RedpwnCTF - Crypto - Easycypher

September 5, 2019

RedpwnCTF 2019 – Easycypher - 50

Category : Crypto Description : This is an easycypher? I heard it’s broken

Write-up

When we land on the website, a prompt for password is displayed We can find the source code for js

<script>var _0x29a9=["\x30\x31\x32\x33\x34\x35\x36\x37\x38\x39\x61\x62\x63\x64\x65\x66","","\x63\x68\x61\x72\x41\x74","\x6C\x65\x6E\x67\x74\x68","\x63\x68\x61\x72\x43\x6F\x64\x65\x41\x74","\x57\x68\x61\x74\x20\x69\x73\x20\x74\x68\x65\x20\x70\x61\x73\x73\x77\x6F\x72\x64","\x61\x61\x34\x32\x62\x32\x33\x34\x63\x62\x30\x35\x39\x31\x35\x37\x31\x36\x63\x31\x34\x33\x34\x30\x35\x38\x66\x65\x31\x61\x65\x65\x31\x36\x63\x31\x34\x33\x34\x30\x63\x62\x30\x35\x39\x31\x35\x37\x61\x61\x34\x32\x62\x32\x33\x34","\x73\x75\x62\x6D\x69\x74\x20\x61\x73\x20\x72\x65\x64\x70\x77\x6E\x63\x74\x66\x7B\x50\x41\x53\x53\x57\x4F\x52\x44\x7D","\x3A\x28"];var hex_chr=_0x29a9[0];function rhex(_0x8e5dx3){str= _0x29a9[1];for(j= 0;j<= 3;j++){str+= hex_chr[_0x29a9[2]]((_0x8e5dx3>> (j* 8+ 4))& 0x0F)+ hex_chr[_0x29a9[2]]((_0x8e5dx3>> (j* 8))& 0x0F)};return str}function str2blks_MD5(_0x8e5dx5){nblk= ((_0x8e5dx5[_0x29a9[3]]+ 8)>> 6)+ 1;blks=  new Array(nblk* 16);for(i= 0;i< nblk* 16;i++){blks[i]= 0};for(i= 0;i< _0x8e5dx5[_0x29a9[3]];i++){blks[i>> 2]|= _0x8e5dx5[_0x29a9[4]](i)<< ((i% 4)* 8)};blks[i>> 2]|= 0x80<< ((i% 4)* 8);blks[nblk* 16- 2]= _0x8e5dx5[_0x29a9[3]]* 8;return blks}function add(_0x8e5dx7,_0x8e5dx8){var _0x8e5dx9=(_0x8e5dx7& 0xFFFF)+ (_0x8e5dx8& 0xFFFF);var _0x8e5dxa=(_0x8e5dx7>> 16)+ (_0x8e5dx8>> 16)+ (_0x8e5dx9>> 16);return (_0x8e5dxa<< 16)| (_0x8e5dx9& 0xFFFF)}function rol(_0x8e5dx3,_0x8e5dxc){return (_0x8e5dx3<< _0x8e5dxc)| (_0x8e5dx3>>> (32- _0x8e5dxc))}function cmn(_0x8e5dxe,_0x8e5dxf,_0x8e5dx10,_0x8e5dx7,_0x8e5dx11,_0x8e5dx12){return add(rol(add(add(_0x8e5dxf,_0x8e5dxe),add(_0x8e5dx7,_0x8e5dx12)),_0x8e5dx11),_0x8e5dx10)}function ff(_0x8e5dxf,_0x8e5dx10,_0x8e5dx14,_0x8e5dx15,_0x8e5dx7,_0x8e5dx11,_0x8e5dx12){return cmn((_0x8e5dx10& _0x8e5dx14)| ((~_0x8e5dx10) & _0x8e5dx15),_0x8e5dxf,_0x8e5dx10,_0x8e5dx7,_0x8e5dx11,_0x8e5dx12)}function gg(_0x8e5dxf,_0x8e5dx10,_0x8e5dx14,_0x8e5dx15,_0x8e5dx7,_0x8e5dx11,_0x8e5dx12){return cmn((_0x8e5dx10& _0x8e5dx15)| (_0x8e5dx14& (~_0x8e5dx15)),_0x8e5dxf,_0x8e5dx10,_0x8e5dx7,_0x8e5dx11,_0x8e5dx12)}function hh(_0x8e5dxf,_0x8e5dx10,_0x8e5dx14,_0x8e5dx15,_0x8e5dx7,_0x8e5dx11,_0x8e5dx12){return cmn(_0x8e5dx10^ _0x8e5dx14^ _0x8e5dx15,_0x8e5dxf,_0x8e5dx10,_0x8e5dx7,_0x8e5dx11,_0x8e5dx12)}function ii(_0x8e5dxf,_0x8e5dx10,_0x8e5dx14,_0x8e5dx15,_0x8e5dx7,_0x8e5dx11,_0x8e5dx12){return cmn(_0x8e5dx14^ (_0x8e5dx10| (~_0x8e5dx15)),_0x8e5dxf,_0x8e5dx10,_0x8e5dx7,_0x8e5dx11,_0x8e5dx12)}function calcMD5(_0x8e5dx5){x= str2blks_MD5(_0x8e5dx5);a= 