/* $OpenBSD: poly1305-donna.c,v 1.3 2014/06/12 15:49:30 deraadt Exp $ */
/*
 * Public Domain poly1305 from Andrew Moon
 * Based on poly1305-donna.c, poly1305-donna-32.h and poly1305-donna.h from:
 *   https://github.com/floodyberry/poly1305-donna
 */

#include <stddef.h>

static inline void poly1305_init(poly1305_context *ctx,
    const unsigned char key[32]);
static inline void poly1305_update(poly1305_context *ctx,
    const unsigned char *m, size_t bytes);
static inline void poly1305_finish(poly1305_context *ctx,
    unsigned char mac[16]);

/*
 * poly1305 implementation using 32 bit * 32 bit = 64 bit multiplication
 * and 64 bit addition.
 */

#define poly1305_block_size 16

/* 17 + sizeof(size_t) + 14*sizeof(unsigned long) */
typedef struct poly1305_state_internal_t {
        unsigned long r[5];
        unsigned long h[5];
        unsigned long pad[4];
        size_t leftover;
        unsigned char buffer[poly1305_block_size];
        unsigned char final;
} poly1305_state_internal_t;

/* interpret four 8 bit unsigned integers as a 32 bit unsigned integer in little endian */
static unsigned long
U8TO32(const unsigned char *p)
{
        return (((unsigned long)(p[0] & 0xff)) |
            ((unsigned long)(p[1] & 0xff) <<  8) |
            ((unsigned long)(p[2] & 0xff) << 16) |
            ((unsigned long)(p[3] & 0xff) << 24));
}

/* store a 32 bit unsigned integer as four 8 bit unsigned integers in little endian */
static void
U32TO8(unsigned char *p, unsigned long v)
{
        p[0] = (v) & 0xff;
        p[1] = (v >>  8) & 0xff;
        p[2] = (v >> 16) & 0xff;
        p[3] = (v >> 24) & 0xff;
}

static inline void
poly1305_init(poly1305_context *ctx, const unsigned char key[32])
{
        poly1305_state_internal_t *st = (poly1305_state_internal_t *)ctx;

        /* r &= 0xffffffc0ffffffc0ffffffc0fffffff */
        st->r[0] = (U8TO32(&key[0])) & 0x3ffffff;
        st->r[1] = (U8TO32(&key[3]) >> 2) & 0x3ffff03;
        st->r[2] = (U8TO32(&key[6]) >> 4) & 0x3ffc0ff;
        st->r[3] = (U8TO32(&key[9]) >> 6) & 0x3f03fff;
        st->r[4] = (U8TO32(&key[12]) >> 8) & 0x00fffff;

        /* h = 0 */
        st->h[0] = 0;
        st->h[1] = 0;
        st->h[2] = 0;
        st->h[3] = 0;
        st->h[4] = 0;

        /* save pad for later */
        st->pad[0] = U8TO32(&key[16]);
        st->pad[1] = U8TO32(&key[20]);
        st->pad[2] = U8TO32(&key[24]);
        st->pad[3] = U8TO32(&key[28]);

        st->leftover = 0;
        st->final = 0;
}

static void
poly1305_blocks(poly1305_state_internal_t *st, const unsigned char *m, size_t bytes)
{
        const unsigned long hibit = (st->final) ? 0 : (1 << 24); /* 1 << 128 */
        unsigned long r0, r1, r2, r3, r4;
        unsigned long s1, s2, s3, s4;
        unsigned long h0, h1, h2, h3, h4;
        unsigned long long d0, d1, d2, d3, d4;
        unsigned long c;

        r0 = st->r[0];
        r1 = st->r[1];
        r2 = st->r[2];
        r3 = st->r[3];
        r4 = st->r[4];

        s1 = r1 * 5;
        s2 = r2 * 5;
        s3 = r3 * 5;
        s4 = r4 * 5;

        h0 = st->h[0];
        h1 = st->h[1];
        h2 = st->h[2];
        h3 = st->h[3];
        h4 = st->h[4];

        while (bytes >= poly1305_block_size) {
                /* h += m[i] */
                h0 += (U8TO32(m + 0)) & 0x3ffffff;
                h1 += (U8TO32(m + 3) >> 2) & 0x3ffffff;
                h2 += (U8TO32(m + 6) >> 4) & 0x3ffffff;
                h3 += (U8TO32(m + 9) >> 6) & 0x3ffffff;
                h4 += (U8TO32(m + 12) >> 8) | hibit;

