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.Dd $Mdocdate: November 11 2015 $
.Dt CMS_DECRYPT 3
.Os
.Sh NAME
.Nm CMS_decrypt
.Nd decrypt content from a CMS envelopedData structure
.Sh SYNOPSIS
.In openssl/cms.h
.Ft int
.Fo CMS_decrypt
.Fa "CMS_ContentInfo *cms"
.Fa "EVP_PKEY *pkey"
.Fa "X509 *cert"
.Fa "BIO *dcont"
.Fa "BIO *out"
.Fa "unsigned int flags"
.Fc
.Sh DESCRIPTION
.Fn CMS_decrypt
extracts and decrypts the content from a CMS EnvelopedData structure.
.Fa pkey
is the private key of the recipient,
.Fa cert
is the recipient's certificate,
.Fa out
is a
.Vt BIO
to write the content to and
.Fa flags
is an optional set of flags.
.Pp
The
.Fa dcont
parameter is used in the rare case where the encrypted content is
detached.
It will normally be set to
.Dv NULL .
.Sh NOTES
.Xr OpenSSL_add_all_algorithms 3
(or equivalent) should be called before using this function or errors
about unknown algorithms will occur.
.Pp
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.
.Pp
If
.Fa cert
is set to
.Dv 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
.Fn 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
.Dv CMS_DEBUG_DECRYPT
is set then the above behaviour is modified and an error
.Em is
returned if no recipient encrypted key can be decrypted
.Em without
generating a random content encryption key.
Applications should use this flag with
.Sy extreme caution
especially in automated gateways as it can leave them open to attack.
.Pp
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
.Xr CMS_set1_pkey 3 .
In this case both
.Fa cert
and
.Fa pkey
should be set to
.Dv NULL .
.Pp
To process KEKRecipientInfo types
.Xr CMS_set1_key 3
or
.Xr CMS_RecipientInfo_set0_key 3
and
.Xr CMS_ReceipientInfo_decrypt 3
should be called before
.Fn CMS_decrypt
and
.Fa cert
and
.Fa pkey
set to
.Dv NULL .
.Pp
The following flags can be passed in the
.Fa flags
parameter:
.Pp
If the
.Dv CMS_TEXT
flag is set MIME headers for type
.Sy text/plain
are deleted from the content.
If the content is not of type
.Sy text/plain
then an error is returned.
.Sh RETURN VALUES
.Fn CMS_decrypt
returns either 1 for success or 0 for failure.
The error can be obtained from
.Xr ERR_get_error 3 .
.Sh BUGS
The lack of single pass processing and the need to hold all data in
memory as mentioned in
.Xr CMS_verify 3
also applies to
.Fn CMS_decrypt .
.Sh SEE ALSO
.Xr CMS_encrypt 3 ,
.Xr ERR_get_error 3
.Sh HISTORY
.Fn CMS_decrypt
was added to OpenSSL 0.9.8.
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