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EUC-EO: Digital Signatures and Certificates

EUC-EO · Work through your exam, one topic at a time.

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Digital Signatures and Certificates

A digital signature is a hash of the message transformed with the signer's private key and verified with the public key. This domain tests what signatures do and do not prove, the main signature schemes, and how X.509 certificates are parsed, validated and revoked.

Core rules

  • Signing provides integrity, authentication of origin and non-repudiation. It does not provide confidentiality; encrypt separately if needed.
  • Schemes: RSA-PSS (preferred over PKCS#1 v1.5), ECDSA (needs a unique random k, since k reuse leaks the private key), Ed25519 (deterministic and misuse-resistant), DSA (legacy).
  • Sign the hash, not the message, for performance; the hash must be collision-resistant.
  • X.509 v3 fields that matter: subject, issuer, serial number, validity dates, public key and algorithm, SAN (the names browsers check, not CN), key usage and extended key usage, basic constraints (CA:TRUE, path length), CRL distribution points and AIA/OCSP URLs.
  • Validation chain: build the path from leaf to trusted root, verify each signature, check validity, check revocation, check name/SAN and EKU, then apply policy constraints.

Worked example

A server presents a leaf certificate for shop.example.com issued by an Intermediate CA, which is itself signed by a Root CA in the client's trust store. The client verifies the intermediate's signature with the root key, then the leaf's with the intermediate key; both validity periods must bracket the current time; OCSP returns 'good'; the SAN must contain shop.example.com and the EKU must permit server authentication. Any failing check terminates the connection, because a valid signature alone is not sufficient.

Traps

A self-signed certificate is not trusted merely because it is encrypted. Wildcards cover one label only: *.example.com matches a.example.com but not a.b.example.com. Revocation checking may soft-fail, so a missing response is not proof of goodness. Certificates bind keys to names, not to good behaviour, and TLS and code-signing EKUs are not interchangeable.