Better tests for SIV

Correct errors in the AEAD tests that assumed process/update always
return something - that isn't true for SIV

Minor optimizations in CMAC and CTR to cache the block size instead
of making a zillion virtual calls for it.

Generalize SIV slightly to where it could support a non-128 bit
cipher, but don't pull the trigger on it since I can't find any
implementations to crosscheck with.

2 files changed, 33 insertions, 22 deletions

diff --git a/src/lib/modes/aead/siv/siv.cpp b/src/lib/modes/aead/siv/siv.cpp
index 1ddfd4d88..8b2cc4ef0 100644
--- a/src/lib/modes/aead/siv/siv.cpp
+++ b/src/lib/modes/aead/siv/siv.cpp
@@ -1,6 +1,6 @@
 /*
 * SIV Mode Encryption
-* (C) 2013 Jack Lloyd
+* (C) 2013,2017 Jack Lloyd
 * (C) 2016 Daniel Neus, Rohde & Schwarz Cybersecurity
 *
 * Botan is released under the Simplified BSD License (see license.txt)
@@ -17,7 +17,8 @@ namespace Botan {
 SIV_Mode::SIV_Mode(BlockCipher* cipher) :
    m_name(cipher->name() + "/SIV"),
    m_ctr(new CTR_BE(cipher->clone())),
-   m_cmac(new CMAC(cipher))
+   m_mac(new CMAC(cipher)),
+   m_bs(cipher->block_size())
    {
    if(cipher->block_size() != 16)
       throw Invalid_Argument("SIV requires a 128 bit block cipher");
@@ -31,7 +32,7 @@ SIV_Mode::~SIV_Mode()
 void SIV_Mode::clear()
    {
    m_ctr->clear();
-   m_cmac->clear();
+   m_mac->clear();
    reset();
    }
 
@@ -65,23 +66,27 @@ size_t SIV_Mode::update_granularity() const
 
 Key_Length_Specification SIV_Mode::key_spec() const
    {
-   return m_cmac->key_spec().multiple(2);
+   return m_mac->key_spec().multiple(2);
    }
 
 void SIV_Mode::key_schedule(const uint8_t key[], size_t length)
    {
    const size_t keylen = length / 2;
-   m_cmac->set_key(key, keylen);
+   m_mac->set_key(key, keylen);
    m_ctr->set_key(key + keylen, keylen);
    m_ad_macs.clear();
    }
 
 void SIV_Mode::set_associated_data_n(size_t n, const uint8_t ad[], size_t length)
    {
+   const size_t max_ads = block_size() * 8 - 2;
+   if(n > max_ads)
+      throw Invalid_Argument(name() + " allows no more than " + std::to_string(max_ads) + " ADs");
+
    if(n >= m_ad_macs.size())
       m_ad_macs.resize(n+1);
 
-   m_ad_macs[n] = m_cmac->process(ad, length);
+   m_ad_macs[n] = m_mac->process(ad, length);
    }
 
 void SIV_Mode::start_msg(const uint8_t nonce[], size_t nonce_len)
@@ -90,7 +95,7 @@ void SIV_Mode::start_msg(const uint8_t nonce[], size_t nonce_len)
       throw Invalid_IV_Length(name(), nonce_len);
 
    if(nonce_len)
-      m_nonce = m_cmac->process(nonce, nonce_len);
+      m_nonce = m_mac->process(nonce, nonce_len);
    else
       m_nonce.clear();
 
@@ -106,9 +111,9 @@ size_t SIV_Mode::process(uint8_t buf[], size_t sz)
 
 secure_vector<uint8_t> SIV_Mode::S2V(const uint8_t* text, size_t text_len)
    {
-   const uint8_t zero[16] = { 0 };
+   const std::vector<uint8_t> zeros(block_size());
 
-   secure_vector<uint8_t> V = m_cmac->process(zero, 16);
+   secure_vector<uint8_t> V = m_mac->process(zeros.data(), zeros.size());
 
    for(size_t i = 0; i != m_ad_macs.size(); ++i)
       {
@@ -122,25 +127,25 @@ secure_vector<uint8_t> SIV_Mode::S2V(const uint8_t* text, size_t text_len)
       V ^= m_nonce;
       }
 
