3 ms·

>With PAKE the keys used to exchange the data are ephemeral, and so this isn't even a possibility. AFAIK this isn't really true. If the adversary captures the

>With PAKE the keys used to exchange the data are ephemeral, and so this isn't even a possibility. AFAIK this isn't really true. If the adversary captures the initial key exchange plus all the data (ie the full transaction), then later discovers your PSK, they'll be able to decrypt. The only case where this helps you is if they capture some packets out of the middle without the initial handshake. >authenticated encryption It doesn't matter if your ciphertext is authenticated if it's both signed and encrypted, which is what I was getting at. In a normal TLS-like encrypted conversation, yes AEAD is very useful. But it's not applicable here.

ta95846893 6y ago
> AFAIK this isn't really true. If the adversary captures the initial key exchange plus all the data (ie the full transaction), then later discovers your PSK, they'll be able to decrypt. The only case where this helps you is if they capture some packets out of the middle without the initial handshake. Someone can correct me if I'm wrong, but I believe the idea behind a PAKE is that the password only authenticates the key exchange and doesn't contribute to it. So if you record all transmitted data you still need to break the key exchange which should have used a bunch of random bytes from both parties that are thrown away after use. The password is only there to prevent MITM, not to derive keys. I believe magic wormhole uses SPAKE2, which has perfect forward secrecy. When using passwords to secure transmitted files it's really important to have forward secrecy otherwise you risk the transmission being recorded and the password being attacked offline which depending on your password strength might lead to trivially decrypting the data.
- bscphil 6y ago
  > Someone can correct me if I'm wrong, but I believe the idea behind a PAKE is that the password only authenticates the key exchange and doesn't contribute to it. That's right. From memory, the passwords are just used to do a DH key exchange. The key is entirely ephemeral. Even if the entire ciphertext is captured, and even if the adversary then gets your password, they can't decrypt. To decrypt you'd have to MITM the key exchange, which would require knowing the password before the file is exchanged.

ta95846893 6y ago

> AFAIK this isn't really true. If the adversary captures the initial key exchange plus all the data (ie the full transaction), then later discovers your PSK, they'll be able to decrypt. The only case where this helps you is if they capture some packets out of the middle without the initial handshake. Someone can correct me if I'm wrong, but I believe the idea behind a PAKE is that the password only authenticates the key exchange and doesn't contribute to it. So if you record all transmitted data you still need to break the key exchange which should have used a bunch of random bytes from both parties that are thrown away after use. The password is only there to prevent MITM, not to derive keys. I believe magic wormhole uses SPAKE2, which has perfect forward secrecy. When using passwords to secure transmitted files it's really important to have forward secrecy otherwise you risk the transmission being recorded and the password being attacked offline which depending on your password strength might lead to trivially decrypting the data.

bscphil 6y ago
> Someone can correct me if I'm wrong, but I believe the idea behind a PAKE is that the password only authenticates the key exchange and doesn't contribute to it. That's right. From memory, the passwords are just used to do a DH key exchange. The key is entirely ephemeral. Even if the entire ciphertext is captured, and even if the adversary then gets your password, they can't decrypt. To decrypt you'd have to MITM the key exchange, which would require knowing the password before the file is exchanged.