CMS_decrypt (3)

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NAME

 CMS_decrypt - decrypt content from a CMS envelopedData structure

SYNOPSIS

 #include <openssl/cms.h>

 int CMS_decrypt(CMS_ContentInfo *cms, EVP_PKEY *pkey, X509 *cert, BIO *dcont, BIO *out, unsigned int flags);

DESCRIPTION

CMS_decrypt() extracts and decrypts the content from a EnvelopedData structure. pkey is the private key of the recipient, cert is the recipient's certificate, out is a 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

NOTES

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

structure.

If cert is set to

all possible recipients are tried. This case however is problematic. To thwart the attack (Bleichenbacher's attack on #1 v1.5 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 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

structure in advance using the utility functions such as CMS_set1_pkey(). In this case both cert and pkey should be set to

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

The following flags can be passed in the flags parameter.

If the

flag is set headers for type text/plain are deleted from the content. If the content is not of type text/plain then an error is returned.

RETURN VALUES

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

BUGS

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().

SEE ALSO

ERR_get_error(3), CMS_encrypt(3)

HISTORY

CMS_decrypt() was added to OpenSSL 0.9.8