Cipher_Mode API improvements

The Cipher_Mode::update API is more general than needed to just
support ciphers (this is due to it previously being an API of
Transform which before 8b85b780515 was Cipher_Mode's base class)

Define a less general interface `process` which either processes the
blocks in-place, producing exactly as much output as there was input,
or (SIV/CCM case) saves the entire message for processing in `finish`.
These two uses cover all current or anticipated cipher modes.

Leaves `update` for compatability with existing callers; all that is
needed is an inline function forwarding to `process`.

Removes the return type from `start` - in all cipher implementations,
this always returned an empty vector.

Adds BOTAN_ARG_CHECK macro; right now BOTAN_ASSERT is being used
for argument checking in some places, which is not right at all.

28 files changed, 243 insertions, 293 deletions

diff --git a/doc/manual/aead.rst b/doc/manual/aead.rst
index 5ab33c849..d216026bb 100644
--- a/doc/manual/aead.rst
+++ b/doc/manual/aead.rst
@@ -41,8 +41,6 @@ support a 128-bit block cipher such as AES. EAX and SIV also support
        Start processing a message, using *nonce* as the unique per-message
        value.
 
-       Returns any initial data that should be emitted (for instance a header).
-
   .. cpp:function:: void update(secure_vector<byte>& buffer, size_t offset = 0)
 
        Continue processing a message. The *buffer* is an in/out parameter and
@@ -79,7 +77,7 @@ support a 128-bit block cipher such as AES. EAX and SIV also support
   .. cpp:function:: size_t update_granularity() const
 
        The AEAD interface requires :cpp:func:`update` be called with blocks of
-       this size.
+       this size. This will be 1, if the mode can process any length inputs.
 
   .. cpp:function:: size_t final_minimum_size() const
 
diff --git a/src/lib/ffi/ffi.cpp b/src/lib/ffi/ffi.cpp
index e42f32234..e8df51fec 100644
--- a/src/lib/ffi/ffi.cpp
+++ b/src/lib/ffi/ffi.cpp
@@ -458,7 +458,7 @@ int botan_cipher_start(botan_cipher_t cipher_obj,
    try
       {
       Botan::Cipher_Mode& cipher = safe_get(cipher_obj);
-      BOTAN_ASSERT(cipher.start(nonce, nonce_len).empty(), "Ciphers have no prefix");
+      cipher.start(nonce, nonce_len);
       cipher_obj->m_buf.reserve(cipher.update_granularity());
       return 0;
       }
diff --git a/src/lib/filters/cipher_filter.cpp b/src/lib/filters/cipher_filter.cpp
index f91cf3aa2..ed82880aa 100644
--- a/src/lib/filters/cipher_filter.cpp
+++ b/src/lib/filters/cipher_filter.cpp
@@ -85,7 +85,7 @@ void Cipher_Mode_Filter::end_msg()
 
 void Cipher_Mode_Filter::start_msg()
    {
-   send(m_mode->start(m_nonce.get()));
+   m_mode->start(m_nonce.get());
    }
 
 void Cipher_Mode_Filter::buffered_block(const byte input[], size_t input_length)
diff --git a/src/lib/modes/aead/ccm/ccm.cpp b/src/lib/modes/aead/ccm/ccm.cpp
index 1f528769e..d5559bfb5 100644
--- a/src/lib/modes/aead/ccm/ccm.cpp
+++ b/src/lib/modes/aead/ccm/ccm.cpp
@@ -11,6 +11,9 @@
 
 namespace Botan {
 
+// 128-bit cipher is intrinsic to CCM definition
+static const size_t CCM_BS = 16;
+
 /*
 * CCM_Mode Constructor
 */
@@ -19,7 +22,7 @@ CCM_Mode::CCM_Mode(BlockCipher* cipher, size_t tag_size, size_t L) :
    m_L(L),
    m_cipher(cipher)
    {
-   if(m_cipher->block_size() != BS)
+   if(m_cipher->block_size() != CCM_BS)
       throw Invalid_Argument(m_cipher->name() + " cannot be used with CCM mode");
 
    if(L < 2 || L > 8)
@@ -84,30 +87,24 @@ void CCM_Mode::set_associated_data(const byte ad[], size_t length)
       m_ad_buf.push_back(get_byte(0, static_cast<u16bit>(length)));
       m_ad_buf.push_back(get_byte(1, static_cast<u16bit>(length)));
       m_ad_buf += std::make_pair(ad, length);
-      while(m_ad_buf.size() % BS)
+      while(m_ad_buf.size() % CCM_BS)
          m_ad_buf.push_back(0); // pad with zeros to full block size
       }
    }
 
-secure_vector<byte> CCM_Mode::start_raw(const byte nonce[], size_t nonce_len)
+void CCM_Mode::start_msg(const byte nonce[], size_t nonce_len)
    {
    if(!valid_nonce_length(nonce_len))
       throw Invalid_IV_Length(name(), nonce_len);
 
    m_nonce.assign(nonce, nonce + nonce_len);
    m_msg_buf.clear();
-
-   return secure_vector<byte>();
    }
 
-void CCM_Mode::update(secure_vector<byte>& buffer, size_t offset)
+size_t CCM_Mode::process(uint8_t buf[], size_t sz)
    {
-   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
-   const size_t sz = buffer.size() - offset;
-   byte* buf = buffer.data() + offset;
-
    m_msg_buf.insert(m_msg_buf.end(), buf, buf + sz);
-   buffer.resize(offset); // truncate msg
+   return 0; // no output until finished
    }
 
 void CCM_Mode::encode_length(size_t len, byte out[])
@@ -131,7 +128,7 @@ void CCM_Mode::inc(secure_vector<byte>& C)
 
 secure_vector<byte> CCM_Mode::format_b0(size_t sz)
    {
-   secure_vector<byte> B0(BS);
+   secure_vector<byte> B0(CCM_BS);
 
    const byte b_flags = (m_ad_buf.size() ? 64 : 0) + (((tag_size()/2)-1) << 3) + (L()-1);
 
@@ -144,7 +141,7 @@ secure_vector<byte> CCM_Mode::format_b0(size_t sz)
 
 secure_vector<byte> CCM_Mode::format_c0()
    {
-   secure_vector<byte> C(BS);
+   secure_vector<byte> C(CCM_BS);
 
    const byte a_flags = L()-1;
 
@@ -164,31 +161,31 @@ void CCM_Encryption::finish(secure_vector<byte>& buffer, size_t offset)
    byte* buf = buffer.data() + offset;
 
    const secure_vector<byte>& ad = ad_buf();
-   BOTAN_ASSERT(ad.size() % BS == 0, "AD is block size multiple");
+   BOTAN_ASSERT(ad.size() % CCM_BS == 0, "AD is block size multiple");
 
    const BlockCipher& E = cipher();
 
-   secure_vector<byte> T(BS);
+   secure_vector<byte> T(CCM_BS);
    E.encrypt(format_b0(sz), T);
 
-   for(size_t i = 0; i != ad.size(); i += BS)
+   for(size_t i = 0; i != ad.size(); i += CCM_BS)
       {
-      xor_buf(T.data(), &ad[i], BS);
+      xor_buf(T.data(), &ad[i], CCM_BS);
       E.encrypt(T);
       }
 
    secure_vector<byte> C = format_c0();
-   secure_vector<byte> S0(BS);
+   secure_vector<byte> S0(CCM_BS);
    E.encrypt(C, S0);
    inc(C);
 
-   secure_vector<byte> X(BS);
+   secure_vector<byte> X(CCM_BS);
 
    const byte* buf_end = &buf[sz];
 
    while(buf != buf_end)
       {
-      const size_t to_proc = std::min<size_t>(BS, buf_end - buf);
+      const size_t to_proc = std::min<size_t>(CCM_BS, buf_end - buf);
 
       xor_buf(T.data(), buf, to_proc);
       E.encrypt(T);
@@ -217,32 +214,32 @@ void CCM_Decryption::finish(secure_vector<byte>& buffer, size_t offset)
    BOTAN_ASSERT(sz >= tag_size(), "We have the tag");
 
    const secure_vector<byte>& ad = ad_buf();
-   BOTAN_ASSERT(ad.size() % BS == 0, "AD is block size multiple");
+   BOTAN_ASSERT(ad.size() % CCM_BS == 0, "AD is block size multiple");
 
    const BlockCipher& E = cipher();
 
-   secure_vector<byte> T(BS);
+   secure_vector<byte> T(CCM_BS);
    E.encrypt(format_b0(sz - tag_size()), T);
 
-   for(size_t i = 0; i != ad.size(); i += BS)
+   for(size_t i = 0; i != ad.size(); i += CCM_BS)
       {
-      xor_buf(T.data(), &ad[i], BS);
+      xor_buf(T.data(), &ad[i], CCM_BS);
       E.encrypt(T);
       }
 
    secure_vector<byte> C = format_c0();
 
-   secure_vector<byte> S0(BS);
+   secure_vector<byte> S0(CCM_BS);
    E.encrypt(C, S0);
    inc(C);
 
