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I know it is possible to use RSASSA-PKCS1-v1_5 with client certificates in TLS 1.3. But what about the other way around: using RSASSA-PSS with client certificates in TLS 1.2?

Is it possible to use RSASSA-PSS signed certificates with TLS 1.2 at all? Do I have to fall-back to RSASSA-PKCS1-v1_5 during handshake?

I searched through the RFC5246 but was not able to find anything that answers my questions.

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    $\begingroup$ You didn't miss anything; RFC 5246 did not have PSS. RFC8446 4.2.3 requires 1.3 implementations that negotiate down to 1.2 support PSS; dupe crypto.stackexchange.com/questions/79665/… . For Java in particular, see stackoverflow.com/questions/61676711/… stackoverflow.com/questions/62688276/… $\endgroup$ Aug 11, 2020 at 3:32
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    $\begingroup$ I've not closed it as a dupe as I think an even more detailed answer would be possible. Client auth is more or less an add-on to the handshake. Generally you would need to discuss with the other party to include client auth anyway (due to the trust of specific client certificate chains). I don't see any specific problems by just using PSS in TLS 1.2 in that case... just test if it works on both sides. $\endgroup$
    – Maarten Bodewes
    Aug 11, 2020 at 8:19

1 Answer 1

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If the client announces in the Signature Algorithm Extension that it supports one of the RSA-PSS:

rsa_pss_rsae_sha256(0x0804),
rsa_pss_rsae_sha384(0x0805),
rsa_pss_rsae_sha512(0x0806),

then server may choose this even in TLS1.2. => Yes, it's possible to use RSA-PSS with TLS1.2.

The main difference beside the different preencoding is, that RSA-PSS in TLS1.2 needs the handshake message content instead of the constructed data (containing TLS 1.3, server CertificateVerify).

If the client does not announce these RSA-PSS signature algorithms, the server must not use these.

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