Added RSA/OAEP implementation.

[BearSSL] / src / rsa / rsa_oaep_unpad.c

diff --git a/src/rsa/rsa_oaep_unpad.c b/src/rsa/rsa_oaep_unpad.c
new file mode 100644 (file)
index 0000000..e172a35
--- /dev/null
+++ b/src/rsa/rsa_oaep_unpad.c
@@ -0,0 +1,145 @@
+/*
+ * 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"
+
+/*
+ * Hash some data and XOR the result into the provided buffer. This is put
+ * as a separate function so that stack allocation of the hash function
+ * context is done only for the duration of the hash.
+ */
+static void
+xor_hash_data(const br_hash_class *dig, void *dst, const void *src, size_t len)
+{
+       br_hash_compat_context hc;
+       unsigned char tmp[64];
+       unsigned char *buf;
+       size_t u, hlen;
+
+       hc.vtable = dig;
+       dig->init(&hc.vtable);
+       dig->update(&hc.vtable, src, len);
+       dig->out(&hc.vtable, tmp);
+       buf = dst;
+       hlen = br_digest_size(dig);
+       for (u = 0; u < hlen; u ++) {
+               buf[u] ^= tmp[u];
+       }
+}
+
+/* see inner.h */
+uint32_t
+br_rsa_oaep_unpad(const br_hash_class *dig,
+       const void *label, size_t label_len,
+       void *data, size_t *len)
+{
+       size_t u, k, hlen;
+       unsigned char *buf;
+       uint32_t r, s, zlen;
+
+       hlen = br_digest_size(dig);
+       k = *len;
+       buf = data;
+
+       /*
+        * There must be room for the padding.
+        */
+       if (k < ((hlen << 1) + 2)) {
+               return 0;
+       }
+
+       /*
+        * Unmask the seed, then the DB value.
+        */
+       br_mgf1_xor(buf + 1, hlen, dig, buf + 1 + hlen, k - hlen - 1);
+       br_mgf1_xor(buf + 1 + hlen, k - hlen - 1, dig, buf + 1, hlen);
+
+       /*
+        * Hash the label and XOR it with the value in the array; if
+        * they are equal then these should yield only zeros.
+        */
+       xor_hash_data(dig, buf + 1 + hlen, label, label_len);
+
+       /*
+        * At that point, if the padding was correct, when we should
+        * have: 0x00 || seed || 0x00 ... 0x00 0x01 || M
+        * Padding is valid as long as:
+        *  - There is at least hlen+1 leading bytes of value 0x00.
+        *  - There is at least one non-zero byte.
+        *  - The first (leftmost) non-zero byte has value 0x01.
+        *
+        * Ultimately, we may leak the resulting message length, i.e.
+        * the position of the byte of value 0x01, but we must take care
+        * to do so only if the number of zero bytes has been verified
+        * to be at least hlen+1.
+        *
+        * The loop below counts the number of bytes of value 0x00, and
+        * checks that the next byte has value 0x01, in constant-time.
+        *
+        *  - If the initial byte (before the seed) is not 0x00, then
+        *    r and s are set to 0, and stay there.
+        *  - Value r is 1 until the first non-zero byte is reached
+        *    (after the seed); it switches to 0 at that point.
+        *  - Value s is set to 1 if and only if the data encountered
+        *    at the time of the transition of r from 1 to 0 has value
+        *    exactly 0x01.
+        *  - Value zlen counts the number of leading bytes of value zero
+        *    (after the seed).
+        */
+       r = 1 - ((buf[0] + 0xFF) >> 8);
+       s = 0;
+       zlen = 0;
+       for (u = hlen + 1; u < k; u ++) {
+               uint32_t w, nz;
+
+               w = buf[u];
+
+               /*
+                * nz == 1 only for the first non-zero byte.
+                */
+               nz = r & ((w + 0xFF) >> 8);
+               s |= nz & EQ(w, 0x01);
+               r &= NOT(nz);
+               zlen += r;
+       }
+
+       /*
+        * Padding is correct only if s == 1, _and_ zlen >= hlen.
+        */
+       s &= GE(zlen, (uint32_t)hlen);
+
+       /*
+        * At that point, padding was verified, and we are now allowed
+        * to make conditional jumps.
+        */
+       if (s) {
+               size_t plen;
+
+               plen = 2 + hlen + zlen;
+               k -= plen;
+               memmove(data, data + plen, k);
+               *len = k;
+       }
+       return s;
+}