DecryptMessage function (sspi.h)

Article
10/13/2021

The DecryptMessage (Digest) function decrypts a message. Some packages do not encrypt and decrypt messages but rather perform and check an integrity hash.

The Digest security support provider (SSP) provides encryption and decryption confidentiality for messages exchanged between client and server 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 DecryptMessage(
  [in]      PCtxtHandle    phContext,
  [in, out] PSecBufferDesc pMessage,
  [in]      unsigned long  MessageSeqNo,
  [out]     unsigned long  *pfQOP
);

Parameters

[in] phContext

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

[in, out] pMessage

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

When using the Digest SSP, on input, the structure references one or more SecBuffer structures. One of these must be of type SECBUFFER_DATA or SECBUFFER_STREAM, and it must contain the encrypted message.

[in] MessageSeqNo

The sequence number expected by the transport application, if any. If the transport application does not maintain sequence numbers, this parameter must be set to zero.

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

[out] pfQOP

A pointer to a variable of type ULONG that receives package-specific flags that indicate the quality of protection.

This parameter can be one of the following flags.

Value	Meaning
SECQOP_WRAP_NO_ENCRYPT	The message was not encrypted, but a header or trailer was produced. Note KERB_WRAP_NO_ENCRYPT has the same value and the same meaning.
SIGN_ONLY	When using the Digest SSP, use this flag when the security context is set to verify the signature only. For more information, see Quality of Protection.

Return value

If the function verifies that the message was received in the correct sequence, the function returns SEC_E_OK.

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

Return code	Description
SEC_E_BUFFER_TOO_SMALL	The message buffer is too small. Used with the Digest SSP.
SEC_E_CRYPTO_SYSTEM_INVALID	The cipher chosen for the security context is not supported. Used with the Digest SSP.
SEC_E_INCOMPLETE_MESSAGE	The data in the input buffer is incomplete. The application needs to read more data from the server and call DecryptMessage (Digest) again.
SEC_E_INVALID_HANDLE	A context handle that is not valid was specified in the phContext parameter. Used with the Digest SSP.
SEC_E_MESSAGE_ALTERED	The message has been altered. Used with the Digest SSP.
SEC_E_OUT_OF_SEQUENCE	The message was not received in the correct sequence.
SEC_E_QOP_NOT_SUPPORTED	Neither confidentiality nor integrity are supported by the security context. Used with the Digest SSP.

Remarks

Sometimes an application will read data from the remote party, attempt to decrypt it by using DecryptMessage (Digest), and discover that DecryptMessage (Digest) succeeded but the output buffers are empty. This is normal behavior, and applications must be able to deal with it.

Windows XP: This function was also known as UnsealMessage. Applications should now use DecryptMessage (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