/*
 * Copyright (c) 2017 Thomas Pornin <pornin@bolet.org>
 *
 * Permission is hereby granted, free of charge, to any person obtaining
 * a copy of this software and associated documentation files (the
 * "Software"), to deal in the Software without restriction, including
 * without limitation the rights to use, copy, modify, merge, publish,
 * distribute, sublicense, and/or sell copies of the Software, and to
 * permit persons to whom the Software is furnished to do so, subject to
 * the following conditions:
 *
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
 * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
 * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

#include "inner.h"

/* see bearssl_rsa.h */
uint32_t br_rsa_pkcs1_sig_unpad(const unsigned char *sig, size_t sig_len, const unsigned char *hash_oid,
                                size_t hash_len, unsigned char *hash_out) {
    static const unsigned char pad1[] = {0x00, 0x01, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF};

    unsigned char pad2[43];
    size_t u, x2, x3, pad_len, zlen;

    if (sig_len < 11) {
        return 0;
    }

    /*
     * Expected format:
     *  00 01 FF ... FF 00 30 x1 30 x2 06 x3 OID [ 05 00 ] 04 x4 HASH
     *
     * with the following rules:
     *
     *  -- Total length is that of the modulus and the signature
     *     (this was already verified by br_rsa_i31_public()).
     *
     *  -- There are at least eight bytes of value 0xFF.
     *
     *  -- x4 is equal to the hash length (hash_len).
     *
     *  -- x3 is equal to the encoded OID value length (so x3 is the
     *     first byte of hash_oid[]).
     *
     *  -- If the "05 00" is present, then x2 == x3 + 4; otherwise,
     *     x2 == x3 + 2.
     *
     *  -- x1 == x2 + x4 + 4.
     *
     * So the total length after the last "FF" is either x3 + x4 + 11
     * (with the "05 00") or x3 + x4 + 9 (without the "05 00").
     */

    /*
     * Constant-time padding verification.
     * Accumulate errors without early exits.
     */
    uint32_t bad = 0;

    /* Check "00 01 FF FF FF FF FF FF FF FF" header */
    for (u = 0; u < sizeof pad1; u++)
        bad |= sig[u] ^ pad1[u];

    /*
     * Find the end of FF bytes: scan all remaining bytes,
     * track position of first non-FF byte in constant time.
     */
    size_t ff_end = sig_len; /* sentinel: no non-FF found */
    for (u = sizeof pad1; u < sig_len; u++) {
        /* ff_end = min(ff_end, u) when sig[u] != 0xFF */
        uint32_t is_not_ff = ((uint32_t)(sig[u] ^ 0xFF) + 0xFF) >> 8; /* 1 if != FF, 0 if == FF */
        uint32_t mask      = -(ff_end > u) & -is_not_ff;              /* pick u only on first non-FF */
        ff_end             = (ff_end & ~(size_t)mask) | (u & (size_t)mask);
    }
    u = ff_end;

    if (hash_oid == NULL) {
        bad |= (sig_len - u != hash_len + 1);
        bad |= sig[u < sig_len ? u : 0]; /* sig[u] must be 0x00 */
    } else {
        x3      = hash_oid[0];
        pad_len = x3 + 9;
        memset(pad2, 0, pad_len);
        zlen = sig_len - u - hash_len;

        /* Accept either pad_len (no NULL params) or pad_len+2 (with "05 00") */
        uint32_t has_null = (zlen == pad_len + 2);
        bad |= (zlen != pad_len) & !has_null;

        if (has_null) {
            x2                = x3 + 4;
            pad_len           = zlen;
            pad2[pad_len - 4] = 0x05;
        } else {
            x2 = x3 + 2;
        }

        pad2[0] = 0x00;
        pad2[1] = 0x30;
        pad2[2] = x2 + hash_len + 4;
        pad2[3] = 0x30;
        pad2[4] = x2;
        pad2[5] = 0x06;
        memcpy(pad2 + 6, hash_oid, x3 + 1);
        pad2[pad_len - 2] = 0x04;
        pad2[pad_len - 1] = hash_len;

        /* Constant-time comparison of padding structure */
        for (size_t i = 0; i < pad_len && u + i < sig_len; i++)
            bad |= pad2[i] ^ sig[u + i];
    }

    if (bad)
        return 0;

    memcpy(hash_out, sig + sig_len - hash_len, hash_len);
    return 1;
}