EncryptMessage function (sspi.h)

Article
10/13/2021

The EncryptMessage (Digest) function encrypts a message to provide privacy. EncryptMessage (Digest) allows the application to choose among cryptographic algorithms supported by the chosen mechanism. The EncryptMessage (Digest) function uses the security context referenced by the context handle. Some packages do not have messages to be encrypted or decrypted but rather provide an integrity hash that can be checked.

This function is available as a SASL mechanism only.

Note EncryptMessage (Digest) and DecryptMessage (Digest) can be called at the same time from two different threads in a single Security Support Provider Interface (SSPI) context if one thread is encrypting and the other is decrypting. If more than one thread is encrypting, or more than one thread is decrypting, each thread should obtain a unique context.

Syntax

SECURITY_STATUS SEC_ENTRY EncryptMessage(
  [in]      PCtxtHandle    phContext,
  [in]      unsigned long  fQOP,
  [in, out] PSecBufferDesc pMessage,
  [in]      unsigned long  MessageSeqNo
);

Parameters

[in] phContext

A handle to the security context to be used to encrypt the message.

[in] fQOP

Package-specific flags that indicate the quality of protection. A security package can use this parameter to enable the selection of cryptographic algorithms.

When using the Digest SSP, this parameter must be set to zero.

[in, out] pMessage

A pointer to a SecBufferDesc structure. On input, the structure references one or more SecBuffer structures that can be of type SECBUFFER_DATA. That buffer contains the message to be encrypted. The message is encrypted in place, overwriting the original contents of the structure.

The function does not process buffers with the SECBUFFER_READONLY attribute.

The length of the SecBuffer structure that contains the message must be no greater than cbMaximumMessage, which is obtained from the QueryContextAttributes (Digest) (SECPKG_ATTR_STREAM_SIZES) function.

When using the Digest SSP, there must be a second buffer of type SECBUFFER_PADDING or SEC_BUFFER_DATA to hold signature information. To get the size of the output buffer, call the QueryContextAttributes (Digest) function and specify SECPKG_ATTR_SIZES. The function will return a SecPkgContext_Sizes structure. The size of the output buffer is the sum of the values in the cbMaxSignature and cbBlockSize members.

Applications that do not use SSL must supply a SecBuffer of type SECBUFFER_PADDING.

[in] MessageSeqNo

The sequence number that the transport application assigned to the message. If the transport application does not maintain sequence numbers, this parameter must be zero.

When using the Digest SSP, this parameter must be set to zero. The Digest SSP manages sequence numbering internally.

Return value

If the function succeeds, the function returns SEC_E_OK.

If the function fails, it returns one of the following error codes.

Return code	Description
SEC_E_BUFFER_TOO_SMALL	The output buffer is too small. For more information, see Remarks.
SEC_E_CONTEXT_EXPIRED	The application is referencing a context that has already been closed. A properly written application should not receive this error.
SEC_E_CRYPTO_SYSTEM_INVALID	The cipher chosen for the security context is not supported.
SEC_E_INSUFFICIENT_MEMORY	There is not enough memory available to complete the requested action.
SEC_E_INVALID_HANDLE	A context handle that is not valid was specified in the phContext parameter.
SEC_E_INVALID_TOKEN	No SECBUFFER_DATA type buffer was found.
SEC_E_QOP_NOT_SUPPORTED	Neither confidentiality nor integrity are supported by the security context.

Remarks

The EncryptMessage (Digest) function encrypts a message based on the message and the session key from a security context.

If the transport application created the security context to support sequence detection and the caller provides a sequence number, the function includes this information with the encrypted message. Including this information protects against replay, insertion, and suppression of messages. The security package incorporates the sequence number passed down from the transport application.

When you use the Digest SSP, get the size of the output buffer by calling the QueryContextAttributes (Digest) function and specifying SECPKG_ATTR_SIZES. The function will return a SecPkgContext_Sizes structure. The size of the output buffer is the sum of the values in the cbMaxSignature and cbBlockSize members.

Note These buffers must be supplied in the order shown.

Buffer type	Description
SECBUFFER_STREAM_HEADER	Used internally. No initialization required.
SECBUFFER_DATA	Contains the plaintext message to be encrypted.
SECBUFFER_STREAM_TRAILER	Used internally. No initialization required.
SECBUFFER_EMPTY	Used internally. No initialization required. Size can be zero.

For optimal performance, the pMessage structures should be allocated from contiguous memory.

Windows XP: This function was also known as SealMessage. Applications should now use EncryptMessage (Digest) only.

Requirements

Requirement	Value
Minimum supported client	Windows XP [desktop apps only]
Minimum supported server	Windows Server 2003 [desktop apps only]
Target Platform	Windows
Header	sspi.h (include Security.h)
Library	Secur32.lib
DLL	Secur32.dll