c - Reading Public/Private Key from Memory with OpenSSL

Question

I am using Public/Private Keys in my project to encrypt/decrypt some data.

I am hosting a public key ("public.pem") on a server.

"public.pem" looks like this:

-----BEGIN PUBLIC KEY-----
.....
.....
-----END PUBLIC KEY-----

I wrote a client side that downloads this public key and save it to disk and then calls OpenSSL's PEM_read_RSA_PUBKEY() with a File descriptor to that file. This operation works great and the result is an RSA object that is ready for encryption.

I would like to avoid writing the public key to disk each time (since i have the buffer in memory already).

How can i do the same operation without saving the buffer to disk? I noticed a function called: PEM_read_bio_RSAPublicKey() but i am not sure of it's usage of BIO structure. Am I on the right path?

So the real question would be: How do I read a public/private key to an RSA object straight from memory and not from a file descriptor.

Answer 1

You are on the right track. You must wrap the PEM key already in memory by means of a BIO buffer via BIO_new_mem_buf(). In other words, something like:

BIO *bufio;
RSA *rsa

bufio = BIO_new_mem_buf((void*)pem_key_buffer, pem_key_buffer_len);
PEM_read_bio_RSAPublicKey(bufio, &rsa, 0, NULL);

The same approach is valid for an RSA private key (via PEM_read_bio_RSAPrivateKey), but in that case you most certainly need to cater for the pass phrase. Check the man page for details.