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sci / sci.crypt / Re: Naive PRNG encryption?

SubjectAuthor
* Naive PRNG encryption?vallor
+- Re: Naive PRNG encryption?Chris M. Thomasson
+- Re: Naive PRNG encryption?Rich
`- Re: Naive PRNG encryption?Phil Carmody

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Subject: Naive PRNG encryption?
From: vallor
Newsgroups: sci.crypt
Organization: A noiseless patient Spider
Date: Sat, 3 Aug 2024 10:41 UTC
Path: eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: vallor@cultnix.org (vallor)
Newsgroups: sci.crypt
Subject: Naive PRNG encryption?
Date: Sat, 3 Aug 2024 10:41:41 -0000 (UTC)
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I saw someone mention this on another 'froup, and I've thought about it
myself. Is it really "uncrackable"?

Take a plaintext and transform each character with the output
of a PRNG, with the seed being the key.

How easy is it to crack the resulting "ciphertext"?

--
-v

Subject: Re: Naive PRNG encryption?
From: Chris M. Thomasson
Newsgroups: sci.crypt
Organization: A noiseless patient Spider
Date: Sat, 3 Aug 2024 18:47 UTC
References: 1
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From: chris.m.thomasson.1@gmail.com (Chris M. Thomasson)
Newsgroups: sci.crypt
Subject: Re: Naive PRNG encryption?
Date: Sat, 3 Aug 2024 11:47:38 -0700
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On 8/3/2024 3:41 AM, vallor wrote:
> I saw someone mention this on another 'froup, and I've thought about it
> myself. Is it really "uncrackable"?
>
> Take a plaintext and transform each character with the output
> of a PRNG, with the seed being the key.
>
> How easy is it to crack the resulting "ciphertext"?
>

Depends on the quality of the PRNG. You would probably want to use a
CSPRNG...

Subject: Re: Naive PRNG encryption?
From: Rich
Newsgroups: sci.crypt
Organization: A noiseless patient Spider
Date: Sat, 3 Aug 2024 20:00 UTC
References: 1
Path: eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: rich@example.invalid (Rich)
Newsgroups: sci.crypt
Subject: Re: Naive PRNG encryption?
Date: Sat, 3 Aug 2024 20:00:36 -0000 (UTC)
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vallor <vallor@cultnix.org> wrote:
> I saw someone mention this on another 'froup, and I've thought about it
> myself. Is it really "uncrackable"?
>
> Take a plaintext and transform each character with the output
> of a PRNG, with the seed being the key.

https://web.pa.msu.edu/reference/pgpdoc1.html

When I was in college in the early seventies, I devised what I
believed was a brilliant encryption scheme. A simple pseudorandom
number stream was added to the plaintext stream to create
ciphertext. This would seemingly thwart any frequency analysis of
the ciphertext, and would be uncrackable even to the most
resourceful Government intelligence agencies. I felt so smug about
my achievement. So cock-sure.

Years later, I discovered this same scheme in several introductory
cryptography texts and tutorial papers. How nice. Other
cryptographers had thought of the same scheme. Unfortunately, the
scheme was presented as a simple homework assignment on how to use
elementary cryptanalytic techniques to trivially crack it. So much for
my brilliant scheme.

Subject: Re: Naive PRNG encryption?
From: Phil Carmody
Newsgroups: sci.crypt
Organization: A noiseless patient Spider
Date: Wed, 21 Aug 2024 17:56 UTC
References: 1
Path: eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: pc+usenet@asdf.org (Phil Carmody)
Newsgroups: sci.crypt
Subject: Re: Naive PRNG encryption?
Date: Wed, 21 Aug 2024 20:56:46 +0300
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vallor <vallor@cultnix.org> writes:
> I saw someone mention this on another 'froup, and I've thought about it
> myself. Is it really "uncrackable"?
>
> Take a plaintext and transform each character with the output
> of a PRNG, with the seed being the key.

That's just a stream cypher.

> How easy is it to crack the resulting "ciphertext"?

If you have more than one encrypted with the same key, probably pretty
easy, depending on the transform. Just "subtract" (the inverse of the
transform) the two streams from each other, and you'll end up with the
difference between the two plain-texts, which will be highly non-uniform.

Phil
--
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