-   secure_vector<byte> X(BS);
+   secure_vector<byte> X(CCM_BS);
 
    const byte* buf_end = &buf[sz - tag_size()];
 
    while(buf != buf_end)
       {
-      const size_t to_proc = std::min<size_t>(BS, buf_end - buf);
+      const size_t to_proc = std::min<size_t>(CCM_BS, buf_end - buf);
 
       E.encrypt(C, X);
       xor_buf(buf, X.data(), to_proc);
diff --git a/src/lib/modes/aead/ccm/ccm.h b/src/lib/modes/aead/ccm/ccm.h
index 8277a8f93..7484b500a 100644
--- a/src/lib/modes/aead/ccm/ccm.h
+++ b/src/lib/modes/aead/ccm/ccm.h
@@ -22,7 +22,7 @@ namespace Botan {
 class BOTAN_DLL CCM_Mode : public AEAD_Mode
    {
    public:
-      void update(secure_vector<byte>& blocks, size_t offset = 0) override;
+      size_t process(uint8_t buf[], size_t sz) override;
 
       void set_associated_data(const byte ad[], size_t ad_len) override;
 
@@ -41,8 +41,6 @@ class BOTAN_DLL CCM_Mode : public AEAD_Mode
       size_t tag_size() const override { return m_tag_size; }
 
    protected:
-      const size_t BS = 16; // intrinsic to CCM definition
-
       CCM_Mode(BlockCipher* cipher, size_t tag_size, size_t L);
 
       size_t L() const { return m_L; }
@@ -60,7 +58,7 @@ class BOTAN_DLL CCM_Mode : public AEAD_Mode
       secure_vector<byte> format_b0(size_t msg_size);
       secure_vector<byte> format_c0();
    private:
-      secure_vector<byte> start_raw(const byte nonce[], size_t nonce_len) override;
+      void start_msg(const byte nonce[], size_t nonce_len) override;
 
       void key_schedule(const byte key[], size_t length) override;
 
diff --git a/src/lib/modes/aead/chacha20poly1305/chacha20poly1305.cpp b/src/lib/modes/aead/chacha20poly1305/chacha20poly1305.cpp
index ca4cc15ed..04326dede 100644
--- a/src/lib/modes/aead/chacha20poly1305/chacha20poly1305.cpp
+++ b/src/lib/modes/aead/chacha20poly1305/chacha20poly1305.cpp
@@ -50,7 +50,7 @@ void ChaCha20Poly1305_Mode::update_len(size_t len)
    m_poly1305->update(len8, 8);
    }
 
-secure_vector<byte> ChaCha20Poly1305_Mode::start_raw(const byte nonce[], size_t nonce_len)
+void ChaCha20Poly1305_Mode::start_msg(const byte nonce[], size_t nonce_len)
    {
    if(!valid_nonce_length(nonce_len))
       throw Invalid_IV_Length(name(), nonce_len);
@@ -80,19 +80,14 @@ secure_vector<byte> ChaCha20Poly1305_Mode::start_raw(const byte nonce[], size_t
       {
       update_len(m_ad.size());
       }
-
-   return secure_vector<byte>();
    }
 
-void ChaCha20Poly1305_Encryption::update(secure_vector<byte>& buffer, size_t offset)
+size_t ChaCha20Poly1305_Encryption::process(uint8_t buf[], size_t sz)
    {
-   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
-   const size_t sz = buffer.size() - offset;
-   byte* buf = buffer.data() + offset;
-
    m_chacha->cipher1(buf, sz);
    m_poly1305->update(buf, sz); // poly1305 of ciphertext
    m_ctext_len += sz;
+   return sz;
    }
 
 void ChaCha20Poly1305_Encryption::finish(secure_vector<byte>& buffer, size_t offset)
@@ -114,15 +109,12 @@ void ChaCha20Poly1305_Encryption::finish(secure_vector<byte>& buffer, size_t off
    m_ctext_len = 0;
    }
 
-void ChaCha20Poly1305_Decryption::update(secure_vector<byte>& buffer, size_t offset)
+size_t ChaCha20Poly1305_Decryption::process(uint8_t buf[], size_t sz)
    {
-   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
-   const size_t sz = buffer.size() - offset;
-   byte* buf = buffer.data() + offset;
-
    m_poly1305->update(buf, sz); // poly1305 of ciphertext
    m_chacha->cipher1(buf, sz);
    m_ctext_len += sz;
+   return sz;
    }
 
 void ChaCha20Poly1305_Decryption::finish(secure_vector<byte>& buffer, size_t offset)
diff --git a/src/lib/modes/aead/chacha20poly1305/chacha20poly1305.h b/src/lib/modes/aead/chacha20poly1305/chacha20poly1305.h
index 5aa2dc010..553508854 100644
--- a/src/lib/modes/aead/chacha20poly1305/chacha20poly1305.h
+++ b/src/lib/modes/aead/chacha20poly1305/chacha20poly1305.h
@@ -50,7 +50,7 @@ class BOTAN_DLL ChaCha20Poly1305_Mode : public AEAD_Mode
       bool cfrg_version() const { return m_nonce_len == 12; }
       void update_len(size_t len);
    private:
-      secure_vector<byte> start_raw(const byte nonce[], size_t nonce_len) override;
+      void start_msg(const byte nonce[], size_t nonce_len) override;
 
       void key_schedule(const byte key[], size_t length) override;
    };
@@ -66,7 +66,7 @@ class BOTAN_DLL ChaCha20Poly1305_Encryption final : public ChaCha20Poly1305_Mode
 
       size_t minimum_final_size() const override { return 0; }
 
-      void update(secure_vector<byte>& blocks, size_t offset = 0) override;
+      size_t process(uint8_t buf[], size_t size) override;
 
       void finish(secure_vector<byte>& final_block, size_t offset = 0) override;
    };
@@ -85,7 +85,7 @@ class BOTAN_DLL ChaCha20Poly1305_Decryption final : public ChaCha20Poly1305_Mode
 
       size_t minimum_final_size() const override { return tag_size(); }
 
-      void update(secure_vector<byte>& blocks, size_t offset = 0) override;
+      size_t process(uint8_t buf[], size_t size) override;
 
       void finish(secure_vector<byte>& final_block, size_t offset = 0) override;
    };
diff --git a/src/lib/modes/aead/eax/eax.cpp b/src/lib/modes/aead/eax/eax.cpp
index 4b928cd31..f26a1eae3 100644
--- a/src/lib/modes/aead/eax/eax.cpp
+++ b/src/lib/modes/aead/eax/eax.cpp
@@ -60,7 +60,7 @@ std::string EAX_Mode::name() const
 
 size_t EAX_Mode::update_granularity() const
    {
-   return 8 * m_cipher->parallel_bytes();
+   return 1;
    }
 
 Key_Length_Specification EAX_Mode::key_spec() const
@@ -91,7 +91,7 @@ void EAX_Mode::set_associated_data(const byte ad[], size_t length)
    m_ad_mac = eax_prf(1, block_size(), *m_cmac, ad, length);
    }
 
-secure_vector<byte> EAX_Mode::start_raw(const byte nonce[], size_t nonce_len)
+void EAX_Mode::start_msg(const byte nonce[], size_t nonce_len)
    {
    if(!valid_nonce_length(nonce_len))
       throw Invalid_IV_Length(name(), nonce_len);
@@ -103,18 +103,13 @@ secure_vector<byte> EAX_Mode::start_raw(const byte nonce[], size_t nonce_len)
    for(size_t i = 0; i != block_size() - 1; ++i)
       m_cmac->update(0);
    m_cmac->update(2);
-
-   return secure_vector<byte>();
    }
 
-void EAX_Encryption::update(secure_vector<byte>& buffer, size_t offset)
+size_t EAX_Encryption::process(uint8_t buf[], size_t sz)
    {
-   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
-   const size_t sz = buffer.size() - offset;
-   byte* buf = buffer.data() + offset;
-
    m_ctr->cipher(buf, buf, sz);
    m_cmac->update(buf, sz);
+   return sz;
    }
 
 void EAX_Encryption::finish(secure_vector<byte>& buffer, size_t offset)
@@ -128,14 +123,11 @@ void EAX_Encryption::finish(secure_vector<byte>& buffer, size_t offset)
    buffer += std::make_pair(data_mac.data(), tag_size());
    }
 
