Re: [AVTCORE] I-D Action: draft-ietf-avtcore-aria-srtp-08.txt

[Magnus Westerlund <[e-mail]>]

Hi,

I have reviewed the now split document and have some few comments:

1. Section 4:

    SRTP_ARIA_128_CTR_HMAC_SHA1_80
            cipher:                   ARIA_128_CTR
            cipher_key_length:        128 bits
            cipher_salt_length:       112 bits
            maximum_lifetime:         2^31 packets
            key derivation function:  ARIA_128_CTR_PRF
            auth_function:            HMAC-SHA1
            auth_key_length:          160 bits
            auth_tag_length:          80 bits

What I reacted to is that all of the CTR mode has a maximum_lifetime of 
2^31 packets. Is there a reason for this, or could this in fact use the 
defaults for SRTP, i.e. RTP 2^48 and RTCP 2^31? I do notice that the 
maximum length is a bit inconsistent defined between the key-management 
functions for other SRTP ciphers. I assume this is an old thing, and not 
a result of the split, but I thought it best to bring up. The reason is 
that 2^31 RTP packets are actually not that many, and can force the need 
for rekeying.

I just want to understand if there is a good reason, or if this is 
taking some values, like from DTLS-SRTP for AES, that actually are 
different from the ones in SRTP (RFC3711) for that cipher.

2. Section 4:
    SRTP_ARIA_256_CTR_HMAC_SHA1_32
            cipher:                   ARIA_256_CTR
            cipher_key_length:        128 bits

I assume this should actually say "256 bits"?

3. Remove SRTP_AEAD_ARIA_128_GCM_8

Based on that AES-GCM with 64 bit tags fails to provide the expected 
security, I am of the opinion that the ARIA counter part cipher needs to 
be removed.

Cheers

Magnus Westerlund

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