43 lines
1.8 KiB
Markdown
43 lines
1.8 KiB
Markdown
# CHALLENGE DecryptThis
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## Writeup
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To decrypt, we first have to understand how the encryption works. The best way to do this would be to try a long string containing one charachter.
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If the input string was long enough, the encrypted version will show a clear pattern. This is a sign that a Vignere-Cipher is probably used. More information about vignere can be found [here](https://www.dcode.fr/vigenere-cipher).
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A second thing we can see is the beginning of the string, here the pattern is broken by _random_ characters, this is the cipher key for Vignere. The last odd thing is the second last character, this is the length of the codeword.
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An example:
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`FFFFFFFFFFFFFFFFFFFF` might be coded to `GAHBICGHIGHIGHIGHIGHIGCH`, where `GHI` is the pattern, `ABC` is the key and `C` (=3) is the key length.
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As the key-word is generated randomly, a string kan be encrypted to multiple strings. The 3 strings in the given file where 3 different encrypted versions of the flag.
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## Decryption Tool
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A (simple) tool could be written to decrypt a string; here is an example of a decryption tool in PHP:
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```php
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function getNumberValue($nr) {
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return chr((($nr + 94) % 94) + 33);
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}
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function decodeChar($char, $offset) {
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return getNumberValue(getLetterValue($char) - getLetterValue($offset));
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}
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function decrypt($code) {
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$len = ord(substr($code, -2, 1)) - 64;
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$code = substr($code, 0, -2) . substr($code, -1);
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$codedMessage = "";
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$codeword = "";
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for ($i = 0; $i < (2 * $len); $i += 2) {
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$codedMessage .= substr($code, $i, 1);
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$codeword .= substr($code, $i + 1, 1);
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}
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$codedMessage .= substr($code, 2 * $len);
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$decrypted = "";
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for ($i = 0; $i < strlen($codedMessage); $i++) {
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$decrypted .= decodeChar(substr($codedMessage, $i, 1), substr($codeword, $i % strlen($codeword), 1));
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}
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return $decrypted;
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}
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```
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