CredUnPackAuthenticationBufferA function (wincred.h)

Article
02/09/2023

The CredUnPackAuthenticationBuffer function converts an authentication buffer returned by a call to the CredUIPromptForWindowsCredentials function into a string user name and password.

Syntax

CREDUIAPI BOOL CredUnPackAuthenticationBufferA(
  [in]      DWORD dwFlags,
  [in]      PVOID pAuthBuffer,
  [in]      DWORD cbAuthBuffer,
  [out]     LPSTR pszUserName,
  [in, out] DWORD *pcchlMaxUserName,
  [out]     LPSTR pszDomainName,
  [in, out] DWORD *pcchMaxDomainName,
  [out]     LPSTR pszPassword,
  [in, out] DWORD *pcchMaxPassword
);

Parameters

[in] dwFlags

Setting the value of this parameter to CRED_PACK_PROTECTED_CREDENTIALS specifies that the function attempts to decrypt the credentials in the authentication buffer. If the credential cannot be decrypted, the function returns FALSE, and a call to the GetLastError function will return the value ERROR_NOT_CAPABLE.

How the decryption is done depends on the format of the authentication buffer.

If the authentication buffer is a SEC_WINNT_AUTH_IDENTITY_EX2 structure, the function can decrypt the buffer if it is encrypted by using SspiEncryptAuthIdentityEx with the SEC_WINNT_AUTH_IDENTITY_ENCRYPT_SAME_LOGON option.

If the authentication buffer is one of the marshaled KERB_*_LOGON structures, the function decrypts the password before returning it in the pszPassword buffer.

[in] pAuthBuffer

A pointer to the authentication buffer to be converted.

This buffer is typically the output of the CredUIPromptForWindowsCredentials or CredPackAuthenticationBuffer function. This must be one of the following types:

A SEC_WINNT_AUTH_IDENTITY_EX2 structure for identity credentials. The function does not accept other SEC_WINNT_AUTH_IDENTITY structures.
A KERB_INTERACTIVE_LOGON or KERB_INTERACTIVE_UNLOCK_LOGON structure for password credentials.
A KERB_CERTIFICATE_LOGON or KERB_CERTIFICATE_UNLOCK_LOGON structure for smart card certificate credentials.
GENERIC_CRED for generic credentials.

[in] cbAuthBuffer

The size, in bytes, of the pAuthBuffer buffer.

[out] pszUserName

A pointer to a null-terminated string that receives the user name.

This string can be a marshaled credential. See Remarks.

[in, out] pcchlMaxUserName

A pointer to a DWORD value that specifies the size, in characters, of the pszUserName buffer. On output, if the buffer is not of sufficient size, specifies the required size, in characters, of the pszUserName buffer. The size includes terminating null character.

[out] pszDomainName

A pointer to a null-terminated string that receives the name of the user's domain.

[in, out] pcchMaxDomainName

A pointer to a DWORD value that specifies the size, in characters, of the pszDomainName buffer. On output, if the buffer is not of sufficient size, specifies the required size, in characters, of the pszDomainName buffer. The size includes the terminating null character. The required size can be zero if there is no domain name.

[out] pszPassword

A pointer to a null-terminated string that receives the password.

[in, out] pcchMaxPassword

A pointer to a DWORD value that specifies the size, in characters, of the pszPassword buffer. On output, if the buffer is not of sufficient size, specifies the required size, in characters, of the pszPassword buffer. The size includes the terminating null character.

This string can be a marshaled credential. See Remarks.

Return value

TRUE if the function succeeds; otherwise, FALSE.

For extended error information, call the GetLastError function. The following table shows common values for the GetLastError function.

Return code/value	Description
ERROR_NOT_CAPABLE	CRED_PACK_PROTECTED_CREDENTIALS was passed as the value of the dwFlags parameter, but this function cannot decrypt the credential because the security context used to protect the password is different from the security context of the caller.
ERROR_INSUFFICIENT_BUFFER	One of the output buffers, pszUserName, pszDomainName, or pszPassword, was of insufficient size.
ERROR_NOT_SUPPORTED	The authentication buffer is not of a supported type.

Remarks

Beginning with Windows 8 and Windows Server 2012, the authentication buffer can be a SEC_WINNT_AUTH_IDENTITY_EX2 structure, which can be optionally encrypted by using the SspiEncryptAuthIdentityEx function with the SEC_WINNT_AUTH_IDENTITY_ENCRYPT_SAME_LOGON option. This credential format is returned by a Credential Provider of an Identity Provider by using the CredUIPromptForWindowsCredentials or SspiPromptForCredentials function. This structure can also be constructed by using the CredPackAuthenticationBuffer function. If the authentication buffer pAuthBuffer represents a nonpassword credential, such as KERB_CERTIFICATE_LOGON or SEC_WINNT_AUTH_IDENTITY_EX2, the function must marshal the credential as character strings, returned as user name, domain name, and password strings. The marshaling is done by using a specific procedure. When dwFlags contains the CRED_PACK_PROTECTED_CREDENTIALS flag, the caller must run in the same logon session in which the credential was encrypted.

Note

The wincred.h header defines CredUnPackAuthenticationBuffer as an alias that automatically selects the ANSI or Unicode version of this function based on the definition of the UNICODE preprocessor constant. Mixing usage of the encoding-neutral alias with code that is not encoding-neutral can lead to mismatches that result in compilation or runtime errors. For more information, see Conventions for Function Prototypes.

Requirements

Requirement	Value
Minimum supported client	Windows Vista [desktop apps only]
Minimum supported server	Windows Server 2008 [desktop apps only]
Target Platform	Windows
Header	wincred.h
Library	Credui.lib
DLL	Credui.dll

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