Glossary

Code Signing

Code signing is the cryptographic process of digitally signing executables, scripts, and software packages to confirm the author's identity and guarantee the code has not been altered or corrupted since it was signed.

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EXECUTABLE AUTHENTICATION

What is Code Signing?

Code signing is a cryptographic process that applies a digital signature to executables, scripts, and software packages to verify the author's identity and guarantee code integrity.

Code signing is the process of digitally signing executables, scripts, and software packages using a private key from a Public Key Infrastructure (PKI) certificate. This cryptographic operation generates a unique digital signature bound to the code, allowing end-user systems to verify the software publisher's identity and confirm the code has not been altered or corrupted since it was signed.

The verification process relies on the corresponding public key to validate the signature against the code's computed hash. If the hash matches and the certificate chains to a trusted root Certificate Authority (CA), the operating system confirms integrity and displays the verified publisher name. This mechanism provides non-repudiation, ensuring the author cannot deny authorship, and is a foundational control in the SLSA Framework for securing the software supply chain.

CRYPTOGRAPHIC INTEGRITY

Key Features of Code Signing

Code signing establishes a verifiable chain of trust from the software publisher to the end-user, ensuring that executables and scripts have not been tampered with since their publication.

Digital Signature Creation

The publisher generates a cryptographic hash of the code and encrypts it with their private key. This creates a unique digital signature bound to both the code and the publisher's identity. The signature is then bundled with the software package, typically using standards like Authenticode for Windows or Mach-O signing for macOS.

Signature Verification Process

When a user launches the software, the operating system:

Decrypts the signature using the publisher's public key from their digital certificate
Independently computes a new hash of the code
Compares the decrypted hash with the newly computed hash A match confirms both authenticity and integrity; a mismatch triggers a security warning.

Timestamping for Long-Term Validity

A Timestamping Authority (TSA) countersigns the code with a trusted timestamp, cryptographically proving the code was signed while the certificate was valid. This ensures the signature remains verifiable even after the original code signing certificate expires, eliminating the need for re-signing legacy software distributions.

Certificate Chain of Trust

Code signing relies on a hierarchical Public Key Infrastructure (PKI). The publisher's certificate is issued by an intermediate Certificate Authority (CA), which is in turn anchored to a trusted root CA pre-installed in operating systems. This chain validates that the publisher's identity was verified by a trusted third party before certificate issuance.

Extended Validation (EV) Code Signing

EV Code Signing certificates require rigorous identity verification of the publishing organization and store the private key on a physical Hardware Security Module (HSM). Software signed with EV certificates benefits from immediate reputation with Microsoft SmartScreen and other trust filters, reducing security warnings for end-users.

Supply Chain Integrity with SBOM

Modern code signing integrates with a Software Bill of Materials (SBOM) to attest to every component in the build pipeline. Frameworks like SLSA and in-toto use code signing to create cryptographically verifiable attestations that the software artifact was produced by a specific, trusted build process without tampering.

CODE SIGNING ESSENTIALS

Frequently Asked Questions

Clear, technically precise answers to the most common questions about code signing, its cryptographic foundations, and its role in software supply chain security.

Code signing is the cryptographic process of digitally signing executables, scripts, and software packages to confirm the software author's identity and guarantee that the code has not been altered or corrupted since it was signed. It works by generating a one-way hash of the code, which is then encrypted with the developer's private key to create a digital signature. This signature is bundled with the software. When a user downloads or executes the software, their operating system uses the developer's corresponding public key—typically distributed via a Public Key Infrastructure (PKI) and embedded in a digital certificate issued by a trusted Certificate Authority—to decrypt the hash and compare it against a freshly computed hash of the received code. If the hashes match, the signature is valid, confirming both data integrity and authenticity. If they differ, the system warns the user that the code may have been tampered with or corrupted.

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CRYPTOGRAPHIC ECOSYSTEM

Related Terms

Code signing relies on a broader infrastructure of cryptographic primitives, trust hierarchies, and verification standards. These related concepts form the technical foundation that makes digital signatures for executables both possible and trustworthy.

Digital Signature

The core cryptographic mechanism underlying code signing. A private key generates a unique digital fingerprint for a software binary, while the corresponding public key allows any user to verify authenticity and integrity. The signature mathematically binds the author's identity to the exact byte sequence of the executable, making any post-signing modification immediately detectable.

FIPS 186-5

NIST Standard

Public Key Infrastructure (PKI)

The trust backbone that validates code signing certificates. A Certificate Authority (CA) vouches for the developer's identity after verification, issuing an X.509 certificate that chains to a trusted root. Key PKI components for code signing:

Registration Authorities that vet organizational identity
Certificate Revocation Lists (CRLs) and OCSP responders for revoking compromised certs
Hardware-backed key storage requirements for Extended Validation (EV) certificates

Hash Function

Before signing, the executable is processed through a cryptographic hash function—typically SHA-256 or SHA-384—to produce a fixed-size digest. The signature is actually computed over this digest, not the full binary. This provides:

Efficiency: signing a 256-bit hash is fast regardless of file size
Integrity verification: any bit flip in the executable produces a completely different hash
Collision resistance: computationally infeasible to craft a malicious binary with the same hash

Timestamping Authority (TSA)

A trusted third-party service that cryptographically binds a code signature to a specific point in time. The TSA issues a timestamp token containing the hash of the signed code and a trusted clock value, countersigned with the TSA's own key. This solves the expired certificate problem: even after the developer's signing certificate expires or is revoked, the timestamp proves the signature was valid when applied, allowing long-term signature verification.

Hardware Security Module (HSM)

A dedicated, tamper-resistant physical device that generates, stores, and manages code signing private keys entirely within its secure boundary. HSMs enforce critical security controls:

Key never leaves the device in plaintext form
Multi-party authorization requiring quorum approval for signing operations
Audit logging of every signing event
FIPS 140-2 Level 3 certification for physical and logical protection This is the gold standard for protecting high-value code signing keys from exfiltration.

Software Bill of Materials (SBOM)

A machine-readable inventory of all components, libraries, and dependencies within a software artifact. When combined with code signing, an SBOM enables supply chain verification: each listed component can be independently validated against its own signature. The SLSA Framework (Supply-chain Levels for Software Artifacts) defines progressive security levels, with Level 3+ requiring hermetic, signed attestations for every build step, creating a verifiable chain from source to binary.

About the author

Prasad Kumkar

CEO & MD, Inference Systems

Prasad Kumkar is the CEO & MD of Inference Systems and writes about AI systems architecture, LLM infrastructure, model serving, evaluation, and production deployment. Over 5+ years, he has worked across computer vision models, L5 autonomous vehicle systems, and LLM research, with a focus on taking complex AI ideas into real-world engineering systems.

His work and writing cover AI systems, large language models, AI agents, multimodal systems, autonomous systems, inference optimization, RAG, evaluation, and production AI engineering.

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