#include <openssl/ec.h> EC_KEY *d2i_ECPrivateKey(EC_KEY **key, const unsigned char **in, long len); int i2d_ECPrivateKey(EC_KEY *key, unsigned char **out); unsigned int EC_KEY_get_enc_flags(const EC_KEY *key); void EC_KEY_set_enc_flags(EC_KEY *eckey, unsigned int flags);
These functions are similar to the d2i_X509() functions, and you should refer to that page for a detailed description (see d2i_X509(3)).
The format of the external representation of the public key written by i2d_ECPrivateKey (such as whether it is stored in a compressed form or not) is described by the point_conversion_form. See EC_GROUP_copy(3) for a description of point_conversion_form.
When reading a private key encoded without an associated public key (e.g. if EC_PKEY_NO_PUBKEY has been used - see below), then d2i_ECPrivateKey generates the missing public key automatically. Private keys encoded without parameters (e.g. if EC_PKEY_NO_PARAMETERS has been used - see below) cannot be loaded using d2i_ECPrivateKey.
The functions EC_KEY_get_enc_flags and EC_KEY_set_enc_flags get and set the value of the encoding flags for the key. There are two encoding flags currently defined - EC_PKEY_NO_PARAMETERS and EC_PKEY_NO_PUBKEY. These flags define the behaviour of how the key is converted into ASN1 in a call to i2d_ECPrivateKey. If EC_PKEY_NO_PARAMETERS is set then the public parameters for the curve are not encoded along with the private key. If EC_PKEY_NO_PUBKEY is set then the public key is not encoded along with the private key.
i2d_ECPrivateKey() returns the number of bytes successfully encoded or a negative value if an error occurs. The error code can be obtained by ERR_get_error(3).