3 files changed, 53 insertions, 14 deletions

diff --git a/src/lib/mac/gmac/gmac.cpp b/src/lib/mac/gmac/gmac.cpp
index 946e22cf0..4461cf370 100644
--- a/src/lib/mac/gmac/gmac.cpp
+++ b/src/lib/mac/gmac/gmac.cpp
@@ -64,17 +64,7 @@ void GMAC::key_schedule(const byte key[], size_t size)
    m_cipher->encrypt(m_H_ad.data(), m_H.data());
    }
 
-void GMAC::start(const std::vector<byte>& nonce)
-   {
-   start(nonce.data(), nonce.size());
-   }
-
-void GMAC::start(const secure_vector<byte>& nonce)
-   {
-   start(nonce.data(), nonce.size());
-   }
-
-void GMAC::start(const byte nonce[], size_t nonce_len)
+void GMAC::start_msg(const byte nonce[], size_t nonce_len)
    {
    secure_vector<byte> y0(GCM_BS);
 
@@ -118,6 +108,6 @@ void GMAC::final_result(byte mac[])
 
 MessageAuthenticationCode* GMAC::clone() const
    {
-   return new GMAC(BlockCipher::create(m_cipher->name()).release());
+   return new GMAC(m_cipher->clone());
    }
 }
diff --git a/src/lib/mac/gmac/gmac.h b/src/lib/mac/gmac/gmac.h
index b651c2e11..b05c5451f 100644
--- a/src/lib/mac/gmac/gmac.h
+++ b/src/lib/mac/gmac/gmac.h
@@ -53,7 +53,7 @@ class BOTAN_DLL GMAC : public MessageAuthenticationCode,
       */
       void start(const std::vector<byte>& nonce);
 
-      Key_Length_Specification key_spec() const
+      Key_Length_Specification key_spec() const override
          {
          return m_cipher->key_spec();
          }
@@ -71,7 +71,7 @@ class BOTAN_DLL GMAC : public MessageAuthenticationCode,
    private:
       void add_data(const byte[], size_t) override;
       void final_result(byte[]) override;
-      void start_msg(const byte nonce[], size_t nonce_len);
+      void start_msg(const byte nonce[], size_t nonce_len) override;
       void key_schedule(const byte key[], size_t size) override;
 
       static const size_t GCM_BS = 16;
diff --git a/src/lib/mac/mac.h b/src/lib/mac/mac.h
index 9c3614f33..d7808c1bf 100644
--- a/src/lib/mac/mac.h
+++ b/src/lib/mac/mac.h
@@ -51,6 +51,55 @@ class BOTAN_DLL MessageAuthenticationCode : public Buffered_Computation,
       virtual ~MessageAuthenticationCode() {}
 
       /**
+      * Prepare for processing a message under the specified nonce
+      *
+      * Most MACs neither require nor support a nonce; for these algorithms
+      * calling `start_msg` is optional and calling it with anything other than
+      * an empty string is an error. One MAC which *requires* a per-message
+      * nonce be specified is GMAC.
+      *
+      * @param nonce the message nonce bytes
+      * @param nonce_len the size of len in bytes
+      * Default implementation simply rejects all non-empty nonces
+      * since most hash/MAC algorithms do not support randomization
+      */
+      virtual void start_msg(const byte nonce[], size_t nonce_len)
+         {
+         BOTAN_UNUSED(nonce);
+         if(nonce_len > 0)
+            throw Invalid_IV_Length(name(), nonce_len);
+         }
+
+      /**
+      * Begin processing a message with a nonce
+      *
+      * @param nonce the per message nonce
+      */
+      template<typename Alloc>
+      void start(const std::vector<byte, Alloc>& nonce)
+         {
+         start_msg(nonce.data(), nonce.size());
+         }
+
+      /**
+      * Begin processing a message.
+      * @param nonce the per message nonce
+      * @param nonce_len length of nonce
+      */
+      void start(const byte nonce[], size_t nonce_len)
+         {
+         start_msg(nonce, nonce_len);
+         }
+
+      /**
+      * Begin processing a message.
+      */
+      void start()
+         {
+         return start_msg(nullptr, 0);
+         }
+
+      /**
       * Verify a MAC.
       * @param in the MAC to verify as a byte array
       * @param length the length of param in