Crypto/rsa encrypter (#35)

* some docker fixes

* add sample values to solve script

* the code is cleaned up, added hint

---------

Co-authored-by: mudasir <96320211+mud-ali@users.noreply.github.com>
Co-authored-by: ZeviCohen <123010327+ZeviCohen@users.noreply.github.com>

RSAEncrypter/Dockerfile

@@ -0,0 +1,26 @@
+FROM --platform=linux/amd64 python:3.12-slim-bookworm AS build
+
+RUN apt-get update -y \
+    && apt-get install -y gcc wget \
+    && rm -rf /var/lib/apt/lists/* \
+    && wget -O ynetd.c https://raw.githubusercontent.com/johnsonjh/ynetd/master/ynetd.c \
+    && gcc -o ynetd ynetd.c
+
+
+FROM --platform=linux/amd64 python:3.12-slim-bookworm AS deployer
+
+RUN useradd -m -d /home/ctf -u 12345 ctf
+WORKDIR /home/ctf
+
+# copy over ynetd
+COPY --from=build ynetd ynetd 
+RUN chmod +x ynetd && chown -R ctf:ctf /home/ctf 
+
+USER ctf
+
+COPY ./flag.txt .
+COPY ./rsa_encrypter.py .
+
+RUN pip install pycryptodome==3.20.0
+
+CMD ["./ynetd", "-p", "3000", "python3 ./rsa_encrypter.py"]

RSAEncrypter/chall.yaml

@@ -0,0 +1,19 @@
+name: RSAEncrypter
+categories:
+  - crypto
+value: 50
+flag:
+  file: ./flag.txt
+description: |-
+  I made an rsa encrypter to send my messages but it seems to be inconsistent...
+hints:
+  - Search up Chinese Remainder Theorem
+files:
+  - src: ./rsa_encrypter.py
+deploy:
+  nc:
+    build: .
+    expose: 3000/tcp
+authors:
+  - Zevi
+visible: true

RSAEncrypter/flag.txt

@@ -0,0 +1 @@
+bcactf{those_were_some_rather_large_numbersosvhb9wrp8ghed}

RSAEncrypter/rsa_encrypter.py

@@ -0,0 +1,18 @@
+from Crypto.Util.number import getPrime, bytes_to_long, long_to_bytes
+
+
+message = open("./flag.txt").read().encode('utf-8')  
+
+
+def encode():
+    n = getPrime(512)*getPrime(512)
+    ciphertext = pow(bytes_to_long(message), 3, n)
+    return (ciphertext, n)
+
+print("Return format: (ciphertext, modulus)")
+print(encode())
+sent = input("Did you recieve the message? (y/n) ")
+while sent=='n':
+    print(encode())
+    sent = input("How about now? (y/n) ")
+print("Message acknowledged.")

RSAEncrypter/solve.py

@@ -0,0 +1,83 @@
+from Crypto.Util.number import long_to_bytes
+import decimal
+
+decimal.setcontext(decimal.Context(prec=2000))
+
+def find_invpow(x,n):
+    high = 1
+    while high ** n < x:
+        high *= 2
+    low = high//2
+    while low < high:
+        mid = (low + high) // 2
+        if low < mid and mid**n < x:
+            low = mid
+        elif high > mid and mid**n > x:
+            high = mid
+        else:
+            return mid
+    return mid + 1
+
+def inv(a, m) :  
+
+    m0 = m  
+    x0 = 0
+    x1 = 1
+
+    if (m == 1) :  
+        return 0
+    while (a > 1) :
+        q = a // m  
+
+        t = m  
+        m = a % m  
+        a = t  
+
+        t = x0  
+
+        x0 = x1 - q * x0  
+
+        x1 = t  
+    if (x1 < 0) :  
+        x1 = x1 + m0  
+
+    return x1  
+
+
+def findMinX(num, rem, k) :  
+
+    # Compute product of all numbers  
+    prod = 1
+    for i in range(0, k) :  
+        prod = prod * num[i]  
+
+    # Initialize result  
+    result = 0
+
+    # Apply above formula  
+    for i in range(0,k):  
+        pp = prod // num[i]  
+        result = result + rem[i] * inv(pp, num[i]) * pp  
+
+
+    return result % prod
+
+# Driver method  
+num = [0, 0, 0]  
+rem = [0, 0, 0]  
+k = 3
+
+#put the first modulus here
+num[0] = 131908813036851855556617106358045477064620329829877608189512978162690578076089028121598550344376968358225755056849060375981974478849098945087519971168073531374196977105208304879275473249566439319959381378193928601404302247638661196533734272630600814429319138350227900384509506222905308341971979186947425519911
+#put the first ciphertext here
+rem[0] = 70532702661706167890859877997371007276988856318339813821969537524428266326504669477070760836591683492705945202125260479709294887786517662375229546627722684960708679755271540180339082167738729017891786945463574672698602848298989362453178123005156463346835632865575274622403875775603255014304408885861155801362
+#put the second modulus here
+num[1] = 92629677957692981976713776010462559714996285454056037348648884205035030518614357189081035607420478026647254856944016800480801066374296632340705745661215789336711368260385738421062856694928822952505996879927591818261842349862490964843539660382381860106527211408680710937025007270030105811711518602544148466807
+#put the second ciphertext here
+rem[1] = 48536443234700218963086199579612631203694527485268834596458116051989289883008903642474640986981657675640655509694020237620599280411153521629460749338421408302210356447315242535215859587519423805298727801435351524865010037331325707546161484151741379507006735932759075256358512118898280302018847672275146255492
+#put the third modulus here
+num[2] = 85329033449618608108507964089134878347417412447615704674475147015755619420897155323888094050574033636273704978399419445583528034870356814916928919096291096811658637221699939527829805578006713205625374508545817277896171145218594199065522064927749204066539610870985057893886772566680826875939414121567948603891
+#put the third ciphertext here
+rem[2] = 37299978314391364505263260265538910643323769703280854308581764743317519463029288393972173302017308085415377808418701915038849127388281256223923463644961767086034343276193047840970348673014594153669711545209664064933843187033436428100916802277441378681203138945091298389726760222966942362076946776706600528506
+
+print(long_to_bytes(find_invpow(decimal.Decimal(findMinX(num, rem, k)),3)).decode('utf-8'))