Verify That a Certificate Supports a Signature Method
This topic describes how to verify that a certificate supports a specific signature method.
The CryptXmlEnumAlgorithmInfo in the Microsoft Crypto API enumerates the properties of a certificate and is used in this code example to enumerate the signature methods that the certificate supports. To use CryptXmlEnumAlgorithmInfo to enumerate the signature methods that the certificate supports, the caller must provide a callback method and a data structure in the call to CryptXmlEnumAlgorithmInfo, allowing it to pass data to the callback method.
The data structure used in the next code example has the following fields:
| Field | Description |
|---|---|
| userSignatureAlgorithmToCheck | An LPWSTR field that points to the string that contains the URI of the signature algorithm to be checked. |
| certificateAlgorithmInfo | A pointer to a CRYPT_OID_INFO structure that contains information about a signature algorithm that is supported by the certificate. |
| userSignatureAlgorithmSupported | A Boolean value that indicates whether the signature algorithm is supported by the certificate. |
struct SignatureMethodData
{
LPCWSTR userSignatureAlgorithmToCheck;
PCCRYPT_OID_INFO certificateAlgorithmInfo;
BOOL userSignatureAlgorithmSupported;
};
The Crypto API method that checks the certificate uses a callback method to return data to the caller. CryptXmlEnumAlgorithmInfo enumerates the signature methods that the certificate supports, and calls the callback method for each signature method until the callback method returns FALSE or until all signature methods in the certificate have been enumerated.
The callback method in the next code example searches for a signature method passed in by CryptXmlEnumAlgorithmInfo that matches the signature method provided by the calling method. When a match is found, the callback method checks whether the signature method is also supported by the system. If the signature methods match and are supported by the system, the signature method is marked as system-supported and the callback method returns FALSE.
BOOL WINAPI
EnumSignatureMethodCallback (
__in const CRYPT_XML_ALGORITHM_INFO *certMethodInfo,
__inout_opt void *userArg
)
{
// MAX_ALG_ID_LEN is used to set the maximum length of the
// algorithm URI in the string comparison. The URI is not
// likely to be longer than 128 characters so a fixed-size
// buffer is used in this example.
// To make this function more robust, you might consider
// setting this value dynamically.
static const size_t MAX_ALG_ID_LEN = 128;
SignatureMethodData *certificateAlgorithmData = NULL;
if (NULL != userArg) {
// Assign user data to local data structure
certificateAlgorithmData = (SignatureMethodData*)userArg;
} else {
// Unable to continue this enumeration
// without data from calling method.
return FALSE;
}
// For each algorithm in the enumeration, check to see if the URI
// of the algorithm supported by the certificate matches the URI
// of the algorithm being tested.
int cmpResult = 0;
cmpResult = wcsncmp(
certMethodInfo->wszAlgorithmURI,
certificateAlgorithmData->userSignatureAlgorithmToCheck,
MAX_ALG_ID_LEN );
if ( 0 == cmpResult )
{
// This is a match...
// Check to see if the algorithm supported by the
// certificate matches any of the supported algorithms
// on the system.
cmpResult = wcsncmp(
certMethodInfo->wszCNGExtraAlgid,
certificateAlgorithmData->certificateAlgorithmInfo->pwszCNGAlgid,
MAX_ALG_ID_LEN );
if ( 0 == cmpResult )
{
// This is also a match so set the field in the data structure
// provided by the calling method.
certificateAlgorithmData->userSignatureAlgorithmSupported = TRUE;
// A match was found so there is no point in continuing
// the enumeration.
return FALSE;
}
}
// The enumeration stops when the callback method returns FALSE.
// If here, then return TRUE because a matching algorithm has
// not been found.
return TRUE;
}
The following code example wraps the validation functionality into a single method. This method returns a Boolean value that indicates whether the certificate supports the signature method and whether the signature method is supported by the system.
BOOL
SupportsSignatureAlgorithm (
__in LPCWSTR signingMethodToCheck,
__in PCCERT_CONTEXT certificateToCheck
)
{
HRESULT hr = S_OK;
// Initialize the structure that contains the
// information about the signature algorithm to check
SignatureMethodData certificateAlgorithmData;
certificateAlgorithmData.userSignatureAlgorithmSupported =
FALSE;
certificateAlgorithmData.userSignatureAlgorithmToCheck =
signingMethodToCheck;
// Call the crypt API to get information about the algorithms
// that are supported by the certificate and initialize
// certificateAlgorithmData
certificateAlgorithmData.certificateAlgorithmInfo = CryptFindOIDInfo (
CRYPT_OID_INFO_OID_KEY,
certificateToCheck->pCertInfo->SubjectPublicKeyInfo.Algorithm.pszObjId,
CRYPT_PUBKEY_ALG_OID_GROUP_ID | CRYPT_OID_PREFER_CNG_ALGID_FLAG);
if (certificateAlgorithmData.certificateAlgorithmInfo != NULL)
{
// Enumerate the algorithms that are supported by the
// certificate, and use our callback method to determine if
// the user supplied signature algorithm is supported by
// the certificate.
//
// Note that CRYPT_XML_GROUP_ID_SIGN is used to enumerate
// the signature methods
hr = CryptXmlEnumAlgorithmInfo(
CRYPT_XML_GROUP_ID_SIGN, // NOTE: CRYPT_XML_GROUP_ID_SIGN
CRYPT_XML_FLAG_DISABLE_EXTENSIONS,
(void*)&certificateAlgorithmData,
EnumSignatureMethodCallback);
// when the enumeration has returned successfully,
// certificateAlgorithmData.userSignatureAlgorithmSupported
// will be TRUE if the signing method is supported by
// the certificate
}
return certificateAlgorithmData.userSignatureAlgorithmSupported;
}
Related topics
Next Steps
Load a Certificate from a File
Verify the System Supports a Digest Method
Embed Certificate Chains in a Document
Used in This Example
CryptXmlEnumAlgorithmInfo
For More Information