# Mailinglist Archive: opensuse-security (757 mails)

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##### RE: [suse-security] which cipher for ssh2

- From: "Reckhard, Tobias" <tobias.reckhard@xxxxxxxxxxx>
- Date: Fri, 25 Jan 2002 11:30:55 +0100
- Message-id: <96C102324EF9D411A49500306E06C8D1A56CF4@xxxxxxxxxxxxxxxxx>

> Wasn't arguing, was just making sure people understand that

> an 80 bit key, a

> 90 bit key and a 112 bit key are _SIGNIFICANTLY_ different

Agreed, people generally have difficulty grasping the exponential increase

in the difficulty to break a key by increasing its number of bits.

> Dah. But then you can do things like 3des which is usually 2

> keys for an

> effective length of 112, but the attacker has to do 3 crypto

> operations, so

> an attack becomes expensive. Imagine the keyscape of 3pgp

> (yes I know pgp is

> a program and not the algorithm used for crypto, but you get

> the idea ;).

I s'pose you mean the asymmetric part in PGP's operation (you and I and many

others, but some not, know that it employs hybrid cryptography). The

question that remains though, is, can it work the way Triple-DES works.

Triple-DES is only good if the order of operations is encrypt with key 1,

*decrypt* with key2, encrypt with key 1 or 3, depending on how many 56-bit

keys you have. I'm not sure that this applies to other algorithms in a

similar manner. Could be you're just doubling the difficulty by encrypting

again with a key of same length as the first, not exponentiating it.

> 99 times out of 100 with modern crypto it's weak

> passphase/mistake in key

> recovery/creation/etc that does it in, the math is rarely

> wrong, unless it's

> a closed system or amateur system (something to be learned from that I

> think).

Agreed.

Tobias

> an 80 bit key, a

> 90 bit key and a 112 bit key are _SIGNIFICANTLY_ different

Agreed, people generally have difficulty grasping the exponential increase

in the difficulty to break a key by increasing its number of bits.

> Dah. But then you can do things like 3des which is usually 2

> keys for an

> effective length of 112, but the attacker has to do 3 crypto

> operations, so

> an attack becomes expensive. Imagine the keyscape of 3pgp

> (yes I know pgp is

> a program and not the algorithm used for crypto, but you get

> the idea ;).

I s'pose you mean the asymmetric part in PGP's operation (you and I and many

others, but some not, know that it employs hybrid cryptography). The

question that remains though, is, can it work the way Triple-DES works.

Triple-DES is only good if the order of operations is encrypt with key 1,

*decrypt* with key2, encrypt with key 1 or 3, depending on how many 56-bit

keys you have. I'm not sure that this applies to other algorithms in a

similar manner. Could be you're just doubling the difficulty by encrypting

again with a key of same length as the first, not exponentiating it.

> 99 times out of 100 with modern crypto it's weak

> passphase/mistake in key

> recovery/creation/etc that does it in, the math is rarely

> wrong, unless it's

> a closed system or amateur system (something to be learned from that I

> think).

Agreed.

Tobias

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