CISSP 7.3 - Digital Signatures

This episode of the ISC2 Certified Information Systems Security Professional (CISSP) exam prep series assembles digital signatures from the Domain 3 pieces built so far, showing how hashing and public key cryptography combine to settle disputes about who did what. It covers how signatures are created and checked, what they can and cannot protect, and the constructions and standards that surround them.

Frequently Asked Questions

What two guarantees does a digital signature give you?

First, nonrepudiation: a signed message assures the recipient it genuinely came from the claimed sender, so that sender cannot later cry forgery. Second, integrity: it proves the message was not altered on its journey, whether by a malicious hand or by simple transmission noise. A signature marries public key cryptography with hashing, using each for what it does best.

How do you actually sign and verify a message?

The sender hashes the message down to a digest, then encrypts only that digest with their own private key — that encrypted digest is the signature, and it rides along with the original message. The recipient decrypts the signature using the sender’s public key to recover the digest, hashes the received message themselves, and compares. A match means the message is authentic and untouched, and signing only the small digest keeps the work light.

Why does signing alone not keep a message secret?

Because a signature is about proof, not privacy — it delivers integrity, authentication, and nonrepudiation, but the message text still travels in the open for anyone to read. If you also need confidentiality, you encrypt the whole signed bundle with the recipient’s public key so only their private key can open it. Sign for proof, then encrypt for secrecy, in that order.

Which key do you reach for in each situation?

Anchor it with four simple rules: to encrypt a confidential message for someone, use that recipient’s public key; to decrypt a message sent to you, use your own private key; to sign a message you are sending, use your own private key; and to verify a signature on a message you received, use the sender’s public key. The pattern is consistent — your private key represents you, and the other party’s public key represents them.

What is HMAC, and what standards govern digital signatures?

HMAC is a keyed hash that acts as a partial signature: using a shared secret key, it guarantees integrity in transit, but because the key is shared rather than private it cannot provide nonrepudiation. It sits between a plain unkeyed hash and a full, computationally heavy signature. For federal use, the Digital Signature Standard requires the SHA-3 family for hashing and approves the RSA signature algorithm, the Elliptic Curve Digital Signature Algorithm with built-in nonrepudiation support, and the Edwards Curve Digital Signature Algorithm.

Reference: This article is based on concepts discussed in CISSP 7.3 - Digital Signatures.