I'm signing mails with the RSASSA-PSS (p1v2.1) certificate. When I click Details, it shows "Signature Algorithm: RSASSA-PSS". openssl x509 -text shows

    Signature Algorithm: rsassaPss
     Hash Algorithm: sha256
     Mask Algorithm: mgf1 with sha256

certutil -dump shows:

1.2.840.113549.1.1.10 RSASSA-PSS

Now, whem I send my signed emails and click on Details in outlook, it shows "Signature Algorithm: RSA (2048)"

I'm using bouncycastle for signing and when I use "SHA256WithRSAEncryption" it works fine, but when I'm using "SHA256WithRSAAndMGF1" Signature, Outlook tells me "The digital signature on this item is Invalid or Not Trusted." and when I click details, I see "Signature Algorithm: RSA (2048)". Is p#1v2.1 still not supported by outlook/microsoft or am I just crazy?

  • $\begingroup$ Why a program does not accept a signature format is off-topic here; I'm thus thinking of migrating to security-SE. That said, one possibility worth checking would be that the hash algorithm used in MGF1 is not the same on both sides: it's not specified by SHA256WithRSAAndMGF1. I have been hit (in encryption, not signature) by Java libraries where MGF1 is by default understood to use SHA-1 even when the algorithm's name mentions SHA-256, see this. $\endgroup$
    – fgrieu
    Mar 14, 2021 at 8:33
  • $\begingroup$ Note: as of 2020 this apparently wasn't supported, see the conversation here with an (initially helpful) Jenifer from Microsoft trying to fill the gaps, but getting wrong footed by her own engineers. $\endgroup$
    – Maarten Bodewes
    Mar 16, 2021 at 13:17

1 Answer 1


Is p#1v2.1 still not supported by outlook/microsoft or am I just crazy?

PSS seems still not introduced to Microsoft, please see here. It's a long discussion, but in the end it seems that you cannot select PSS as a signature scheme when sending, I presume that also means that PSS signatures are not accepted.

RSA-OAEP based encryption is generally supported earlier than PSS. There are padding oracle attacks against the PKCS#1 v1.5 encryption scheme. The PKCS#1 v1.5 signature scheme is still considered secure, so there is much less need for upgrading it.


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