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

Asked 2010-01-06T11:46:08.973
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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)

  1. Alice is connected to Bob over a network
  2. Alice and Bob want secure communications
  3. Alice generates a public / private key pair and sends public key to Bob
  4. 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
  5. Alice decrypts the ciphertext with the originally generated private key and obtains the symmetric blowfish key
  6. 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 cer

c++ openssl rsa public-key
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Actually, no problem, I have just read that basically, the RSA object is a structure that contains both public and private fields. One can extract the public field data and only send that to Bob.

I.e. basically, to extract the public fields from rsa and store each in two different buffers (which are char arrays and can then be sent to Bob), you do:

sprintf(keyBufferA,"%s\r\n",BN_bn2hex(rsa->n));
sprintf(keyBufferB,"%s\r\n",BN_bn2hex(rsa->e));

And then Bob, on the receiving end, reconstructs as follows:

rsa = RSA_new();
BN_hex2bn(&rsa->n, keybufA);
BN_hex2bn(&rsa->e, keybufB);

Bob can then use rsa* to publicly encrypt the symmetric cypher key which can then be sent to Alice. Alice can then decrypt with the private key

Ben.

answered 2010-01-06T12:11:30.127

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