Write private/public keys in DER format

Question

I need to write a C program that generates an RSA key, and saves an X.509 public key in DER format and a PKCS#8 private key in DER format. I've used Google, but haven't really found much. What I have so far is this:

#include <stdio.h>
#include <openssl/rsa.h>
#include <openssl/pem.h>

void main() {
    int ret = 0;
    RSA *r = NULL;
    BIGNUM *bne = NULL;
    BIO *bp_public = NULL, *bp_private = NULL;
    int bits = 2048;
    unsigned long e = RSA_F4;

    // Generate the RSA key
    printf("Generating RSA key...\n");
    bne = BN_new();
    ret = BN_set_word(bne, e);
    if(ret != 1) {
        goto free_all;
    }
    r = RSA_new();
    ret = RSA_generate_key_ex(r, bits, bne, NULL);
    if(ret != 1) {
        goto free_all;
    }

    // Save the public key in PEM format
    printf("Writing key files...\n");
    bp_public = BIO_new_file("public.pem", "w+");
    ret = PEM_write_bio_RSAPublicKey(bp_public, r);
    if(ret != 1) {
        goto free_all;
    }

    // Save the private key in PEM format
    bp_private = BIO_new_file("private.pem", "w+");
    ret = PEM_write_bio_RSAPrivateKey(bp_private, r, NULL, NULL, 0, NULL, NULL);

    // Free everything
    free_all:
    BIO_free_all(bp_public);
    BIO_free_all(bp_private);
    RSA_free(r);
    BN_free(bne);
    printf("Done!\n");
}

This is obviously writing the keys in PEM format. I also need to be able to actually have the data in memory in the code, not just write it directly to a file, as there's some other stuff I need to do with the public key.

Thank you for any help

You already have key(r) in memory that you are writing to file — Pras, Jan 18 '18 at 15:47
[Look for methods](https://www.openssl.org/docs/manmaster/man3/) that start out with the letters 'i2d', which I think means "internal to der". For example, [this](https://www.openssl.org/docs/manmaster/man3/i2d_PKCS8PrivateKey_bio.html) may be one of the methods you are looking for. — President James K. Polk, Jan 18 '18 at 16:02
Ok, I've read through the links. Is there any more clear documentation? It's difficult to understand what the various arguments are supposed to be — atoms118, Jan 18 '18 at 16:30

score 2 · Answer 1 · answered Jan 18 '18 at 17:01

I think you're going to need to convert your key to an EVP_PKEY using EVP_PKEY_assign_RSA. Then you can use i2d_PUBKEY_bio to write out to a bio.

The following modification of your code works for me:

include the header file <openssl/evp.h>
declare BIO for writing out the public_der
create EVP_PKEY structure with EVP_PKEY_new()
convert using EVP_PKEY_assign_RSA
write out to bio with i2d_PUBKEY_bio

In context:

#include <stdio.h>
#include <openssl/rsa.h>
#include <openssl/pem.h>
#include <openssl/evp.h>

void main() {
    int ret = 0;
    RSA *r = NULL;
    BIGNUM *bne = NULL;
    BIO *bp_public = NULL, *bp_private = NULL, *bp_public_der = NULL;
    int bits = 2048;
    unsigned long e = RSA_F4;

    // Generate the RSA key
    printf("Generating RSA key...\n");
    bne = BN_new();
    ret = BN_set_word(bne, e);
    if(ret != 1) {
        goto free_all;
    }
    r = RSA_new();
    ret = RSA_generate_key_ex(r, bits, bne, NULL);
    if(ret != 1) {
        goto free_all;
    }

    // Save the public key in PEM format
    printf("Writing key files...\n");
    bp_public = BIO_new_file("public.pem", "w+");
    ret = PEM_write_bio_RSAPublicKey(bp_public, r);
    if(ret != 1) {
        goto free_all;
    }

    // Save the private key in PEM format
    bp_private = BIO_new_file("private.pem", "w+");
    ret = PEM_write_bio_RSAPrivateKey(bp_private, r, NULL, NULL, 0, NULL, NULL);

    // Save in DER
    EVP_PKEY *evp = EVP_PKEY_new();
    ret = EVP_PKEY_assign_RSA(evp, r);
    if(ret != 1){
        printf("failure %i\n", ret);
    }
    bp_public_der = BIO_new_file("public.key", "w+");
    ret = i2d_PUBKEY_bio(bp_public_der, evp);



    // Free everything
    free_all:
    BIO_free_all(bp_public);
    BIO_free_all(bp_public_der);
    BIO_free_all(bp_private);
    RSA_free(r);
    BN_free(bne);
    printf("Done!\n");
}

You can now find the DER of the public key in public.key. And you should be able to do the same thing for the private.

Hope this helps.

Thank you, this is more-or-less exactly what I'm looking for! — atoms118, Jan 19 '18 at 13:03

score 2 · Accepted Answer · answered Jan 18 '18 at 18:04

Your question is a little ambiguous in what you actually mean by "saves an X.509 public key in DER format". Assuming you actually mean "save it as a SubjectPublicKeyInfo structure" (which is the bit of an X.509 certificate that holds public keys) then you should use i2d_RSA_PUBKEY (or i2d_RSA_PUBKEY_fp or i2d_RSA_PUBKEY_bio) to write it out (no need to convert it to an EVP_PKEY first).

For the PKCS#8 private key in DER format, your current method is incorrect for PEM format. The PEM_write_bio_RSAPrivateKey() function will write this out in traditional format (not PKCS#8).

I assume you don't want to do anything complicated like encrypting the key first. For this one you will need to convert it to an EVP_PKEY (using EVP_PKEY_assign_RSA() as mentioned by @JawguyChooser). Next you obtain a PKCS8_PRIV_KEY_INFO structure using the (sadly undocumented) function EVP_PKEY2PKCS8.

PKCS8_PRIV_KEY_INFO *EVP_PKEY2PKCS8(EVP_PKEY *pkey);

You need to free this structure when your done using PKCS8_PRIV_KEY_INFO_free(). Next write out the PKCS8 DER using i2d_PKCS8_PRIV_KEY_INFO() (or i2d_PKCS8_PRIV_KEY_INFO_fp() or i2d_PKCS8_PRIV_KEY_INFO_bio).

See the man page for info on various of these functions:

https://www.openssl.org/docs/man1.1.0/crypto/i2d_RSAPublicKey.html

Thank you! I never would've figured this out... – atoms118 Jan 19 '18 at 13:27 — atoms118, Jan 19 '18 at 13:27

Write private/public keys in DER format

2 Answers2