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CMS_decrypt(3) OpenSSL CMS_decrypt(3)
CMS_decrypt - decrypt content from a CMS envelopedData structure
int CMS_decrypt(CMS_ContentInfo *cms, EVP_PKEY *pkey, X509 *cert, BIO *dcont, BIO *out, unsigned int flags);
CMS_decrypt() extracts and decrypts the content from a CMS
EnvelopedData structure. pkey is the private key of the recipient, cert
is the recipient's certificate, out is a BIO to write the content to
and flags is an optional set of flags.
The dcont parameter is used in the rare case where the encrypted
content is detached. It will normally be set to NULL.
OpenSSL_add_all_algorithms() (or equivalent) should be called before
using this function or errors about unknown algorithms will occur.
Although the recipients certificate is not needed to decrypt the data
it is needed to locate the appropriate (of possible several) recipients
in the CMS structure.
If cert is set to NULL all possible recipients are tried. This case
however is problematic. To thwart the MMA attack (Bleichenbacher's
attack on PKCS #1 v1.5 RSA padding) all recipients are tried whether
they succeed or not. If no recipient succeeds then a random symmetric
key is used to decrypt the content: this will typically output garbage
and may (but is not guaranteed to) ultimately return a padding error
only. If CMS_decrypt() just returned an error when all recipient
encrypted keys failed to decrypt an attacker could use this in a timing
attack. If the special flag CMS_DEBUG_DECRYPT is set then the above
behaviour is modified and an error is returned if no recipient
encrypted key can be decrypted without generating a random content
encryption key. Applications should use this flag with extreme caution
especially in automated gateways as it can leave them open to attack.
It is possible to determine the correct recipient key by other means
(for example looking them up in a database) and setting them in the CMS
structure in advance using the CMS utility functions such as
CMS_set1_pkey(). In this case both cert and pkey should be set to NULL.
To process KEKRecipientInfo types CMS_set1_key() or
CMS_RecipientInfo_set0_key() and CMS_ReceipientInfo_decrypt() should be
called before CMS_decrypt() and cert and pkey set to NULL.
The following flags can be passed in the flags parameter.
If the CMS_TEXT flag is set MIME headers for type text/plain are
deleted from the content. If the content is not of type text/plain then
an error is returned.
CMS_decrypt() returns either 1 for success or 0 for failure. The error
can be obtained from ERR_get_error(3)
The lack of single pass processing and the need to hold all data in
memory as mentioned in CMS_verify() also applies to CMS_decrypt().
CMS_decrypt() was added to OpenSSL 0.9.8
1.0.2h 2016-05-03 CMS_decrypt(3)