-void EAX_Decryption::update(secure_vector<byte>& buffer, size_t offset)
+size_t EAX_Decryption::process(uint8_t buf[], size_t sz)
    {
-   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
-   const size_t sz = buffer.size() - offset;
-   byte* buf = buffer.data() + offset;
-
    m_cmac->update(buf, sz);
    m_ctr->cipher(buf, buf, sz);
+   return sz;
    }
 
 void EAX_Decryption::finish(secure_vector<byte>& buffer, size_t offset)
diff --git a/src/lib/modes/aead/eax/eax.h b/src/lib/modes/aead/eax/eax.h
index e3d942d5e..0dedefe07 100644
--- a/src/lib/modes/aead/eax/eax.h
+++ b/src/lib/modes/aead/eax/eax.h
@@ -54,7 +54,7 @@ class BOTAN_DLL EAX_Mode : public AEAD_Mode
 
       secure_vector<byte> m_nonce_mac;
    private:
-      secure_vector<byte> start_raw(const byte nonce[], size_t nonce_len) override;
+      void start_msg(const byte nonce[], size_t nonce_len) override;
 
       void key_schedule(const byte key[], size_t length) override;
    };
@@ -77,7 +77,7 @@ class BOTAN_DLL EAX_Encryption final : public EAX_Mode
 
       size_t minimum_final_size() const override { return 0; }
 
-      void update(secure_vector<byte>& blocks, size_t offset = 0) override;
+      size_t process(uint8_t buf[], size_t size) override;
 
       void finish(secure_vector<byte>& final_block, size_t offset = 0) override;
    };
@@ -103,7 +103,7 @@ class BOTAN_DLL EAX_Decryption final : public EAX_Mode
 
       size_t minimum_final_size() const override { return tag_size(); }
 
-      void update(secure_vector<byte>& blocks, size_t offset = 0) override;
+      size_t process(uint8_t buf[], size_t size) override;
 
       void finish(secure_vector<byte>& final_block, size_t offset = 0) override;
    };
diff --git a/src/lib/modes/aead/gcm/gcm.cpp b/src/lib/modes/aead/gcm/gcm.cpp
index e23551cb4..78792b79a 100644
--- a/src/lib/modes/aead/gcm/gcm.cpp
+++ b/src/lib/modes/aead/gcm/gcm.cpp
@@ -17,6 +17,8 @@
 
 namespace Botan {
 
+static const size_t GCM_BS = 16;
+
 void GHASH::gcm_multiply(secure_vector<byte>& x) const
    {
 #if defined(BOTAN_HAS_GCM_CLMUL)
@@ -66,15 +68,13 @@ void GHASH::gcm_multiply(secure_vector<byte>& x) const
 void GHASH::ghash_update(secure_vector<byte>& ghash,
                          const byte input[], size_t length)
    {
-   const size_t BS = 16;
-
    /*
    This assumes if less than block size input then we're just on the
    final block and should pad with zeros
    */
    while(length)
       {
-      const size_t to_proc = std::min(length, BS);
+      const size_t to_proc = std::min(length, GCM_BS);
 
       xor_buf(ghash.data(), input, to_proc);
 
@@ -88,7 +88,7 @@ void GHASH::ghash_update(secure_vector<byte>& ghash,
 void GHASH::key_schedule(const byte key[], size_t length)
    {
    m_H.assign(key, key+length);
-   m_H_ad.resize(16);
+   m_H_ad.resize(GCM_BS);
    m_ad_len = 0;
    m_text_len = 0;
    }
@@ -109,7 +109,7 @@ void GHASH::set_associated_data(const byte input[], size_t length)
 
 void GHASH::update(const byte input[], size_t length)
    {
-   BOTAN_ASSERT(m_ghash.size() == 16, "Key was set");
+   BOTAN_ASSERT(m_ghash.size() == GCM_BS, "Key was set");
 
    m_text_len += length;
 
@@ -119,7 +119,7 @@ void GHASH::update(const byte input[], size_t length)
 void GHASH::add_final_block(secure_vector<byte>& hash,
                             size_t ad_len, size_t text_len)
    {
-   secure_vector<byte> final_block(16);
+   secure_vector<byte> final_block(GCM_BS);
    store_be<u64bit>(final_block.data(), 8*ad_len, 8*text_len);
    ghash_update(hash, final_block.data(), final_block.size());
    }
@@ -139,7 +139,7 @@ secure_vector<byte> GHASH::final()
 secure_vector<byte> GHASH::nonce_hash(const byte nonce[], size_t nonce_len)
    {
    BOTAN_ASSERT(m_ghash.size() == 0, "nonce_hash called during wrong time");
-   secure_vector<byte> y0(16);
+   secure_vector<byte> y0(GCM_BS);
 
    ghash_update(y0, nonce, nonce_len);
    add_final_block(y0, 0, nonce_len);
@@ -162,15 +162,14 @@ GCM_Mode::GCM_Mode(BlockCipher* cipher, size_t tag_size) :
    m_tag_size(tag_size),
    m_cipher_name(cipher->name())
    {
-   if(cipher->block_size() != m_BS)
-      throw Invalid_Argument("GCM requires a 128 bit cipher so cannot be used with " +
-                                  cipher->name());
+   if(cipher->block_size() != GCM_BS)
+      throw Invalid_Argument("Invalid block cipher for GCM");
 
    m_ghash.reset(new GHASH);
 
    m_ctr.reset(new CTR_BE(cipher, 4)); // CTR_BE takes ownership of cipher
 
-   if(m_tag_size != 8 && m_tag_size != 16)
+   if(m_tag_size != 8 && m_tag_size != GCM_BS)
       throw Invalid_Argument(name() + ": Bad tag size " + std::to_string(m_tag_size));
    }
 
@@ -187,7 +186,7 @@ std::string GCM_Mode::name() const
 
 size_t GCM_Mode::update_granularity() const
    {
-   return m_BS;
+   return GCM_BS;
    }
 
 Key_Length_Specification GCM_Mode::key_spec() const
@@ -199,10 +198,10 @@ void GCM_Mode::key_schedule(const byte key[], size_t keylen)
    {
    m_ctr->set_key(key, keylen);
 
-   const std::vector<byte> zeros(m_BS);
+   const std::vector<byte> zeros(GCM_BS);
    m_ctr->set_iv(zeros.data(), zeros.size());
 
-   secure_vector<byte> H(m_BS);
+   secure_vector<byte> H(GCM_BS);
    m_ctr->encipher(H);
    m_ghash->set_key(H);
    }
@@ -212,12 +211,12 @@ void GCM_Mode::set_associated_data(const byte ad[], size_t ad_len)
    m_ghash->set_associated_data(ad, ad_len);
    }
 
-secure_vector<byte> GCM_Mode::start_raw(const byte nonce[], size_t nonce_len)
+void GCM_Mode::start_msg(const byte nonce[], size_t nonce_len)
    {
    if(!valid_nonce_length(nonce_len))
       throw Invalid_IV_Length(name(), nonce_len);
 
-   secure_vector<byte> y0(m_BS);
+   secure_vector<byte> y0(GCM_BS);
 
    if(nonce_len == 12)
       {
@@ -231,48 +230,48 @@ secure_vector<byte> GCM_Mode::start_raw(const byte nonce[], size_t nonce_len)
 
    m_ctr->set_iv(y0.data(), y0.size());
 
-   secure_vector<byte> m_enc_y0(m_BS);
+   secure_vector<byte> m_enc_y0(GCM_BS);
    m_ctr->encipher(m_enc_y0);
 
    m_ghash->start(m_enc_y0.data(), m_enc_y0.size());
-
-   return secure_vector<byte>();
    }
 
-void GCM_Encryption::update(secure_vector<byte>& buffer, size_t offset)
+size_t GCM_Encryption::process(uint8_t buf[], size_t sz)
    {
-   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
-   const size_t sz = buffer.size() - offset;
-   byte* buf = buffer.data() + offset;
-
+   BOTAN_ARG_CHECK(sz % update_granularity() == 0);
    m_ctr->cipher(buf, buf, sz);
    m_ghash->update(buf, sz);
+   return sz;
    }
 
 void GCM_Encryption::finish(secure_vector<byte>& buffer, size_t offset)
    {
-   update(buffer, offset);
+   BOTAN_ARG_CHECK(offset <= buffer.size());
+   const size_t sz = buffer.size() - offset;
+   byte* buf = buffer.data() + offset;
+
+   m_ctr->cipher(buf, buf, sz);
+   m_ghash->update(buf, sz);
    auto mac = m_ghash->final();
    buffer += std::make_pair(mac.data(), tag_size());
    }
 
-void GCM_Decryption::update(secure_vector<byte>& buffer, size_t offset)
+size_t GCM_Decryption::process(uint8_t buf[], size_t sz)
    {
-   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
-   const size_t sz = buffer.size() - offset;
-   byte* buf = buffer.data() + offset;
-
+   BOTAN_ARG_CHECK(sz % update_granularity() == 0);
    m_ghash->update(buf, sz);
    m_ctr->cipher(buf, buf, sz);
+   return sz;
    }
 
 void GCM_Decryption::finish(secure_vector<byte>& buffer, size_t offset)
    {
-   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
+   BOTAN_ARG_CHECK(offset <= buffer.size());
    const size_t sz = buffer.size() - offset;
    byte* buf = buffer.data() + offset;
 
