Simple crypto
1. ARKLN{EekwcxiXxgprwbwlMsariMmzorcklKaxzej}
2. Go to https://www.dcode.fr/vigenere-cipher
3. type the encrypted message and known key (thisisasecretkey)
4. HKCTF{MessageEncryptedUsingVigenereCipher}


Simple Crypto 2
1. wzr%uL>6DD28606?4CJAE650FD:?80C@EcfN
2. noticed that wzr%uL maps to HKCTF (shifted 47bit)
3. Go to https://www.dcode.fr/rot-47-cipher and decrypt
4. HKCTF{message_encrypted_using_rot47}

Encrypted message
1. Go https://www.guballa.de/vigenere-solver
2. result is I can't beliele this is such an uasy problem in HACTF. It's almost ai if I solved a prorlem already! Okao. Here's the flag: HACTF{substitutien_ciphers_are_toe_easy_cipher_to_selve}
3. manually change the error bit
4. HKCTF{substitution_ciphers_are_too_easy_cipher_to_solve}

RSA 1
c: 1267512865767235284579628962679981517661651162006242932307504395229069157817213250932198187
n: 1441341319160614646189772775947458689224268167522603816085381857689639120119532883988453931
e: 65537

1a. go to http://factordb.com and check found that it is not factorized (now is available because i reported the factor)
1b. go to https://www.alpertron.com.ar/ECM.HTM and wait for the factorization
2. Get 1441341319160614646189772775947458689224268167522603816085381857689639120119532883988453931 <91> = 1334556412516773775580925026039868529270309<43> · 1080015281214272920245486385411085443503979257359<49>

3a. find d, the private key:
3b. Go to https://www.cryptool.org/en/cto-highlights/rsa-step-by-step
3c. Enter p,q and e. Get d=
1114748423704076698222698821954755917736704851677922639992477593197483107298411669566550537
4. decrypt the cipher
m= c^d mod n
m= 498954823561354940679082867578948974184995796436288108951260273636356989

change m to hex, then hex to ascii

using https://www.rapidtables.com/convert/number/decimal-to-hex.html?x=16 & https://www.rapidtables.com/convert/number/hex-to-ascii.html

hex(m)= 484B4354467B7273405F6372797074305F7573335F31303830313139337D
ascii(hex(m))= HKCTF{rs@_crypt0_us3_10801193}

RSA 2:
n:123011419727242929605859484379712787224119427868122185028414426038747211967728126687082223191959583800124030930442533997557997625495312608148837196827665382944411142837816321635710707548473070155845149369804586838545770200114861944189730393681376431146673015470955622822572616394605670811484761576817673309001

c:52127047932811110668013864133349571790867805534855543297048613670965313472772560575182413718393507843165300390227330555558736664829280206143700900677133430280286900207970264306716622456672164011284966405111997950821662174402552515880677459669874493401227056820044471828999856535032257525489909965248714298022

e:20370827750732677953101194500404700852089173301382884082478321647291201786559551992537091540692087873762090234342322985115231826746732803397154738501467143140654322623572067396058860233911575260540468106570172920030157403146163826401598627492164762337650828047117823273414399019740348998847585859920303350373

1. Get Rsactftool
2. ./RsaCtfTool.py --createpub -n 123011419727242929605859484379712787224119427868122185028414426038747211967728126687082223191959583800124030930442533997557997625495312608148837196827665382944411142837816321635710707548473070155845149369804586838545770200114861944189730393681376431146673015470955622822572616394605670811484761576817673309001 -e 20370827750732677953101194500404700852089173301382884082478321647291201786559551992537091540692087873762090234342322985115231826746732803397154738501467143140654322623572067396058860233911575260540468106570172920030157403146163826401598627492164762337650828047117823273414399019740348998847585859920303350373 > key.pub
3. ./RsaCtfTool.py --publickey key.pub --private > key.priv
4. c=52127047932811110668013864133349571790867805534855543297048613670965313472772560575182413718393507843165300390227330555558736664829280206143700900677133430280286900207970264306716622456672164011284966405111997950821662174402552515880677459669874493401227056820044471828999856535032257525489909965248714298022
echo "obase=16; $c" | BC_LINE_LENGTH=0 bc | awk '{ print (length($0) % 2 == 0) ? $0 : 0$0; }' | xxd -p -r > c.bin
xxd c.bin
5.
./RsaCtfTool.py --publickey key.pub --private --uncipherfile c.bin

6. See https://imgur.com/a/HEllo6T
The flag is HKCTF{w@tch_y0ur_rs@_c@r3fully_87027243203}

Buffer overflow
1. ./vuln aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
2. HKCTF{S1mpl13_Buff3r_0v3rfl0w_016dc68c}

Admin Access
1. use ac:user & pw: 1234 to login
2. get auth cookie and put into base64 decode
3. See https://imgur.com/64cEvKp
4. Change admin = FALSE7 to admin = True
5. encode base64 and edit the cookie
6. (due to the site http://47.56.165.70:32954/login.php is not accessible after ctf ended, flag not retrieved)

Good Image
1. Download the file and open it with notepad++
2. Search for HKCTF
3. HKCTF{Open2019_b07209aJ}

Simple Forensics
1. Download the file and open it with notepad++
2. Search for HKCTF
3. HKCTF{grep_and_you_will_find_me}

Simple Forensics 2
1. Download the file and open it with notepad++
2. Search for HKCTF
3. HKCTF{sTrIngS_sAVeS_Time_4c987dcwxq}
4. sorry i know the proper way should be[ strings file | grep HKCTF ]

Welcome
1. open the image
2. HKCTF{Welcome}

Welcome again
1. 484b4354467b57656c636f6d65215f3235363037617d
2. hex to ascii (https://www.rapidtables.com/convert/number/hex-to-ascii.html)
3. HKCTF{Welcome!_25607a}

Connect
1. nc 47.56.165.70 22222
2. (due to the site 47.56.165.70:22222 is not accessible after ctf ended, flag not retrieved)