Encrypt a message with a Public Key: BER decode error

I have a problem regarding the use of a public key to encrypt a message. I would like to encrypt a string (let us say "test") with a public key RSA-4096 contained in a string. Let us say:

To do so, I tried to use the CryptoPP library in C++ (https://www.cryptopp.com/wiki/Keys_and_formats), but when I try to read and decode my public key, I get the following error: BER decode error. Here is the full source code that I am using:

#include <string>
#include <iomanip> 
#include <iostream>
#include <exception>
#include <fstream>

#include <cryptopp/queue.h>
using CryptoPP::ByteQueue;

#include <cryptopp/filters.h>
using CryptoPP::Redirector;

#include <cryptopp/files.h>
using CryptoPP::FileSource;
using CryptoPP::FileSink;

#include <cryptopp/cryptlib.h>
using CryptoPP::PrivateKey;
using CryptoPP::PublicKey;
using CryptoPP::BufferedTransformation;

#include <cryptopp/sha.h>
#include <cryptopp/rsa.h>
using CryptoPP::RSA;

#include <cryptopp/base64.h>
using CryptoPP::Base64Encoder;
using CryptoPP::Base64Decoder;

#include <cryptopp/osrng.h>
using CryptoPP::AutoSeededRandomPool;

using namespace std;

int main(int argc, char** argv) {

    ByteQueue queue;

    string RSA_PUBLIC_KEY ="-----BEGIN PUBLIC KEY-----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-----END PUBLIC KEY-----";
    static string HEADER = "-----BEGIN PUBLIC KEY-----";
    static string FOOTER = "-----END PUBLIC KEY-----";

    size_t pos1 = RSA_PUBLIC_KEY.find(HEADER);
    if(pos1 == string::npos) throw runtime_error("PEM header not found");

    size_t pos2 = RSA_PUBLIC_KEY.find(FOOTER, pos1+1);
    if(pos2 == string::npos) throw runtime_error("PEM footer not found");

    // Start position and length
    pos1 = pos1 + HEADER.length();
    pos2 = pos2 - pos1;

    string keystr = RSA_PUBLIC_KEY.substr(pos1, pos2);    
    /*Base64Encoder decoder;
    decoder.Attach(new Redirector(queue));*/
    queue.Put((const byte*)keystr.data(), keystr.length());
    queue.MessageEnd();

    cout << keystr << endl;

    try {

        RSA::PublicKey public_key;
        public_key.BERDecodePublicKey(queue, false /*paramsPresent*/, queue.MaxRetrievable());

        if(queue.IsEmpty()) {
            cerr << "The queue is empty...";
        }

        AutoSeededRandomPool prng;
        bool valid = public_key.Validate(prng, 3);
        if(!valid) cerr << "RSA public key is not valid" << endl;

        cout << "N:" << public_key.GetModulus() << endl;
        cout << "E:" << public_key.GetPublicExponent() << endl;

    } catch (exception& e) {

        printf( "Caught exception: %s\n", e.what() );
        exit (1);
    } 
}

Many thanks in advance for those who can help me understand why the public key is not correctly read. For those speaking golang, I am basically trying to reproduce the following function:

func WriteEncryptedWithPublicKeyInformation(filename string, information string, publicKey string) error {

    f, err := os.Create(filename)

    block, _ := pem.Decode([]byte(publicKey))
    if block == nil {
        return errors.New("Failed to parse PEM block containing the public key")
    }

    pub, err := x509.ParsePKIXPublicKey(block.Bytes)
    if err != nil {
        return errors.New("Failed to parse DER encoded public key: " + err.Error())
    }

    key := pub.(*(rsa.PublicKey))
    ciphertext, err := rsa.EncryptOAEP(sha256.New(), rand.Reader, key, []byte(information), nil)
    if err != nil {
        return err
    }

    _, err = f.WriteString(b64.StdEncoding.EncodeToString(ciphertext))
    if err != nil {
        return err

    }
    f.Close()
    return nil
}

Where filename is my output file, information is the string that I want to encrypt and publicKey is the string containing the public key.

Solution

The way your program is decoding base64 is not correct and the function RSAFunction::BERDecodePublicKey according to reference

BERDecodePublicKey() the decodes subjectPublicKey part of subjectPublicKeyInfo, without the BIT STRING header.

You should use X509PublicKey::BERDecode function since class RSA::PublicKey inherits from X509PublicKey

Try this program it should work.

#include <string>
#include <iomanip>
#include <iostream>
#include <exception>
#include <fstream>


#include <cryptopp/queue.h>
using CryptoPP::ByteQueue;

#include <cryptopp/filters.h>
using CryptoPP::Redirector;

#include <cryptopp/files.h>
using CryptoPP::FileSource;
using CryptoPP::FileSink;

#include <cryptopp/cryptlib.h>
using CryptoPP::PrivateKey;
using CryptoPP::PublicKey;
using CryptoPP::BufferedTransformation;

#include <cryptopp/sha.h>
#include <cryptopp/rsa.h>
#include <cryptopp/asn.h>

using CryptoPP::RSA;

#include <cryptopp/base64.h>
using CryptoPP::Base64Encoder;
using CryptoPP::Base64Decoder;

#include <cryptopp/osrng.h>
using CryptoPP::AutoSeededRandomPool;

using namespace std;

int main() {

    ByteQueue queue;

    string RSA_PUBLIC_KEY ="-----BEGIN PUBLIC KEY-----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-----END PUBLIC KEY-----";
    static string HEADER = "-----BEGIN PUBLIC KEY-----";
    static string FOOTER = "-----END PUBLIC KEY-----";

    size_t pos1 = RSA_PUBLIC_KEY.find(HEADER);
    if(pos1 == string::npos) throw runtime_error("PEM header not found");

    size_t pos2 = RSA_PUBLIC_KEY.find(FOOTER, pos1+1);
    if(pos2 == string::npos) throw runtime_error("PEM footer not found");

    // Start position and length
    pos1 = pos1 + HEADER.length();
    pos2 = pos2 - pos1;

    string keystr = RSA_PUBLIC_KEY.substr(pos1, pos2);

    CryptoPP::StringSource ss{keystr.c_str(), true};

    Base64Decoder decoder;
    decoder.Attach(new Redirector(queue));
    ss.TransferTo(decoder);
    decoder.MessageEnd();

    cout << keystr << endl;

    try {

        RSA::PublicKey public_key;

        if(queue.IsEmpty()) {
            cerr << "The queue is empty...";
        }

        public_key.BERDecode(queue);

        AutoSeededRandomPool prng;
        bool valid = public_key.Validate(prng, 3);
        if(!valid) cerr << "RSA public key is not valid" << endl;

        cout << "N:" << public_key.GetModulus() << endl;
        cout << "E:" << public_key.GetPublicExponent() << endl;

    } catch (exception& e) {

        printf( "Caught exception: %s\n", e.what() );
        exit (1);
    }
}