-   BOTAN_ASSERT(sz >= tag_size(), "Have the tag as part of final input");
+   if(sz < tag_size())
+      throw Exception("Insufficient input for GCM decryption, tag missing");
 
    const size_t remaining = sz - tag_size();
 
diff --git a/src/lib/modes/aead/gcm/gcm.h b/src/lib/modes/aead/gcm/gcm.h
index ba0d6cad8..0110436b2 100644
--- a/src/lib/modes/aead/gcm/gcm.h
+++ b/src/lib/modes/aead/gcm/gcm.h
@@ -47,7 +47,7 @@ class BOTAN_DLL GCM_Mode : public AEAD_Mode
       std::unique_ptr<StreamCipher> m_ctr;
       std::unique_ptr<GHASH> m_ghash;
    private:
-      secure_vector<byte> start_raw(const byte nonce[], size_t nonce_len) override;
+      void start_msg(const byte nonce[], size_t nonce_len) override;
 
       void key_schedule(const byte key[], size_t length) override;
    };
@@ -70,7 +70,7 @@ class BOTAN_DLL GCM_Encryption final : public GCM_Mode
 
       size_t minimum_final_size() const override { return 0; }
 
-      void update(secure_vector<byte>& blocks, size_t offset = 0) override;
+      size_t process(uint8_t buf[], size_t size) override;
 
       void finish(secure_vector<byte>& final_block, size_t offset = 0) override;
    };
@@ -96,7 +96,7 @@ class BOTAN_DLL GCM_Decryption final : public GCM_Mode
 
       size_t minimum_final_size() const override { return tag_size(); }
 
-      void update(secure_vector<byte>& blocks, size_t offset = 0) override;
+      size_t process(uint8_t buf[], size_t size) override;
 
       void finish(secure_vector<byte>& final_block, size_t offset = 0) override;
    };
diff --git a/src/lib/modes/aead/ocb/ocb.cpp b/src/lib/modes/aead/ocb/ocb.cpp
index 77126ec7a..e21749f3b 100644
--- a/src/lib/modes/aead/ocb/ocb.cpp
+++ b/src/lib/modes/aead/ocb/ocb.cpp
@@ -33,13 +33,12 @@ class L_computer
                                                  size_t block_index,
                                                  size_t blocks) const
          {
-         const size_t BS = m_L_star.size();
-         m_offset_buf.resize(blocks * BS);
+         m_offset_buf.resize(blocks * 16);
 
          for(size_t i = 0; i != blocks; ++i)
             { // could be done in parallel
             offset ^= get(ctz(block_index + 1 + i));
-            copy_mem(&m_offset_buf[BS*i], offset.data(), BS);
+            copy_mem(&m_offset_buf[16*i], offset.data(), 16);
             }
 
          return m_offset_buf;
@@ -73,15 +72,13 @@ secure_vector<byte> ocb_hash(const L_computer& L,
                              const BlockCipher& cipher,
                              const byte ad[], size_t ad_len)
    {
-   const size_t BS = cipher.block_size();
+   secure_vector<byte> sum(16);
+   secure_vector<byte> offset(16);
 
-   secure_vector<byte> sum(BS);
-   secure_vector<byte> offset(BS);
+   secure_vector<byte> buf(16);
 
-   secure_vector<byte> buf(BS);
-
-   const size_t ad_blocks = (ad_len / BS);
-   const size_t ad_remainder = (ad_len % BS);
+   const size_t ad_blocks = (ad_len / 16);
+   const size_t ad_remainder = (ad_len % 16);
 
    for(size_t i = 0; i != ad_blocks; ++i)
       {
@@ -89,7 +86,7 @@ secure_vector<byte> ocb_hash(const L_computer& L,
       offset ^= L(ctz(i+1));
 
       buf = offset;
-      xor_buf(buf.data(), &ad[BS*i], BS);
+      xor_buf(buf.data(), &ad[16*i], 16);
 
       cipher.encrypt(buf);
 
@@ -101,8 +98,8 @@ secure_vector<byte> ocb_hash(const L_computer& L,
       offset ^= L.star();
 
       buf = offset;
-      xor_buf(buf.data(), &ad[BS*ad_blocks], ad_remainder);
-      buf[ad_len % BS] ^= 0x80;
+      xor_buf(buf.data(), &ad[16*ad_blocks], ad_remainder);
+      buf[ad_len % 16] ^= 0x80;
 
       cipher.encrypt(buf);
 
@@ -116,19 +113,16 @@ secure_vector<byte> ocb_hash(const L_computer& L,
 
 OCB_Mode::OCB_Mode(BlockCipher* cipher, size_t tag_size) :
    m_cipher(cipher),
-   m_BS(m_cipher->block_size()),
    m_checksum(m_cipher->parallel_bytes()),
-   m_offset(m_BS),
-   m_ad_hash(m_BS),
+   m_offset(16),
+   m_ad_hash(16),
    m_tag_size(tag_size)
    {
-   if(BS() != 16)
-      throw Invalid_Argument("OCB is not compatible with " + m_cipher->name());
-
-   if(m_tag_size % 4 != 0 || m_tag_size < 8 || m_tag_size > BS())
-      throw Invalid_Argument("OCB cannot produce a " + std::to_string(m_tag_size) +
-                                  " byte tag");
+   if(m_cipher->block_size() != 16)
+      throw Invalid_Argument("OCB requires 128 bit cipher");
 
+   if(m_tag_size % 4 != 0 || m_tag_size < 8 || m_tag_size > 16)
+      throw Invalid_Argument("Invalid OCB tag length");
    }
 
 OCB_Mode::~OCB_Mode() { /* for unique_ptr destructor */ }
@@ -178,16 +172,16 @@ void OCB_Mode::set_associated_data(const byte ad[], size_t ad_len)
 secure_vector<byte>
 OCB_Mode::update_nonce(const byte nonce[], size_t nonce_len)
    {
-   BOTAN_ASSERT(nonce_len < BS(), "OCB nonce is less than cipher block size");
+   BOTAN_ASSERT(nonce_len < 16, "OCB nonce is less than cipher block size");
 
-   secure_vector<byte> nonce_buf(BS());
+   secure_vector<byte> nonce_buf(16);
 
-   copy_mem(&nonce_buf[BS() - nonce_len], nonce, nonce_len);
+   copy_mem(&nonce_buf[16 - nonce_len], nonce, nonce_len);
    nonce_buf[0] = ((tag_size() * 8) % 128) << 1;
-   nonce_buf[BS() - nonce_len - 1] = 1;
+   nonce_buf[16 - nonce_len - 1] = 1;
 
-   const byte bottom = nonce_buf[BS()-1] & 0x3F;
-   nonce_buf[BS()-1] &= 0xC0;
+   const byte bottom = nonce_buf[16-1] & 0x3F;
+   nonce_buf[16-1] &= 0xC0;
 
    const bool need_new_stretch = (m_last_nonce != nonce_buf);
 
@@ -197,7 +191,7 @@ OCB_Mode::update_nonce(const byte nonce[], size_t nonce_len)
 
       m_cipher->encrypt(nonce_buf);
 
-      for(size_t i = 0; i != BS() / 2; ++i)
+      for(size_t i = 0; i != 16 / 2; ++i)
          nonce_buf.push_back(nonce_buf[i] ^ nonce_buf[i+1]);
 
       m_stretch = nonce_buf;
@@ -208,8 +202,8 @@ OCB_Mode::update_nonce(const byte nonce[], size_t nonce_len)
    const size_t shift_bytes = bottom / 8;
    const size_t shift_bits  = bottom % 8;
 
-   secure_vector<byte> offset(BS());
-   for(size_t i = 0; i != BS(); ++i)
+   secure_vector<byte> offset(16);
+   for(size_t i = 0; i != 16; ++i)
       {
       offset[i]  = (m_stretch[i+shift_bytes] << shift_bits);
       offset[i] |= (m_stretch[i+shift_bytes+1] >> (8-shift_bits));
@@ -218,7 +212,7 @@ OCB_Mode::update_nonce(const byte nonce[], size_t nonce_len)
    return offset;
    }
 
-secure_vector<byte> OCB_Mode::start_raw(const byte nonce[], size_t nonce_len)
+void OCB_Mode::start_msg(const byte nonce[], size_t nonce_len)
    {
    if(!valid_nonce_length(nonce_len))
       throw Invalid_IV_Length(name(), nonce_len);
@@ -228,18 +222,16 @@ secure_vector<byte> OCB_Mode::start_raw(const byte nonce[], size_t nonce_len)
    m_offset = update_nonce(nonce, nonce_len);
    zeroise(m_checksum);
    m_block_index = 0;
-
-   return secure_vector<byte>();
    }
 
 void OCB_Encryption::encrypt(byte buffer[], size_t blocks)
    {
-   const size_t par_blocks = m_checksum.size() / BS();
+   const size_t par_blocks = m_checksum.size() / 16;
 
    while(blocks)
       {
       const size_t proc_blocks = std::min(blocks, par_blocks);
-      const size_t proc_bytes = proc_blocks * BS();
+      const size_t proc_bytes = proc_blocks * 16;
 
       const auto& offsets = m_L->compute_offsets(m_offset, m_block_index, proc_blocks);
 
@@ -255,14 +247,11 @@ void OCB_Encryption::encrypt(byte buffer[], size_t blocks)
       }
    }
 
