--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/pkcs_1_oaep_decode.c	Mon May 31 18:21:40 2004 +0000
@@ -0,0 +1,135 @@
+/* LibTomCrypt, modular cryptographic library -- Tom St Denis
+ *
+ * LibTomCrypt is a library that provides various cryptographic
+ * algorithms in a highly modular and flexible manner.
+ *
+ * The library is free for all purposes without any express
+ * guarantee it works.
+ *
+ * Tom St Denis, [email protected], http://libtomcrypt.org
+ */
+#include "mycrypt.h"
+
+/* OAEP Padding for PKCS #1 -- Tom St Denis */
+
+#ifdef PKCS_1
+
+int pkcs_1_oaep_decode(const unsigned char *msg,    unsigned long msglen,
+                        const unsigned char *lparam, unsigned long lparamlen,
+                              unsigned long modulus_bitlen, int hash_idx,
+                              unsigned char *out,    unsigned long *outlen)
+{
+   unsigned char DB[1024], seed[MAXBLOCKSIZE], mask[sizeof(DB)];
+   unsigned long hLen, x, y, modulus_len;
+   int           err;
+
+   _ARGCHK(msg    != NULL);
+   _ARGCHK(out    != NULL);
+   _ARGCHK(outlen != NULL);
+   
+   /* test valid hash */
+   if ((err = hash_is_valid(hash_idx)) != CRYPT_OK) { 
+      return err;
+   }
+   hLen        = hash_descriptor[hash_idx].hashsize;
+   modulus_len = (modulus_bitlen >> 3) + (modulus_bitlen & 7 ? 1 : 0);
+
+   /* test message size */
+   if (modulus_len >= sizeof(DB) || msglen != modulus_len) {
+      return CRYPT_PK_INVALID_SIZE;
+   }
+
+   /* ok so it's now in the form
+  
+      0x00  || maskedseed || maskedDB 
+  
+       1    ||   hLen     ||  modulus_len - hLen - 1
+   
+    */
+
+   /* must have leading 0x00 byte */
+   if (msg[0] != 0x00) {
+      return CRYPT_INVALID_PACKET;
+   }
+
+   /* now read the masked seed */
+   for (x = 1, y = 0; y < hLen; y++) {
+      seed[y] = msg[x++];
+   }
+
+   /* now read the masked DB */
+   for (y = 0; y < modulus_len - hLen - 1; y++) {
+      DB[y] = msg[x++];
+   }
+
+   /* compute MGF1 of maskedDB (hLen) */ 
+   if ((err = pkcs_1_mgf1(DB, modulus_len - hLen - 1, hash_idx, mask, hLen)) != CRYPT_OK) {
+      return err;
+   }
+
+   /* XOR against seed */
+   for (y = 0; y < hLen; y++) {
+      seed[y] ^= mask[y];
+   }
+
+   /* compute MGF1 of seed (k - hlen - 1) */
+   if ((err = pkcs_1_mgf1(seed, hLen, hash_idx, mask, modulus_len - hLen - 1)) != CRYPT_OK) {
+      return err;
+   }
+
+   /* xor against DB */
+   for (y = 0; y < (modulus_len - hLen - 1); y++) {
+       DB[y] ^= mask[y]; 
+   }
+
+   /* now DB == lhash || PS || 0x01 || M, PS == k - mlen - 2hlen - 2 zeroes */
+
+   /* compute lhash and store it in seed [reuse temps!] */
+   x = sizeof(seed);
+   if (lparam != NULL) {
+      if ((err = hash_memory(hash_idx, lparam, lparamlen, seed, &x)) != CRYPT_OK) {
+         return err;
+      }
+   } else {
+      /* can't pass hash_memory a NULL so use DB with zero length */
+      if ((err = hash_memory(hash_idx, DB, 0, seed, &x)) != CRYPT_OK) {
+         return err;
+      }
+   }
+
+   /* compare the lhash'es */
+   if (memcmp(seed, DB, hLen) != 0) {
+      return CRYPT_INVALID_PACKET;
+   }
+
+   /* now zeroes before a 0x01 */
+   for (x = hLen; x < (modulus_len - hLen - 1) && DB[x] == 0x00; x++) {
+      /* step... */
+   }
+
+   /* error out if wasn't 0x01 */
+   if (x == (modulus_len - hLen - 1) || DB[x] != 0x01) {
+      return CRYPT_INVALID_PACKET;
+   }
+
+   /* rest is the message (and skip 0x01) */
+   if (msglen - ++x > *outlen) {
+      return CRYPT_BUFFER_OVERFLOW;
+   }
+
+   /* copy message */
+   *outlen = (modulus_len - hLen - 1) - x;
+   for (y = 0; x != (modulus_len - hLen - 1); ) {
+       out[y++] = DB[x++];
+   }
+
+#ifdef CLEAN_STACK
+   zeromem(DB,   sizeof(DB));
+   zeromem(seed, sizeof(seed));
+   zeromem(mask, sizeof(mask));
+#endif
+
+   return CRYPT_OK;
+}
+
+#endif /* PKCS_1 */