/* $OpenBSD: rsa_oaep.c,v 1.29 2018/08/19 20:17:20 tb Exp $ */
/* Written by Ulf Moeller. This software is distributed on an "AS IS"
   basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. */

/* EME-OAEP as defined in RFC 2437 (PKCS #1 v2.0) */

/* See Victor Shoup, "OAEP reconsidered," Nov. 2000,
 * <URL: http://www.shoup.net/papers/oaep.ps.Z>
 * for problems with the security proof for the
 * original OAEP scheme, which EME-OAEP is based on.
 *
 * A new proof can be found in E. Fujisaki, T. Okamoto,
 * D. Pointcheval, J. Stern, "RSA-OEAP is Still Alive!",
 * Dec. 2000, <URL: http://eprint.iacr.org/2000/061/>.
 * The new proof has stronger requirements for the
 * underlying permutation: "partial-one-wayness" instead
 * of one-wayness.  For the RSA function, this is
 * an equivalent notion.
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <openssl/opensslconf.h>

#if !defined(OPENSSL_NO_SHA) && !defined(OPENSSL_NO_SHA1)

#include <openssl/bn.h>
#include <openssl/err.h>
#include <openssl/evp.h>
#include <openssl/rsa.h>
#include <openssl/sha.h>

static int MGF1(unsigned char *mask, long len, const unsigned char *seed,
    long seedlen);

int
RSA_padding_add_PKCS1_OAEP(unsigned char *to, int tlen,
    const unsigned char *from, int flen, const unsigned char *param, int plen)
{
	int i, emlen = tlen - 1;
	unsigned char *db, *seed;
	unsigned char *dbmask, seedmask[SHA_DIGEST_LENGTH];

	if (flen > emlen - 2 * SHA_DIGEST_LENGTH - 1) {
		RSAerror(RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
		return 0;
	}

	if (emlen < 2 * SHA_DIGEST_LENGTH + 1) {
		RSAerror(RSA_R_KEY_SIZE_TOO_SMALL);
		return 0;
	}

	to[0] = 0;
	seed = to + 1;
	db = to + SHA_DIGEST_LENGTH + 1;

	if (!EVP_Digest((void *)param, plen, db, NULL, EVP_sha1(), NULL))
		return 0;
	memset(db + SHA_DIGEST_LENGTH, 0,
	    emlen - flen - 2 * SHA_DIGEST_LENGTH - 1);
	db[emlen - flen - SHA_DIGEST_LENGTH - 1] = 0x01;
	memcpy(db + emlen - flen - SHA_DIGEST_LENGTH, from, flen);
	arc4random_buf(seed, SHA_DIGEST_LENGTH);

	dbmask = malloc(emlen - SHA_DIGEST_LENGTH);
	if (dbmask == NULL) {
		RSAerror(ERR_R_MALLOC_FAILURE);
		return 0;
	}

	if (MGF1(dbmask, emlen - SHA_DIGEST_LENGTH, seed,
	    SHA_DIGEST_LENGTH) < 0) {
		free(dbmask);
		return 0;
	}
	for (i = 0; i < emlen - SHA_DIGEST_LENGTH; i++)
		db[i] ^= dbmask[i];

	if (MGF1(seedmask, SHA_DIGEST_LENGTH, db,
	    emlen - SHA_DIGEST_LENGTH) < 0) {
		free(dbmask);
		return 0;
	}
	for (i = 0; i < SHA_DIGEST_LENGTH; i++)
		seed[i] ^= seedmask[i];

	free(dbmask);
	return 1;
}

int
RSA_padding_check_PKCS1_OAEP(unsigned char *to, int tlen,
    const unsigned char *from, int flen, int num, const unsigned char *param,
    int plen)
{
	int i, dblen, mlen = -1;
	const unsigned char *maskeddb;
	int lzero;
	unsigned char *db = NULL;
	unsigned char seed[SHA_DIGEST_LENGTH], phash[SHA_DIGEST_LENGTH];
	unsigned char *padded_from;
	int bad = 0;

	if (--num < 2 * SHA_DIGEST_LENGTH + 1)
		/*
		 * 'num' is the length of the modulus, i.e. does not depend
		 * on the particular ciphertext.
		 */
		goto decoding_err;

