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EVP_OPENINIT(3) Library Functions Manual EVP_OPENINIT(3)

EVP_OpenInit, EVP_OpenUpdate, EVP_OpenFinal
EVP envelope decryption

#include <openssl/evp.h>
int
EVP_OpenInit(EVP_CIPHER_CTX *ctx, EVP_CIPHER *type, unsigned char *ek, int ekl, unsigned char *iv, EVP_PKEY *priv);
int
EVP_OpenUpdate(EVP_CIPHER_CTX *ctx, unsigned char *out, int *outl, unsigned char *in, int inl);
int
EVP_OpenFinal(EVP_CIPHER_CTX *ctx, unsigned char *out, int *outl);

The EVP envelope routines are a high level interface to envelope decryption. They decrypt a public key encrypted symmetric key and then decrypt data using it.
EVP_OpenInit() initializes a cipher context ctx for decryption with cipher type. It decrypts the encrypted symmetric key of length ekl bytes passed in the ek parameter using the private key priv. The IV is supplied in the iv parameter.
EVP_OpenUpdate() and EVP_OpenFinal() have exactly the same properties as the EVP_DecryptUpdate(3) and EVP_DecryptFinal(3) routines.
It is possible to call EVP_OpenInit() twice in the same way as EVP_DecryptInit(3). The first call should have priv set to NULL and (after setting any cipher parameters) it should be called again with type set to NULL.
If the cipher passed in the type parameter is a variable length cipher then the key length will be set to the value of the recovered key length. If the cipher is a fixed length cipher then the recovered key length must match the fixed cipher length.

EVP_OpenInit() returns 0 on error or a non-zero integer (actually the recovered secret key size) if successful.
EVP_OpenUpdate() returns 1 for success or 0 for failure.
EVP_OpenFinal() returns 0 if the decrypt failed or 1 for success.

evp(3), EVP_EncryptInit(3), EVP_SealInit(3), RAND_bytes(3)

EVP_OpenInit(), EVP_OpenUpdate(), and EVP_OpenFinal() first appeared in SSLeay 0.5.1 and have been available since OpenBSD 2.4.
March 27, 2018 OpenBSD-current