What is used for digital signatures?

• A. RSA and SHA1
• B. AES and HMAC
• C. MD5 and SHA1
• D. RSA and MD5

Answer: (A)

Digital signatures rely on public-key cryptography to provide nonrepudiation and integrity. The signing process uses a private key to create a signature over a hash of the message, and anyone can verify it using the corresponding public key. RSA is a common public-key algorithm used for creating signing keys, and a hash function like SHA-1 is used to generate the digest that gets signed. So pairing RSA with SHA-1 fits the standard signing workflow: hash the message with SHA-1, sign that hash with the private RSA key, and verify with the public RSA key. The other options don’t fit because AES with HMAC is a symmetric construction producing a MAC, not a nonrepudiable signature; hash functions alone (MD5 or SHA1) don’t create a signature without a private key; and RSA with MD5 uses a weak hash for signing. Therefore, RSA and SHA1 is the best choice for digital signatures.