                /* h *= r */
                d0 = ((unsigned long long)h0 * r0) +
                    ((unsigned long long)h1 * s4) +
                    ((unsigned long long)h2 * s3) +
                    ((unsigned long long)h3 * s2) +
                    ((unsigned long long)h4 * s1);
                d1 = ((unsigned long long)h0 * r1) +
                    ((unsigned long long)h1 * r0) +
                    ((unsigned long long)h2 * s4) +
                    ((unsigned long long)h3 * s3) +
                    ((unsigned long long)h4 * s2);
                d2 = ((unsigned long long)h0 * r2) +
                    ((unsigned long long)h1 * r1) +
                    ((unsigned long long)h2 * r0) +
                    ((unsigned long long)h3 * s4) +
                    ((unsigned long long)h4 * s3);
                d3 = ((unsigned long long)h0 * r3) +
                    ((unsigned long long)h1 * r2) +
                    ((unsigned long long)h2 * r1) +
                    ((unsigned long long)h3 * r0) +
                    ((unsigned long long)h4 * s4);
                d4 = ((unsigned long long)h0 * r4) +
                    ((unsigned long long)h1 * r3) +
                    ((unsigned long long)h2 * r2) +
                    ((unsigned long long)h3 * r1) +
                    ((unsigned long long)h4 * r0);

                /* (partial) h %= p */
                c = (unsigned long)(d0 >> 26);
                h0 = (unsigned long)d0 & 0x3ffffff;
                d1 += c;
                c = (unsigned long)(d1 >> 26);
                h1 = (unsigned long)d1 & 0x3ffffff;
                d2 += c;
                c = (unsigned long)(d2 >> 26);
                h2 = (unsigned long)d2 & 0x3ffffff;
                d3 += c;
                c = (unsigned long)(d3 >> 26);
                h3 = (unsigned long)d3 & 0x3ffffff;
                d4 += c;
                c = (unsigned long)(d4 >> 26);
                h4 = (unsigned long)d4 & 0x3ffffff;
                h0 += c * 5;
                c = (h0 >> 26);
                h0 = h0 & 0x3ffffff;
                h1 += c;

                m += poly1305_block_size;
                bytes -= poly1305_block_size;
        }

        st->h[0] = h0;
        st->h[1] = h1;
        st->h[2] = h2;
        st->h[3] = h3;
        st->h[4] = h4;
}

static inline void
poly1305_update(poly1305_context *ctx, const unsigned char *m, size_t bytes)
{
        poly1305_state_internal_t *st = (poly1305_state_internal_t *)ctx;
        size_t i;

        /* handle leftover */
        if (st->leftover) {
                size_t want = (poly1305_block_size - st->leftover);
                if (want > bytes)
                        want = bytes;
                for (i = 0; i < want; i++)
                        st->buffer[st->leftover + i] = m[i];
                bytes -= want;
                m += want;
                st->leftover += want;
                if (st->leftover < poly1305_block_size)
                        return;
                poly1305_blocks(st, st->buffer, poly1305_block_size);
                st->leftover = 0;
        }

        /* process full blocks */
        if (bytes >= poly1305_block_size) {
                size_t want = (bytes & ~(poly1305_block_size - 1));
                poly1305_blocks(st, m, want);
                m += want;
                bytes -= want;
        }

