Tags: crypto
Rating:
**RSA 2 - Crypto - 350**
Enoncé :
```
Can you decrypt these ciphertexts ?
```
[Sources](src/)
__Résolution :__
On récupère les informations contenues dans la clé publique :
```BASH
openssl rsa -in pubkey.pem -pubin -text -modulus
```
```
Public-Key: (7470 bit)
Modulus:
3f:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:
ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:
[...]
00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:
00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:
00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:
00:00:00:01
Exponent: 65537 (0x10001)
Modulus=3FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF[...]0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001
-----BEGIN PUBLIC KEY-----
MIIDxzANBgkqhkiG9w0BAQEFAAOCA7QAMIIDrwKCA6Y/////////////////////
////////////////////////////////////////////////////////////////
[...]
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAABAgMBAAE=
-----END PUBLIC KEY-----
```
On passe le modulo de hexa à int :
```
sage : n=int("mon_modulo_en_hexa",16)
```
La longueur du modulo est trop élevé pour être factorisé directement
On va voir si il a déjà été factorisé sur [factordb](http://factordb.com/)
=> full factorized : on a les deux facteurs p & q
On recréé les clé publique avec rsatool :
```BASH
python rsatool.py -p mon_p -q mon_q -f PEM -o privkey.pem
```
On déchiffre :
```BASH
openssl rsautl -decrypt -inkey privkey.pem -in cipher.txt
```
YUBITSEC{G00D_J0B_BRO_Y0U_MUST_KN0W_H0W_D03S_1T_W0RKS}
By team Beers4Flags
```
________
| |
| #BFF |
|________|
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( | )
]~,"-.-~~[
.=] Beers ([
| ]) 4 ([
'=]) Flags [
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~~----~~
```
