In Microsoft Office XP, digital certificates can be used to sign files (such as documents, presentations and workbooks) and macro projects. When using a signed file or macro project, a digital signature can provide additional reliability protection for the file or project. If the whole file is signed, a certificate helps to ensure that the file has not been modified since the date of signing. Similarly, if the file contains a signed macro, the certificate used for macro signing can ensure that the macro has not been tampered with since the date of signing. Using signed macros can also add macro developers to the list of trusted sources, so that you can open macros from this source without receiving warning messages. Viewing Certificates When you view a signed file or receive a file containing a signed macro project, you'd better check the attached certificate to determine whether it is valid. You should check the certificate before deciding to trust the document content, add the macro developer to the list of trusted sources, or enable macros. Certificates contain a lot of information. Although some of this information looks quite complicated, there are still some relatively simple contents that you can (and should) check to ensure the validity of the certificate. For example, when auditing a certificate, you can check whether the certificate is issued by a reliable institution; Do you know or trust the grantee of the certificate? Whether the certificate is within the validity period, etc. To view a file's certificate, on the Tools menu, click Options. Click the Security tab. Click the digital signature. To view the details of the certificate, select the name of the signer, and then click View Certificate. View the certificate of macro project If your macro security is set to Medium or High, you will receive a warning when the file you want to open contains macros signed with suspicious certificates. In the warning dialog box, you can click Details to view the properties of the certificate. To view the certificate for opening a file, do the following: On the Tools menu, point to Macros, and then click Visual Basic Editor. Use the Project Manager to select the desired macro project. In the Visual Basic editor, click Digital Signature on the Tools menu. In the Digital Signature dialog box, click Details to view the details of the certificate. Note that the Details button only appears when a digital certificate is attached to the project. Check whether there is a red X in the upper left corner of a valid certificate, indicating that the certificate is flawless. The certificate in question shows a red X on the image. The certificate is marked with a red X for various reasons, including: the signed file or macro has been tampered with. The certificate was not issued by a trusted certificate authority (CA). The certificate was issued without verification (for example, it was provided by CA as a free trial download). This certificate is invalid when used to sign files or macros. The issuer certificate can be issued by a certificate authority (such as VeriSign or electronic lock). They can also be published by organizations or created by individuals. You should check the Issuer field to determine whether you trust the CA (or organization or individual) that issued the certificate. Some cas issue free, unverified certificates. The note part of the certifiCAte will record this unverified situation, and its content is similar to: Verisign Class 1 CA individual subscriber-individual unverified (Verisign Class 1 CA individual user-unverified). A certificate created by an individual using the Selfcert.exe tool included in Office XP contains a note similar to the following: The CA root certificate is not trusted. To enable trust, install the certificate into the trusted root certification authority store. Generally, do not trust certificates with comments, such as demonstrations, tests, or demonstrations. Just as you should have a certain degree of trust in the CA that issued the certificate, you should also know who the certificate is issued to (individual or organization). Validity documents or macro projects should be signed within the validity period of digital certificates. The method of verifying the signature date varies depending on whether you are checking a file or a macro. Verify the signature date of the file Compare the validity period of the certificate with the current date or the date of receipt of the file. If the certificate is invalid at these times, you should not believe that the signature is valid in most cases. Verify the signing date of the macro project If your macro security is set to Medium or High, you will receive a warning when the file you want to open contains macros signed with suspicious certificates. In the warning dialog box, you can click Details to view the properties of the certificate. Click the "Details" button. Please view it in the General tab of the Digital Signature Details dialog box. In the signer information, check the date displayed in the signature time box. You can verify whether the certificate is still valid after the file or macro is signed by clicking View Certificate. To do this, click the General tab of the certificate and check the date next to the effective date. Compare the signature date of the file or macro with the "effective date" on the certificate.