Why is IDEA so secure"

Why is IDEA so secure"

Post by Thorsten Manegol » Sun, 23 Feb 1997 04:00:00



Hi All!
I'm just wondering. But why is an IDEA key of 128 bit more secure than
an RSA key of the same length? Can't I just use brute force on an 128
bit key?
Sure 128bit mean 2^128 combinations (right?) which is a lot. And further
I could only crack one single message of PGP with that since it's used
only once and does not give me the RSA secret key.

TIA
Thorsten

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Why is IDEA so secure"

Post by burd.. » Tue, 25 Feb 1997 04:00:00



Quote:> Hi All!
> I'm just wondering. But why is an IDEA key of 128 bit more secure than
> an RSA key of the same length? Can't I just use brute force on an 128
> bit key?
> Sure 128bit mean 2^128 combinations (right?) which is a lot. And further
> I could only crack one single message of PGP with that since it's used
> only once and does not give me the RSA secret key.

If you have some spare time left, why not buy a book on the subject ? "applied
cryptography" would be a good start ...

 
 
 

Why is IDEA so secure"

Post by Ed Pu » Tue, 25 Feb 1997 04:00:00


Hi.


> Hi All!
> I'm just wondering. But why is an IDEA key of 128 bit more secure than
> an RSA key of the same length? Can't I just use brute force on an 128
> bit key?
> Sure 128bit mean 2^128 combinations (right?) which is a lot. And further
> I could only crack one single message of PGP with that since it's used
> only once and does not give me the RSA secret key.

The thing you are missing, Thorsten, is that the only known way to
"break" an IDEA cipher (AFAIK) is to brute force the key (i.e. try
every key in the keyspace until you can decipher the message), BUT,
to break RSA with a key of the same length (128-bits) all you need
to do is factor the 128-bit number into its two 64-bit prime factors.
This is very do-able within a short amount of time.  (Probably minutes
or seconds with a fast computer?  Others will know exactly how long
it would take.)

So, a 128-bit IDEA key is plenty big enough to safeguard IDEA encrypted
messages.  But to ensure that it is "infeasible" to factor your key
modulus within, say, the next 500 years or so, you will want a larger
RSA key, say 1024 or 2048 bits.  (I am "hand-waving" with the "500
years or so" figure.  Again, others may probably know how long it
takes to factor 1024 or 2048 bit RSA key moduli.)

Best regards,
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Why is IDEA so secure"

Post by Galactu » Wed, 26 Feb 1997 04:00:00


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> I'm just wondering. But why is an IDEA key of 128 bit more secure than
> an RSA key of the same length?

Because factoring a 128 bits number (16 digits) is quite easy. There
is no known way to 'crack' a 128 bits IDEA key, other than trying
out all possible values.

Quote:> Can't I just use brute force on an 128
> bit key?

Goodness no. 2^128 combinations is a _lot_ of keys to try. Assuming
that we have a chip which can try one billion (10^9) keys per second,
and we have one billion such chips (this is WAAAY beyond our reach)
it takes 3.4*10^20 seconds, or 10^13 years to try them all.

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Why is IDEA so secure"

Post by ejnb.. » Thu, 27 Feb 1997 04:00:00


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writes:

Quote:>>I'm just wondering. But why is an IDEA key of 128 bit more

secure than an RSA key of the same length? Can't I just use brute
force on an 128 bit key?<<

See:
http://axion.physics.ubc.ca/pgp-attack.html
which, I think, will give you all the answers you need.

Regards from UK,       Noel
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| ftp://users.aol.com/EJNBell/EJNBell.asc KeyID: 1024/20015B9D |
| Fingerprint: 9E A7 36 69 3A 66 49 CB 74 FA 6C 5F 28 37 9A 76 |
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