Re: An hash-Encryption algorithm

Kristian Gjøsteen <kristiag+news@xxxxxxxxxxxx> wrote in news:ctbpa3-
7hk1.ln1@xxxxxxxxxxxxxxxxxxxxx:

> Neo <neoscandal@xxxxxxxxx> wrote:
>>Sebastian Gottschalk <seppi@xxxxxxxxx> wrote in news:43ucb4F1p9v0dU1
>>@news.dfncis.de:
>>
>>> Neo wrote:
>>>
>>> [a stateless streamcipher]
>>>
>>> Once I find a known plaintext (Px,Cx), I can decrypt all following
>>> blocks. Insecure as hell. Not to mention stream repeating...
>>>
>>
>>hmm.. how about changing the key generation a little.
>>
>>K(n) = MD5 ( K(n) + K(n-1) + K(n-2) + ... + K(1))
>>
>>that would add feedback. and solve the problms you've brought forward.
>
> That doesn't make sense. You would have to solve for K(n), which
> would be at least hard, probably impossible. If you take away K(n)
> on the right hand side, the attack above still works (given P1,C1).
>
> Second, the stream cipher would slow down as the MD5 step would
> have to process longer and longer strings.
>
> The right way to do this is to use MD5 as a pseudo-random function
> and use either counter mode, output feedback mode or ciphertext
> feedback mode. (You need to make sure that something the adversary
> _does not_ know is included in every application of MD5.)
>

Thank you for your suggestions.

Further R & D:

> Second, the stream cipher would slow down as the MD5 step would
> have to process longer and longer strings.

this is rectified below

-----------------------
K(0) = user entered key + salt (which is added plainly into the
encrypted file header)

K(1) = MD5(K(0))

C1 = K(1) XOR P1

for n = 2 onwards..

K(n) = MD5( K(n-1) + C(n-1)) <----------ciphertxt f/b
C(n) = K(n) XOR P(n)
-----------------------

Provided the attacker has ONE (Cn,Pn) pair.. n = 1 or n != 1 .. what are
the attacks possible.

Sincerely
.

Relevant Pages

  • Re: An hash-Encryption algorithm
    ... the stream cipher would slow down as the MD5 step would ... have to process longer and longer strings. ... The right way to do this is to use MD5 as a pseudo-random function ... output feedback mode or ciphertext ...
    (sci.crypt)
  • Re: Maybe a stupid question regarding input output fed hash algorithms
    ... > stream/chain of MD5 fed loop. ... do MD5 carry such features that it is possible to ... stream cipher from a hash is an CTR mode e.g. ...
    (sci.crypt)
  • Re: Maybe a stupid question regarding input output fed hash algorithms
    ... To create a pseudo random stream from a plain password and MD5 that could ...
    (sci.crypt)
  • Re: MD5 Algorithm
    ... I have down loaded the Forth code for MD5 but I don't ... decrypt asymetric encryptions coming from the server or, ... 2- Client generates a random stream B ...
    (comp.lang.forth)
  • Re: CRC
    ... or does someone have a code snippet I can have? ... you could MD5 the stream. ... includes an Adler32 implementation at ...
    (microsoft.public.dotnet.framework)