Cipher_Mode and AEAD_Mode improvements

See PR #552

- Add Cipher_Mode::reset() which resets just the message specific state and allows encrypting again under the existing key
- In Cipher_Mode::clear() (at some planes) use cipher->clear() instead of resetting the pointer which would make the cipher object unusable
- EAX_Decryption::output_length() bugfix?! Now its possible to decrypt an empty ciphertext (just a tag)
- Bugfix for GCM_Decryption::finish()
- set tag length in GCM_Mode::name()
- Cipher_Mode tests: add tests for reset()and process()
- AEAD_Mode tests: add tests for reset(), clear(), update() and process()

2 files changed, 13 insertions, 2 deletions

diff --git a/src/lib/modes/aead/ocb/ocb.cpp b/src/lib/modes/aead/ocb/ocb.cpp
index 0ce2b6f00..c530dda5d 100644
--- a/src/lib/modes/aead/ocb/ocb.cpp
+++ b/src/lib/modes/aead/ocb/ocb.cpp
@@ -1,6 +1,7 @@
 /*
 * OCB Mode
 * (C) 2013 Jack Lloyd
+* (C) 2016 Daniel Neus, Rohde & Schwarz Cybersecurity
 *
 * Botan is released under the Simplified BSD License (see license.txt)
 */
@@ -129,12 +130,19 @@ OCB_Mode::~OCB_Mode() { /* for unique_ptr destructor */ }
 
 void OCB_Mode::clear()
    {
-   m_cipher.reset();
-   m_L.reset();
+   m_cipher->clear();
+   m_L.reset(); // add clear here?
+   reset();
+   }
 
+void OCB_Mode::reset()
+   {
+   m_block_index = 0;
    zeroise(m_ad_hash);
    zeroise(m_offset);
    zeroise(m_checksum);
+   m_last_nonce.clear();
+   m_stretch.clear();
    }
 
 bool OCB_Mode::valid_nonce_length(size_t length) const
diff --git a/src/lib/modes/aead/ocb/ocb.h b/src/lib/modes/aead/ocb/ocb.h
index 4daa7a81b..ce9d29f1b 100644
--- a/src/lib/modes/aead/ocb/ocb.h
+++ b/src/lib/modes/aead/ocb/ocb.h
@@ -1,6 +1,7 @@
 /*
 * OCB Mode
 * (C) 2013,2014 Jack Lloyd
+* (C) 2016 Daniel Neus, Rohde & Schwarz Cybersecurity
 *
 * Botan is released under the Simplified BSD License (see license.txt)
 */
@@ -41,6 +42,8 @@ class BOTAN_DLL OCB_Mode : public AEAD_Mode
 
       void clear() override;
 
+      void reset() override;
+
       ~OCB_Mode();
    protected:
       /**