1732584193;b=  -271733879;c=  -1732584194;d= 271733878;for(i= 0;i< x[_0x29a9[3]];i+= 16){olda= a;oldb= b;oldc= c;oldd= d;a= ff(a,b,c,d,x[i+ 0],7,-680876936);d= ff(d,a,b,c,x[i+ 1],12,-389564586);c= ff(c,d,a,b,x[i+ 2],17,606105819);b= ff(b,c,d,a,x[i+ 3],22,-1044525330);a= ff(a,b,c,d,x[i+ 4],7,-176418897);d= ff(d,a,b,c,x[i+ 5],12,1200080426);c= ff(c,d,a,b,x[i+ 6],17,-1473231341);b= ff(b,c,d,a,x[i+ 7],22,-45705983);a= ff(a,b,c,d,x[i+ 8],7,1770035416);d= ff(d,a,b,c,x[i+ 9],12,-1958414417);c= ff(c,d,a,b,x[i+ 10],17,-42063);b= ff(b,c,d,a,x[i+ 11],22,-1990404162);a= ff(a,b,c,d,x[i+ 12],7,1804603682);d= ff(d,a,b,c,x[i+ 13],12,-40341101);c= ff(c,d,a,b,x[i+ 14],17,-1502002290);b= ff(b,c,d,a,x[i+ 15],22,1236535329);a= gg(a,b,c,d,x[i+ 1],5,-165796510);d= gg(d,a,b,c,x[i+ 6],9,-1069501632);c= gg(c,d,a,b,x[i+ 11],14,643717713);b= gg(b,c,d,a,x[i+ 0],20,-373897302);a= gg(a,b,c,d,x[i+ 5],5,-701558691);d= gg(d,a,b,c,x[i+ 10],9,38016083);c= gg(c,d,a,b,x[i+ 15],14,-660478335);b= gg(b,c,d,a,x[i+ 4],20,-405537848);a= gg(a,b,c,d,x[i+ 9],5,568446438);d= gg(d,a,b,c,x[i+ 14],9,-1019803690);c= gg(c,d,a,b,x[i+ 3],14,-187363961);b= gg(b,c,d,a,x[i+ 8],20,1163531501);a= gg(a,b,c,d,x[i+ 13],5,-1444681467);d= gg(d,a,b,c,x[i+ 2],9,-51403784);c= gg(c,d,a,b,x[i+ 7],14,1735328473);b= gg(b,c,d,a,x[i+ 12],20,-1926607734);a= hh(a,b,c,d,x[i+ 5],4,-378558);d= hh(d,a,b,c,x[i+ 8],11,-2022574463);c= hh(c,d,a,b,x[i+ 11],16,1839030562);b= hh(b,c,d,a,x[i+ 14],23,-35309556);a= hh(a,b,c,d,x[i+ 1],4,-1530992060);d= hh(d,a,b,c,x[i+ 4],11,1272893353);c= hh(c,d,a,b,x[i+ 7],16,-155497632);b= hh(b,c,d,a,x[i+ 10],23,-1094730640);a= hh(a,b,c,d,x[i+ 13],4,681279174);d= hh(d,a,b,c,x[i+ 0],11,-358537222);c= hh(c,d,a,b,x[i+ 3],16,-722521979);b= hh(b,c,d,a,x[i+ 6],23,76029189);a= hh(a,b,c,d,x[i+ 9],4,-640364487);d= hh(d,a,b,c,x[i+ 12],11,-421815835);c= hh(c,d,a,b,x[i+ 15],16,530742520);b= hh(b,c,d,a,x[i+ 2],23,-995338651);a= ii(a,b,c,d,x[i+ 0],6,-198630844);d= ii(d,a,b,c,x[i+ 7],10,1126891415);c= ii(c,d,a,b,x[i+ 14],15,-1416354905);b= ii(b,c,d,a,x[i+ 5],21,-57434055);a= ii(a,b,c,d,x[i+ 12],6,1700485571);d= ii(d,a,b,c,x[i+ 3],10,-1894986606);c= ii(c,d,a,b,x[i+ 10],15,-1051523);b= ii(b,c,d,a,x[i+ 1],21,-2054922799);a= ii(a,b,c,d,x[i+ 8],6,1873313359);d= ii(d,a,b,c,x[i+ 15],10,-30611744);c= ii(c,d,a,b,x[i+ 6],15,-1560198380);b= ii(b,c,d,a,x[i+ 13],21,1309151649);a= ii(a,b,c,d,x[i+ 4],6,-145523070);d= ii(d,a,b,c,x[i+ 11],10,-1120210379);c= ii(c,d,a,b,x[i+ 2],15,718787259);b= ii(b,c,d,a,x[i+ 9],21,-343485551);a= add(a,olda);b= add(b,oldb);c= add(c,oldc);d= add(d,oldd)};return rhex(a)+ rhex(b)+ rhex(c)+ rhex(d)+ rhex(c)+ rhex(b)+ rhex(a)}if(calcMD5(prompt(_0x29a9[5]))=== _0x29a9[6]){alert(_0x29a9[7])}else {alert(_0x29a9[8])}</script>

