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Re: Photuris questions
> > If both AH and ESP are indicated by one SPI, then how are the keys for
> > the two modes assigned? Are there two separate keys?
> >
> The same key bitstuff is reused. There are two separate session-keys
> only when two separate SPIs. SPI => session-key, remember?
Re-using the same key with two different algorithms (or the same algorithm
in two different modes) is a really bad thing to do:
- If the same key is used in two modes, there often attacks when one mode
is used to break the other (particularly if one mode is a confidentiality
mode and the other is an authentication mode),
- If the same key is used with two algorithms, one algorithm can be used to
break the other. Suppose I'm using RC4-40 for ESP confidentiality (which, for
the sake of argument, imagine I don't care too much about) and a DES MAC
for AH integrity (which I do care about). An attacker can break the RC4-40
by exhaustive search, and then use their new-found knowlege of 40 bits of
the DES key to break the DES key by exhaustive search too. This would be bad.
If multiple use of the same key is to be permitted at all, the specification
should contain some warning about why you shouldn;t do it.
Mike
References: