Decrypting RSA with the key

Zoro Roronoa

12/21/23, 10:29 PM

I have a

1024-bits modulus n,
the key d which is 1022 bit long,
public exponent 65537,
two factor p and q,
and the ciphertext y 1023 bits which is all in numbers.

How can we perform decryption and work out the plaintext? I have tried CrypTool, tried entering the key d, two factor p and q, but it always give me an error saying

Output block size = 127 is too small must be 128

141

1 + 1

Maarten Bodewes

5/21/24, 4:16 PM

The output size is the (minimum) size of the modulus in bytes. So you could enter 128 and then simply ignore the leftmost bit.

Score:1

Crypto

mti2935

12/21/23, 11:13 PM

Being that you have all of the necessary ingredients (p, q, n, d, and e) as integers, you can use rsatool to create a .pem file containing the private key in PEM format.

Then, you can use openssl rsautil -decrypt to decrypt the ciphertext, using the private key in PEM format. See https://www.openssl.org/docs/man3.0/man1/openssl-rsautl.html for more info.

Note that openssl operates on raw bytes. You say that you have ciphertext y 1023 bits which is all in numbers, so you may need to convert these number to bytes for openssl to operate on.

+ 0

Elon Musk

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

EN: Decrypting RSA with the key

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