-void OCB_Encryption::update(secure_vector<byte>& buffer, size_t offset)
+size_t OCB_Encryption::process(uint8_t buf[], size_t sz)
    {
-   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
-   const size_t sz = buffer.size() - offset;
-   byte* buf = buffer.data() + offset;
-
-   BOTAN_ASSERT(sz % BS() == 0, "Input length is an even number of blocks");
-   encrypt(buf, sz / BS());
+   BOTAN_ASSERT(sz % 16 == 0, "Invalid OCB input size");
+   encrypt(buf, sz / 16);
+   return sz;
    }
 
 void OCB_Encryption::finish(secure_vector<byte>& buffer, size_t offset)
@@ -273,14 +262,14 @@ void OCB_Encryption::finish(secure_vector<byte>& buffer, size_t offset)
 
    if(sz)
       {
-      const size_t final_full_blocks = sz / BS();
-      const size_t remainder_bytes = sz - (final_full_blocks * BS());
+      const size_t final_full_blocks = sz / 16;
+      const size_t remainder_bytes = sz - (final_full_blocks * 16);
 
       encrypt(buf, final_full_blocks);
 
       if(remainder_bytes)
          {
-         BOTAN_ASSERT(remainder_bytes < BS(), "Only a partial block left");
+         BOTAN_ASSERT(remainder_bytes < 16, "Only a partial block left");
          byte* remainder = &buf[sz - remainder_bytes];
 
          xor_buf(m_checksum.data(), remainder, remainder_bytes);
@@ -288,13 +277,13 @@ void OCB_Encryption::finish(secure_vector<byte>& buffer, size_t offset)
 
          m_offset ^= m_L->star(); // Offset_*
 
-         secure_vector<byte> zeros(BS());
+         secure_vector<byte> zeros(16);
          m_cipher->encrypt(m_offset, zeros);
          xor_buf(remainder, zeros.data(), remainder_bytes);
          }
       }
 
-   secure_vector<byte> checksum(BS());
+   secure_vector<byte> checksum(16);
 
    // fold checksum
    for(size_t i = 0; i != m_checksum.size(); ++i)
@@ -320,14 +309,14 @@ void OCB_Decryption::decrypt(byte buffer[], size_t blocks)
    {
    const size_t par_bytes = m_cipher->parallel_bytes();
 
-   BOTAN_ASSERT(par_bytes % BS() == 0, "Cipher is parallel in full blocks");
+   BOTAN_ASSERT(par_bytes % 16 == 0, "Cipher is parallel in full blocks");
 
-   const size_t par_blocks = par_bytes / BS();
+   const size_t par_blocks = par_bytes / 16;
 
    while(blocks)
       {
       const size_t proc_blocks = std::min(blocks, par_blocks);
-      const size_t proc_bytes = proc_blocks * BS();
+      const size_t proc_bytes = proc_blocks * 16;
 
       const auto& offsets = m_L->compute_offsets(m_offset, m_block_index, proc_blocks);
 
@@ -343,15 +332,11 @@ void OCB_Decryption::decrypt(byte buffer[], size_t blocks)
       }
    }
 
-void OCB_Decryption::update(secure_vector<byte>& buffer, size_t offset)
+size_t OCB_Decryption::process(uint8_t buf[], size_t sz)
    {
-   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
-   const size_t sz = buffer.size() - offset;
-   byte* buf = buffer.data() + offset;
-
-   BOTAN_ASSERT(sz % BS() == 0, "Input length is an even number of blocks");
-
-   decrypt(buf, sz / BS());
+   BOTAN_ASSERT(sz % 16 == 0, "Invalid OCB input size");
+   decrypt(buf, sz / 16);
+   return sz;
    }
 
 void OCB_Decryption::finish(secure_vector<byte>& buffer, size_t offset)
@@ -366,20 +351,20 @@ void OCB_Decryption::finish(secure_vector<byte>& buffer, size_t offset)
 
    if(remaining)
       {
-      const size_t final_full_blocks = remaining / BS();
-      const size_t final_bytes = remaining - (final_full_blocks * BS());
+      const size_t final_full_blocks = remaining / 16;
+      const size_t final_bytes = remaining - (final_full_blocks * 16);
 
       decrypt(buf, final_full_blocks);
 
       if(final_bytes)
          {
-         BOTAN_ASSERT(final_bytes < BS(), "Only a partial block left");
+         BOTAN_ASSERT(final_bytes < 16, "Only a partial block left");
 
          byte* remainder = &buf[remaining - final_bytes];
 
          m_offset ^= m_L->star(); // Offset_*
 
-         secure_vector<byte> pad(BS());
+         secure_vector<byte> pad(16);
          m_cipher->encrypt(m_offset, pad); // P_*
 
          xor_buf(remainder, pad.data(), final_bytes);
@@ -389,7 +374,7 @@ void OCB_Decryption::finish(secure_vector<byte>& buffer, size_t offset)
          }
       }
 
-   secure_vector<byte> checksum(BS());
+   secure_vector<byte> checksum(16);
 
    // fold checksum
    for(size_t i = 0; i != m_checksum.size(); ++i)
diff --git a/src/lib/modes/aead/ocb/ocb.h b/src/lib/modes/aead/ocb/ocb.h
index 92edce970..4daa7a81b 100644
--- a/src/lib/modes/aead/ocb/ocb.h
+++ b/src/lib/modes/aead/ocb/ocb.h
@@ -49,20 +49,17 @@ class BOTAN_DLL OCB_Mode : public AEAD_Mode
       */
       OCB_Mode(BlockCipher* cipher, size_t tag_size);
 
-      size_t BS() const { return m_BS; }
-
       // fixme make these private
       std::unique_ptr<BlockCipher> m_cipher;
       std::unique_ptr<L_computer> m_L;
 
-      size_t m_BS;
       size_t m_block_index = 0;
 
       secure_vector<byte> m_checksum;
       secure_vector<byte> m_offset;
       secure_vector<byte> m_ad_hash;
    private:
-      secure_vector<byte> start_raw(const byte nonce[], size_t nonce_len) override;
+      void start_msg(const byte nonce[], size_t nonce_len) override;
 
       void key_schedule(const byte key[], size_t length) override;
 
@@ -88,7 +85,7 @@ class BOTAN_DLL OCB_Encryption final : public OCB_Mode
 
       size_t minimum_final_size() const override { return 0; }
 
-      void update(secure_vector<byte>& blocks, size_t offset = 0) override;
+      size_t process(uint8_t buf[], size_t size) override;
 
       void finish(secure_vector<byte>& final_block, size_t offset = 0) override;
    private:
@@ -113,7 +110,7 @@ class BOTAN_DLL OCB_Decryption final : public OCB_Mode
 
       size_t minimum_final_size() const override { return tag_size(); }
 
-      void update(secure_vector<byte>& blocks, size_t offset = 0) override;
+      size_t process(uint8_t buf[], size_t size) override;
 
       void finish(secure_vector<byte>& final_block, size_t offset = 0) override;
    private:
diff --git a/src/lib/modes/aead/siv/siv.cpp b/src/lib/modes/aead/siv/siv.cpp
index a4cb65a94..9e638b659 100644
--- a/src/lib/modes/aead/siv/siv.cpp
+++ b/src/lib/modes/aead/siv/siv.cpp
@@ -70,7 +70,7 @@ void SIV_Mode::set_associated_data_n(size_t n, const byte ad[], size_t length)
    m_ad_macs[n] = m_cmac->process(ad, length);
    }
 
-secure_vector<byte> SIV_Mode::start_raw(const byte nonce[], size_t nonce_len)
+void SIV_Mode::start_msg(const byte nonce[], size_t nonce_len)
    {
    if(!valid_nonce_length(nonce_len))
       throw Invalid_IV_Length(name(), nonce_len);
@@ -81,18 +81,13 @@ secure_vector<byte> SIV_Mode::start_raw(const byte nonce[], size_t nonce_len)
       m_nonce.clear();
 
    m_msg_buf.clear();
-
-   return secure_vector<byte>();
    }
 
-void SIV_Mode::update(secure_vector<byte>& buffer, size_t offset)
+size_t SIV_Mode::process(uint8_t buf[], size_t sz)
    {
-   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
-   const size_t sz = buffer.size() - offset;
-   byte* buf = buffer.data() + offset;
-
+   // all output is saved for processing in finish
    m_msg_buf.insert(m_msg_buf.end(), buf, buf + sz);
-   buffer.resize(offset); // truncate msg
+   return 0;
    }
 
 secure_vector<byte> SIV_Mode::S2V(const byte* text, size_t text_len)
diff --git a/src/lib/modes/aead/siv/siv.h b/src/lib/modes/aead/siv/siv.h
index d3e4c5270..6acfe515a 100644
--- a/src/lib/modes/aead/siv/siv.h
+++ b/src/lib/modes/aead/siv/siv.h
@@ -21,7 +21,7 @@ namespace Botan {
 class BOTAN_DLL SIV_Mode : public AEAD_Mode
    {
    public:
-      void update(secure_vector<byte>& blocks, size_t offset = 0) override;
+      size_t process(uint8_t buf[], size_t size) override;
 
       void set_associated_data_n(size_t n, const byte ad[], size_t ad_len);
 