After beautifiying the source code and translating hex code we have this result

var _0x29a9 = [
  "\x30\x31\x32\x33\x34\x35\x36\x37\x38\x39\x61\x62\x63\x64\x65\x66", //0123456789abcdef
  "",
  "\x63\x68\x61\x72\x41\x74", // charAt
  "\x6C\x65\x6E\x67\x74\x68", //length
  "\x63\x68\x61\x72\x43\x6F\x64\x65\x41\x74", //charCodeAt
  "\x57\x68\x61\x74\x20\x69\x73\x20\x74\x68\x65\x20\x70\x61\x73\x73\x77\x6F\x72\x64", //What is the password
  //aa42b234cb05915716c1434058fe1aee16c14340cb059157aa42b234
  "\x61\x61\x34\x32\x62\x32\x33\x34\x63\x62\x30\x35\x39\x31\x35\x37\x31\x36\x63\x31\x34\x33\x34\x30\x35\x38\x66\x65\x31\x61\x65\x65\x31\x36\x63\x31\x34\x33\x34\x30\x63\x62\x30\x35\x39\x31\x35\x37\x61\x61\x34\x32\x62\x32\x33\x34",
  //submitasredpwnctf{PASSWORD}
  "\x73\x75\x62\x6D\x69\x74\x20\x61\x73\x20\x72\x65\x64\x70\x77\x6E\x63\x74\x66\x7B\x50\x41\x53\x53\x57\x4F\x52\x44\x7D",
  "\x3A\x28" //:(
];
var hex_chr = _0x29a9[0];

function rhex(_0x8e5dx3) {
  str = _0x29a9[1];

  for (j = 0; j <= 3; j++) {
      str += hex_chr[_0x29a9[2]]((_0x8e5dx3 >> (j * 8 + 4)) & 0x0F) + hex_chr[_0x29a9[2]]((_0x8e5dx3 >> (j * 8)) & 0x0F)
  };
  return str
}

function str2blks_MD5(_0x8e5dx5) {
  nblk = ((_0x8e5dx5[_0x29a9[3]] + 8) >> 6) + 1;
  blks = new Array(nblk * 16);

  for (i = 0; i < nblk * 16; i++) {
      blks[i] = 0
  };

  for (i = 0; i < _0x8e5dx5[_0x29a9[3]]; i++) {
      blks[i >> 2] |= _0x8e5dx5[_0x29a9[4]](i) << ((i % 4) * 8)
  };
  blks[i >> 2] |= 0x80 << ((i % 4) * 8);
  blks[nblk * 16 - 2] = _0x8e5dx5[_0x29a9[3]] * 8;
  return blks
}

