Allow setting CTR width via string

Prohibit very small counter widths (under 4 bytes), since they lead
to trivial keystream reuse.

Add tests.

Fix clone which always returned an object with a block-wide counter.

3 files changed, 14 insertions, 6 deletions

diff --git a/src/lib/stream/ctr/ctr.cpp b/src/lib/stream/ctr/ctr.cpp
index d0b44589b..99a589bb9 100644
--- a/src/lib/stream/ctr/ctr.cpp
+++ b/src/lib/stream/ctr/ctr.cpp
@@ -32,7 +32,7 @@ CTR_BE::CTR_BE(BlockCipher* cipher, size_t ctr_size) :
    m_iv(m_cipher->block_size()),
    m_pad_pos(0)
    {
-   if(m_ctr_size == 0 || m_ctr_size > m_block_size)
+   if(m_ctr_size < 4 || m_ctr_size > m_block_size)
       throw Invalid_Argument("Invalid CTR-BE counter size");
    }
 
@@ -55,7 +55,11 @@ void CTR_BE::key_schedule(const uint8_t key[], size_t key_len)
 
 std::string CTR_BE::name() const
    {
-   return ("CTR-BE(" + m_cipher->name() + ")");
+   if(m_ctr_size == m_block_size)
+      return ("CTR-BE(" + m_cipher->name() + ")");
+   else
+      return ("CTR-BE(" + m_cipher->name() + "," + std::to_string(m_ctr_size) + ")");
+
    }
 
 void CTR_BE::cipher(const uint8_t in[], uint8_t out[], size_t length)
diff --git a/src/lib/stream/ctr/ctr.h b/src/lib/stream/ctr/ctr.h
index 3ff63b8e5..c4c598161 100644
--- a/src/lib/stream/ctr/ctr.h
+++ b/src/lib/stream/ctr/ctr.h
@@ -34,7 +34,7 @@ class BOTAN_PUBLIC_API(2,0) CTR_BE final : public StreamCipher
       std::string name() const override;
 
       CTR_BE* clone() const override
-         { return new CTR_BE(m_cipher->clone()); }
+         { return new CTR_BE(m_cipher->clone(), m_ctr_size); }
 
       void clear() override;
 
diff --git a/src/lib/stream/stream_cipher.cpp b/src/lib/stream/stream_cipher.cpp
index f33d68296..c0e75c0a8 100644
--- a/src/lib/stream/stream_cipher.cpp
+++ b/src/lib/stream/stream_cipher.cpp
@@ -44,12 +44,16 @@ std::unique_ptr<StreamCipher> StreamCipher::create(const std::string& algo_spec,
    const SCAN_Name req(algo_spec);
 
 #if defined(BOTAN_HAS_CTR_BE)
-   if(req.algo_name() == "CTR-BE" && req.arg_count() == 1)
+   if((req.algo_name() == "CTR-BE" || req.algo_name() == "CTR") && req.arg_count_between(1,2))
       {
       if(provider.empty() || provider == "base")
          {
-         if(auto c = BlockCipher::create(req.arg(0)))
-            return std::unique_ptr<StreamCipher>(new CTR_BE(c.release()));
+         auto cipher = BlockCipher::create(req.arg(0));
+         if(cipher)
+            {
+            size_t ctr_size = req.arg_as_integer(1, cipher->block_size());
+            return std::unique_ptr<StreamCipher>(new CTR_BE(cipher.release(), ctr_size));
+            }
          }
       }
 #endif