        /* store leftover */
        if (bytes) {
                for (i = 0; i < bytes; i++)
                        st->buffer[st->leftover + i] = m[i];
                st->leftover += bytes;
        }
}

static inline void
poly1305_finish(poly1305_context *ctx, unsigned char mac[16])
{
        poly1305_state_internal_t *st = (poly1305_state_internal_t *)ctx;
        unsigned long h0, h1, h2, h3, h4, c;
        unsigned long g0, g1, g2, g3, g4;
        unsigned long long f;
        unsigned long mask;

        /* process the remaining block */
        if (st->leftover) {
                size_t i = st->leftover;
                st->buffer[i++] = 1;
                for (; i < poly1305_block_size; i++)
                        st->buffer[i] = 0;
                st->final = 1;
                poly1305_blocks(st, st->buffer, poly1305_block_size);
        }

        /* fully carry h */
        h0 = st->h[0];
        h1 = st->h[1];
        h2 = st->h[2];
        h3 = st->h[3];
        h4 = st->h[4];

        c = h1 >> 26;
        h1 = h1 & 0x3ffffff;
        h2 += c;
        c = h2 >> 26;
        h2 = h2 & 0x3ffffff;
        h3 += c;
        c = h3 >> 26;
        h3 = h3 & 0x3ffffff;
        h4 += c;
        c = h4 >> 26;
        h4 = h4 & 0x3ffffff;
        h0 += c * 5;
        c = h0 >> 26;
        h0 = h0 & 0x3ffffff;
        h1 += c;

        /* compute h + -p */
        g0 = h0 + 5;
        c = g0 >> 26;
        g0 &= 0x3ffffff;
        g1 = h1 + c;
        c = g1 >> 26;
        g1 &= 0x3ffffff;
        g2 = h2 + c;
        c = g2 >> 26;
        g2 &= 0x3ffffff;
        g3 = h3 + c;
        c = g3 >> 26;
        g3 &= 0x3ffffff;
        g4 = h4 + c - (1 << 26);

        /* select h if h < p, or h + -p if h >= p */
        mask = (g4 >> ((sizeof(unsigned long) * 8) - 1)) - 1;
        g0 &= mask;
        g1 &= mask;
        g2 &= mask;
        g3 &= mask;
        g4 &= mask;
        mask = ~mask;
        h0 = (h0 & mask) | g0;
        h1 = (h1 & mask) | g1;
        h2 = (h2 & mask) | g2;
        h3 = (h3 & mask) | g3;
        h4 = (h4 & mask) | g4;

        /* h = h % (2^128) */
        h0 = ((h0) | (h1 << 26)) & 0xffffffff;
        h1 = ((h1 >>  6) | (h2 << 20)) & 0xffffffff;
        h2 = ((h2 >> 12) | (h3 << 14)) & 0xffffffff;
        h3 = ((h3 >> 18) | (h4 <<  8)) & 0xffffffff;

        /* mac = (h + pad) % (2^128) */
        f = (unsigned long long)h0 + st->pad[0];
        h0 = (unsigned long)f;
        f = (unsigned long long)h1 + st->pad[1] + (f >> 32);
        h1 = (unsigned long)f;
        f = (unsigned long long)h2 + st->pad[2] + (f >> 32);
        h2 = (unsigned long)f;
        f = (unsigned long long)h3 + st->pad[3] + (f >> 32);
        h3 = (unsigned long)f;

        U32TO8(mac +  0, h0);
        U32TO8(mac +  4, h1);
        U32TO8(mac +  8, h2);
        U32TO8(mac + 12, h3);

        /* zero out the state */
        st->h[0] = 0;
        st->h[1] = 0;
        st->h[2] = 0;
        st->h[3] = 0;
        st->h[4] = 0;
        st->r[0] = 0;
        st->r[1] = 0;
        st->r[2] = 0;
        st->r[3] = 0;
        st->r[4] = 0;
        st->pad[0] = 0;
        st->pad[1] = 0;
        st->pad[2] = 0;
        st->pad[3] = 0;
}