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Can you help me get my head around openssl public key encryption with rsa.h in c++?

- by Ben

Hi there, I am trying to get my head around public key encryption using the openssl implementation of rsa in C++. Can you help? So far these are my thoughts (please do correct if necessary) Alice is connected to Bob over a network Alice and Bob want secure communications Alice generates a public / private key pair and sends public key to Bob Bob receives public key and encrypts a randomly generated symmetric cypher key (e.g. blowfish) with the public key and sends the result to Alice Alice decrypts the ciphertext with the originally generated private key and obtains the symmetric blowfish key Alice and Bob now both have knowledge of symmetric blowfish key and can establish a secure communication channel Now, I have looked at the openssl/rsa.h rsa implementation (since I already have practical experience with openssl/blowfish.h), and I see these two functions: int RSA_public_encrypt(int flen, unsigned char *from, unsigned char *to, RSA *rsa, int padding); int RSA_private_decrypt(int flen, unsigned char *from, unsigned char *to, RSA *rsa, int padding); If Alice is to generate *rsa, how does this yield the rsa key pair? Is there something like rsa_public and rsa_private which are derived from rsa? Does *rsa contain both public and private key and the above function automatically strips out the necessary key depending on whether it requires the public or private part? Should two unique *rsa pointers be generated so that actually, we have the following: int RSA_public_encrypt(int flen, unsigned char *from, unsigned char *to, RSA *rsa_public, int padding); int RSA_private_decrypt(int flen, unsigned char *from, unsigned char *to, RSA *rsa_private, int padding); Secondly, in what format should the *rsa public key be sent to Bob? Must it be reinterpreted in to a character array and then sent the standard way? I've heard something about certificates -- are they anything to do with it? Sorry for all the questions, Best Wishes, Ben. EDIT: Coe I am currently employing: /* * theEncryptor.cpp * * * Created by ben on 14/01/2010. * Copyright 2010 __MyCompanyName__. All rights reserved. * */ #include "theEncryptor.h" #include <iostream> #include <sys/socket.h> #include <sstream> theEncryptor::theEncryptor() { } void theEncryptor::blowfish(unsigned char *data, int data_len, unsigned char* key, int enc) { // hash the key first! unsigned char obuf[20]; bzero(obuf,20); SHA1((const unsigned char*)key, 64, obuf); BF_KEY bfkey; int keySize = 16;//strlen((char*)key); BF_set_key(&bfkey, keySize, obuf); unsigned char ivec[16]; memset(ivec, 0, 16); unsigned char* out=(unsigned char*) malloc(data_len); bzero(out,data_len); int num = 0; BF_cfb64_encrypt(data, out, data_len, &bfkey, ivec, &num, enc); //for(int i = 0;i<data_len;i++)data[i]=out[i]; memcpy(data, out, data_len); free(out); } void theEncryptor::generateRSAKeyPair(int bits) { rsa = RSA_generate_key(bits, 65537, NULL, NULL); } int theEncryptor::publicEncrypt(unsigned char* data, unsigned char* dataEncrypted,int dataLen) { return RSA_public_encrypt(dataLen, data, dataEncrypted, rsa, RSA_PKCS1_OAEP_PADDING); } int theEncryptor::privateDecrypt(unsigned char* dataEncrypted, unsigned char* dataDecrypted) { return RSA_private_decrypt(RSA_size(rsa), dataEncrypted, dataDecrypted, rsa, RSA_PKCS1_OAEP_PADDING); } void theEncryptor::receivePublicKeyAndSetRSA(int sock, int bits) { int max_hex_size = (bits / 4) + 1; char keybufA[max_hex_size]; bzero(keybufA,max_hex_size); char keybufB[max_hex_size]; bzero(keybufB,max_hex_size); int n = recv(sock,keybufA,max_hex_size,0); n = send(sock,"OK",2,0); n = recv(sock,keybufB,max_hex_size,0); n = send(sock,"OK",2,0); rsa = RSA_new(); BN_hex2bn(&rsa->n, keybufA); BN_hex2bn(&rsa->e, keybufB); } void theEncryptor::transmitPublicKey(int sock, int bits) { const int max_hex_size = (bits / 4) + 1; long size = max_hex_size; char keyBufferA[size]; char keyBufferB[size]; bzero(keyBufferA,size); bzero(keyBufferB,size); sprintf(keyBufferA,"%s\r\n",BN_bn2hex(rsa->n)); sprintf(keyBufferB,"%s\r\n",BN_bn2hex(rsa->e)); int n = send(sock,keyBufferA,size,0); char recBuf[2]; n = recv(sock,recBuf,2,0); n = send(sock,keyBufferB,size,0); n = recv(sock,recBuf,2,0); } void theEncryptor::generateRandomBlowfishKey(unsigned char* key, int bytes) { /* srand( (unsigned)time( NULL ) ); std::ostringstream stm; for(int i = 0;i<bytes;i++){ int randomValue = 65 + rand()% 26; stm << (char)((int)randomValue); } std::string str(stm.str()); const char* strs = str.c_str(); for(int i = 0;bytes;i++)key[i]=strs[i]; */ int n = RAND_bytes(key, bytes); if(n==0)std::cout<<"Warning key was generated with bad entropy. You should not consider communication to be secure"<<std::endl; } theEncryptor::~theEncryptor(){}

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