custom streambuf - not working with >> but works with << and .get()

Hello,
my custom streambuf works fine with output via <<
and with input via .get()
but fails to input via >> or getline... any idea why?

--------------------

A custom stream buffer (for encryption, currently it is doing "nothing")

it almost works, it supports << any_stringable_object
it supports input via .get()
but other inputs - using >> char_variable and getline etc.
fail (program hangs / wait)

Why?

operator>> or getline uses more from underlying streabuf, then .getc()
needs?

PROBLEM is at end of code.

--------------------

#include <streambuf>
#include <istream>
#include <algorithm>
#include <iostream>
#include <fstream>

namespace nUpt {
using namespace std;

// http://www.cplusplus.com/ref/iostream/streambuf/sgetc.html
typedef int uint_type ; // TODO

class decrypting_streambuf : public std::streambuf {
        typedef std::streambuf::int_type int_type;

        int_type m_key; // this filter (decryption) specyfic
        pos_type m_pos; // this filter (decryption) specyfic
        std::streambuf *m_sbuf; // underlying streambuf (only-pointed, not owned
but this)
        
  public:
    decrypting_streambuf(std::streambuf *buf, uint_type header_key)
     : m_key(header_key),
                        m_sbuf(buf),
                        m_pos(0)
                        //m_pos( m_sbuf->pubseekoff(0,std::ios::cur) )
                {
                        cerr<<" "<<__LINE__<<" CTOR! "<<endl;
        m_sbuf->pubseekpos( 0 ); // ???
    }
                
    // std::streambuf *rdbuf() { return m_sbuf; }
                
  private:
    inline char_type decode(char_type c, pos_type const &pos) {
                        return c;// ^ (pos^m_key);
    }
                
    char_type encode(char_type c, pos_type const pos) {
                        return c;// ^ (pos^m_key);
    }
                
  protected:
    virtual int_type underflow() { // XXX underfload: READ
                        cout<<':';
                        int_type fromUnder = m_sbuf->sbumpc(); // read from underlying buf
                        if (fromUnder = traits_type::eof()) return fromUnder; // EOF
                        int_type coded = traits_type::to_int_type(
                                decode(traits_type::to_char_type( fromUnder ), m_pos)
                        );
                        m_pos+=1;
                        return coded;
    }
                
    virtual int_type uflow() { // XXX underflow: READ
                        cout<<'>';
                        int_type fromUnder = m_sbuf->sbumpc(); // read from underlying buf
                        if (fromUnder == traits_type::eof()) return fromUnder; // EOF
                        int_type coded = traits_type::to_int_type(
                                decode(traits_type::to_char_type( fromUnder ), m_pos)
                        );
                        m_pos+=1;
                        return coded;
    }
                
    virtual int_type overflow(int_type c = traits_type::eof()) { // XXX
WRITE
                        cout<<'<';
        if ( c == traits_type::eof() ) return c; // EOF
        c = m_sbuf->sputc(
                                encode(traits_type::to_char_type(c), m_pos )
                        );
        m_pos+=1;
        return c;
                }
};

} // NAMESPACE nUpt

void Test() {
        using namespace std;
        cout << "\nWRITTING...: " << endl;
        {
                ofstream plik("test.txt");
                nUpt::decrypting_streambuf buf( plik.rdbuf() , 1234 ); // underlying
                ostream out( &buf );
                
                out << "abc";
                if (1) {
                        out << "def" << ' ' << 2*64*2 << "\n" ;
                        out << "Hello" << endl;
                        out << "World!" << endl;
                        out << "123" << flush << 456;
                }
                plik.close();
        }
        
        cout << "\nFILE: " << endl;
        {
                ifstream plik("test.txt");
                while (!!plik) {
                        string s;
                        getline(plik,s);
                        cout<<s<<endl;
                }
        }
        
        cout << "\nDECODE: " << endl;
        {
                ifstream plik("test.txt"); // underlying device
                nUpt::decrypting_streambuf buf( plik.rdbuf() , 1234 ); // underlying
buffer/filter
                istream in( &buf ); // the stream
                int i=30;
                while (1) {
                        cout<<"*";

// HERE ***********************************
                        if (!(i--)) break;
                        char c = in.get(); // THIS WORKS OK
                        // char c; cin>>c; // THIS FAILS!
                        cout<<c;
                }
        }
}

int main() { Test(); return 0; }

==================================================
Output (working)

WRITTING...:
   31 CTOR!
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
FILE:
abcdef 256
Hello
World!
123456

DECODE:
   31 CTOR!
*>a*>b*>c*>d*>e*>f*> *>2*>5*>6*>
*>H*>e*>l*>l*>o*>
*>W*>o*>r*>l*>d*>!*>
*>1*>2*>3*>4*>5*>6*

==================================================
Do not work - using operator>> at end of code above

WRITTING...:
   31 CTOR!
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
FILE:
abcdef 256
Hello
World!
123456

DECODE:
   31 CTOR!
*

and program "hangs" waits here.

--
RafaV Maj