# Is there parameter recommendation of DSA or RSA?

I know there are recommendations of elliptic curve parameters (such as NIST P-xxx).

Are there recommendations of DSA (or DLP such as Schnorr signature) or RSA parameters?

If not, why?

• Do you ask for keylengths or for concrete parameters that you can "just use" (like with ECC) – SEJPM Mar 17 '17 at 8:00
• @SEJPM I meant concrete parameters. – takita Mar 17 '17 at 8:11
• I can't answer right now but the TL;DR is "yes, but" for DSA and "no, but" for RSA. – SEJPM Mar 17 '17 at 8:13

Yes, but you really want to verify these parameters yourself. DSA uses - as parameters - $(p,q,g)$ such that $q\mid p-1$ and $\operatorname{ord}(g)=q$. Among other things you want to verify that $q$ is prime, that $p$ is prime, that $q\mid p-1$ indeed holds, that $g^q\bmod p=1$ (ie that $g$ has the alleged order), that $p>2^{2047}$ and that $q>2^{255}$.
No, but you can re-use the public RSA exponent safely. To understand what can be shared and what not, we first need to understand the relevant RSA parameters which are $(N,e,d)$ such that $ed\equiv 1\pmod{\lambda(N)}$ where $\lambda(\cdot)$ is the carmichael function. Now note that $(N,e)$ forms the public key and given just $N$ recovering $e$ or $d$ from each other is equally hard (ie as hard as brute-forcing the value). Note however that given a valid $(N,e,d)$ one can recover the factorization of $N$ (by applying the classical part of Shor's algorithm) using which one can break all other key-pairs $(N,e')$ (by the same way you'd normally find $d$ when generating the key), so sharing $N$ is out, but sharing $e$ is fine and commonly done with the value $e=65537=2^{16}+1=F_4$, where $F_4$ is the fourth fermat number.
• @minkyung: in RSA, there are no specific values of $p$ or $q$ recommended, because these numbers must remain secret, even is their product $n=p\cdot q$ is public. It follows that for actual systems, $p$ and $q$ must be large (150 decimal digits is barely adequate). Small $p$ and $q$ like you cite are used in illustrative examples only. – fgrieu Mar 18 '17 at 10:29