What is Code Signing?

How Code Signing Works

Code signing relies on Public Key Infrastructure (PKI) to create verifiable digital signatures. The developer generates a public/private key pair and obtains a code signing certificate from a trusted Certificate Authority (CA) after identity verification. When signing code, the developer creates a cryptographic hash of the software using SHA-256, encrypts that hash with their private key to produce the digital signature, and bundles the signature and certificate with the software package.

When a user downloads or installs the signed software, their operating system uses the CA's trusted public key to verify the certificate's authenticity. The system then extracts the developer's public key from the certificate, decrypts the signature to retrieve the original hash, and recomputes the hash from the downloaded file. If both hashes match, the code is verified as authentic and unaltered. If any byte has been modified, the hashes will not match and the system displays a security warning or blocks execution. Organizations with 24/7 security monitoring infrastructure can integrate code signing validation into their threat detection workflows to catch anomalous certificate usage patterns.

Timestamping adds long-term validity to signatures. By embedding a cryptographic timestamp from a Timestamp Authority (TSA) at the moment of signing, developers ensure signatures remain valid even after the signing certificate expires. Without timestamping, code signed with an expired certificate triggers security warnings despite being legitimately signed.

Why Code Signing Matters

Without code signing, software supply chains remain vulnerable to tampering at every distribution point. The 2020 SolarWinds attack demonstrated this risk when attackers compromised signing keys and inserted malicious code into legitimate updates signed with SolarWinds' own certificates, affecting approximately 18,000 organizations (U.S. Government Accountability Office, 2021). The attack succeeded because the malware carried a valid digital signature that security tools trusted.

Code signing authenticates publisher identity, ensures code integrity during distribution, and eliminates security warning dialogs that condition users to ignore real threats. As of 2024, 54% of organizations had implemented code signing, with 87% planning further adoption. The CA/Browser Forum's June 2023 mandate requiring Hardware Security Module (HSM) storage for all code signing private keys reflects the industry's recognition that key compromise represents the single point of failure in code signing security.

For environments managing software deployments across multiple locations or supporting BYOD policies, code signing provides a verifiable chain of trust from development through deployment. Organizations protecting against ransomware and supply chain attacks use code signing as a foundational control to prevent execution of unsigned or improperly signed executables.

Code Signing vs. Digital Signatures

Code signing is a specialized application of digital signature technology specifically designed for executable code. While both use PKI and cryptographic hashing, code signing includes platform-specific requirements like timestamping, certificate validation by operating systems, and mandatory HSM storage for private keys as of June 2023.

Common Misconceptions

Code signing does not prevent malware execution if the signed code itself is malicious. Code signing only verifies that the code came from the stated publisher and has not been altered since signing. It does not analyze the code's behavior or guarantee the software is safe to run.

Code signing does not make expired certificates invalidate old signatures if timestamping was used. A properly timestamped signature remains valid indefinitely because the timestamp proves the code was signed when the certificate was still valid, preserving trust in software releases even years after the certificate expires.