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

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开发者_JAVA技巧 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?

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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.