Re: [TLS] I-D Action: draft-ietf-tls-ecdhe-psk-aead-00.txt

Martin Thomson <> Tue, 08 November 2016 03:50 UTC

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From: Martin Thomson <>
Date: Tue, 08 Nov 2016 14:50:35 +1100
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To: Daniel Migault <>
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Subject: Re: [TLS] I-D Action: draft-ietf-tls-ecdhe-psk-aead-00.txt
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I don't understand this.  I have no problems with a recommendation
(i.e., SHOULD), but you will find that many implementations will not
comply with these requirements when pushed.

On 8 November 2016 at 14:09, Daniel Migault <> wrote:
>    The cipher suites defined in this document are based on the AES-GCM
>    and AES-CCM Authenticated Encryption with Associated Data (AEAD)
>    algorithms AEAD_AES_128_GCM, AEAD_AES_256_GCM, AEAD_AES_128_CCM, and
>    AEAD_AES_256_CCM defined in [RFC5116], AEAD_AES_128_CCM_8 and
>    AEAD_AES_256_CCM_8 defined in [RFC6655].
>    For the AES-128 cipher suites, the TLS Pseudorandom Function (PRF)
>    with SHA-256 as the hash function SHALL be used and Clients and
>    Servers MUST NOT negotiate curves of less than 255 bits.

This is two statements:

1. The PRF hash for these suites is SHA-256.  That's not a
requirement, you just define it, no 2119 language.

2. Using a weak key negotiation means that your key negotiation is the
weak point.  Here, my preference is to avoid stating a requirement,
but that's only because it's very hard to get right.  Otherwise, use a
SHOULD and copy from HTTP/2:

"When choosing these cipher suites, servers SHOULD select a key
exchange that is at least 2048 bits for cipher suites that use
ephemeral finite field Diffie-Hellman (DHE) [TLS12] and 224 bits for
cipher suites that use ephemeral elliptic curve Diffie-Hellman (ECDHE)

Getting into the exact number of bits is a bit self-defeating.  4Q
isn't 255 bits.  25519 isn't either if you count them all.  Both seem
to be more than adequate in strength though.

>    For the AES-256 cipher suites, the TLS PRF with SHA-384 as the hash
>    function SHALL be used and Clients and Servers MUST NOT negotiate
>    curves of less than 384 bits.

As above, but with bigger numbers.

>    TLS enable curve negotiation but not for code point.  This makes
>    restrictions on code points hard to implement.  As a result Endpoints
>    MAY treat negotiation of key sizes smaller than the lower limits as a
>    connection error of type insufficient_security(71) for TLS1.2 and
>    TLS1.3.

Nits: 'enables'; there is a space between "TLS" and the version number.