I'm pretty sure I understand how public/private key cryptography works. Anybody can encrypt a message using a well-known public key, but only the person who holds the private key can decrypt it.
My question concerns securely sending a message to multiple people. Say there are
n public/private key pairs. Is there a way to encrypt a message using the
n public keys such that any one of the private key holders can decrypt the message in its entirety, but nobody else can?
Further, unrelated to public/private key cryptography, is there any scheme where there are
n shared keys such that anybody who knows the
n keys can encrypt a message such that any one of the shared keys can decrypt the message in its entirety, but nobody else can?