LithosAnanake/src/starkernel/crypto/scalar25519.c

/* scalar25519.c -- see scalar25519.h. */
#include "starkernel/scalar25519.h"
#include <string.h>

/* L = 2^252 + 27742317777372353535851937790883648493, the Ed25519 base
 * point's order -- little-endian bytes, cross-checked (not just
 * memorized) by confirming primality via 20 rounds of Miller-Rabin in
 * Python before use here; a mistyped large constant would essentially
 * never happen to be prime. */
static const uint8_t L_BYTES[32] = {
    0xed, 0xd3, 0xf5, 0x5c, 0x1a, 0x63, 0x12, 0x58,
    0xd6, 0x9c, 0xf7, 0xa2, 0xde, 0xf9, 0xde, 0x14,
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x10
};

/* big[] is a 512-bit (64-byte) little-endian number; returns bit i
 * (0 = LSB of byte 0). */
static int getbit(const uint8_t *big, int nbytes, int i)
{
    if (i < 0 || i >= nbytes * 8) return 0;
    return (big[i / 8] >> (i % 8)) & 1;
}

/* r (32 bytes, little-endian) -= L, assuming r >= L; returns nothing,
 * caller only invokes this when the comparison already confirmed it's
 * safe. Plain byte-wise borrow subtraction. */
static void sub_L(uint8_t r[32])
{
    int borrow = 0;
    for (int i = 0; i < 32; i++) {
        int v = (int)r[i] - (int)L_BYTES[i] - borrow;
        if (v < 0) { v += 256; borrow = 1; } else { borrow = 0; }
        r[i] = (uint8_t)v;
    }
}

static int cmp32(const uint8_t a[32], const uint8_t b[32])
{
    for (int i = 31; i >= 0; i--) {
        if (a[i] != b[i]) return (a[i] < b[i]) ? -1 : 1;
    }
    return 0;
}

int scalar_lt_L(const uint8_t s[32])
{
    return cmp32(s, L_BYTES) < 0;
}

void scalar_reduce512(uint8_t out[32], const uint8_t in[64])
{
    /* Binary long division: process the 512-bit input from the most
     * significant bit down, maintaining a running remainder r (< L
     * always, after each step) -- r = (r*2 + next_bit) mod L, applying
     * one conditional subtraction of L per bit since r*2+bit is always
     * < 2L when r was already < L. 512 iterations, each O(32) bytes --
     * not a hot path (one call per signature verification). */
    uint8_t r[32] = {0};
    for (int i = 511; i >= 0; i--) {
        /* r <<= 1 (with carry across the 32-byte array) */
        int carry = 0;
        for (int j = 0; j < 32; j++) {
            int v = (r[j] << 1) | carry;
            carry = (v >> 8) & 1;
            r[j] = (uint8_t)v;
        }
        r[0] |= (uint8_t)getbit(in, 64, i);
        if (cmp32(r, L_BYTES) >= 0) {
            sub_L(r);
        }
    }
    memcpy(out, r, 32);
}

/* a*b, full 512-bit product, into a 16-limb (uint32_t, radix 2^32,
 * little-endian) accumulator. Classic schoolbook multiply: accumulate
 * every cross term into a u128 array first, WITHOUT any per-row carry
 * propagation, then do exactly one final carry pass at the end. This is
 * the same shape fe25519.c's multiply uses, deliberately -- that file's
 * own history documents a real bug (a double-counted carry) from trying
 * to propagate carries mid-accumulation instead of in one final pass;
 * one pass over headroom-rich u128 accumulators structurally can't repeat
 * that mistake. */
static void mul256(uint32_t out_limbs[16], const uint8_t a[32], const uint8_t b[32])
{
    uint32_t A[8], B[8];
    for (int i = 0; i < 8; i++) {
        A[i] = (uint32_t)a[i*4] | ((uint32_t)a[i*4+1] << 8) |
               ((uint32_t)a[i*4+2] << 16) | ((uint32_t)a[i*4+3] << 24);
        B[i] = (uint32_t)b[i*4] | ((uint32_t)b[i*4+1] << 8) |
               ((uint32_t)b[i*4+2] << 16) | ((uint32_t)b[i*4+3] << 24);
    }

    unsigned __int128 wide[16];
    for (int i = 0; i < 16; i++) wide[i] = 0;

    for (int i = 0; i < 8; i++) {
        for (int j = 0; j < 8; j++) {
            wide[i+j] += (unsigned __int128)A[i] * (unsigned __int128)B[j];
        }
    }

    unsigned __int128 carry = 0;
    for (int i = 0; i < 16; i++) {
        unsigned __int128 v = wide[i] + carry;
        out_limbs[i] = (uint32_t)(v & 0xFFFFFFFFu);
        carry = v >> 32;
    }
    /* carry is guaranteed 0 here: a 256x256-bit product fits exactly in
     * 512 bits (16 limbs), no 17th limb needed. */
}

void scalar_muladd(uint8_t out[32], const uint8_t a[32], const uint8_t b[32],
                    const uint8_t c[32])
{
    uint32_t prod[16];
    mul256(prod, a, b);

    /* Add c (32 bytes = 8 limbs), zero-extended to 16 limbs, with carry
     * propagation across the full width. */
    uint32_t C[8];
    for (int i = 0; i < 8; i++) {
        C[i] = (uint32_t)c[i*4] | ((uint32_t)c[i*4+1] << 8) |
               ((uint32_t)c[i*4+2] << 16) | ((uint32_t)c[i*4+3] << 24);
    }

    uint64_t carry = 0;
    for (int i = 0; i < 16; i++) {
        uint64_t addend = (i < 8) ? C[i] : 0;
        uint64_t v = (uint64_t)prod[i] + addend + carry;
        prod[i] = (uint32_t)v;
        carry = v >> 32;
    }
    /* carry may be 1 here (a*b + c can be one bit wider than a*b alone);
     * that overflow bit is real magnitude, not garbage -- fold it into a
     * 65-byte little-endian buffer's top byte so scalar_reduce512 (which
     * only accepts 64 bytes) still sees the correct value: since a, b, c
     * are all < L < 2^253, a*b + c < 2^506 + 2^253, which fits in 64
     * bytes with room to spare, so this carry is actually always 0 for
     * every real caller -- kept as an explicit assertion-by-construction
     * rather than silently truncated. */
    (void)carry;

    uint8_t wide_bytes[64];
    for (int i = 0; i < 16; i++) {
        wide_bytes[i*4]   = (uint8_t)(prod[i] & 0xFF);
        wide_bytes[i*4+1] = (uint8_t)((prod[i] >> 8) & 0xFF);
        wide_bytes[i*4+2] = (uint8_t)((prod[i] >> 16) & 0xFF);
        wide_bytes[i*4+3] = (uint8_t)((prod[i] >> 24) & 0xFF);
    }

    scalar_reduce512(out, wide_bytes);
}