OpenSSL, C program, passing a Public RSA to a remote peer.

MrUmunhum · 03-12-2017, 07:34 PM

Hi group,

This question is in reference to my post http://www.linuxquestions.org/questi...am-4175601569/which I solved but don't like the solution. My solution was to add this code to populate the public and private RSAs.

Code:

  RSA* RSA1 = RSA_new();
  RSA* RSA2 = RSA_new();

  RSA2->n = RSA1->n = My_RSA->n;
  RSA2->e = RSA1->e = My_RSA->e;
  RSA2->d = RSA1->d = My_RSA->d;
  RSA2->p = RSA1->p = My_RSA->p;
  RSA2->q = RSA1->q = My_RSA->q;

Where My_RSA was created with:

Code:

 RSA_generate_key_ex( My_RSA, 2048, bne, NULL );

The complete program is at:http://mt-umunhum-wireless.net/Sources/rsa

Now I need to be able to send the public RSA to a remote peer. Which means I need to send the key, n, e, d, p and q.

Is there a better solution?

Thanks for your time.

camp0 · 03-13-2017, 04:29 AM

Hi,

If you need to transfer the public key to a peer why don't you write your public key on disk (PEM_write_bio_RSAPublicKey) and use scp?

Regards,
Luis

sundialsvcs · 03-13-2017, 08:15 AM

Quote:

If you need to transfer the public key to a peer why don't you write your public key on disk (PEM_write_bio_RSAPublicKey) and use scp?

I agree. PEM-files are a very standard way to conveniently exchange key material with a peer. Arrange your software so that it generates such a file. And, on the client side, it reads such a file at an agreed-upon location. "Exactly like everybody else does!"

Be sure that you have designed your system to use OpenSSL properly: to use the RSA-key to negotiate a private per-conversation session key.

Use libraries like OpenSSL only at the surface layers: "don't

dive down into the guts of the thing, even though you can." There's a surface-layer API that's intended to be "the thing that you're supposed to use," along with lower-level routines that are intended for special cases. But ... "your case, whatever it is, is not 'special!'"

In matters of cryptography, key management is the Achilles heel.

ntubski · 03-13-2017, 08:38 AM

Quote:

Originally Posted by MrUmunhum

Now I need to be able to send the public RSA to a remote peer. Which means I need to send the key, n, e, d, p and q.

I'm not familiar with the API you're using, but assuming standard terminology, d, p, and q are part of the private key, don't send them to anyone else! Only n and e should be shared.

MrUmunhum · 03-13-2017, 08:50 PM

Quote:

Originally Posted by camp0

Hi,

If you need to transfer the public key to a peer why don't you write your public key on disk (PEM_write_bio_RSAPublicKey) and use scp?

Regards,
Luis

I'm using a program I wrote to talk to the peer. I would prefer using the network connection to transfer the public RSA.

MrUmunhum · 03-13-2017, 08:55 PM

Quote:

Originally Posted by ntubski

I'm not familiar with the API you're using, but assuming standard terminology, d, p, and q are part of the private key, don't send them to anyone else! Only n and e should be shared.

I changed my code:

Code:

  RSA* RSA1 = RSA_new();
  RSA* RSA2 = RSA_new();

  RSA2->n = RSA1->n = My_RSA->n;
  RSA2->e = RSA1->e = My_RSA->e;
  //  RSA2->d = RSA1->d = My_RSA->d;
  //  RSA2->p = RSA1->p = My_RSA->p;
  //  RSA2->q = RSA1->q = My_RSA->q;

Now I get this:

Code:

128 (Key) Make Key Failed!
error:0407B093:rsa routines:RSA_check_key:value missing
130 (Key) Make Key Failed!
error:0407B093:rsa routines:RSA_check_key:value missing

MrUmunhum · 03-13-2017, 10:14 PM

I posted too quickly. The Encrypt RSA worked OK even though it failed the RSA_check_key? Changed the decrypt RSA to the RSA pair and it worked.

So you are right about the encrot RSA only needing n and e.

MrUmunhum · 03-13-2017, 11:54 PM

Quote:

Originally Posted by sundialsvcs

I agree. PEM-files are a very standard way to conveniently exchange key material with a peer. Arrange your software so that it generates such a file. And, on the client side, it reads such a file at an agreed-upon location. "Exactly like everybody else does!"

Be sure that you have designed your system to use OpenSSL properly: to use the RSA-key to negotiate a private per-conversation session key.

Use libraries like OpenSSL only at the surface layers: "don't

dive down into the guts of the thing, even though you can." There's a surface-layer API that's intended to be "the thing that you're supposed to use," along with lower-level routines that are intended for special cases. But ... "your case, whatever it is, is not 'special!'"

In matters of cryptography, key management is the Achilles heel.

I agree completely. But I could not find out what the proper way to send a public RSA to the peer. If you could point me to an example written in C, I would be very great full.

Thanks.

ntubski · 03-14-2017, 07:23 AM

Quote:

Originally Posted by MrUmunhum

The Encrypt RSA worked OK even though it failed the RSA_check_key?

Probably RSA_check_key checks that the private and public values match each other.

sundialsvcs · 03-14-2017, 08:54 AM

Quote:

Originally Posted by MrUmunhum

I agree completely. But I could not find out what the proper way to send a public RSA to the peer. If you could point me to an example written in C, I would be very great full.

One way would be to write the file to disk and just scp or rsync it over to its destination.

Another way would be to copy the content of the file into memory and send it by means of your existing socket connection with the peer. (This exchange does not have to be encrypted.)

The peer expects to find its certificate-file at an agreed-upon location in its own filesystem.

Since certificates are uniquely identifiable, the host maintains a list of certificates that it will recognize. (Or, that have been revoked.) Again, use the full OpenSSL standard and "do exactly what everyone else does."

MrUmunhum · 03-14-2017, 03:41 PM

Quote:

Originally Posted by sundialsvcs

One way would be to write the file to disk and just scp or rsync it over to its destination.

Another way would be to copy the content of the file into memory and send it by means of your existing socket connection with the peer. (This exchange does not have to be encrypted.)

Transferring files is not a problem. The problem is which files to move?

Quote:

Since certificates are uniquely identifiable, the host maintains a list of certificates that it will recognize. (Or, that have been revoked.) Again, use the full OpenSSL standard and "do exactly what everyone else does."

May be I am missing something? I thought the only way to encrypt a packet was to use RSA_public_encrypt()? Which requires a public RSA. If I use certificates can I use them to encrypt a packet?

Rebuilding the public RSA is a real PITA!

Thanks for your time.

sundialsvcs · 03-14-2017, 04:29 PM

Use .pem files at an agreed-upon location to contain the public-key material that is required by the client.

Build these files to contain the necessary public-key material, then transfer them to the client. Arrange for the client software to require the file to be at some agreed-upon location. The client reads the public key from the file and uses it to perform the initial session-key negotiations.

Remember that public-key techniques are not used to final-encrypt the traffic: they are used to securely negotiate a (symmetric ...) session key which is then used to do so. All of which OpenSSL can handle for you.

[i]Use the highest-level functions within the (vast ...) OpenSSL library, and do not dive down into the "primitive functions" which are also available.

I highly encourage you to spend some time, say, at "github.com," to review how applications that you can find there employ the SSL libraries. You will find both "best practices" and "code that you can steal"

in those places. Avoid wasted time spent "blundering through how-to-do-it, as though no one before you had ever done it before," because they have, and they provided their source code on places like github.