> If the browser's symmetric key is encrypted with icicibank's public key

For Diffie-Helman key exchange, atleast...

Encrypting/"locking" uses the public key.

Unlocking a public-key-encrypted-symmetric-key happens with icicibank's private key, which the sniffer does not have.

Here's another diagram: https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exc...