[Cryptography] A Fun Trick: The Little MAC Attack

[Ray Dillinger]

On 05/10/2015 04:19 AM, Jerry Leichter wrote:

> Most proposals of the day simply used MD5(key || message), a few might go for the "more secure" MD5(key || message || key).  

If I map || onto XOR, then (key || message || key) == message, which
makes the use of the key pointless.  If I map || onto OR, then
(key || message || key) == (key || message), which makes the second
use of key pointless.

Was the above MD5(key || message || key) a typo for

MD5(key || message) || key

?

The above form (even if MD5 were secure which it ain't)
is still brittle as hell in terms of what its security is
applicable to, but it at least isn't obviously broken, and
makes less nonsense than what you typed?

				Bear

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