From: John Mattsson <john.mattsson@ericsson.com>
To: "<tls@ietf.org>" <tls@ietf.org>
Thread-Topic: Deployability of DTLS 1.3, HRR, and MLKEM512X25519
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Date: Mon, 31 Aug 2026 10:38:54 +0000
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Hi,

We frequently conduct interoperability testing using our internal test suit=
e, CipherSnake. We recently expanded the test suite to cover DTLS 1.3 and H=
elloRetryRequest (HRR).

* DTLS 1.3 appears essentially undeployable in its current state. As far as=
 I know, BoringSSL and wolfSSL are currently the only libraries claiming su=
pport for RFC 9147, and in our tests they do not interoperate. I assume we =
will have to wait for RFC 9147bis and subsequent implementation work before=
 DTLS 1.3 can realistically be deployed.

* Relying on HRR for middlebox traversal of large ClientHellos is questiona=
ble. When discussing the need for ML-KEM-512, several people argued that it=
 was unnecessary because HRR could be used instead. However, after testing =
HRR interoperability across 11 TLS libraries, our conclusion is that severa=
l libraries do not interoperate, making reliance on HRR problematic. It is =
therefore good to see that MLKEM512X25519 has recently been registered, alt=
hough future library support remains uncertain. In contrast, support for st=
andalone ML-KEM-512 appears to be good.

Cheers,
John Preu=DF Mattsson

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<html>
<head>
<meta http-equiv=3D"Content-Type" content=3D"text/html; charset=3Diso-8859-=
1">
</head>
<body>
<div style=3D"direction: ltr; font-family: Aptos, Arial, Helvetica, sans-se=
rif; font-size: 12pt; color: rgb(0, 0, 0);">
Hi,</div>
<div style=3D"direction: ltr; font-family: Aptos, Arial, Helvetica, sans-se=
rif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div style=3D"font-family: Aptos, Arial, Helvetica, sans-serif; font-size: =
12pt; color: rgb(0, 0, 0);">
We frequently conduct interoperability testing using our internal test suit=
e, CipherSnake. We recently expanded the test suite to cover DTLS 1.3 and H=
elloRetryRequest (HRR).</div>
<div style=3D"direction: ltr; font-family: Aptos, Arial, Helvetica, sans-se=
rif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div style=3D"font-family: Aptos, Arial, Helvetica, sans-serif; font-size: =
12pt; color: rgb(0, 0, 0);">
* DTLS 1.3 appears essentially undeployable in its current state. As far as=
 I know, BoringSSL and wolfSSL are currently the only libraries claiming su=
pport for RFC 9147, and in our tests they do not interoperate. I assume we =
will have to wait for RFC 9147bis
 and subsequent implementation work before DTLS 1.3 can realistically be de=
ployed.</div>
<div style=3D"direction: ltr; font-family: Aptos, Arial, Helvetica, sans-se=
rif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div style=3D"font-family: Aptos, Arial, Helvetica, sans-serif; font-size: =
12pt; color: rgb(0, 0, 0);">
* Relying on HRR for middlebox traversal of large ClientHellos is questiona=
ble. When discussing the need for ML-KEM-512, several people argued that it=
 was unnecessary because HRR could be used instead. However, after testing =
HRR interoperability across 11 TLS
 libraries, our conclusion is that several libraries do not interoperate, m=
aking reliance on HRR problematic. It is therefore good to see that MLKEM51=
2X25519 has recently been registered, although future library support remai=
ns uncertain. In contrast, support
 for standalone ML-KEM-512 appears to be good.</div>
<div style=3D"direction: ltr; font-family: Aptos, Arial, Helvetica, sans-se=
rif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div style=3D"font-family: Aptos, Arial, Helvetica, sans-serif; font-size: =
12pt; color: rgb(0, 0, 0);">
Cheers,</div>
<div style=3D"font-family: Aptos, Arial, Helvetica, sans-serif; font-size: =
12pt; color: rgb(0, 0, 0);">
John Preu=DF Mattsson</div>
</body>
</html>

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