RSA Decryption in C++, Encryption in PHP - php

Question

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-----\n"\
...
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).