A code-signing certificate connects a software publisher’s identity to a public key, allowing a verifier to check who signed a program and whether it changed after signing. It is one part of a larger signing system: the certificate is not the private key, and a valid signature does not prove software is safe.
What a code-signing certificate is
A code-signing certificate is a public-key certificate used in software signing. It contains a public key and identifying information; a trusted issuer, commonly a certificate authority (CA), digitally signs the certificate to bind that key to the identified signer. NIST defines a certificate as information that binds a public key to an identity, and describes a content-signing certificate as one used to sign information so its author and integrity can be checked.
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In the signing process, the publisher uses the corresponding private key to create a digital signature. A verifier uses the public key to check that signature. The private key is separate from the certificate and must be protected. NIST certificate glossary · NIST content-signing certificate glossary · NIST signature-certificate glossary
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| Item | Role |
|---|---|
| Certificate | Binds the signer’s identity to a public key through an issuer’s signature. |
| Private key | Used by the signer to create the software signature; it is not contained in the certificate. |
| Digital signature | Created with the private key and checked with the corresponding public key to detect changes and verify the signing identity. |
These are distinct parts of the system, not interchangeable names for the same thing. NIST’s definitions of certificates and digital signatures describe those separate roles.
How code signing works
A typical workflow connects identity verification, key custody, signing, and verification. NIST also describes timestamping as a possible additional step.
- Protect the private key. The signer creates or controls a key pair and safeguards the private key used to sign releases.
- Obtain a certificate. The signer provides the public key to a CA, which verifies identity and issues a certificate signed by the CA.
- Sign authorized code. The publisher uses the private key to sign the software artifact it intends to release.
- Verify the signature. A recipient or platform checks the signature using the certificate’s public key. A successful check supports confirmation of the signer and that the signed content has not changed.
- Optionally timestamp the signature. A timestamp can be included as another part of the signing workflow; it is not the certificate itself.
If no CA is used, NIST says the public key must instead be distributed through a trusted out-of-band mechanism. That shifts the trust-establishment task to how the recipient obtains and confirms the key. NIST ITL Bulletin, “Protecting Software Integrity Through Code Signing” (May 2018)
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What a code signature tells you—and what it cannot tell you
NIST summarizes the purpose of code signing this way: “Digitally signing code provides both data integrity to prove that the code was not modified, and source authentication to identify who signed the code.” The statement appears in NIST’s Security Considerations for Code Signing, published January 26, 2018. NIST CSWP 5
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsThose checks are meaningful only when the signing and verification system is properly designed and operated. A valid signature does not establish that the program is harmless, free of vulnerabilities, high quality, or suitable for your needs. It identifies a signer and helps detect post-signing changes; it is not a safety certification.
Code signing is used in software contexts including firmware, operating systems, mobile applications, and application container images. Across these contexts, the exact implementation can differ, but the certificate’s basic role remains binding identity to a public key. NIST CSWP 5
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The certificate does not prevent misuse of the private key. If an attacker or unauthorized process can use that key, they may be able to sign code as the certificate’s holder. NIST’s guidance therefore treats code signing as a system-security problem, not just a certificate-purchase step.
- Restrict access to signing keys and protect them against theft or unauthorized use.
- Control which code is approved for signing, so an otherwise legitimate signing process is not used to release unreviewed or malicious artifacts.
- Ensure verification is part of the recipient’s or platform’s process; creating a signature alone does not make anyone check it.
NIST discusses key protection and controls over what code is signed in Security Considerations for Code Signing and its May 2018 ITL Bulletin.
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