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PKCS7_DECRYPT(3) Library Functions Manual PKCS7_DECRYPT(3)

PKCS7_decrypt
decrypt content from a PKCS#7 envelopedData structure

#include <openssl/pkcs7.h>
int
PKCS7_decrypt(PKCS7 *p7, EVP_PKEY *pkey, X509 *cert, BIO *data, int flags);

PKCS7_decrypt() extracts and decrypts the content from a PKCS#7 envelopedData structure. pkey is the private key of the recipient, cert is the recipient's certificate, data is a BIO to write the content to and flags is an optional set of flags.
OpenSSL_add_all_algorithms(3) (or equivalent) should be called before using this function or errors about unknown algorithms will occur.
Although the recipient's certificate is not needed to decrypt the data, it is needed to locate the appropriate recipients in the PKCS#7 structure.
If the PKCS7_TEXT flag is set, MIME headers for type text/plain are deleted from the content. If the content is not of type text/plain, an error is returned.

PKCS7_decrypt() returns 1 for success or 0 for failure.
The error can be obtained from ERR_get_error(3).

ERR_get_error(3), PKCS7_encrypt(3), PKCS7_new(3)

PKCS7_decrypt() first appeared in OpenSSL 0.9.5 and has been available since OpenBSD 2.7.

PKCS7_decrypt() must be passed the correct recipient key and certificate. It would be better if it could look up the correct key and certificate from a database.
The lack of single pass processing and need to hold all data in memory as mentioned in PKCS7_sign(3) also applies to PKCS7_decrypt().
March 22, 2018 OpenBSD-current