CryptMsgCalculateEncodedLength function

The CryptMsgCalculateEncodedLength function calculates the maximum number of bytes needed for an encoded cryptographic message given the message type, encoding parameters, and total length of the data to be encoded. Note that the result will always be greater than or equal to the actual number of bytes needed.

Syntax

DWORD CryptMsgCalculateEncodedLength(
  DWORD      dwMsgEncodingType,
  DWORD      dwFlags,
  DWORD      dwMsgType,
  void const *pvMsgEncodeInfo,
  LPSTR      pszInnerContentObjID,
  DWORD      cbData
);

Parameters

dwMsgEncodingType

Specifies the encoding type used. It is always acceptable to specify both the certificate and message encoding types by combining them with a bitwise-OR operation as shown in the following example:

X509_ASN_ENCODING | PKCS_7_ASN_ENCODING

Currently defined encoding types are:

  • X509_ASN_ENCODING
  • PKCS_7_ASN_ENCODING

dwFlags

Currently defined flags are shown in the following table.

Value Meaning
CMSG_BARE_CONTENT_FLAG
Indicates that streamed output will not have an outer ContentInfo wrapper (as defined by PKCS #7). This makes it suitable to be streamed into an enclosing message.
CMSG_DETACHED_FLAG
Indicates that there is detached data being supplied for the subsequent calls to CryptMsgUpdate.
CMSG_CONTENTS_OCTETS_FLAG
Used to calculate the size of a DER encoding of a message to be nested inside an enveloped message. This is particularly useful when streaming is being performed.
CMSG_CMS_ENCAPSULATED_CONTENT_FLAG
Non-Data type inner content is encapsulated within an OCTET STRING. This flag is applicable for both Signed and Enveloped messages.

dwMsgType

Currently defined message types are shown in the following table.

Value Meaning
CMSG_DATA
An octet (BYTE) string.
CMSG_SIGNED

CMSG_SIGNED_ENCODE_INFO

CMSG_ENVELOPED

CMSG_ENVELOPED_ENCODE_INFO

CMSG_SIGNED_AND_ENVELOPED
Not implemented.
CMSG_HASHED

CMSG_HASHED_ENCODE_INFO

CMSG_ENCRYPTED
Not implemented.

pvMsgEncodeInfo

A pointer to the data to be encoded. The type of data pointed to depends on the value of dwMsgType. For details, see the dwMsgType table.

pszInnerContentObjID

When calling CryptMsgCalculateEncodedLength with data provided to CryptMsgUpdate already encoded, the appropriate object identifier is passed in pszInnerContentObjID. If pszInnerContentObjID is NULL, the inner content type is assumed not to have been previously encoded, and is encoded as an octet string and given the type CMSG_DATA.

When streaming is being used, pszInnerContentObjID must be either NULL or szOID_RSA_data.

The following algorithm object identifiers are commonly used:

  • szOID_RSA_data
  • szOID_RSA_signedData
  • szOID_RSA_envelopedData
  • szOID_RSA_signEnvData
  • szOID_RSA_digestedData
  • szOID_RSA_encryptedData
  • SPC_INDIRECT_DATA_OBJID
A user can define new inner content usage. The user must ensure that the sender and receiver of the message agree upon the semantics associated with the object identifier.

cbData

The size, in bytes, of the content.

Return Value

Returns the required length for an encoded cryptographic message. This length might not be the exact length but it will not be less than the required length. Zero is returned if the function fails.

To retrieve extended error information, use the GetLastError function. The following table lists the error codes most commonly returned.

Return code Description
CRYPT_E_INVALID_MSG_TYPE
The message type is not valid.
CRYPT_E_UNKNOWN_ALGO
The cryptographic algorithm is unknown.
E_INVALIDARG
One or more arguments are not valid.

Requirements

   
Minimum supported client Windows XP [desktop apps | UWP apps]
Minimum supported server Windows Server 2003 [desktop apps | UWP apps]
Target Platform Windows
Header wincrypt.h
Library Crypt32.lib
DLL Crypt32.dll

See Also

CryptMsgOpenToEncode

Low-level Message Functions

Simplified Message Functions