C ++中的RSA解密，PHP中的加密

I am working on an application which generates RSA encrypted session keys and stores them in a database. Later theses keys are transfered to a C++ application via Javascript. Therefore I want to use the OpenSSL library. I generated a 2048 bit key pair with openssl, which is used in both methods.

My PHP functions works like this:

function encrypt_with_public_key($input, $key)
{   
    openssl_public_encrypt($input, $crypttext, $key, OPENSSL_PKCS1_OAEP_PADDING); 
    return $crypttext;          
}

and

$fp = fopen("public.pem","r");
$public_pem = fread($fp,8192);
fclose($fp);

$public_key = openssl_get_publickey($public_pem);

$sessionKey = ...;
$encSessionKey = encrypt_with_public_key($sessionKey, $public_key);

I tested this part successfully. The part I have trouble with, is the C++ part. I use MS Visual Studio 2013. Edit: added hex encoding (using Crypto++)

#include <openssl/pem.h>
#include <openssl/rsa.h>
#include <openssl/err.h>
#include <openssl/crypto.h>
...
string decrypted, encoded;
decrypted.clear();

char privateKey[] =
    "-----BEGIN RSA PRIVATE KEY-----
"\
    ...

RSA *rsa = NULL;
BIO *keybio;
keybio = BIO_new_mem_buf(privateKey, strlen(privateKey));

rsa = PEM_read_bio_RSAPrivateKey(keybio, &rsa, NULL, NULL);

StringSource ss2(input, true,
    new HexEncoder(
        new StringSink(encoded)
    )
);

RSA_private_decrypt(encoded.length(), (unsigned char *)encoded.data(), (unsigned char *)decrypted.data(), rsa, RSA_PKCS1_OAEP_PADDING);

FBLOG_INFO("", ERR_error_string(ERR_get_error(), NULL));

return decrypted;

Note that the private key is not read from a file.

OpenSSL returns the following error: 0406506C: lib(4): func(101): reason(108).

It means afaik that my input data is longer than the modulus length (please correct me if I'm wrong). Anyone who knows how to handle this? I thought such problems are solved through the padding parameters.

The input data is the direct output of the php encrypt function (no base64 oder anything).