Digital signatures — SY0-701

WHAT IT IS

A digital signature is the result of a cryptographic transformation of data that, when properly implemented, provides a mechanism to verify origin authenticity and data integrity and to enforce signatory non-repudiation. (FIPS 186-5, via NIST CSRC Glossary)

The transformation uses asymmetric cryptography: the signer uses a private key to compute the signature, and any recipient uses the corresponding public key to verify it.

Mental model

Think of a wax seal on an envelope — but one that is mathematically impossible to forge and that also proves the letter was not altered after sealing.

• Only the owner of the private key can create the seal (origin authentication).
• Anyone with the public key can check whether the seal is intact (integrity verification).
• Because only one entity holds the private key, that entity cannot later deny having sealed the letter (non-repudiation).

When to use it

The exam frequently tests digital signatures against message authentication codes (MACs). Both protect integrity, but they differ critically on who can verify and whether repudiation is possible.

Property	Digital Signature	Message Authentication Code (MAC)
Key type used	Asymmetric (private key signs, public key verifies)	Symmetric (same shared key on both sides)
Who can verify	Anyone with the public key	Only parties that share the secret key
Data integrity	Yes	Yes
Origin authentication	Yes	Yes
Non-repudiation	Yes	No — either party could have produced the MAC

Source for MAC properties: NIST SP 800-63-4, which explicitly states that MACs "provide authenticity and integrity protection but not non-repudiation protection."

Sign / Verify sequence

COMMON MISCONCEPTION

"The private key encrypts the message for confidentiality."

This conflates two distinct operations. In a digital signature scheme, the private key is used to compute a signature over a hash of the message — not to encrypt the message contents for secrecy. NIST SP 800-57 Part 1 Rev. 5 defines the private key's signing role separately from its use in decryption. A signed message is not automatically confidential; anyone can read it. Confidentiality requires a separate encryption step.

A related trap: candidates sometimes assume that because the public key verifies the signature, the public key "decrypts" in the same sense it would in an encryption scenario. The operation is verification against a hash output, not message decryption.

How it shows up on the exam

The cognitive target is distinguishing the three security properties a digital signature provides — origin authentication, data integrity, and non-repudiation — and recognizing which of those properties a MAC or a symmetric mechanism cannot deliver.

Signal phrases to watch for:

• "…proves the sender cannot deny…" — points toward non-repudiation, a property digital signatures provide and MACs do not
• "…verify the source and confirm the message was not altered…" — both properties together; digital signature is the mechanism
• "…shared secret key…" combined with "…cannot deny…" — a shared key means either party could have produced the token, so non-repudiation is not available

Candidates often confuse signing (private key produces signature) with encrypting for confidentiality (recipient's public key produces ciphertext). Keeping the key roles distinct — private key signs, public key verifies — resolves most scenario questions in this area.

Related concepts

• Public Key Infrastructure — the trust framework that binds public keys to identities, making signature verification meaningful
• Asymmetric Encryption — the underlying key-pair mechanism; digital signatures are one application of it
• Symmetric Encryption — contrast with the shared-key model; understanding the difference clarifies why MACs cannot provide non-repudiation