x1[2];
	x3[3] = x1[3];
	x3[4] = x1[4];

	x2[0] = z3[0] = 1;
	x2[1] = z3[1] = 0;
	x2[2] = z3[2] = 0;
	x2[3] = z3[3] = 0;
	x2[4] = z3[4] = 0;

	for (i = 254; i >= 0; --i) {
		unsigned int k_t = 1 & (s[i / 8] >> (i & 7));
		fe51 a, b, c, d, e;
		fe51 da, cb, aa, bb;
		fe51 dacb_p, dacb_m;

		swap ^= k_t;
		x25519_cswap(x2, x3, swap);
		x25519_cswap(z2, z3, swap);
		swap = k_t;

		fsub(b, x2, z2);		// B = x_2 - z_2
		fadd(a, x2, z2);		// A = x_2 + z_2
		fsub(d, x3, z3);		// D = x_3 - z_3
		fadd(c, x3, z3);		// C = x_3 + z_3

		fsqr(bb, b);			// BB = B^2
		fsqr(aa, a);			// AA = A^2
		fmul(da, d, a);			// DA = D * A
		fmul(cb, c, b);			// CB = C * B

		fsub(e, aa, bb);		// E = AA - BB
		fmul(x2, aa, bb);		// x2 = AA * BB
		fadd(dacb_p, da, cb);		// DA + CB
		fsub(dacb_m, da, cb);		// DA - CB

		fmul121666(z3, e);		// 121666 * E
		fsqr(z2, dacb_m);		// (DA - CB)^2
		fsqr(x3, dacb_p);		// x3 = (DA + CB)^2
		fadd(b, bb, z3);		// BB + 121666 * E
		fmul(z3, x1, z2);		// z3 = x1 * (DA - CB)^2
		fmul(z2, e, b);		// z2 = e * (BB + (DA + CB)^2)
	}

	finv(z2, z2);
	fmul(x2, x2, z2);
	fe51_tobytes(out, x2);
}

static void curve25519_arch(u8 mypublic[CURVE25519_KEY_SIZE],
			    const u8 secret[CURVE25519_KEY_SIZE],
			    const u8 basepoint[CURVE25519_KEY_SIZE])
{
	curve25519_fe51(mypublic, secret, basepoint);
}

static void curve25519_base_arch(u8 pub[CURVE25519_KEY_SIZE],
				 const u8 secret[CURVE25519_KEY_SIZE])
{
	curve25519_fe51(pub, secret, curve25519_base_point);
}