1 files changed, 105 insertions, 0 deletions
diff --git a/src/ltc/encauth/ocb3/ocb3_decrypt_last.c b/src/ltc/encauth/ocb3/ocb3_decrypt_last.c
new file mode 100644
index 00000000..a932d537
--- /dev/null
+++ b/src/ltc/encauth/ocb3/ocb3_decrypt_last.c
@@ -0,0 +1,105 @@
+/* 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.
+ */
+
+/**
+   @file ocb3_decrypt_last.c
+   OCB implementation, internal helper, by Karel Miko
+*/
+#include "tomcrypt.h"
+
+#ifdef LTC_OCB3_MODE
+
+/**
+   Finish an OCB (decryption) stream
+   @param ocb    The OCB state
+   @param ct     The remaining ciphertext
+   @param ctlen  The length of the ciphertext (octets)
+   @param pt     [out] The output buffer
+   @return CRYPT_OK if successful
+*/
+int ocb3_decrypt_last(ocb3_state *ocb, const unsigned char *ct, unsigned long ctlen, unsigned char *pt)
+{
+   unsigned char iOffset_star[MAXBLOCKSIZE];
+   unsigned char iPad[MAXBLOCKSIZE];
+   int err, x, full_blocks, full_blocks_len, last_block_len;
+
+   LTC_ARGCHK(ocb != NULL);
+   LTC_ARGCHK(ct  != NULL);
+   if ((err = cipher_is_valid(ocb->cipher)) != CRYPT_OK) {
+      goto LBL_ERR;
+   }
+
+   full_blocks = ctlen/ocb->block_len;
+   full_blocks_len = full_blocks * ocb->block_len;
+   last_block_len = ctlen - full_blocks_len;
+
+   /* process full blocks first */
+   if (full_blocks>0) {
+     if ((err = ocb3_decrypt(ocb, ct, full_blocks_len, pt)) != CRYPT_OK) {
+       goto LBL_ERR;
+     }
+   }
+
+   if (last_block_len>0) {
+     /* Offset_* = Offset_m xor L_* */
+     ocb3_int_xor_blocks(iOffset_star, ocb->Offset_current, ocb->L_star, ocb->block_len);
+
+     /* Pad = ENCIPHER(K, Offset_*) */
+     if ((err = cipher_descriptor[ocb->cipher].ecb_encrypt(iOffset_star, iPad, &ocb->key)) != CRYPT_OK) {
+       goto LBL_ERR;
+     }
+
+     /* P_* = C_* xor Pad[1..bitlen(C_*)] */
+     ocb3_int_xor_blocks(pt+full_blocks_len, (unsigned char *)ct+full_blocks_len, iPad, last_block_len);
+
+     /* Checksum_* = Checksum_m xor (P_* || 1 || zeros(127-bitlen(P_*))) */
+     ocb3_int_xor_blocks(ocb->checksum, ocb->checksum, pt+full_blocks_len, last_block_len);
+     for(x=last_block_len; x<ocb->block_len; x++) {
+       if (x == last_block_len)
+         ocb->checksum[x] ^= 0x80;
+       else
+         ocb->checksum[x] ^= 0x00;
+     }
+
+     /* Tag = ENCIPHER(K, Checksum_* xor Offset_* xor L_$) xor HASH(K,A) */
+     /* at this point we calculate only: Tag_part = ENCIPHER(K, Checksum_* xor Offset_* xor L_$) */
+     for(x=0; x<ocb->block_len; x++) {
+       ocb->tag_part[x] = (ocb->checksum[x] ^ iOffset_star[x]) ^ ocb->L_dollar[x];
+     }
+     if ((err = cipher_descriptor[ocb->cipher].ecb_encrypt(ocb->tag_part, ocb->tag_part, &ocb->key)) != CRYPT_OK) {
+       goto LBL_ERR;
+     }
+   }
+   else {
+     /* Tag = ENCIPHER(K, Checksum_m xor Offset_m xor L_$) xor HASH(K,A) */
+     /* at this point we calculate only: Tag_part = ENCIPHER(K, Checksum_m xor Offset_m xor L_$) */
+     for(x=0; x<ocb->block_len; x++) {
+       ocb->tag_part[x] = (ocb->checksum[x] ^ ocb->Offset_current[x]) ^ ocb->L_dollar[x];
+     }
+     if ((err = cipher_descriptor[ocb->cipher].ecb_encrypt(ocb->tag_part, ocb->tag_part, &ocb->key)) != CRYPT_OK) {
+       goto LBL_ERR;
+     }
+   }
+
+   err = CRYPT_OK;
+
+LBL_ERR:
+#ifdef LTC_CLEAN_STACK
+   zeromem(iOffset_star, MAXBLOCKSIZE);
+   zeromem(iPad, MAXBLOCKSIZE);
+#endif
+
+   return err;
+}
+
+#endif
+
+/* $Source$ */
+/* $Revision$ */
+/* $Date$ */