OpenSSL - How to determine the right length of an rsa encrypted string? - c++

Question

I use OpenSSL to encryt a string. After that I want to encode the encrypted string with base64 algorithm also using OpenSSL. So I found the following code snipped: ( bit.ly/adUSEw )
char *base64(const unsigned char *input, int length) {
BIO *bmem, *b64;
BUF_MEM *bptr;
b64 = BIO_new(BIO_f_base64());
bmem = BIO_new(BIO_s_mem());
b64 = BIO_push(b64, bmem);
BIO_write(b64, input, length);
BIO_flush(b64);
BIO_get_mem_ptr(b64, &bptr);
char *buff = (char*)malloc(bptr->length);
memcpy(buff, bptr->data, bptr->length - 1);
buff[bptr->length - 1] = 0;
BIO_free_all(b64);
return buff;
}
int main(int argc, char **argv) {
char *message = "TEST";
char *encryptedString = Encrypt(message);
if (encryptedString == NULL) {
return 0;
}
else {
char *output = base64(encryptedString, strlen(encryptedString));
cout << output << endl;
} }
I noticed that strlen(encryptedString) isn't working properly in this case. Sometimes it returns the right lenght but mostly not. So whats is the proper way to determine the correct lenght?

Answer 1

The size of the encrypted message is exactly the size of the modulus in the private key. You have to get the information from there.
You cannot use strlen because
the buffer with the encrypted message is likely not null-terminated, and
the encrypted message may contain (and will likely contain) null bytes.