/* rsa.c
**
** Copyright 2012, The Android Open Source Project
**
** Redistribution and use in source and binary forms, with or without
** modification, are permitted provided that the following conditions are met:
**     * Redistributions of source code must retain the above copyright
**       notice, this list of conditions and the following disclaimer.
**     * Redistributions in binary form must reproduce the above copyright
**       notice, this list of conditions and the following disclaimer in the
**       documentation and/or other materials provided with the distribution.
**     * Neither the name of Google Inc. nor the names of its contributors may
**       be used to endorse or promote products derived from this software
**       without specific prior written permission.
**
** THIS SOFTWARE IS PROVIDED BY Google Inc. ``AS IS'' AND ANY EXPRESS OR
** IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
** MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO
** EVENT SHALL Google Inc. BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
** SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
** PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
** OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
** WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
** OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
** ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

#include "mincrypt/rsa.h"
#include "mincrypt/sha.h"
#include "mincrypt/sha256.h"

// a[] -= mod
static void subM(const RSAPublicKey* key,
                 uint32_t* a) {
    int64_t A = 0;
    int i;
    for (i = 0; i < key->len; ++i) {
        A += (uint64_t)a[i] - key->n[i];
        a[i] = (uint32_t)A;
        A >>= 32;
    }
}

// return a[] >= mod
static int geM(const RSAPublicKey* key,
               const uint32_t* a) {
    int i;
    for (i = key->len; i;) {
        --i;
        if (a[i] < key->n[i]) return 0;
        if (a[i] > key->n[i]) return 1;
    }
    return 1;  // equal
}

// montgomery c[] += a * b[] / R % mod
static void montMulAdd(const RSAPublicKey* key,
                       uint32_t* c,
                       const uint32_t a,
                       const uint32_t* b) {
    uint64_t A = (uint64_t)a * b[0] + c[0];
    uint32_t d0 = (uint32_t)A * key->n0inv;
    uint64_t B = (uint64_t)d0 * key->n[0] + (uint32_t)A;
    int i;

    for (i = 1; i < key->len; ++i) {
        A = (A >> 32) + (uint64_t)a * b[i] + c[i];
        B = (B >> 32) + (uint64_t)d0 * key->n[i] + (uint32_t)A;
        c[i - 1] = (uint32_t)B;
    }

    A = (A >> 32) + (B >> 32);

    c[i - 1] = (uint32_t)A;

    if (A >> 32) {
        subM(key, c);
    }
}

// montgomery c[] = a[] * b[] / R % mod
static void montMul(const RSAPublicKey* key,
                    uint32_t* c,
                    const uint32_t* a,
                    const uint32_t* b) {
    int i;
    for (i = 0; i < key->len; ++i) {
        c[i] = 0;
    }
    for (i = 0; i < key->len; ++i) {
        montMulAdd(key, c, a[i], b);
    }
}

// In-place public exponentiation.
// Input and output big-endian byte array in inout.
static void modpow(const RSAPublicKey* key,
                   uint8_t* inout) {
    uint32_t a[RSANUMWORDS];
    uint32_t aR[RSANUMWORDS];
    uint32_t aaR[RSANUMWORDS];
    uint32_t* aaa = 0;
    int i;

    // Convert from big endian byte array to little endian word array.
    for (i = 0; i < key->len; ++i) {
        uint32_t tmp =
            (inout[((key->len - 1 - i) * 4) + 0] << 24) |
            (inout[((key->len - 1 - i) * 4) + 1] << 16) |
            (inout[((key->len - 1 - i) * 4) + 2] << 8) |
            (inout[((key->len - 1 - i) * 4) + 3] << 0);
        a[i] = tmp;
    }

    if (key->exponent == 65537) {
        aaa = aaR;  // Re-use location.
        montMul(key, aR, a, key->rr);  // aR = a * RR / R mod M
        for (i = 0; i < 16; i += 2) {
            montMul(key, aaR, aR, aR);  // aaR = aR * aR / R mod M
            montMul(key, aR, aaR, aaR);  // aR = aaR * aaR / R mod M
        }
        montMul(key, aaa, aR, a);  // aaa = aR * a / R mod M
    } else if (key->exponent == 3) {
        aaa = aR;  // Re-use location.
        montMul(key, aR, a, key->rr);  /* aR = a * RR / R mod M   */
        montMul(key, aaR, aR, aR);     /* aaR = aR * aR / R mod M */
        montMul(key, aaa, aaR, a);     /* aaa = aaR * a / R mod M */
    } else {
        return; // avoid NULL aaa caused exceptions
    }

    // Make sure aaa < mod; aaa is at most 1x mod too large.
    if (geM(key, aaa)) {
        subM(key, aaa);
    }

    // Convert to bigendian byte array
    for (i = key->len - 1; i >= 0; --i) {
        uint32_t tmp = aaa[i];
        *inout++ = tmp >> 24;
        *inout++ = tmp >> 16;
        *inout++ = tmp >> 8;
        *inout++ = tmp >> 0;
    }
}

// Expected PKCS1.5 signature padding bytes, for a keytool RSA signature.
// Has the 0-length optional parameter encoded in the ASN1 (as opposed to the
// other flavor which omits the optional parameter entirely). This code does not
// accept signatures without the optional parameter.

