Score:-1

Crypto

How to decrypt a file using RSA and just a public.pem file?

pee2pee

3/27/23, 12:18 AM

I have an encoded file and a public.pem file. Is it possible to decode the file using the public.pem file or do I have to start looking at private keys?

I tried https://github.com/Ganapati/RsaCtfTool with no luck. The public key (pem) is as below

288

0 + 0

rsa

openssl

decryption

kelalaka

3/27/23, 12:18 PM

What is the origin of this Q?

Score:3

Crypto

MostlyResults

3/27/23, 8:10 PM

Adding on to the above, the modulus and public exponent can be extracted from the public.pem. The public exponent e is 10001.

It turns out that this is a constructed modulus which is very weak and was not generated using recommended security guidelines.

The 2048 bit, 617 decimal digit modulus N=pq can be factored immediately because it is a square. That's right p=q and N = p^2.

phi(N) = p(p-1)

The method to find d the private exponent when e, p and q are known has been shown many times on this forum. Then the file can be decrypted.

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dave_thompson_085

3/28/23, 5:02 AM

Good catch on the square, but phi(p^2) is NOT (p-1)x(p-1). See https://en.wikipedia.org/wiki/Euler%27s_totient_function#Value_of_phi_for_a_prime_power_argument .

MostlyResults

3/28/23, 3:15 PM

Corrected. Thanks. The error could have led someone down a time consuming path.

pee2pee

4/3/23, 9:23 PM

Any advice on a good question/answer that would highlight how to find the numbers @MostlyResults

MostlyResults

4/5/23, 2:30 PM

If your ctf question insists that a file or text can be decrypted from the public.pem even though p and q are the same, look at the answer to the second question in this link: https://crypto.stackexchange.com/questions/52240/why-does-this-rsa-example-break-when-1-setting-p-or-q-as-non-prime-or-2-sett

Score:2

Crypto

Ilmari Karonen

3/27/23, 2:05 AM

Is it possible to decode the file using the public.pem file

No.

or do I have to start looking at private keys?

Yes.

RSA is an asymmetric encryption algorithm. That means that its keys come in pairs, containing a public key and a private key, and that data encrypted with the public key can only be decrypted with the private key.

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fgrieu

3/27/23, 10:46 AM

[Dr. Spock](https://en.wikipedia.org/wiki/Spock) would qualify _"data encrypted with the public key can only be decrypted with the private key"_ as technically incorrect. Poorly thought RSA padding (as common in CTFs and introductions to RSA, including textbook RSA) may allow decryption without private key. So can access to a decryption device in Bleichenbacher's attack on RSAES-PKCS1-v1_5. And in all RSA encryption modes of PKCS#1, any of multiple private keys allow decryption.

pee2pee

3/29/23, 11:07 AM

Thanks - it as indeed for a CTF and was possible

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: How to decrypt a file using RSA and just a public.pem file?

TH: จะถอดรหัสไฟล์โดยใช้ RSA และไฟล์ public.pem ได้อย่างไร

RO: Cum să decriptați un fișier folosind RSA și doar un fișier public.pem?

RU: Как расшифровать файл с помощью RSA и просто файла public.pem?

VI: Làm cách nào để giải mã một tệp bằng RSA và chỉ một tệp public.pem?

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