How to cast a char* to a byte* in c++? - c++

Question

I am very new in c++ and want to cast a char* from a std::string to a byte*.
Here is my code:
inline string XOR(const string &value, const string &key) {
string retval(value);
CryptoPP::xorbuf(&retval[0], &key[0], retval.length());
return retval;
}
In g++, the output is:
AESXCBC128.cpp: In function ‘std::string CryptoPP::XOR(const string&, const string&)’:
AESXCBC128.cpp:79:48: error: invalid conversion from ‘char*’ to ‘byte* {aka unsigned char*}’ [-fpermissive]
xorbuf(&retval[0], &key[0], retval.length());
^
AESXCBC128.cpp:45:6: error: initializing argument 1 of ‘void CryptoPP::xorbuf(byte*, const byte*, size_t)’ [-fpermissive]
void xorbuf(byte *buf, const byte *mask, size_t count)
^
AESXCBC128.cpp:79:48: error: invalid conversion from ‘const char*’ to ‘const byte* {aka const unsigned char*}’ [-fpermissive]
xorbuf(&retval[0], &key[0], retval.length());
^
AESXCBC128.cpp:45:6: error: initializing argument 2 of ‘void CryptoPP::xorbuf(byte*, const byte*, size_t)’ [-fpermissive]
void xorbuf(byte *buf, const byte *mask, size_t count)

Answer 1

inline string XOR(const string &value, const string &key) {
string retval(value);
CryptoPP::xorbuf(&retval[0], &key[0], retval.length());
return retval;
}
Crypto++ typedefs a byte in confg.h:
typedef unsigned char byte;
You can use something like:
CryptoPP::xorbuf(
reinterpret_cast<byte*>(&retval[0]),
reinterpret_cast<const byte*>(&key[0]),
retval.length());
Or, you can do it with C-style casts:
CryptoPP::xorbuf((byte*)&retval[0], (const byte*)&key[0], retval.length());
Here are some similar questions for SecByteBlock, which is a byte array rather than a char array:
How to convert SecByteBlock to string?
string to SecByteBlock conversion
Here are some references on C++ casting:
When should static_cast, dynamic_cast, const_cast and reinterpret_cast be used?
Regular cast vs. static_cast vs. dynamic_cast