RC4 fix only (#81)

* Stream factory allows 0 size of dummy stream

Used for RC4's IV, but in future, it can be usefull elsewhere as well

* Fix reinitialization of RC4 internal state after each function call

* Fix intended reinitialization of RC4

streams.cc

@@ -217,7 +217,7 @@ std::unique_ptr<stream> make_stream(const json &config,
                                     core::optional<stream *> stream) {
     const std::string type = config.at("type");
 
-    if (osize == 0)
+    if (osize == 0 and type != "dummy_stream") // we allow dummy stream with 0 size
         throw std::runtime_error("Stream " + type + " cannot have osize 0.");
 
     // trivial source only streams

streams/stream_ciphers/other/rc4/rc4.cpp

@@ -38,13 +38,13 @@ static void arcfour_key_setup(std::uint8_t state[state_size], const std::uint8_t
 
 // This does not hold state between calls. It always generates the
 // stream starting from the first  output byte.
-static void arcfour_generate_stream(std::uint8_t state[], std::uint8_t out[], const size_t len)
+// indices i and j has to be part of internal state - they were added to the former API
+static void arcfour_generate_stream(std::uint8_t state[], std::uint8_t out[], const size_t len, int &i, int &j)
 {
-    int i, j;
     size_t idx;
     std::uint8_t t;
 
-    for (idx = 0, i = 0, j = 0; idx < len; ++idx)  {
+    for (idx = 0; idx < len; ++idx)  {
         i = (i + 1) % 256;
         j = (j + state[i]) % 256;
         t = state[i];
@@ -55,6 +55,8 @@ static void arcfour_generate_stream(std::uint8_t state[], std::uint8_t out[], co
 }
 
 void rc4::keysetup(const u8* key, const u32 key_bitsize, const u32 iv_bitsize) {
+    _ctx.i = 0;
+    _ctx.j = 0;
     _ctx.key = std::make_unique<std::uint8_t[]>(key_bitsize / 8);
     std::copy_n(key, key_bitsize / 8, _ctx.key.get());
     arcfour_key_setup(_ctx.state, _ctx.key.get(), int(key_bitsize / 8));
@@ -67,7 +69,7 @@ void rc4::ivsetup(const u8* iv) { }
 void rc4::encrypt_bytes(const u8 *plaintext, u8 *ciphertext, const u32 ptx_size) {
     // prepare stream
     auto stream = std::make_unique<std::uint8_t[]>(ptx_size);
-    arcfour_generate_stream(_ctx.state, stream.get(), ptx_size);
+    arcfour_generate_stream(_ctx.state, stream.get(), ptx_size, _ctx.i, _ctx.j);
     // xor it with input
     for (unsigned i = 0; i < ptx_size; ++i) {
         ciphertext[i] = plaintext[i] ^ stream[i];

streams/stream_ciphers/other/rc4/rc4.h

@@ -17,12 +17,12 @@ class rc4 : public stream_interface {
     struct rc4_ctx {
 
         rc4_ctx()
-            : state{0}
-            , key_size(0) {}
+            : state{0} {}
 
         std::uint8_t state[256];
         std::unique_ptr<std::uint8_t[]> key;
         std::uint32_t key_size;
+        int i, j;
     } _ctx;
 
 public: