Crypto++ GetModulus() 问题

Question

My goal is the generation of a RSA 2048 bits keys pair to be used to get access to a SSH server.

The issue is that the modulus got from GetModulus() is not the real key's modulus. If I export the key in DER format the content of the modulus field is different from GetModulus() function result.

Below is my code for generating the keys pair:

// Generate keys pair
AutoSeededRandomPool lRnd;
InvertibleRSAFunction lPrms;
lPrms.GenerateRandomWithKeySize(lRnd, 2048);
RSA::PrivateKey lPrvKey(lPrms);
RSA::PublicKey lPubKey(lPrms);

// Convert private key to DER base64
Base64Encoder lB64PrvEncoder;
AlgorithmParameters lPrvPrms = MakeParameters(Name::MaxLineLength (), 64);
lB64PrvEncoder.IsolatedInitialize(lPrvPrms);
lPrvKey.DEREncodePrivateKey(lB64PrvEncoder);
lB64PrvEncoder.MessageEnd();

// Convert public key to base64 openSSH format
Integer lN = lPubKey.GetModulus();
Integer lE = lPubKey.GetPublicExponent();
QByteArray lSSHPubKey;
PubKeyToSSHFormat(lE, lN, lSSHPubKey);

Above, lN is different from the modulus found in the DER formatted private key.

Any help will be really appreciated.

Answer 1

Thank you all for the answers. I found out what the issue is and I would like to ask you an opinion about the library implementation. GetModulus() returns the modulus in the "Integer" class. That big number is stored using the endianess of the machine (Intel in my case) even tough the number is a sequence of bytes, I didn't manage the endianess. that's why the modulus I put in the file got rejected, I didn't think about the endianess. based on my knowledge it applies only to variables with the same size of CPU's word length (or less). In this case we have a large integer stored as a sequence of bytes. Any opinion is appreciated.

Best Regards

/Alessandro