@@ -53,7 +53,7 @@ class BOTAN_DLL SIV_Mode : public AEAD_Mode
 
       secure_vector<byte> S2V(const byte text[], size_t text_len);
    private:
-      secure_vector<byte> start_raw(const byte nonce[], size_t nonce_len) override;
+      void start_msg(const byte nonce[], size_t nonce_len) override;
 
       void key_schedule(const byte key[], size_t length) override;
 
diff --git a/src/lib/modes/cbc/cbc.cpp b/src/lib/modes/cbc/cbc.cpp
index fedeaf20d..8066dae12 100644
--- a/src/lib/modes/cbc/cbc.cpp
+++ b/src/lib/modes/cbc/cbc.cpp
@@ -61,7 +61,7 @@ void CBC_Mode::key_schedule(const byte key[], size_t length)
    m_cipher->set_key(key, length);
    }
 
-secure_vector<byte> CBC_Mode::start_raw(const byte nonce[], size_t nonce_len)
+void CBC_Mode::start_msg(const byte nonce[], size_t nonce_len)
    {
    if(!valid_nonce_length(nonce_len))
       throw Invalid_IV_Length(name(), nonce_len);
@@ -73,8 +73,6 @@ secure_vector<byte> CBC_Mode::start_raw(const byte nonce[], size_t nonce_len)
    */
    if(nonce_len)
       m_state.assign(nonce, nonce + nonce_len);
-
-   return secure_vector<byte>();
    }
 
 size_t CBC_Encryption::minimum_final_size() const
@@ -90,12 +88,8 @@ size_t CBC_Encryption::output_length(size_t input_length) const
       return round_up(input_length, cipher().block_size());
    }
 
-void CBC_Encryption::update(secure_vector<byte>& buffer, size_t offset)
+size_t CBC_Encryption::process(uint8_t buf[], size_t sz)
    {
-   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
-   const size_t sz = buffer.size() - offset;
-   byte* buf = buffer.data() + offset;
-
    const size_t BS = cipher().block_size();
 
    BOTAN_ASSERT(sz % BS == 0, "CBC input is full blocks");
@@ -114,6 +108,8 @@ void CBC_Encryption::update(secure_vector<byte>& buffer, size_t offset)
 
       state().assign(&buf[BS*(blocks-1)], &buf[BS*blocks]);
       }
+
+   return sz;
    }
 
 void CBC_Encryption::finish(secure_vector<byte>& buffer, size_t offset)
@@ -201,12 +197,8 @@ size_t CBC_Decryption::minimum_final_size() const
    return cipher().block_size();
    }
 
-void CBC_Decryption::update(secure_vector<byte>& buffer, size_t offset)
+size_t CBC_Decryption::process(uint8_t buf[], size_t sz)
    {
-   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
-   const size_t sz = buffer.size() - offset;
-   byte* buf = buffer.data() + offset;
-
    const size_t BS = cipher().block_size();
 
    BOTAN_ASSERT(sz % BS == 0, "Input is full blocks");
@@ -227,6 +219,8 @@ void CBC_Decryption::update(secure_vector<byte>& buffer, size_t offset)
       buf += to_proc;
       blocks -= to_proc / BS;
       }
+
+   return sz;
    }
 
 void CBC_Decryption::finish(secure_vector<byte>& buffer, size_t offset)
diff --git a/src/lib/modes/cbc/cbc.h b/src/lib/modes/cbc/cbc.h
index 961991d4a..caad102d4 100644
--- a/src/lib/modes/cbc/cbc.h
+++ b/src/lib/modes/cbc/cbc.h
@@ -47,7 +47,7 @@ class BOTAN_DLL CBC_Mode : public Cipher_Mode
       byte* state_ptr() { return m_state.data(); }
 
    private:
-      secure_vector<byte> start_raw(const byte nonce[], size_t nonce_len) override;
+      void start_msg(const byte nonce[], size_t nonce_len) override;
 
       void key_schedule(const byte key[], size_t length) override;
 
@@ -65,7 +65,7 @@ class BOTAN_DLL CBC_Encryption : public CBC_Mode
       CBC_Encryption(BlockCipher* cipher, BlockCipherModePaddingMethod* padding) :
          CBC_Mode(cipher, padding) {}
 
-      void update(secure_vector<byte>& blocks, size_t offset = 0) override;
+      size_t process(uint8_t buf[], size_t size) override;
 
       void finish(secure_vector<byte>& final_block, size_t offset = 0) override;
 
@@ -100,7 +100,7 @@ class BOTAN_DLL CBC_Decryption : public CBC_Mode
       CBC_Decryption(BlockCipher* cipher, BlockCipherModePaddingMethod* padding) :
          CBC_Mode(cipher, padding), m_tempbuf(update_granularity()) {}
 
-      void update(secure_vector<byte>& blocks, size_t offset = 0) override;
+      size_t process(uint8_t buf[], size_t size) override;
 
       void finish(secure_vector<byte>& final_block, size_t offset = 0) override;
 
diff --git a/src/lib/modes/cfb/cfb.cpp b/src/lib/modes/cfb/cfb.cpp
index 6c9239e73..cc5e9bae7 100644
--- a/src/lib/modes/cfb/cfb.cpp
+++ b/src/lib/modes/cfb/cfb.cpp
@@ -69,7 +69,7 @@ void CFB_Mode::key_schedule(const byte key[], size_t length)
    m_cipher->set_key(key, length);
    }
 
-secure_vector<byte> CFB_Mode::start_raw(const byte nonce[], size_t nonce_len)
+void CFB_Mode::start_msg(const byte nonce[], size_t nonce_len)
    {
    if(!valid_nonce_length(nonce_len))
       throw Invalid_IV_Length(name(), nonce_len);
@@ -77,24 +77,19 @@ secure_vector<byte> CFB_Mode::start_raw(const byte nonce[], size_t nonce_len)
    m_shift_register.assign(nonce, nonce + nonce_len);
    m_keystream_buf.resize(m_shift_register.size());
    cipher().encrypt(m_shift_register, m_keystream_buf);
-
-   return secure_vector<byte>();
    }
 
-void CFB_Encryption::update(secure_vector<byte>& buffer, size_t offset)
+size_t CFB_Encryption::process(uint8_t buf[], size_t sz)
    {
-   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
-   size_t sz = buffer.size() - offset;
-   byte* buf = buffer.data() + offset;
-
    const size_t BS = cipher().block_size();
 
    secure_vector<byte>& state = shift_register();
    const size_t shift = feedback();
+   size_t left = sz;
 
-   while(sz)
+   while(left)
       {
-      const size_t took = std::min(shift, sz);
+      const size_t took = std::min(shift, left);
       xor_buf(buf, &keystream_buf()[0], took);
 
       // Assumes feedback-sized block except for last input
@@ -106,8 +101,9 @@ void CFB_Encryption::update(secure_vector<byte>& buffer, size_t offset)
       cipher().encrypt(state, keystream_buf());
 
       buf += took;
-      sz -= took;
+      left -= took;
       }
+   return sz;
    }
 
 void CFB_Encryption::finish(secure_vector<byte>& buffer, size_t offset)
@@ -115,20 +111,17 @@ void CFB_Encryption::finish(secure_vector<byte>& buffer, size_t offset)
    update(buffer, offset);
    }
 
-void CFB_Decryption::update(secure_vector<byte>& buffer, size_t offset)
+size_t CFB_Decryption::process(uint8_t buf[], size_t sz)
    {
-   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
-   size_t sz = buffer.size() - offset;
-   byte* buf = buffer.data() + offset;
-
    const size_t BS = cipher().block_size();
 
    secure_vector<byte>& state = shift_register();
    const size_t shift = feedback();
+   size_t left = sz;
 
-   while(sz)
+   while(left)
       {
-      const size_t took = std::min(shift, sz);
+      const size_t took = std::min(shift, left);
 
       // first update shift register with ciphertext
       if (BS - shift > 0)
@@ -144,8 +137,9 @@ void CFB_Decryption::update(secure_vector<byte>& buffer, size_t offset)
       cipher().encrypt(state, keystream_buf());
 
       buf += took;
-      sz -= took;
+      left -= took;
       }
+   return sz;
    }
 
 void CFB_Decryption::finish(secure_vector<byte>& buffer, size_t offset)
diff --git a/src/lib/modes/cfb/cfb.h b/src/lib/modes/cfb/cfb.h
index 49321a1c9..2ec87244e 100644
--- a/src/lib/modes/cfb/cfb.h
+++ b/src/lib/modes/cfb/cfb.h
@@ -46,7 +46,7 @@ class BOTAN_DLL CFB_Mode : public Cipher_Mode
       secure_vector<byte>& keystream_buf() { return m_keystream_buf; }
 
    private:
-      secure_vector<byte> start_raw(const byte nonce[], size_t nonce_len) override;
+      void start_msg(const byte nonce[], size_t nonce_len) override;
       void key_schedule(const byte key[], size_t length) override;
 
       std::unique_ptr<BlockCipher> m_cipher;
@@ -64,7 +64,7 @@ class BOTAN_DLL CFB_Encryption final : public CFB_Mode
       CFB_Encryption(BlockCipher* cipher, size_t feedback_bits) :
          CFB_Mode(cipher, feedback_bits) {}
 