	lzero = num - flen;
	if (lzero < 0) {
		/*
		 * signalling this error immediately after detection might allow
		 * for side-channel attacks (e.g. timing if 'plen' is huge
		 * -- cf. James H. Manger, "A Chosen Ciphertext Attack on RSA
		 * Optimal Asymmetric Encryption Padding (OAEP) [...]",
		 * CRYPTO 2001), so we use a 'bad' flag
		 */
		bad = 1;
		lzero = 0;
		flen = num; /* don't overflow the memcpy to padded_from */
	}

	dblen = num - SHA_DIGEST_LENGTH;
	if ((db = malloc(dblen + num)) == NULL) {
		RSAerror(ERR_R_MALLOC_FAILURE);
		return -1;
	}

	/*
	 * Always do this zero-padding copy (even when lzero == 0)
	 * to avoid leaking timing info about the value of lzero.
	 */
	padded_from = db + dblen;
	memset(padded_from, 0, lzero);
	memcpy(padded_from + lzero, from, flen);

	maskeddb = padded_from + SHA_DIGEST_LENGTH;

	if (MGF1(seed, SHA_DIGEST_LENGTH, maskeddb, dblen))
		goto err;
	for (i = 0; i < SHA_DIGEST_LENGTH; i++)
		seed[i] ^= padded_from[i];

	if (MGF1(db, dblen, seed, SHA_DIGEST_LENGTH))
		goto err;
	for (i = 0; i < dblen; i++)
		db[i] ^= maskeddb[i];

	if (!EVP_Digest((void *)param, plen, phash, NULL, EVP_sha1(), NULL))
		goto err;

	if (timingsafe_memcmp(db, phash, SHA_DIGEST_LENGTH) != 0 || bad)
		goto decoding_err;
	else {
		for (i = SHA_DIGEST_LENGTH; i < dblen; i++)
			if (db[i] != 0x00)
				break;
		if (i == dblen || db[i] != 0x01)
			goto decoding_err;
		else {
			/* everything looks OK */

			mlen = dblen - ++i;
			if (tlen < mlen) {
				RSAerror(RSA_R_DATA_TOO_LARGE);
				mlen = -1;
			} else
				memcpy(to, db + i, mlen);
		}
	}
	free(db);
	return mlen;

 decoding_err:
	/*
	 * To avoid chosen ciphertext attacks, the error message should not
	 * reveal which kind of decoding error happened
	 */
	RSAerror(RSA_R_OAEP_DECODING_ERROR);
 err:
	free(db);
	return -1;
}

int
PKCS1_MGF1(unsigned char *mask, long len, const unsigned char *seed,
    long seedlen, const EVP_MD *dgst)
{
	long i, outlen = 0;
	unsigned char cnt[4];
	EVP_MD_CTX c;
	unsigned char md[EVP_MAX_MD_SIZE];
	int mdlen;
	int rv = -1;

	EVP_MD_CTX_init(&c);
	mdlen = EVP_MD_size(dgst);
	if (mdlen < 0)
		goto err;
	for (i = 0; outlen < len; i++) {
		cnt[0] = (unsigned char)((i >> 24) & 255);
		cnt[1] = (unsigned char)((i >> 16) & 255);
		cnt[2] = (unsigned char)((i >> 8)) & 255;
		cnt[3] = (unsigned char)(i & 255);
		if (!EVP_DigestInit_ex(&c, dgst, NULL) ||
		    !EVP_DigestUpdate(&c, seed, seedlen) ||
		    !EVP_DigestUpdate(&c, cnt, 4))
			goto err;
		if (outlen + mdlen <= len) {
			if (!EVP_DigestFinal_ex(&c, mask + outlen, NULL))
				goto err;
			outlen += mdlen;
		} else {
			if (!EVP_DigestFinal_ex(&c, md, NULL))
				goto err;
			memcpy(mask + outlen, md, len - outlen);
			outlen = len;
		}
	}
	rv = 0;
 err:
	EVP_MD_CTX_cleanup(&c);
	return rv;
}

static int
MGF1(unsigned char *mask, long len, const unsigned char *seed, long seedlen)
{
	return PKCS1_MGF1(mask, len, seed, seedlen, EVP_sha1());
}
#endif
