Extract public key from EVP_PKEY keypair?

Question

I am implementing an encryption / decryption scheme in my application using OpenSSL's high-level EVP_*() functions, so I can easily switch the actual algorithms used without having to change the API calls.

I can create a key pair with relative ease:

// dumbed down, no error checking for brevity
EVP_PKEY * pkey;
// can change EVP_PKEY_RSA to something else here
EVP_PKEY_CTX * context = EVP_PKEY_CTX_new_id( EVP_PKEY_RSA, NULL );
EVP_PKEY_keygen_init( ctx );
// could set parameters here
EVP_PKEY_keygen( context, &pkey );
// ...
EVP_PKEY_CTX_free( context );

pkey now holds a key pair, i.e. both secret and public key. That's fine for the secret side of things, but obviously I would like to extract only the public key component for use on the public side of things.

I was able to find RSA-specific functions, but nothing using the high-level EVP_*() API.

Help?

Fishbolt

What is your question? "extract only the public key component for use on the public side of things" is kind of ambiguous. Do you want to know how to get a public key from a PKEY? Or do you want to know how to save a public key? Or something else? – Inkerman

@jww: I thought the question was unambiguous... I would like to get an EVP_PKEY data structure that contains only the public key component. – Fishbolt

"I would like to get an EVP_PKEY data structure that contains only the public key component" - that's going to take some work because OpenSSL is not setup that way for the EVP_* gear (it can be done, but it takes some work). Usually you write out a DER or PEM encoded key (public or private). Have you looked at the functions in pem(3) gear. With the *_bio functions, you can write to file or memory. Take a look at, for example, <openssl src>/apps/rsautl.c (for the RSA case). – Inkerman

@jwiw: Will do, thanks for the pointers. (This is my first close encounter with OpenSSL, so it's entirely possible that I am barking up the wrong tree here.) – Fishbolt

In essence, you have to round trip a EVP_PKEY. During the trip, you have to get the key in its native format (like RSA* or DSA*) and convert it from private to public. Then you send it back to an EVP_PKEY. But like I said, if all you need is an DER or PEM encoded key to distribute, use the stuff in pem(3). – Inkerman

how did you make out on this? I have some free time and can show you how to round trip a key if interested. I don't have code handy, so I'd have to write it. – Inkerman

@jww: This is currently nine-to-five, and I have decided to go with symmetric encoding for the time being. I'll probably write up my own answer once I get around to it. (I'm not particularly quick, but persistant about these things.) – Fishbolt

Answer 1

You could use following methods to separate public key and private key for future use.

int PEM_write_bio_PrivateKey(BIO *bp, EVP_PKEY *x, const EVP_CIPHER *enc,
                    unsigned char *kstr, int klen,
                    pem_password_cb *cb, void *u);

 int PEM_write_PrivateKey(FILE *fp, EVP_PKEY *x, const EVP_CIPHER *enc,
                    unsigned char *kstr, int klen,
                    pem_password_cb *cb, void *u);
EVP_PKEY *PEM_read_bio_PUBKEY(BIO *bp, EVP_PKEY **x,
                    pem_password_cb *cb, void *u);

 EVP_PKEY *PEM_read_PUBKEY(FILE *fp, EVP_PKEY **x,
                    pem_password_cb *cb, void *u);

 int PEM_write_bio_PUBKEY(BIO *bp, EVP_PKEY *x);
 int PEM_write_PUBKEY(FILE *fp, EVP_PKEY *x);

For detailed information, please refer to <openssl/pem.h>.

Terryl

Answer 2

It's may be, you should use i2d_PUBKEY_bio() or similar function.

Swear