Added writeup for crypto-100 of trend-micro-ctf-2015

Author: chaitan94

trend-micro-ctf-2015/crypto/crypto-100/PublicKey.pem

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+-----BEGIN PUBLIC KEY-----
+MDwwDQYJKoZIhvcNAQEBBQADKwAwKAIhALYtzp8lgWNXI9trGI8S8EacvuDLxdrL
+NsNuDJa26nv8AgMBAAE=
+-----END PUBLIC KEY-----

trend-micro-ctf-2015/crypto/crypto-100/chaitan94/README.md

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+[](ctf=trend-micro-ctf-2015)
+[](type=crypto)
+[](tags=rsa)
+[](tools=factordb,rsatool,openssl)
+[](techniques=rsa)
+
+# crypto-100
+
+Problem description
+> You're given an RSA public key and an encrypted message which contains a flag. Get the flag.
+>
+> There's also a hint about "1bit" being wrong in the public key.
+>
+> message: kPmDFLk5b/torG53sThWwEeNm0AIpEQek0rVG3vCttc=
+
+So lets go ahead and get the modulus (n) and exponent (e) from the given [public key](../PublicKey.pem)
+```bash
+$ openssl rsa -pubin -inform PEM -text -noout < PublicKey.pem
+Public-Key: (256 bit)
+Modulus:
+    00:b6:2d:ce:9f:25:81:63:57:23:db:6b:18:8f:12:
+    f0:46:9c:be:e0:cb:c5:da:cb:36:c3:6e:0c:96:b6:
+    ea:7b:fc
+Exponent: 65537 (0x10001)
+```
+n's value in base 10 is 82401872610398250859431855480217685317486932934710222647212042489320711027708
+
+Hmm, n is even? Question says 1 bit is wrong.. it has to be last bit!
+
+So, n must be 82401872610398250859431855480217685317486932934710222647212042489320711027709
+
+Lets try http://www.factordb.com/index.php?query=82401872610398250859431855480217685317486932934710222647212042489320711027709
+
+Perfect! Got p and q.
+
+Let's generate private key and decrypt
+```bash
+$ python2 rsatool.py -f PEM -o private.pem -p 279125332373073513017147096164124452877 -q 295214597363242917440342570226980714417
+$ echo "kPmDFLk5b/torG53sThWwEeNm0AIpEQek0rVG3vCttc=" > flag.enc 
+$ base64 -d flag.enc | openssl rsautl -decrypt -inkey private.pem
+```
+
+Aand we get out flag
+> TMCTF{$@!zbo4+qt9=5}