Why I can't read openssl generated RSA pub key with PEM_read_RSAPublicKey?

Question

I'm trying to read a RSA public key generated with openssl like this:

Private Key:
    openssl genrsa -out mykey.pem 1024

Public Key afterwards:
    openssl rsa -in mykey.pem -pubout > somewhere.pub

Then I try to read:

FILE *keyfile = fopen("somewhere.pub", "r");
RSA *rsa_pub = PEM_read_RSAPublicKey(keyfile, NULL, NULL, NULL);
//rsa_pub == NULL!

When I'm reading the private key it works

FILE *keyfile = fopen("mykey.pem", "r");
RSA *rsa_pri = PEM_read_RSAPrivateKey(keyfile, NULL, NULL, NULL);
//all good

Any ideas?

I've read that openssl generate a X509 key of the RSA public key. But I could not manage to load even a X509 pub key.

Thanks

Answer 1

You might try PEM_read_RSA_PUBKEY() instead of PEM_read_RSAPublicKey().

This is all about formats.

The default public key file format generated by openssl is the PEM format.

PEM_read_RSA_PUBKEY() reads the PEM format. PEM_read_RSAPublicKey() reads the PKCS#1 format.

So if you want to stick to PEM_read_RSAPublicKey() you could generate the public key file using the PKCS#1 format by specifying the -outform DER option when generating the public key.

Answer 2

it seems there are two format of rsa public key, with different encoding.

A. RSA_PUBKEY

RSA* rsaPubKey = PEM_read_bio_RSA_PUBKEY( bio, NULL, 0, pass ) ;

read PUBKEY with this format

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

generated by

$ openssl rsa -in key.pri -pubout -out key.pub1

B. RSAPublicKey

RSA* rsaPubKey = PEM_read_bio_RSAPublicKey( bio, NULL, 0, pass ) ;

read PublicKey with this format

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

generated by

$ openssl rsa -in key.pri -RSAPublicKey_out -out key.pub2

convert

A to B format

$ openssl rsa -in key.pub1 -pubin -RSAPublicKey_out -out key.pub2_

B to A format

$ openssl rsa -in key.pub2 -RSAPublicKey_in -pubout -out key.pub1_

Answer 3

The openssl rsa utility saves the public key using the function PEM_write_bio_RSA_PUBKEY and not PEM_write_bio_RSAPubicKey. So, if you want your program to be compatible with its output, then you should use PEM_write_bio_RSA_PUBKEY and PEM_read_bio_RSA_PUBKEY for saving/loading public key files.

http://openssl.6102.n7.nabble.com/RSA-public-private-keys-only-work-when-created-programatically-td12532.html