function add(_0x8e5dx7, _0x8e5dx8) {
  var _0x8e5dx9 = (_0x8e5dx7 & 0xFFFF) + (_0x8e5dx8 & 0xFFFF);
  var _0x8e5dxa = (_0x8e5dx7 >> 16) + (_0x8e5dx8 >> 16) + (_0x8e5dx9 >> 16);
  return (_0x8e5dxa << 16) | (_0x8e5dx9 & 0xFFFF)
}

function rol(_0x8e5dx3, _0x8e5dxc) {
  return (_0x8e5dx3 << _0x8e5dxc) | (_0x8e5dx3 >>> (32 - _0x8e5dxc))
}

function cmn(_0x8e5dxe, _0x8e5dxf, _0x8e5dx10, _0x8e5dx7, _0x8e5dx11, _0x8e5dx12) {
  return add(rol(add(add(_0x8e5dxf, _0x8e5dxe), add(_0x8e5dx7, _0x8e5dx12)), _0x8e5dx11), _0x8e5dx10)
}

function ff(_0x8e5dxf, _0x8e5dx10, _0x8e5dx14, _0x8e5dx15, _0x8e5dx7, _0x8e5dx11, _0x8e5dx12) {
  return cmn((_0x8e5dx10 & _0x8e5dx14) | ((~_0x8e5dx10) & _0x8e5dx15), _0x8e5dxf, _0x8e5dx10, _0x8e5dx7, _0x8e5dx11, _0x8e5dx12)
}

function gg(_0x8e5dxf, _0x8e5dx10, _0x8e5dx14, _0x8e5dx15, _0x8e5dx7, _0x8e5dx11, _0x8e5dx12) {
  return cmn((_0x8e5dx10 & _0x8e5dx15) | (_0x8e5dx14 & (~_0x8e5dx15)), _0x8e5dxf, _0x8e5dx10, _0x8e5dx7, _0x8e5dx11, _0x8e5dx12)
}

function hh(_0x8e5dxf, _0x8e5dx10, _0x8e5dx14, _0x8e5dx15, _0x8e5dx7, _0x8e5dx11, _0x8e5dx12) {
  return cmn(_0x8e5dx10 ^ _0x8e5dx14 ^ _0x8e5dx15, _0x8e5dxf, _0x8e5dx10, _0x8e5dx7, _0x8e5dx11, _0x8e5dx12)
}

function ii(_0x8e5dxf, _0x8e5dx10, _0x8e5dx14, _0x8e5dx15, _0x8e5dx7, _0x8e5dx11, _0x8e5dx12) {
  return cmn(_0x8e5dx14 ^ (_0x8e5dx10 | (~_0x8e5dx15)), _0x8e5dxf, _0x8e5dx10, _0x8e5dx7, _0x8e5dx11, _0x8e5dx12)
}

