The phrase "The identity used to sign the executable is no longer valid" refers to a critical issue that can occur when running software applications, particularly in environments where code signing is crucial for security and integrity. This situation typically arises when the digital certificate used to sign a software executable becomes invalid.

1. Understanding Code Signing

1.1 What is Code Signing?

Code signing is a process that uses digital certificates to sign executable programs and scripts. This signature certifies that the code has not been altered or corrupted since it was signed and confirms the identity of the signer.

1.2 Importance of Code Signing

• Authenticity: Ensures that the software comes from a legitimate source.
• Integrity: Verifies that the software has not been tampered with since its signing.
• Trust: Provides users with confidence to download and install software.

2. Key Reasons for Invalid Signatures

2.1 Expired Certificates

One of the most common reasons for a signature to become invalid is that the signing certificate has expired. Certificates have a specified validity period, after which they must be renewed or replaced.

2.2 Revoked Certificates

Certificates may be revoked by the issuer before their expiration date, typically due to a security breach or compromise of the certificate.

2.3 Certificate Mismatch

A mismatch can occur if the executable is signed with a certificate that doesn't match the expected certificate, often due to procedural errors in the signing process.

2.4 Incorrect Timestamping

Even if a certificate expires, a correct timestamp associated with the signing process can maintain signature validity. If timestamping is missed or incorrect, the signature can become invalid when the certificate expires.

3. Technical Aspects

3.1 Digital Certificate Structure

Digital certificates follow the X.509 standard, which comprises:

• Version: Indicates the version of the certificate format.
• Serial Number: A unique number assigned by the certificate authority.
• Algorithm Information: Specifies the cryptographic algorithm used.
• Issuer Name: Identifies the certificate authority (CA) that issued the certificate.
• Validity Period: Defines the start and end dates when the certificate is valid.
• Subject Name: Identifies the entity that owns the certificate.
• Public Key: The key used to verify the signatures made by its corresponding private key.

3.2 Timestamping

Timestamping a signature allows the executable to prove that it was signed during the period of validity of the code signing. The timestamp itself is also signed by the CA.

3.3 Example of Code Signing

Consider a Windows executable signed using `signtool`, Microsoft's tool for code signing.

• `/a`: Automatically selects the best certificate.
• `/t`: Specifies the timestamp server URL.
• `/v`: Produces verbose output, useful for troubleshooting.