X-Git-Url: https://www.bearssl.org/gitweb//home/git/?p=BearSSL;a=blobdiff_plain;f=src%2Frsa%2Frsa_i31_pubexp.c;fp=src%2Frsa%2Frsa_i31_pubexp.c;h=f26537d8548fb2f4e561ec804b7e3f2bc02931a3;hp=0000000000000000000000000000000000000000;hb=d8fa415fcabecb7d7c8461d8edbe2f265911e72c;hpb=cfbc702d3d64c209784b664eeab8867b603f4d4c

diff --git a/src/rsa/rsa_i31_pubexp.c b/src/rsa/rsa_i31_pubexp.c
new file mode 100644
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+++ b/src/rsa/rsa_i31_pubexp.c
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+/*
+ * Copyright (c) 2018 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"
+
+/*
+ * Recompute public exponent, based on factor p and reduced private
+ * exponent dp.
+ */
+static uint32_t
+get_pubexp(const unsigned char *pbuf, size_t plen,
+	const unsigned char *dpbuf, size_t dplen)
+{
+	/*
+	 * dp is the inverse of e modulo p-1. If p = 3 mod 4, then
+	 * p-1 = 2*((p-1)/2). Taken modulo 2, e is odd and has inverse 1;
+	 * thus, dp must be odd.
+	 *
+	 * We compute the inverse of dp modulo (p-1)/2. This requires
+	 * first reducing dp modulo (p-1)/2 (this can be done with a
+	 * conditional subtract, no need to use the generic modular
+	 * reduction function); then, we use moddiv.
+	 */
+
+	uint32_t tmp[6 * ((BR_MAX_RSA_FACTOR + 61) / 31)];
+	uint32_t *p, *dp, *x;
+	size_t len;
+	uint32_t e;
+
+	/*
+	 * Compute actual factor length (in bytes) and check that it fits
+	 * under our size constraints.
+	 */
+	while (plen > 0 && *pbuf == 0) {
+		pbuf ++;
+		plen --;
+	}
+	if (plen == 0 || plen < 5 || plen > (BR_MAX_RSA_FACTOR / 8)) {
+		return 0;
+	}
+
+	/*
+	 * Compute actual reduced exponent length (in bytes) and check that
+	 * it is not longer than p.
+	 */
+	while (dplen > 0 && *dpbuf == 0) {
+		dpbuf ++;
+		dplen --;
+	}
+	if (dplen > plen || dplen == 0
+		|| (dplen == plen && dpbuf[0] > pbuf[0]))
+	{
+		return 0;
+	}
+
+	/*
+	 * Verify that p = 3 mod 4 and that dp is odd.
+	 */
+	if ((pbuf[plen - 1] & 3) != 3 || (dpbuf[dplen - 1] & 1) != 1) {
+		return 0;
+	}
+
+	/*
+	 * Decode p and compute (p-1)/2.
+	 */
+	p = tmp;
+	br_i31_decode(p, pbuf, plen);
+	len = (p[0] + 63) >> 5;
+	br_i31_rshift(p, 1);
+
+	/*
+	 * Decode dp and make sure its announced bit length matches that of
+	 * p (we already know that the size of dp, in bits, does not exceed
+	 * the size of p, so we just have to copy the header word).
+	 */
+	dp = p + len;
+	memset(dp, 0, len * sizeof *dp);
+	br_i31_decode(dp, dpbuf, dplen);
+	dp[0] = p[0];
+
+	/*
+	 * Subtract (p-1)/2 from dp if necessary.
+	 */
+	br_i31_sub(dp, p, NOT(br_i31_sub(dp, p, 0)));
+
+	/*
+	 * If another subtraction is needed, then this means that the
+	 * value was invalid. We don't care to leak information about
+	 * invalid keys.
+	 */
+	if (br_i31_sub(dp, p, 0) == 0) {
+		return 0;
+	}
+
+	/*
+	 * Invert dp modulo (p-1)/2. If the inversion fails, then the
+	 * key value was invalid.
+	 */
+	x = dp + len;
+	br_i31_zero(x, p[0]);
+	x[1] = 1;
+	if (br_i31_moddiv(x, dp, p, br_i31_ninv31(p[1]), x + len) == 0) {
+		return 0;
+	}
+
+	/*
+	 * We now have an inverse. We must set it to zero (error) if its
+	 * length is greater than 32 bits and/or if it is an even integer.
+	 * Take care that the bit_length function returns an encoded
+	 * bit length.
+	 */
+	e = (uint32_t)x[1] | ((uint32_t)x[2] << 31);
+	e &= -LT(br_i31_bit_length(x + 1, len - 1), 34);
+	e &= -(e & 1);
+	return e;
+}
+
+/* see bearssl_rsa.h */
+uint32_t
+br_rsa_i31_compute_pubexp(const br_rsa_private_key *sk)
+{
+	/*
+	 * Get the public exponent from both p and q. This is the right
+	 * exponent if we get twice the same value.
+	 */
+	uint32_t ep, eq;
+
+	ep = get_pubexp(sk->p, sk->plen, sk->dp, sk->dplen);
+	eq = get_pubexp(sk->q, sk->qlen, sk->dq, sk->dqlen);
+	return ep & -EQ(ep, eq);
+}