function calcMD5(_0x8e5dx5) {
  x = str2blks_MD5(_0x8e5dx5);
  a = 1732584193;
  b = -271733879;
  c = -1732584194;
  d = 271733878;
  for (i = 0; i < x[_0x29a9[3]]; i += 16) {
      olda = a;
      oldb = b;
      oldc = c;
      oldd = d;
      a = ff(a, b, c, d, x[i + 0], 7, -680876936);
      d = ff(d, a, b, c, x[i + 1], 12, -389564586);
      c = ff(c, d, a, b, x[i + 2], 17, 606105819);
      b = ff(b, c, d, a, x[i + 3], 22, -1044525330);
      a = ff(a, b, c, d, x[i + 4], 7, -176418897);
      d = ff(d, a, b, c, x[i + 5], 12, 1200080426);
      c = ff(c, d, a, b, x[i + 6], 17, -1473231341);
      b = ff(b, c, d, a, x[i + 7], 22, -45705983);
      a = ff(a, b, c, d, x[i + 8], 7, 1770035416);
      d = ff(d, a, b, c, x[i + 9], 12, -1958414417);
      c = ff(c, d, a, b, x[i + 10], 17, -42063);
      b = ff(b, c, d, a, x[i + 11], 22, -1990404162);
      a = ff(a, b, c, d, x[i + 12], 7, 1804603682);
      d = ff(d, a, b, c, x[i + 13], 12, -40341101);
      c = ff(c, d, a, b, x[i + 14], 17, -1502002290);
      b = ff(b, c, d, a, x[i + 15], 22, 1236535329);
      a = gg(a, b, c, d, x[i + 1], 5, -165796510);
      d = gg(d, a, b, c, x[i + 6], 9, -1069501632);
      c = gg(c, d, a, b, x[i + 11], 14, 643717713);
      b = gg(b, c, d, a, x[i + 0], 20, -373897302);
      a = gg(a, b, c, d, x[i + 5], 5, -701558691);
      d = gg(d, a, b, c, x[i + 10], 9, 38016083);
      c = gg(c, d, a, b, x[i + 15], 14, -660478335);
      b = gg(b, c, d, a, x[i + 4], 20, -405537848);
      a = gg(a, b, c, d, x[i + 9], 5, 568446438);
      d = gg(d, a, b, c, x[i + 14], 9, -1019803690);
      c = gg(c, d, a, b, x[i + 3], 14, -187363961);
      b = gg(b, c, d, a, x[i + 8], 20, 1163531501);
      a = gg(a, b, c, d, x[i + 13], 5, -1444681467);
      d = gg(d, a, b, c, x[i + 2], 9, -51403784);
      c = gg(c, d, a, b, x[i + 7], 14, 1735328473);
      b = gg(b, c, d, a, x[i + 12], 20, -1926607734);
      a = hh(a, b, c, d, x[i + 5], 4, -378558);
      d = hh(d, a, b, c, x[i + 8], 11, -2022574463);
      c = hh(c, d, a, b, x[i + 11], 16, 1839030562);
      b = hh(b, c, d, a, x[i + 14], 23, -35309556);
      a = hh(a, b, c, d, x[i + 1], 4, -1530992060);
      d = hh(d, a, b, c, x[i + 4], 11, 1272893353);
      c = hh(c, d, a, b, x[i + 7], 16, -155497632);
      b = hh(b, c, d, a, x[i + 10], 23, -1094730640);
      a = hh(a, b, c, d, x[i + 13], 4, 681279174);
      d = hh(d, a, b, c, x[i + 0], 11, -358537222);
      c = hh(c, d, a, b, x[i + 3], 16, -722521979);
      b = hh(b, c, d, a, x[i + 6], 23, 76029189);
      a = hh(a, b, c, d, x[i + 9], 4, -640364487);
      d = hh(d, a, b, c, x[i + 12], 11, -421815835);
      c = hh(c, d, a, b, x[i + 15], 16, 530742520);
      b = hh(b, c, d, a, x[i + 2], 23, -995338651);
      a = ii(a, b, c, d, x[i + 0], 6, -198630844);
      d = ii(d, a, b, c, x[i + 7], 10, 1126891415);
      c = ii(c, d, a, b, x[i + 14], 15, -1416354905);
      b = ii(b, c, d, a, x[i + 5], 21, -57434055);
      a = ii(a, b, c, d, x[i + 12], 6, 1700485571);
      d = ii(d, a, b, c, x[i + 3], 10, -1894986606);
      c = ii(c, d, a, b, x[i + 10], 15, -1051523);
      b = ii(b, c, d, a, x[i + 1], 21, -2054922799);
      a = ii(a, b, c, d, x[i + 8], 6, 1873313359);
      d = ii(d, a, b, c, x[i + 15], 10, -30611744);
      c = ii(c, d, a, b, x[i + 6], 15, -1560198380);
      b = ii(b, c, d, a, x[i + 13], 21, 1309151649);
      a = ii(a, b, c, d, x[i + 4], 6, -145523070);
      d = ii(d, a, b, c, x[i + 11], 10, -1120210379);
      c = ii(c, d, a, b, x[i + 2], 15, 718787259);
      b = ii(b, c, d, a, x[i + 9], 21, -343485551);
      a = add(a, olda);
      b = add(b, oldb);
      c = add(c, oldc);
      d = add(d, oldd)
  };
  return rhex(a) + rhex(b) + rhex(c) + rhex(d) + rhex(c) + rhex(b) + rhex(a)
}
if (calcMD5(prompt(_0x29a9[5])) === _0x29a9[6]) {
  alert(_0x29a9[7])
} else {
  alert(_0x29a9[8])
}

The control of the password is located here :
if (calcMD5(prompt(_0x29a9[5])) === _0x29a9[6]) {
  alert(_0x29a9[7])
} else {
  alert(_0x29a9[8])
}

The interesting part: If calcMD5(entered value) === ‘aa42b234cb05915716c1434058fe1aee16c14340cb059157aa42b234’ So display submitasredpwnctf{PASSWORD} Else display :(

calcMD5 don’t change the value of the passed parameter so we can assume that the flag is: aa42b234cb05915716c1434058fe1aee16c14340cb059157aa42b234