-      void update(secure_vector<byte>& blocks, size_t offset = 0) override;
+      size_t process(uint8_t buf[], size_t size) override;
 
       void finish(secure_vector<byte>& final_block, size_t offset = 0) override;
    };
@@ -78,7 +78,7 @@ class BOTAN_DLL CFB_Decryption final : public CFB_Mode
       CFB_Decryption(BlockCipher* cipher, size_t feedback_bits) :
          CFB_Mode(cipher, feedback_bits) {}
 
-      void update(secure_vector<byte>& blocks, size_t offset = 0) override;
+      size_t process(uint8_t buf[], size_t size) override;
 
       void finish(secure_vector<byte>& final_block, size_t offset = 0) override;
    };
diff --git a/src/lib/modes/cipher_mode.h b/src/lib/modes/cipher_mode.h
index 73a5f7d96..5781ebbee 100644
--- a/src/lib/modes/cipher_mode.h
+++ b/src/lib/modes/cipher_mode.h
@@ -1,6 +1,6 @@
 /*
 * Cipher Modes
-* (C) 2013 Jack Lloyd
+* (C) 2013,2016 Jack Lloyd
 *
 * Botan is released under the Simplified BSD License (see license.txt)
 */
@@ -28,25 +28,19 @@ class BOTAN_DLL Cipher_Mode
 
       virtual ~Cipher_Mode() {}
 
-      /**
-      * Begin processing a message.
-      * @param nonce the per message nonce
+      /*
+      * Prepare for processing a message under the specified nonce
       */
-      template<typename Alloc>
-      secure_vector<byte> start(const std::vector<byte, Alloc>& nonce)
-         {
-         return start(nonce.data(), nonce.size());
-         }
+      virtual void start_msg(const byte nonce[], size_t nonce_len) = 0;
 
       /**
       * Begin processing a message.
       * @param nonce the per message nonce
       */
       template<typename Alloc>
-      BOTAN_DEPRECATED("Use Transform::start")
-      secure_vector<byte> start_vec(const std::vector<byte, Alloc>& nonce)
+      void start(const std::vector<byte, Alloc>& nonce)
          {
-         return start(nonce.data(), nonce.size());
+         start_msg(nonce.data(), nonce.size());
          }
 
       /**
@@ -54,27 +48,45 @@ class BOTAN_DLL Cipher_Mode
       * @param nonce the per message nonce
       * @param nonce_len length of nonce
       */
-      secure_vector<byte> start(const byte nonce[], size_t nonce_len)
+      void start(const byte nonce[], size_t nonce_len)
          {
-         return start_raw(nonce, nonce_len);
+         start_msg(nonce, nonce_len);
          }
 
       /**
       * Begin processing a message.
       */
-      secure_vector<byte> start()
+      void start()
          {
-         return start_raw(nullptr, 0);
+         return start_msg(nullptr, 0);
          }
 
-      virtual secure_vector<byte> start_raw(const byte nonce[], size_t nonce_len) = 0;
+      /**
+      * Process message blocks
+      *
+      * Input must be a multiple of update_granularity
+      *
+      * Processes msg in place and returns bytes written. Normally
+      * this will be either msg_len (indicating the entire message was
+      * processes) or for certain AEAD modes zero (indicating that the
+      * mode requires the entire message be processed in one pass.
+      */
+      virtual size_t process(uint8_t msg[], size_t msg_len) = 0;
 
       /**
       * Process some data. Input must be in size update_granularity() byte blocks.
       * @param blocks in/out parameter which will possibly be resized
       * @param offset an offset into blocks to begin processing
       */
-      virtual void update(secure_vector<byte>& blocks, size_t offset = 0) = 0;
+      void update(secure_vector<byte>& buffer, size_t offset = 0)
+         {
+         BOTAN_ASSERT(buffer.size() >= offset, "Offset ok");
+         byte* buf = buffer.data() + offset;
+         const size_t buf_size = buffer.size() - offset;
+
+         const size_t written = process(buf, buf_size);
+         buffer.resize(offset + written);
+         }
 
       /**
       * Complete processing of a message.
diff --git a/src/lib/modes/ecb/ecb.cpp b/src/lib/modes/ecb/ecb.cpp
index 407b5c582..650cfedf1 100644
--- a/src/lib/modes/ecb/ecb.cpp
+++ b/src/lib/modes/ecb/ecb.cpp
@@ -55,12 +55,10 @@ void ECB_Mode::key_schedule(const byte key[], size_t length)
    m_cipher->set_key(key, length);
    }
 
-secure_vector<byte> ECB_Mode::start_raw(const byte[], size_t nonce_len)
+void ECB_Mode::start_msg(const byte[], size_t nonce_len)
    {
-   if(!valid_nonce_length(nonce_len))
+   if(nonce_len != 0)
       throw Invalid_IV_Length(name(), nonce_len);
-
-   return secure_vector<byte>();
    }
 
 size_t ECB_Encryption::minimum_final_size() const
@@ -76,18 +74,13 @@ size_t ECB_Encryption::output_length(size_t input_length) const
       return round_up(input_length, cipher().block_size());
    }
 
-void ECB_Encryption::update(secure_vector<byte>& buffer, size_t offset)
+size_t ECB_Encryption::process(uint8_t buf[], size_t sz)
    {
-   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
-   const size_t sz = buffer.size() - offset;
-   byte* buf = buffer.data() + offset;
-
    const size_t BS = cipher().block_size();
-
    BOTAN_ASSERT(sz % BS == 0, "ECB input is full blocks");
    const size_t blocks = sz / BS;
-
    cipher().encrypt_n(buf, buf, blocks);
+   return sz;
    }
 
 void ECB_Encryption::finish(secure_vector<byte>& buffer, size_t offset)
@@ -117,18 +110,13 @@ size_t ECB_Decryption::minimum_final_size() const
    return cipher().block_size();
    }
 
-void ECB_Decryption::update(secure_vector<byte>& buffer, size_t offset)
+size_t ECB_Decryption::process(uint8_t buf[], size_t sz)
    {
-   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
-   const size_t sz = buffer.size() - offset;
-   byte* buf = buffer.data() + offset;
-
    const size_t BS = cipher().block_size();
-
    BOTAN_ASSERT(sz % BS == 0, "Input is full blocks");
    size_t blocks = sz / BS;
-
    cipher().decrypt_n(buf, buf, blocks);
+   return sz;
    }
 
 void ECB_Decryption::finish(secure_vector<byte>& buffer, size_t offset)
diff --git a/src/lib/modes/ecb/ecb.h b/src/lib/modes/ecb/ecb.h
index aebd4c1a5..9ebbf76a4 100644
--- a/src/lib/modes/ecb/ecb.h
+++ b/src/lib/modes/ecb/ecb.h
@@ -39,7 +39,7 @@ class BOTAN_DLL ECB_Mode : public Cipher_Mode
       const BlockCipherModePaddingMethod& padding() const { return *m_padding; }
 
    private:
-      secure_vector<byte> start_raw(const byte nonce[], size_t nonce_len) override;
+      void start_msg(const byte nonce[], size_t nonce_len) override;
       void key_schedule(const byte key[], size_t length) override;
 
       std::unique_ptr<BlockCipher> m_cipher;
@@ -55,7 +55,7 @@ class BOTAN_DLL ECB_Encryption final : public ECB_Mode
       ECB_Encryption(BlockCipher* cipher, BlockCipherModePaddingMethod* padding) :
          ECB_Mode(cipher, padding) {}
 
-      void update(secure_vector<byte>& blocks, size_t offset = 0) override;
+      size_t process(uint8_t buf[], size_t size) override;
 
       void finish(secure_vector<byte>& final_block, size_t offset = 0) override;
 
@@ -73,7 +73,7 @@ class BOTAN_DLL ECB_Decryption final : public ECB_Mode
       ECB_Decryption(BlockCipher* cipher, BlockCipherModePaddingMethod* padding) :
          ECB_Mode(cipher, padding) {}
 
-      void update(secure_vector<byte>& blocks, size_t offset = 0) override;
+      size_t process(uint8_t buf[], size_t size) override;
 
       void finish(secure_vector<byte>& final_block, size_t offset = 0) override;
 
diff --git a/src/lib/modes/stream_mode.h b/src/lib/modes/stream_mode.h
index f5f1aa33a..f59f6d9ba 100644
--- a/src/lib/modes/stream_mode.h
+++ b/src/lib/modes/stream_mode.h
@@ -17,10 +17,10 @@ class BOTAN_DLL Stream_Cipher_Mode : public Cipher_Mode
    public:
       explicit Stream_Cipher_Mode(StreamCipher* cipher) : m_cipher(cipher) {}
 
