CMS_decrypt, CMS_decrypt_set1_pkey_and_peer, CMS_decrypt_set1_pkey, CMS_decrypt_set1_password - decrypt content from a CMS envelopedData structure
#include <openssl/cms.h> int CMS_decrypt(CMS_ContentInfo *cms, EVP_PKEY *pkey, X509 *cert, BIO *dcont, BIO *out, unsigned int flags); int CMS_decrypt_set1_pkey_and_peer(CMS_ContentInfo *cms, EVP_PKEY *pk, X509 *cert, X509 *peer); int CMS_decrypt_set1_pkey(CMS_ContentInfo *cms, EVP_PKEY *pk, X509 *cert); int CMS_decrypt_set1_password(CMS_ContentInfo *cms, unsigned char *pass, ossl_ssize_t passlen);
CMS_decrypt() extracts the decrypted content from a CMS EnvelopedData or AuthEnvelopedData structure. It uses CMS_decrypt_set1_pkey() to decrypt the content with the recipient private key pkey if pkey is not NULL. In this case, it is recommended to provide the associated certificate in cert - see the NOTES below. out is a BIO to write the content to and flags is an optional set of flags. If pkey is NULL the function assumes that decryption was already done (e.g., using CMS_decrypt_set1_pkey() or CMS_decrypt_set1_password()) and just provides the content unless cert, dcont, and out are NULL as well. The dcont parameter is used in the rare case where the encrypted content is detached. It will normally be set to NULL.
CMS_decrypt_set1_pkey_and_peer() decrypts the CMS_ContentInfo structure cms using the private key pkey, the corresponding certificate cert, which is recommended to be supplied but may be NULL, and the (optional) originator certificate peer. On success, it also records in cms the decryption key pkey, and this should be followed by "CMS_decrypt(cms, NULL, NULL, dcont, out, flags)". This call deallocates any decryption key stored in cms.
CMS_decrypt_set1_pkey() is the same as CMS_decrypt_set1_pkey_and_peer() with peer being NULL.
CMS_decrypt_set1_password() decrypts the CMS_ContentInfo structure cms using the secret pass of length passlen. On success, it also records in cms the decryption key used, and this should be followed by "CMS_decrypt(cms, NULL, NULL, dcont, out, flags)". This call deallocates any decryption key stored in cms.
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(), or use CMS_decrypt_set1_password() if the recipient has a symmetric key. In these cases both cert and pkey should be set to NULL.
To process KEKRecipientInfo types CMS_set1_key() or CMS_RecipientInfo_set0_key() and CMS_RecipientInfo_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(), CMS_decrypt_set1_pkey_and_peer(), CMS_decrypt_set1_pkey(), and CMS_decrypt_set1_password() return either 1 for success or 0 for failure. The error can be obtained from ERR_get_error(3).
The set1_ part of these function names is misleading and should better read: with_.
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_set1_pkey_and_peer() and CMS_decrypt_set1_password() were added in OpenSSL 3.0.
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