Add CyBRICS CTF

skyf0l · Jul 28, 2020 · 6b06e36 · 6b06e36
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diff --git a/CyBRICS_2020/README.md b/CyBRICS_2020/README.md
@@ -0,0 +1,52 @@
+# Crypto
+
+## Broken Invitation
+
+### [Description as pdf](files/broken_invitation.pdf)
+
+We can attack the RSA cipher with the low exponent attack because `e = 3` and we the message is encrypted 3 times with different keys
+
+We use for it my [rsa solver](https://github.com/skyf0l/RsaSolver):
+
+```
+$ ./rsasolver.py 
+Chose an attack:
+ 1/ Primes known (p, q, e, c)
+ 2/ Factorization (n, e, c)
+ 3/ Low exponent (e = 3, n1, n2, n3, c1, c2, c3)
+> 3
+n1: 924506488821656685683910901697171383575761384058997452768161613244316449994435541406042874502024337501621283644549497446327156438552952982774526792356194523541927862677535193330297876054850415513120023262998063090052673978470859715791539316871
+n2: 88950937117255391223977435698486265468789676087383749025900580476857958577458361251855358598960638495873663408330100969812759959637583297211068274793121379054729169786199319454344007481804946263873110263761707375758247409
+n3: 46120424124283407631877739918717497745499448442081604908717069311339764302716539899549382470988469546914660420190473379187397425725302899111432304753418508501904277711772373006543099077921097373552317823052570252978144835744949941108416471431004677
+c1: 388825822870813587493154615238012547494666151428446904627095554917874019374474234421038941934804209410745453928513883448152675699305596595130706561989245940306390625802518940063853046813376063232724848204735684760377804361178651844505881089386
+c2: 4132099145786478580573701281040504422332184017792293421890701268012883566853254627860193724809808999005233349057847375798626123207766954266507411969802654226242300965967704040276250440511648395550180630597000941240639594
+c3: 43690392479478733802175619151519523453201200942800536494806512990350504964044289998495399805335942227586694852363272883331080188161308470522306485983861114557449204887644890409995598852299488628159224012730372865280540944897915435604154376354144428
+
+-> m(dec): 949557364767986162692541204888383714648410089749288993554212847615599100096583727459
+```
+
+Then, we have the message but it is encoded with the function:
+
+``` python
+def func(str):
+    sum = 0
+    for i in range(len(str)):
+        sum += ord(str[i]) * pow(2, 8 * i)
+    return sum
+```
+
+We decode the message with the function inverse and we get the flag:
+``` python
+>>> def func_reverse(sum):
+...     str = ''
+...     while sum > 0:
+...         str += chr(sum % pow(2, 8))
+...         sum //= pow(2, 8)
+...     return str
+... 
+>>> m = 949557364767986162692541204888383714648410089749288993554212847615599100096583727459
+>>> print func_reverse(m)
+cybrics{h3y_guY5_c0m3_t0_my_p4rtY!}
+```
+
+The flag is: `cybrics{h3y_guY5_c0m3_t0_my_p4rtY!}`
diff --git a/CyBRICS_2020/files/broken_invitation.pdf b/CyBRICS_2020/files/broken_invitation.pdf
diff --git a/README.md b/README.md
@@ -1,6 +1,14 @@
 # CTF
 Some wrrite-ups
 
+## CyBRICS CTF 2020
+
+Sat, 25 July 2020, 12:00 CEST — Sun, 26 July 2020, 12:00 CEST
+
+**Ranked 462th on 786**
+
+[Write-ups](CyBRICS/README.md)
+
 ## RedPwnCTF 2 2020
 
 Sun, 21 June 2020, 22:00 CEST — Thu, 25 June 2020, 22:00 CEST