Earlier quoted context omitted.
> Not on the analogue phone. And not on a consumer cell phone, either. > But you can easily get a TLS-enabled or ZRTP-enabled hardware or software VoIP phone. In theory, arbitrarily strong/usable encryption products can be marketed within the USA. In practice, from this list: A) Usable by non-experts (requires no software fiddling, hardware mix-and-match, or other wastes of time) B) Based on uncrackable/de-facto uncr…
How would a phone use an OTP? Would you send the key by carrier pidgeon to the guy every time you wanted to make a phone call? There's absolutely no way to get security without some sort of verification by the user. At the very least, you need someone to verify that the keys are correct "can you read me your phone's security serial?". It shouldn't be too hard to create affordable, usable phones based on PKC. The prob…
You can use a symmetric cipher (AES is the current standard), and exchange keys via Diffie-Hellman key exchange, which is a method of securely generating a shared private cipher key over a public channel.
As I understand it, the primary advantage of public key cryptography is that it makes encryption and decryption asymmetric processes - i.e. everyone can encrypt something using your public key, but only you can decrypt it, because only you know your private key. In the phone case, the asymmetry would actually be annoying, because both parties want to send encrypted messages to each other, so you'd have to deal with two key pairs for each phone conversation.
See http://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exch... and http://en.wikipedia.org/wiki/Advanced_Encryption_Standard .