-   if(text_len < 16)
+   if(text_len < block_size())
       {
       poly_double_n(V.data(), V.size());
       xor_buf(V.data(), text, text_len);
       V[text_len] ^= 0x80;
-      return m_cmac->process(V);
+      return m_mac->process(V);
       }
 
-   m_cmac->update(text, text_len - 16);
-   xor_buf(V.data(), &text[text_len - 16], 16);
-   m_cmac->update(V);
+   m_mac->update(text, text_len - block_size());
+   xor_buf(V.data(), &text[text_len - block_size()], block_size());
+   m_mac->update(V);
 
-   return m_cmac->final();
+   return m_mac->final();
    }
 
 void SIV_Mode::set_ctr_iv(secure_vector<uint8_t> V)
    {
-   V[8] &= 0x7F;
-   V[12] &= 0x7F;
+   V[m_bs-8] &= 0x7F;
+   V[m_bs-4] &= 0x7F;
 
    ctr().set_iv(V.data(), V.size());
    }
@@ -150,8 +155,9 @@ void SIV_Encryption::finish(secure_vector<uint8_t>& buffer, size_t offset)
    BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
 
    buffer.insert(buffer.begin() + offset, msg_buf().begin(), msg_buf().end());
+   msg_buf().clear();
 
-   secure_vector<uint8_t> V = S2V(buffer.data() + offset, buffer.size() - offset);
+   const secure_vector<uint8_t> V = S2V(buffer.data() + offset, buffer.size() - offset);
 
    buffer.insert(buffer.begin() + offset, V.begin(), V.end());
 
@@ -164,12 +170,14 @@ void SIV_Decryption::finish(secure_vector<uint8_t>& buffer, size_t offset)
    BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
 
    buffer.insert(buffer.begin() + offset, msg_buf().begin(), msg_buf().end());
+   msg_buf().clear();
 
    const size_t sz = buffer.size() - offset;
 
    BOTAN_ASSERT(sz >= tag_size(), "We have the tag");
 
-   secure_vector<uint8_t> V(buffer.data() + offset, buffer.data() + offset + 16);
+   secure_vector<uint8_t> V(buffer.data() + offset,
+                            buffer.data() + offset + block_size());
 
    set_ctr_iv(V);
 
@@ -177,9 +185,9 @@ void SIV_Decryption::finish(secure_vector<uint8_t>& buffer, size_t offset)
                 buffer.data() + offset,
                 buffer.size() - offset - V.size());
 
-   secure_vector<uint8_t> T = S2V(buffer.data() + offset, buffer.size() - offset - V.size());
+   const secure_vector<uint8_t> T = S2V(buffer.data() + offset, buffer.size() - offset - V.size());
 
-   if(T != V)
+   if(!constant_time_compare(T.data(), V.data(), T.size()))
       throw Integrity_Failure("SIV tag check failed");
 
    buffer.resize(buffer.size() - tag_size());
diff --git a/src/lib/modes/aead/siv/siv.h b/src/lib/modes/aead/siv/siv.h
index 255e4a277..098aa1f38 100644
--- a/src/lib/modes/aead/siv/siv.h
+++ b/src/lib/modes/aead/siv/siv.h
@@ -57,6 +57,8 @@ class BOTAN_PUBLIC_API(2,0) SIV_Mode : public AEAD_Mode
 
       ~SIV_Mode();
 
+      size_t block_size() const { return m_bs; }
+
       StreamCipher& ctr() { return *m_ctr; }
 
       void set_ctr_iv(secure_vector<uint8_t> V);
@@ -71,9 +73,10 @@ class BOTAN_PUBLIC_API(2,0) SIV_Mode : public AEAD_Mode
 
       const std::string m_name;
       std::unique_ptr<StreamCipher> m_ctr;
-      std::unique_ptr<MessageAuthenticationCode> m_cmac;
+      std::unique_ptr<MessageAuthenticationCode> m_mac;
       secure_vector<uint8_t> m_nonce, m_msg_buf;
       std::vector<secure_vector<uint8_t>> m_ad_macs;
+      const size_t m_bs;
    };
 
 /**