Re: [TLS] Question for TLS1.2 AEAD cipher

Hi Eric,

Thank you very much for your quick response.

>> 2. The additional data for encryption and decryption should be the 
same. 
>> While we find that the additional data for encryption contains seq_num 
and 
>> the additional data for decryption does not contain seq_num.

>I don't see that in the text.

It is on page 23, draft-ietf-tls-rfc4346-bis-03.txt or 
draft-ietf-tls-rfc4346-bis-04.txt.

 additional_data = seq_num + TLSCompressed.type +
                        TLSCompressed.version + TLSCompressed.length;
 AEADEncrypted = AEAD-Encrypt(key, IV, plaintext,
                      additional_data)

 TLSCompressed.fragment = AEAD-Decrypt(write_key, IV, AEADEncrypted,
                   TLSCiphertext.type + TLSCiphertext.version +
                   TLSCiphertext.length);

>> 3. IV length and Tag length. Is there standard IV length and Tag 
length? 
>> If not, when the receiver get the message, how does he know which part 
is 
>> IV and which part is aead_output? Also, the receiver needs to know the 
tag 
>> length for decryption.

>It's defined by the cipher suite.

For example, cipher suite "TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256", GCM 
allows Tag length from 8 to 16 bytes, IV length from 1 to 2^64 bytes(the 
recommended IV length is 12 bytes). What Tag length and IV length should 
we choose? The recommended length?

Thank you.

Dana