VerifySignedByCert (Transact-SQL)
Tests whether digitally signed data has been changed since it was signed.
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Transact-SQL Syntax Conventions
Syntax
VerifySignedByCert( Cert_ID , signed_data , signature )
Arguments
- Cert_ID
Is the ID of a certificate in the database. int
- signed_data
Is a variable of type nvarchar, char, varchar, or nchar that contains data that has been signed with a certificate.
- signature
Is the signature that was attached to the signed data. varbinary.
Return Types
int
Returns 1 if signed data is unchanged, otherwise 0.
Remarks
VerifySignedByCert derives a signature from previously signed data by using a certificate stored in the database. VerifySignedByCert compares the newly derived signature to the signature that was attached to the signed data when the data was retrieved. If the newly derived signature is the same as the signature that was attached to the signed data, this proves that the data was not changed after it was signed. The signature calculation is based on all characters in the module that is being verified, including leading spaces, trailing spaces, carriage returns, and line feeds.
Permissions
Requires VIEW DEFINITION permission on the certificate.
Examples
A. Verifying that signed data has not been tampered with
The following example tests whether the information in Signed_Data has been changed since it was signed with the certificate called Shipping04. The signature is stored in DataSignature. The certificate, Shipping04, is passed to Cert_ID, which returns the ID of the certificate in the database. If VerifySignedByCert returns 1, the signature is correct. If VerifySignedByCert returns 0, the data in Signed_Data is not the data that was used to generate DataSignature. In this case, either Signed_Data has been changed since it was signed or Signed_Data was signed with a different certificate.
SELECT Data, VerifySignedByCert( Cert_Id( 'Shipping04' ),
Signed_Data, DataSignature ) AS IsSignatureValid
FROM [AdventureWorks].[SignedData04]
WHERE Description = N'data signed by certificate ''Shipping04''';
GO
B. Returning only records that have a valid signature
This query returns only records that have not been changed since they were signed using certificate Shipping04.
SELECT Data FROM [AdventureWorks].[SignedData04]
WHERE VerifySignedByCert( Cert_Id( 'Shipping04' ), Data,
DataSignature ) = 1
AND Description = N'data signed by certificate ''Shipping04''';
GO
See Also
Reference
Cert_ID (Transact-SQL)
SignByCert (Transact-SQL)
CREATE CERTIFICATE (Transact-SQL)
ALTER CERTIFICATE (Transact-SQL)
DROP CERTIFICATE (Transact-SQL)
BACKUP CERTIFICATE (Transact-SQL)