OpenSSL i2d_ECPrivateKey() crashes - c

Question

I have the following program. It successfully obtains the raw 32 byte EC private key data and then creates a EC_KEY from it. But i2d_ECPrivateKey fails to give the size of the DER encoded private key, as it crashes. Does anyone know why and how to fix this?
#include "CBWIF.h"
#include <openssl/ssl.h>
int main(int argc, char * argv[]) {
CBWIF wif;
if (argc != 2)
return EXIT_FAILURE;
// Decode WIF string
CBByteArray str;
CBInitByteArrayFromString(&str, argv[1], false);
CBInitWIFFromString(&wif, &str, false);
CBDestroyByteArray(&str);
// Get key
uint8_t key[32];
CBWIFGetPrivateKey(&wif, key);
CBDestroyWIF(&wif);
// Create OpenSSL key
EC_KEY * eckey = EC_KEY_new_by_curve_name(NID_secp256k1);
BIGNUM * bn = BN_bin2bn(key, CB_PRIVKEY_SIZE, NULL);
if (!EC_KEY_set_private_key(eckey, bn))
return EXIT_FAILURE;
// Convert key to DER format
int len = i2d_ECPrivateKey(eckey, NULL); // <-- CRASH HERE
unsigned char derkey[len];
i2d_ECPrivateKey(eckey, (unsigned char **)&derkey);
EC_KEY_free(eckey);
// Encode DER key as hex
char out[len*2+1];
CBBytesToString(derkey, 0, len, out, false);
// Print to stdout
puts(out);
return EXIT_SUCCESS;
}
CB_PRIVKEY_SIZE is 32. I verified that the key data from CBWIFGetPrivateKey is correct. The program crashes with the following stacktrace:
#0 0x00007ffff766cb03 in EC_POINT_point2oct () from /usr/lib/x86_64-linux-gnu/libcrypto.so.1.0.0
#1 0x00007ffff7658124 in i2d_ECPrivateKey () from /usr/lib/x86_64-linux-gnu/libcrypto.so.1.0.0
#2 0x0000000000400bf6 in main (argc=2, argv=0x7fffffffe038) at examples/WIF2DER.c:46

Answer 1

The reason is that you cannot run i2d_ECPrivateKey without setting a public key, probably due to a bug in OpenSSL. I generated the public key and solved a few other issues an now the program works:
#include "CBWIF.h"
#include <openssl/ssl.h>
int main(int argc, char * argv[]) {
CBWIF wif;
if (argc != 2)
return EXIT_FAILURE;
// Decode WIF string
CBByteArray str;
CBInitByteArrayFromString(&str, argv[1], false);
CBInitWIFFromString(&wif, &str, false);
CBDestroyByteArray(&str);
// Get key
uint8_t key[32];
CBWIFGetPrivateKey(&wif, key);
CBDestroyWIF(&wif);
// Create OpenSSL key
EC_KEY * eckey = EC_KEY_new_by_curve_name(NID_secp256k1);
BIGNUM * bn = BN_bin2bn(key, CB_PRIVKEY_SIZE, NULL);
if (!EC_KEY_set_private_key(eckey, bn))
return EXIT_FAILURE;
// Create public key as OpenSSL cannot do this easily
EC_GROUP * group = EC_GROUP_new_by_curve_name(NID_secp256k1);
EC_POINT * point = EC_POINT_new(group);
BN_CTX * ctx = BN_CTX_new();
EC_POINT_mul(group, point, bn, NULL, NULL, ctx);
BN_CTX_free(ctx);
EC_GROUP_free(group);
BN_free(bn);
if (!EC_KEY_set_public_key(eckey, point))
return EXIT_FAILURE;
EC_POINT_free(point);
// Check the key
if (!EC_KEY_check_key(eckey))
return EXIT_FAILURE;
// Convert key to DER format
int len = i2d_ECPrivateKey(eckey, NULL);
unsigned char derkey[len];
unsigned char * derkeyPtr = derkey;
i2d_ECPrivateKey(eckey, &derkeyPtr);
// Freeing the EC_KEY here crashes for some reason???
// Encode DER key as hex
char out[len*2+1];
CBBytesToString(derkey, 0, len, out, false);
// Print to stdout
puts(out);
return EXIT_SUCCESS;
}

Answer 2

Here's how OpenSSL uses it in <openssl src/crypto/ec/ec_ameth.c. All the other similar uses in the library utilize i2d_ECPrivateKey_bio. Also, you might take a quick look at how the OPENSSL_EC_NAMED_CURVE flag is used with V_ASN1_OBJECT.
unsigned char *ep, *p;
int eplen, ptype;
unsigned int tmp_flags, old_flags;
...
old_flags = EC_KEY_get_enc_flags(ec_key);
tmp_flags = old_flags | EC_PKEY_NO_PARAMETERS;
...
eplen = i2d_ECPrivateKey(ec_key, NULL);
if (!eplen)
{
EC_KEY_set_enc_flags(ec_key, old_flags);
ECerr(EC_F_ECKEY_PRIV_ENCODE, ERR_R_EC_LIB);
return 0;
}
ep = (unsigned char *) OPENSSL_malloc(eplen);
if (!ep)
{
EC_KEY_set_enc_flags(ec_key, old_flags);
ECerr(EC_F_ECKEY_PRIV_ENCODE, ERR_R_MALLOC_FAILURE);
return 0;
}
p = ep;
if (!i2d_ECPrivateKey(ec_key, &p))
{
EC_KEY_set_enc_flags(ec_key, old_flags);
OPENSSL_free(ep);
ECerr(EC_F_ECKEY_PRIV_ENCODE, ERR_R_EC_LIB);
return 0;
}
...
Also take a look at Avoid a NULL dereference in i2d_ECPrivateKey() when an EC_KEY lacks the public key member from OpenBSD's LibReSSL project.