[cryptography] RSA Moduli (NetLock Minositett Kozjegyzoi Certificate)

Adam Back adam at cypherspace.org
Fri Mar 23 10:05:48 EDT 2012

I presume its implied (too much tongue in cheek stuff for my literal brain
to interpret) but a self-signed CA cert is a serious thing - thats a sub-CA
cert typically.  How that came to be signed with a bizarre though legal e
parameter is scary - what library or who wrote the code etc.

Usual reason to use primes of form 2^n+1 and co-prime to carmichael(n) is
low hamming weight.

Other than that typically p, q are strong primes P=(p-1)/2, Q=(q-1)/2 also
prime, so any odd (non-even) e is pretty much guaranteed to work as carm(n)
= 2*P*Q where P = (p-1)/2, Q = (q-1)/2.  Or if using Lim-Lee primes, at
least B-smooth, meaning P=P1*P2*...Pn where |Pi|>B for all Pi.  And e would
typically be smaller than B-bits anyway for performance.

(If e is not-coprime to carm(n) then d doesnt exist, as modinv(a,x) requires
gcd(a,x)==1, so its not like it will be insecure, it just wont work!)

e should also not be too small or other attacks kick in.

Dan Boneh has a good summary of RSA limitations:



ps carm(n) = phi(n)/2 = (p-1)*(q-1)/2.

On Fri, Mar 23, 2012 at 06:51:51AM -0700, Jon Callas wrote:
>Hash: SHA1
>On Mar 23, 2012, at 6:39 AM, Peter Gutmann wrote:
>> Jon Callas <jon at callas.org> writes:
>>> On Mar 23, 2012, at 6:03 AM, Peter Gutmann wrote:
>>>> Jeffrey Walton <noloader at gmail.com> writes:
>>>>> Is there any benefit to using an exponent that factors? I always thought low
>>>>> hamming weights and primality were the desired attributes for public
>>>>> exponents. And I'm not sure about primality.
>>>> Seeing a CA put a key like this in a cert is a bit like walking down the
>>>> street and noticing someone coming towards you wearing their underpants on
>>>> their head, there's nothing inherently bad about this but you do tend to want
>>>> to cross the street to make sure that you avoid them.
>>> But Peter, CAs don't *precisely* put keys into certs. CAs certify a key that
>>> the key creator wants to have in their cert.
>> This is a self-signed cert from the CA, so the key creator was the CA.
>So it's like issuing yourself an Artistic License card with a color printer and laminator. :-) Good for lots of laughs.
>	Jon
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