Improve polynomial doubling code, move to util

Now does 64-bits at a time instead of 8 bits, and avoids conditional
timing channel on the XOR carry. Confirmed that at least GCC 7 and
Clang 4 on x86-64 compile the functions without conditional jumps.

Also removes CMAC as a dependency of OCB, which only needed it in
order to call CMAC::poly_double

1 files changed, 6 insertions, 36 deletions

diff --git a/src/lib/mac/cmac/cmac.cpp b/src/lib/mac/cmac/cmac.cpp
index bb862196f..4d76e4a20 100644
--- a/src/lib/mac/cmac/cmac.cpp
+++ b/src/lib/mac/cmac/cmac.cpp
@@ -6,6 +6,7 @@
 */
 
 #include <botan/cmac.h>
+#include <botan/internal/poly_dbl.h>
 
 namespace Botan {
 
@@ -14,41 +15,8 @@ namespace Botan {
 */
 secure_vector<uint8_t> CMAC::poly_double(const secure_vector<uint8_t>& in)
    {
-   const bool top_carry = static_cast<bool>((in[0] & 0x80) != 0);
-
    secure_vector<uint8_t> out = in;
-
-   uint8_t carry = 0;
-   for(size_t i = out.size(); i != 0; --i)
-      {
-      uint8_t temp = out[i-1];
-      out[i-1] = (temp << 1) | carry;
-      carry = (temp >> 7);
-      }
-
-   if(top_carry)
-      {
-      switch(in.size())
-         {
-         case 8:
-            out[out.size()-1] ^= 0x1B;
-            break;
-         case 16:
-            out[out.size()-1] ^= 0x87;
-            break;
-         case 32:
-            out[out.size()-2] ^= 0x4;
-            out[out.size()-1] ^= 0x25;
-            break;
-         case 64:
-            out[out.size()-2] ^= 0x1;
-            out[out.size()-1] ^= 0x25;
-            break;
-         default:
-            throw Exception("Unsupported CMAC size " + std::to_string(in.size()));
-         }
-      }
-
+   poly_double_n(out.data(), out.size());
    return out;
    }
 
@@ -112,8 +80,10 @@ void CMAC::key_schedule(const uint8_t key[], size_t length)
    clear();
    m_cipher->set_key(key, length);
    m_cipher->encrypt(m_B);
-   m_B = poly_double(m_B);
-   m_P = poly_double(m_B);
+   poly_double_n(m_B.data(), m_B.size());
+
+   m_P = m_B;
+   poly_double_n(m_P.data(), m_P.size());
    }
 
 /*