[EXTERNAL] - Re: Question about TLS 1.3 and openssl -cipher aNULL option

[Yury Mazin]

Thank you Benjamin,

According to OpenSSL , aNULL stands for no-authentication.
NULL-ciphers that you mention would be part of eNULL group, that offer no encryption.
Does it mean that all 3 default protocols of TLS 1.3 offer no authentication (because they are listed under command
openssl ciphers -v -s aNULL
TLS_AES_256_GCM_SHA384  TLSv1.3 Kx=any      Au=any  Enc=AESGCM(256) Mac=AEAD
TLS_CHACHA20_POLY1305_SHA256 TLSv1.3 Kx=any      Au=any  Enc=CHACHA20/POLY1305(256) Mac=AEAD
TLS_AES_128_GCM_SHA256  TLSv1.3 Kx=any      Au=any  Enc=AESGCM(128) Mac=AEAD

Thank you,

Yury


________________________________
From: Benjamin Kaduk <bkaduk at akamai.com>
Sent: Thursday, September 3, 2020 5:12 PM
To: Yury Mazin <ymazin at opentext.com>
Cc: openssl-users at openssl.org <openssl-users at openssl.org>
Subject: [EXTERNAL] - Re: Question about TLS 1.3 and openssl -cipher aNULL option

On Thu, Sep 03, 2020 at 11:45:28PM +0000, Yury Mazin via openssl-users wrote:
> Hello,
>
> We have a server was originaly using OpenSSL 1.0.2h.
> Server is configured to use SSL ciphers as following
> ALL:!aNULL:!ADH:!EDH:!eNULL:!EXPORT
> When openssl client tries to connect to this server with command
> openssl s_client -connect localhost:8101-cipher aNULL
> it fails, because any aNULL ciphers are not available per server configuration.
> We have now upgraded server to use OpenSSL 1.1.1f.
> The current behavior is this:  client can connect using the same command
> openssl s_client -connect localhost:8101 -cipher aNULL
> or
> openssl s_client -tls1_3 -connect localhost:8101 -cipher aNULL
>
> while the same connect attempt using TLS1.2 protocol would still fail
>
> openssl s_client -tls1_2 -connect localhost:8001-cipher aNULL
>
> Would the fact that I can connect to the server using TLS 1.3 using the following command (specifically, using -cipher aNULL, while server is configured to exclude all aNULL cipher suites) considered a security violation?
>
> openssl s_client -tls1_3 -connect localhost:8001 -cipher aNULL
>
> Also, if this a security violation, how this can be addressed in the server configuration?
> Lastly, if this is not a security violation, please explain.

It is not a security violation, because you are using TLS 1.3 ciphers,
and there are not any NULL-encryption TLS 1.3 ciphers.

Configuration of TLS 1.3 ciphers and ciphers for previous versions
of TLS are separate (since, at a protocol level, they serve different roles).
See the documentation for s_client/s_server -ciphersuites for more
information about TLS 1.3 ciphers.

-Ben
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