Subject: [MSEC] RSA Signature padding recommendations

From: Brian Weis <[email-masked]>
Date: Fri, 15 Apr 2005 17:17:26 -0700

When an RSA signature is created, the signature algorithm input 
encapsulates the hash output with a padding method. RFC 3447 (PKCS #1 
v2.1) specifies two methods of padding: EMSA-PKCS1-v1_5 and EMSA-PSS. 
EMSA-PKCS1-v1_5 is the traditional method of padding, but since attacks 
have been found on that method PSS was added to PKCS#1 v2.1.

Due to its improved security properties, new protocols using RSA 
signatures are being given the advice to adopt PSS as a MUST. However, 
there are some complications with using PSS.

1. The base PSS specification has intellectual property claimed on it. 
Whether or not the construction of PSS specified in RFC 3447 is covered 
is not clear. (No IPR disclosure has been filed with the IETF. However, 
the only publicly available statement regarding licensing its use 
applies to IEEE P1363, not the IETF.)

2. Commonly used crypto toolkits and RSA hardware accelerators that I 
have investigated do not typically support PSS padding.

So while PSS padding is a better security method, specifying it as a 
required method will likely not result in those standards being adopted 
until these issues are sorted out.

RFC 3447 suggests that no known attacks are known against the 
EMSA-PKCS-v1_5 encoding method, yet "a gradual transition to EMSA-PSS is 
recommended as a precaution against future developments". It doesn't 
seem to me as if such a transition is yet possible, but I'd be 
interested in other hearing other opinions.

Thanks,
Brian

-- 
Brian Weis
Advanced Security Development, Security Technology Group, Cisco Systems
Telephone: +1 408 526 4796
Email: [email-masked]
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