Convert PEM key to C bytes array

ed25519-read-der.c

/*-
 * Read Ed25519 public and private keys in DER format.
 */

#include <stdio.h>
#include <openssl/err.h>
#include <openssl/evp.h>
#include <openssl/x509.h>

#include "private.h" /* generated by 'pem-to-c.sh' */


int main(void)
{
    BIO *bp = BIO_new_fp(stderr, 0);
    if (bp == NULL) {
        fprintf(stderr, "ERROR: BIO_new_fp() failed\n");
        return 1;
    }

    const unsigned char *p;
    long len;

    p = _PRIVKEY;
    len = (long)sizeof(_PRIVKEY);

    EVP_PKEY *pkey = d2i_PrivateKey(EVP_PKEY_ED25519, NULL, &p, len);
    if (pkey == NULL) {
        fprintf(stderr, "ERROR: d2i_PrivateKey() failed\n");
        ERR_print_errors(bp);
        return 1;
    }
    printf("Private Key:\n");
    EVP_PKEY_print_private(bp, pkey, 0, NULL);
    EVP_PKEY_free(pkey);

    p = _PUBKEY;
    len = (long)sizeof(_PUBKEY);
    /* NOTE: cannot use d2i_PublicKey(EVP_PKEY_ED25519, ...) */
    pkey = d2i_PUBKEY(NULL, &p, len);
    if (pkey == NULL) {
        fprintf(stderr, "ERROR: d2i_PUBKEY() failed\n");
        ERR_print_errors(bp);
        return 1;
    }
    printf("Public Key:\n");
    EVP_PKEY_print_public(bp, pkey, 0, NULL);
    EVP_PKEY_free(pkey);

    BIO_free(bp);

    return 0;
}

pem-to-c.sh

#!/bin/sh
#
# Convert PEM key to C bytes array for easier use and more security.
#

set -o pipefail
set -e

c_hex() {
    od -t x1 |
    sed -E \
        -e 's|^([[:xdigit:]]+)|/* \1 */ |' \
        -e 's|\b([[:xdigit:]][[:xdigit:]])\b|0x\1,|g'
}

case $1 in
'' | -h | --help)
    echo "usage: ${0##*/} <private.key>" >&2
    exit 1
    ;;
esac

keyfile="$1"

v_privkey=$(openssl rand -hex 8)
v_pubkey=$(openssl rand -hex 8)

privkey=$(openssl pkey -in ${keyfile} -outform DER | c_hex) || exit $?
pubkey=$(openssl pkey -in ${keyfile} -pubout -outform DER | c_hex) || exit $?

cat << _EOF_
#define _PUBKEY _${v_pubkey}
static const unsigned char _${v_pubkey}[] = {
${pubkey}
};

#define _PRIVKEY _${v_privkey}
static const unsigned char _${v_privkey}[] = {
${privkey}
};
_EOF_

test.md

1. Generate Ed25519 private key.

openssl genpkey -algorithm ED25519 -out ed25519.key

2. Convert to C bytes array.

./pem-to-c.sh ed25519.key

Example output:

#define _PUBKEY _b653c2afa57d511f
static const unsigned char _b653c2afa57d511f[] = {
/* 0000000 */  0x30, 0x2a, 0x30, 0x05, 0x06, 0x03, 0x2b, 0x65, 0x70, 0x03, 0x21, 0x00, 0x2a, 0x5c, 0x90, 0x18,
/* 0000020 */  0x01, 0x29, 0x58, 0xd4, 0x1f, 0x13, 0x64, 0x93, 0xa1, 0x54, 0x80, 0x13, 0x96, 0x77, 0x5a, 0x0f,
/* 0000040 */  0xdf, 0xf7, 0x5b, 0x15, 0x17, 0xfb, 0x4d, 0x4f, 0x20, 0x5e, 0x4f, 0x12,
/* 0000054 */ 
};

#define _PRIVKEY _7a88403e83fa8560
static const unsigned char _7a88403e83fa8560[] = {
/* 0000000 */  0x30, 0x2e, 0x02, 0x01, 0x00, 0x30, 0x05, 0x06, 0x03, 0x2b, 0x65, 0x70, 0x04, 0x22, 0x04, 0x20,
/* 0000020 */  0x49, 0x72, 0x57, 0xc7, 0x9c, 0x74, 0xe4, 0x1f, 0xdd, 0xee, 0x42, 0x50, 0xb8, 0x1d, 0x40, 0x1f,
/* 0000040 */  0x4a, 0xd4, 0x3b, 0x8f, 0x29, 0x70, 0x05, 0x7e, 0x13, 0x2e, 0x37, 0xcb, 0x9c, 0x68, 0xa1, 0xcb,
/* 0000060 */ 
};