/*
static const uint8_t sha_padding[RSANUMBYTES] = {
    0x00, 0x01, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0x00, 0x30, 0x21, 0x30,
    0x09, 0x06, 0x05, 0x2b, 0x0e, 0x03, 0x02, 0x1a,
    0x05, 0x00, 0x04, 0x14,

    // 20 bytes of hash go here.
    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
};
*/

// SHA-1 of PKCS1.5 signature sha_padding for 2048 bit, as above.
// At the location of the bytes of the hash all 00 are hashed.
static const uint8_t kExpectedPadShaRsa2048[SHA_DIGEST_SIZE] = {
    0xdc, 0xbd, 0xbe, 0x42, 0xd5, 0xf5, 0xa7, 0x2e,
    0x6e, 0xfc, 0xf5, 0x5d, 0xaf, 0x9d, 0xea, 0x68,
    0x7c, 0xfb, 0xf1, 0x67
};

/*
static const uint8_t sha256_padding[RSANUMBYTES] = {
    0x00, 0x01, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff, 0x00, 0x30, 0x31, 0x30,
    0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01, 0x65,
    0x03, 0x04, 0x02, 0x01, 0x05, 0x00, 0x04, 0x20,

    // 32 bytes of hash go here.
    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
};
*/

// SHA-256 of PKCS1.5 signature sha256_padding for 2048 bit, as above.
// At the location of the bytes of the hash all 00 are hashed.
static const uint8_t kExpectedPadSha256Rsa2048[SHA256_DIGEST_SIZE] = {
    0xab, 0x28, 0x8d, 0x8a, 0xd7, 0xd9, 0x59, 0x92,
    0xba, 0xcc, 0xf8, 0x67, 0x20, 0xe1, 0x15, 0x2e,
    0x39, 0x8d, 0x80, 0x36, 0xd6, 0x6f, 0xf0, 0xfd,
    0x90, 0xe8, 0x7d, 0x8b, 0xe1, 0x7c, 0x87, 0x59,
};

// Verify a 2048-bit RSA PKCS1.5 signature against an expected hash.
// Both e=3 and e=65537 are supported.  hash_len may be
// SHA_DIGEST_SIZE (== 20) to indicate a SHA-1 hash, or
// SHA256_DIGEST_SIZE (== 32) to indicate a SHA-256 hash.  No other
// values are supported.
//
// Returns 1 on successful verification, 0 on failure.
int RSA_verify(const RSAPublicKey *key,
               const uint8_t *signature,
               const int len,
               const uint8_t *hash,
               const int hash_len) {
    uint8_t buf[RSANUMBYTES];
    int i;
    const uint8_t* padding_hash;

    if (key->len != RSANUMWORDS) {
        return 0;  // Wrong key passed in.
    }

    if (len != sizeof(buf)) {
        return 0;  // Wrong input length.
    }

    if (hash_len != SHA_DIGEST_SIZE &&
        hash_len != SHA256_DIGEST_SIZE) {
        return 0;  // Unsupported hash.
    }

    if (key->exponent != 3 && key->exponent != 65537) {
        return 0;  // Unsupported exponent.
    }

    for (i = 0; i < len; ++i) {  // Copy input to local workspace.
        buf[i] = signature[i];
    }

    modpow(key, buf);  // In-place exponentiation.

    // Xor sha portion, so it all becomes 00 iff equal.
    for (i = len - hash_len; i < len; ++i) {
        buf[i] ^= *hash++;
    }

    // Hash resulting buf, in-place.
    switch (hash_len) {
        case SHA_DIGEST_SIZE:
            padding_hash = kExpectedPadShaRsa2048;
            SHA_hash(buf, len, buf);
            break;
        case SHA256_DIGEST_SIZE:
            padding_hash = kExpectedPadSha256Rsa2048;
            SHA256_hash(buf, len, buf);
            break;
        default:
            return 0;
    }

    // Compare against expected hash value.
    for (i = 0; i < hash_len; ++i) {
        if (buf[i] != padding_hash[i]) {
            return 0;
        }
    }

    return 1;  // All checked out OK.
}