Quote from: Shevek on June 20, 2013, 03:24:44 PM

Quote from: oleganza on June 20, 2013, 11:37:47 AM

How about using ECC point multiplication and AES:

1. Get recipient's public key R (R = r*G, r is private key)
2. For each message, generate unique keypair S,s: S = s*G.
3. Create a shared secret: K = R*s
4. Compute 256-bit encryption key out of that shared secret: key = SHA256(SHA256(K))
5. Encrypt the message with that key and send the message together with unique pubkey S.
6. Recipient gets the message and computes key using his private key r and S: key = SHA256(SHA256(r*S))
7. Recipient's key turns out to be the same because r*S = R*s = r*s*G.

I like this scheme more because it allows to efficiently encrypt messages of arbitrary sizes.

It's OK but:

1) There is no need of doubling SHA256 to obtain the symmetric key. SHA256^2 is a bitcoinish thing that does not improve the security of SHA256. In fact, there is no need of SHA256. Take "Kx" as symmetric key and it's ok.
2) It is better to use "S" as the shared secret and send "K" to the recipient. She can get the secret "S" as S = r^(-1) * K. This way, you can send the same encrypted message to many recipients: just create the K1, K2, K3, etc, pubkeys for de recipient's keys R1, R2, R3, etc

2. Indeed!
1. Wouldn't you lose one bit of entropy? (As only half of x values between 0 and n-1 leads to a pair of points)
Using Kx+(Ky&1) should make it a bit better right?