Digital Signatures and Certificates

A digital signature proves that a document or message was created by a specific private key holder and has not been modified. Unlike a handwritten signature, it is mathematically bound to the content — any change invalidates the signature.

Signing process:

1. Hash the document: H = SHA256(document)
2. Encrypt the hash with the private key: sig = RSA_encrypt(H, private_key)
3. Attach the signature to the document

Verification: decrypt sig with public key, recompute hash, compare.

Modern signature algorithms:

• ECDSA (with P-256) — used in TLS certificates and Bitcoin
• Ed25519 (EdDSA) — faster, simpler, no per-signature randomness vulnerability, used in SSH, Signal

Prefer Ed25519 for new applications.

A digital certificate (X.509) binds a public key to an identity. It contains: subject name, public key, validity period, issuer, and the CA's digital signature. When you visit HTTPS sites, the server presents its certificate to prove its identity.

A Certificate Authority (CA) is a trusted entity that signs certificates after verifying the applicant's identity. Browsers and OS trust stores contain root CA certificates. Trust chains: root CA → intermediate CA → end-entity certificate.

Let's Encrypt is a free, automated CA. It issues Domain Validated (DV) certificates valid for 90 days, automatically renewable via the ACME protocol. It has dramatically increased HTTPS adoption since 2015.

Certificate validation levels:

• DV (Domain Validated) — proves domain control only
• OV (Organization Validated) — verifies organization exists
• EV (Extended Validation) — rigorous identity verification (green bar, now deprecated in browsers)

Compromised certificates must be revoked:

• CRL (Certificate Revocation List) — downloadable list of revoked serial numbers
• OCSP (Online Certificate Status Protocol) — real-time single-certificate check
• OCSP Stapling — server attaches CA-signed OCSP response to TLS handshake

Certificate Transparency logs are public, append-only records of every issued certificate. Browsers require CT proof in certificates. This enables detection of mis-issued certificates within hours. Search: crt.sh

Software publishers sign executables with code signing certificates. The OS verifies signatures before execution. This prevents tampered binaries from running silently. macOS Gatekeeper and Windows SmartScreen enforce code signing policies.

Certificate pinning hardcodes expected certificates or public keys in applications. Even if a rogue CA issues a fraudulent certificate, pinned apps reject it. Used in mobile apps and high-security services. Difficult to update — use with care.

Which mechanism allows browsers to check in real-time whether a specific certificate has been revoked?

Digital signatures prove authenticity and integrity using private keys. Certificates bind public keys to identities, signed by Certificate Authorities. Let's Encrypt provides free DV certificates. Revocation uses CRL or OCSP. Certificate Transparency logs detect mis-issuance. Ed25519 is the modern signature algorithm of choice.