std::string and sha256 hashing

[a sea of c]

Hi,

I'm having a bit of a struggle getting a SHA hash routine working with a string input.

The problem seems to lie with the update element(SHA256_Update being openssl's SHA256_Update routine) :

typedef unsigned char Byte;
void mynamespace::Sha256::Update(mynamespace::Byte *buffer, int bytesRead) {
    SHA256_Update(&sha256, buffer, bytesRead);
}

When called in the context of reading a file, it works without problem :

 while ((bytes_read = fread(pBuffer, 1, mynamespace::kBufferSize, f))) {
        Update(pBuffer, bytes_read);
    }

But no matter what I try I can't get it to work with string input :

std::string canocJPFinal;
mynamespace::Sha256 sha_calculator;
    sha_calculator.Update(canocJPFinal.c_str(),canocJPFinal.size());
    sha_calculator.Final();
myprog.cpp:74: error: invalid conversion from 'const char*' to 'unsigned char*'
myprog.cpp:74: error:   initializing argument 1 of 'void mynamespace::Sha256::Update(unsigned char*, size_t)'

I've tried all sorts of things to fix it .... casting, copying the string to a byte array, I just can't get it to work ! I'm sure I'm doing something stupid, but I think I've reached the point where I need a second pair of eyes on the problem.

I've even tried to copy the function argumentconcept from openssl's sha.h, but that doesn't help either :

void mynamespace::Sha256::Update(unsigned void* buffer, std::size_t bytesRead) {
    SHA256_Update(&sha256, buffer, bytesRead);
}

[Peter87]

.c_str() and .data() return const char*. You need a char*. You can use & on the first element in the string to get that.
sha_calculator.Update(&canocJPFinal[0],canocJPFinal.size());

[modoran]

I'd use this:
sha_calculator.Update(reinterpret_cast<unsigned char *>(canocJPFinal.data()), canocJPFinal.size());

[Peter87]

Ah, yes you'll have to cast the pointer because it's unsigned char, not char.

[a sea of c]

Hi Peter87 and modoran,

modoran, re: your answer, the compiler gives :
error: reinterpret_cast from type 'const char*' to type 'unsigned char*' casts away constness.

Am going to try adding a < const_cast> and will feedback. Will also try peter's idea.

[a sea of c]

Same errorr with this:

sha_calculator.Update(const_cast<unsigned char*>(reinterpret_cast<unsigned char *>(canocJPFinal.data())),canocJPFinal.size());

Peter's gives :

 error: invalid conversion from 'char*' to 'unsigned char*'
 error:   initializing argument 1 of 'void mynamespace::Sha256::Update(unsigned char*, size_t)'

[Peter87]

sha_calculator.Update(reinterpret_cast<unsigned char*>(&canocJPFinal[0]), canocJPFinal.size());

[Texan40]

I think you need to deal with the const-ness first, then cast the data type

sha_calculator.Update(reinterpret_cast<unsigned char*>(const_cast<char*>(canocJPFinal.data())),canocJPFinal.size());

[a sea of c]

That did the trick.. Thanks Texan40.