-      void update(secure_vector<byte>& buf, size_t offset) override
+      size_t process(uint8_t buf[], size_t sz) override
          {
-         if(offset < buf.size())
-            m_cipher->cipher1(&buf[offset], buf.size() - offset);
+         m_cipher->cipher1(buf, sz);
+         return sz;
          }
 
       void finish(secure_vector<byte>& buf, size_t offset) override
@@ -28,7 +28,7 @@ class BOTAN_DLL Stream_Cipher_Mode : public Cipher_Mode
 
       size_t output_length(size_t input_length) const override { return input_length; }
 
-      size_t update_granularity() const override { return 64; /* arbitrary */ }
+      size_t update_granularity() const override { return 1; }
 
       size_t minimum_final_size() const override { return 0; }
 
@@ -44,10 +44,9 @@ class BOTAN_DLL Stream_Cipher_Mode : public Cipher_Mode
       void clear() override { return m_cipher->clear(); }
 
    private:
-      secure_vector<byte> start_raw(const byte nonce[], size_t nonce_len) override
+      void start_msg(const byte nonce[], size_t nonce_len) override
          {
          m_cipher->set_iv(nonce, nonce_len);
-         return secure_vector<byte>();
          }
 
       void key_schedule(const byte key[], size_t length) override
diff --git a/src/lib/modes/xts/xts.cpp b/src/lib/modes/xts/xts.cpp
index b369fde29..1993bf15f 100644
--- a/src/lib/modes/xts/xts.cpp
+++ b/src/lib/modes/xts/xts.cpp
@@ -105,7 +105,7 @@ void XTS_Mode::key_schedule(const byte key[], size_t length)
    m_tweak_cipher->set_key(&key[key_half], key_half);
    }
 
-secure_vector<byte> XTS_Mode::start_raw(const byte nonce[], size_t nonce_len)
+void XTS_Mode::start_msg(const byte nonce[], size_t nonce_len)
    {
    if(!valid_nonce_length(nonce_len))
       throw Invalid_IV_Length(name(), nonce_len);
@@ -114,8 +114,6 @@ secure_vector<byte> XTS_Mode::start_raw(const byte nonce[], size_t nonce_len)
    m_tweak_cipher->encrypt(m_tweak.data());
 
    update_tweak(0);
-
-   return secure_vector<byte>();
    }
 
 void XTS_Mode::update_tweak(size_t which)
@@ -136,12 +134,8 @@ size_t XTS_Encryption::output_length(size_t input_length) const
    return input_length;
    }
 
-void XTS_Encryption::update(secure_vector<byte>& buffer, size_t offset)
+size_t XTS_Encryption::process(uint8_t buf[], size_t sz)
    {
-   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
-   const size_t sz = buffer.size() - offset;
-   byte* buf = buffer.data() + offset;
-
    const size_t BS = cipher().block_size();
 
    BOTAN_ASSERT(sz % BS == 0, "Input is full blocks");
@@ -163,6 +157,8 @@ void XTS_Encryption::update(secure_vector<byte>& buffer, size_t offset)
 
       update_tweak(to_proc);
       }
+
+   return sz;
    }
 
 void XTS_Encryption::finish(secure_vector<byte>& buffer, size_t offset)
@@ -214,12 +210,8 @@ size_t XTS_Decryption::output_length(size_t input_length) const
    return input_length;
    }
 
-void XTS_Decryption::update(secure_vector<byte>& buffer, size_t offset)
+size_t XTS_Decryption::process(uint8_t buf[], size_t sz)
    {
-   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
-   const size_t sz = buffer.size() - offset;
-   byte* buf = buffer.data() + offset;
-
    const size_t BS = cipher().block_size();
 
    BOTAN_ASSERT(sz % BS == 0, "Input is full blocks");
@@ -241,6 +233,8 @@ void XTS_Decryption::update(secure_vector<byte>& buffer, size_t offset)
 
       update_tweak(to_proc);
       }
+
+   return sz;
    }
 
 void XTS_Decryption::finish(secure_vector<byte>& buffer, size_t offset)
diff --git a/src/lib/modes/xts/xts.h b/src/lib/modes/xts/xts.h
index e751b1644..6c4ba8d99 100644
--- a/src/lib/modes/xts/xts.h
+++ b/src/lib/modes/xts/xts.h
@@ -42,7 +42,7 @@ class BOTAN_DLL XTS_Mode : public Cipher_Mode
       void update_tweak(size_t last_used);
 
    private:
-      secure_vector<byte> start_raw(const byte nonce[], size_t nonce_len) override;
+      void start_msg(const byte nonce[], size_t nonce_len) override;
       void key_schedule(const byte key[], size_t length) override;
 
       std::unique_ptr<BlockCipher> m_cipher, m_tweak_cipher;
@@ -57,7 +57,7 @@ class BOTAN_DLL XTS_Encryption final : public XTS_Mode
    public:
       explicit XTS_Encryption(BlockCipher* cipher) : XTS_Mode(cipher) {}
 
-      void update(secure_vector<byte>& blocks, size_t offset = 0) override;
+      size_t process(uint8_t buf[], size_t size) override;
 
       void finish(secure_vector<byte>& final_block, size_t offset = 0) override;
 
@@ -72,7 +72,7 @@ class BOTAN_DLL XTS_Decryption final : public XTS_Mode
    public:
       explicit XTS_Decryption(BlockCipher* cipher) : XTS_Mode(cipher) {}
 
-      void update(secure_vector<byte>& blocks, size_t offset = 0) override;
+      size_t process(uint8_t buf[], size_t size) override;
 
       void finish(secure_vector<byte>& final_block, size_t offset = 0) override;
 
diff --git a/src/lib/tls/tls_record.cpp b/src/lib/tls/tls_record.cpp
index 877b81b41..4a52aa4a9 100644
--- a/src/lib/tls/tls_record.cpp
+++ b/src/lib/tls/tls_record.cpp
@@ -190,7 +190,6 @@ void write_record(secure_vector<byte>& output,
 
       const std::vector<byte> nonce = cs->aead_nonce(seq);
 
-      // wrong if start returns something
       const size_t rec_size = ctext_size + cs->nonce_bytes_from_record();
 
       BOTAN_ASSERT(rec_size <= 0xFFFF, "Ciphertext length fits in field");
@@ -203,13 +202,11 @@ void write_record(secure_vector<byte>& output,
          {
          output += std::make_pair(&nonce[cs->nonce_bytes_from_handshake()], cs->nonce_bytes_from_record());
          }
-      BOTAN_ASSERT(aead->start(nonce).empty(), "AEAD doesn't return anything from start");
-
-      const size_t offset = output.size();
+      const size_t header_size = output.size();
       output += std::make_pair(msg.get_data(), msg.get_size());
-      aead->finish(output, offset);
 
-      BOTAN_ASSERT(output.size() == offset + ctext_size, "Expected size");
+      aead->start(nonce);
+      aead->finish(output, header_size);
 
       BOTAN_ASSERT(output.size() < MAX_CIPHERTEXT_SIZE,
                    "Produced ciphertext larger than protocol allows");
@@ -469,7 +466,7 @@ void decrypt_record(secure_vector<byte>& output,
          cs.format_ad(record_sequence, record_type, record_version, static_cast<u16bit>(ptext_size))
          );
 
-      output += aead->start(nonce);
+      aead->start(nonce);
 
       const size_t offset = output.size();
       output += std::make_pair(msg, msg_length);
diff --git a/src/lib/utils/assert.h b/src/lib/utils/assert.h
index f80f9b170..c49ae62ee 100644
--- a/src/lib/utils/assert.h
+++ b/src/lib/utils/assert.h
@@ -35,6 +35,19 @@ BOTAN_NORETURN void BOTAN_DLL assertion_failure(const char* expr_str,
    } while(0)
 
 /**
+* Make an assertion
+*/
+#define BOTAN_ASSERT_NOMSG(expr)                          \
+   do {                                                   \
+      if(!(expr))                                         \
+         Botan::assertion_failure(#expr,                  \
+                                  "",                     \
+                                  BOTAN_CURRENT_FUNCTION, \
+                                  __FILE__,               \
+                                  __LINE__);              \
+   } while(0)
+
+/**
 * Assert that value1 == value2
 */
 #define BOTAN_ASSERT_EQUAL(expr1, expr2, assertion_made)   \
diff --git a/src/lib/utils/exceptn.h b/src/lib/utils/exceptn.h
index 193d78ce9..a3cb11f81 100644
--- a/src/lib/utils/exceptn.h
+++ b/src/lib/utils/exceptn.h
@@ -29,14 +29,20 @@ class BOTAN_DLL Exception : public std::exception
    };
 
 /**
-* An invalid argument which caused
+* An invalid argument
 */
 class BOTAN_DLL Invalid_Argument : public Exception
    {
    public:
       explicit Invalid_Argument(const std::string& msg) :
          Exception("Invalid argument", msg) {}
-   };
+
+      explicit Invalid_Argument(const std::string& msg, const std::string& where) :
+         Exception("Invalid argument", msg + " in " + where) {}
+};
+
+#define BOTAN_ARG_CHECK(expr) \
+   do { if(!(expr)) throw Invalid_Argument(#expr, BOTAN_CURRENT_FUNCTION); } while(0)
 
 /**
 * Unsupported_Argument Exception