Consider the following statements with respect to digital signatures:
S1: Digital signature ensures authenticity of the sender but not integrity of the message.
S2: A single secret key can be used by the sender to sign multiple documents for multiple receiver.
S3: Using a public-key system, the signer signs using his private key and the verifier verifies using his public key.
S4: Both RSA cryptosystem and RSA digital signature scheme uses same set of keys.
Which of the following is true?