EasyCTF_2018: RSA Returns

Category: Cryptography Points: 400 Description:

It's the return of everyone's favorite cryptosystem! Crack it for another flag. Help me decipher file.

Write-up

This challenge reeks of ROCA and even signature checks confirmed it. However, there were no PoCs for ROCA. Instead, I came across NECA. I love open source.

$ ./neca 8729581225262922855975327965201482091621603739716646047811615593091116970536029316597901441665882719811658068143200342813247447157083464709878924070575467
NECA - Not Even Coppersmith's Attack
ROCA weak RSA key attack by Jannis Harder (me@jix.one)

 *** Currently only 512-bit keys are supported ***

 *** OpenMP support enabled ***

N = 8729581225262922855975327965201482091621603739716646047811615593091116970536029316597901441665882719811658068143200342813247447157083464709878924070575467
Factoring...

 [=                       ]  5.40% elapsed: 92s left: 1610.35s total: 1702.36s

Factorization found:
N = 98156620780003086692513114529458599425385116396074158691207247964958060681783 * 88935225722861812850195157838313383408060631426601900565225384215559424240749

Now that we have our factors, just simply decrypt c

$ ./solve.py 
easyctf{kmwmv8wnhnap5old1p}

Therefore, the flag is easyctf{kmwmv8wnhnap5old1p}.

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