Load public key to create rsa object for public encryption

Question

I am trying to load a rsa object from a generated public key. I used PEM_write_bio_RSAPublicKey to generate the public key string. Then I used PEM_read_bio_RSA_PUBKEY to load the rsa object from the public key string. The problem is the rsa object is null. The generated string looks okay as far as I can tell. Any ideas?

gcc test_public_private_key.c -lcrypto -o test

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <openssl/rsa.h>
#include <openssl/pem.h>
#include <openssl/err.h>

char* get_public_key() {

        RSA* rsa = RSA_new();
        int kbits = 2048;
        BIGNUM* bne = BN_new();
        BN_set_word(bne, RSA_F4);

        int status = RSA_generate_key_ex(rsa, kbits, bne, NULL);
        if(status == 0) {
                fprintf(stderr, "rsa key generation failed\n");
        }


        BIO *bio = BIO_new(BIO_s_mem());
        PEM_write_bio_RSAPublicKey(bio, rsa);

        int length = BIO_pending(bio);

        char* pem_key = malloc(length+1);
        memset(pem_key, '\0', length+1);

        BIO_read(bio, pem_key, length);

        return pem_key;

}

int main(int argc, char* argv[]) {

        char* public_key = get_public_key();

        printf("%s", public_key);

        BIO* keybio = BIO_new_mem_buf(public_key, -1);

        if (keybio == NULL) {
                fprintf(stderr, "failed to create key BIO");
        }

        printf("keybio: %p\n", keybio);
        RSA* rsa = PEM_read_bio_RSA_PUBKEY(keybio, NULL, NULL, NULL);
        printf("rsa result %p\n", rsa);
        BIO_free(keybio);

        free(public_key);

        return 0;
}

I remember seeing a working example with the ```BIO_FLAGS_BASE64_NO_NL``` flag set. Maybe try setting this flag with ```BIO_set_flags( bio, BIO_FLAGS_BASE64_NO_NL );``` — Daniel, Jul 16 '17 at 02:00
@Daniel I am not sure if it should contain the word RSA....its the library's own function that generates the string. I saw the NL example through my google searches but wouldn't that mean that I should strip the new lines from the generated string if I use that option. — eat_a_lemon, Jul 16 '17 at 02:03
I tried the NO_NL option with and without stripping the newlines and it still did not work. — eat_a_lemon, Jul 16 '17 at 02:11
After looking a bit more I'm pretty certain that the RSA word should be there. As for the flag, I saw it used with a key that had new lines, but looking at it again and reading what BIO_set_flags does i think it might be irrelevant since I think it's used only in BIO_write methods. You can try it anyways and see if it works. You should also try using a key generated with the openssl commands and hardcode it and see if it works with it — Daniel, Jul 16 '17 at 02:11
I used the open ssl command to generate a public key and the RSA word was not there. I tried that and I am getting the same null result. — eat_a_lemon, Jul 16 '17 at 02:20
I got it! I used the openssl command and added newlines to the output. I didn't use the NL option. It must be the RSA word. It comes from there own api! — eat_a_lemon, Jul 16 '17 at 02:26
Interesting, I was about to suggest that u try using ```PEM_read_RSAPublicKey``` instead of ```PEM_read_bio_RSA_PUBKEY``` and that ought to solve it for u for the first key. refer to Nikolai's comment here: https://stackoverflow.com/a/7825076/5513449 He talks exactly about what we just discussed. — Daniel, Jul 16 '17 at 02:28
Related, see [Use OpenSSL RSA key with .Net](https://stackoverflow.com/q/30475758/608639). It goes through the four types of key/encoding combinations. The founr combinations are {ASN.1, DER} x {PublicKey, subjectPublicKeyInfo}. — jww, Jul 16 '17 at 19:52

Daniel · Accepted Answer · 2017-07-17T01:02:27.910

3

I thought it would be good to add that the reason this didn't work is because PEM_read_RSA_PUBKEY expects a SubjectPublicKeyInfo structure, which starts with BEGIN PUBLIC KEY, while PEM_read_RSAPublicKey expects a RSAPublicKey structure, which starts with BEGIN RSA PUBLIC KEY.
PEM_write_bio_RSAPublicKey generated the latter which corresponds to the second function, PEM_read_RSAPublicKey.

edited Jul 17 '17 at 01:02

answered Jul 16 '17 at 02:43

Daniel

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`RSA_generate_key_ex` creates the in-memory form; it is `PEM_write_[bio_]RSAPublicKey` (in OP's `get_public_key`) that creates the PKCS1 form which is readable by `PEM_read_[bio_]RSAPublicKey` but not `PEM_read_[bio_][RSA_]PUBKEY` – dave_thompson_085 Jul 17 '17 at 00:55
@dave_thompson_085 Ah yes, my bad. Edited my answer, thank you very much. – Daniel Jul 17 '17 at 01:02

Load public key to create rsa object for public encryption

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