Digital Signatures and Key Management
Digital Signatures and Key Management
This domain tests the signature operation, the standards behind it, and the lifecycle controls that keep private keys trustworthy.
Core rules
- Signing. Hash the message, then apply the private key to the digest (RSA) or use the digest inside a signature scheme (DSA/ECDSA). Verification recomputes the digest and checks it with the public key. Properties delivered: integrity, authenticity, non-repudiation.
- Schemes. RSA-PSS is the modern RSA padding for signatures (PKCS#1 v1.5 remains widely deployed). DSA and ECDSA come from FIPS 186; EdDSA offers deterministic, misuse-resistant signing.
- The k value matters. ECDSA/DSA require a unique, unpredictable nonce per signature. Reusing k across two signatures exposes the private key algebraically - the PlayStation 3 failure is the classic case.
- Key lifecycle. Generation in a validated module, secure distribution, activation, rotation before cryptoperiod expiry, archival, and destruction. NIST SP 800-57 frames these phases.
- Escrow and recovery. Escrow lets a third party recover a key (useful for confidentiality, fatal for non-repudiation). Split-knowledge schemes require M of N holders.
Worked example
An engineer signs a software release. She hashes the build with SHA-256, signs the digest with an ECDSA P-256 key held in an HSM, and publishes the signature beside the artefact. A user re-hashes the download, verifies the signature with the vendor's public key, and confirms the build was not altered. Because the private key never left the HSM, the vendor cannot credibly deny signing.
Traps
- Signing by "encrypting with the private key" and believing raw RSA is secure - padding is required.
- Claiming non-repudiation when keys are escrowed to the organisation.
- Reusing an ECDSA nonce.
- Treating revocation as retroactive: signatures made before compromise may still stand.
- Confusing encryption of a document with signing it.