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Encoding Used in Computing

This section describes the different encoding used by a computer.

Alphabet Encoding

  • A-Z or a-z corresponds to 1-26 or 0-25

Tools

ASCII encoding

ASCII - Wikipedia

Features

We use ASCII encoding for printable characters, such as the following:

  • 0-9, 49-57

  • A-Z, 65-90

  • a-z, 97-122

ASCII Conversion

Binary

Convert ASCII decimal to binary representation.

  • Only contains 0 and 1
  • No more than 8 bits, 7 bits are also possible because printable characters are up to 127
  • Actually just another form of ASCII code

Hexadecimal

Convert letters to ASCII hexadecimal representation.

  • A-Z → 41-5a
  • a-z → 61-7a

Tools

CTF Example

ascii

2018 DEFCON Quals - Ghettohackers: Throwback

The challenge details:

Anyo!e!howouldsacrificepo!icyforexecu!!onspeedthink!securityisacomm!ditytop!urintoasy!tem!

Here we split the given string by ! mark, then the length of each string corresponds to a letter. For example, length 1 = a, length 2 = b, and so on.

ori = 'Anyo!e!howouldsacrificepo!icyforexecu!!onspeedthink!securityisacomm!ditytop!urintoasy!tem!'
sp = ori.split('!')
# split original text by !
print(''.join(chr(97+len(s)-1) if s else ' ' for s in sp))
# get length of each word and add offset, then convert it to letter with chr()

We assumed that 0 characters are spaces because it makes the flag readable.

The flag is: dark logic

Base Encoding

the xx in basexx indicates how many characters are used for encoding. For example, base64 uses 64 characters to encode since 2 to the sixth power is 64, each 6 bit is an element that corresponds to a printable character. Three bytes have 24 bits, corresponding to four Base64 elements, so three bytes are needed to represent four printable characters. The printable characters in Base64 include the letters A-Z, a-z, numbers 0-9 that are 62 characters and two printable characters differ in different systems.

base64

Base64 - Wikipedia.

Encoding the word man:

base64 Man

If the numbers of bytes are not divisible by 3, there will be 1 or 2 extra bytes at the end.

  • Fill the remaining with value 0 at the end so that it can be divisible by 3, then encode to base64.
  • After it's encoded to base64, you need to add one or two = at the end. When there are 8 bits (one byte) remaining and the last 6 base64 bits contain four values of 0, you need to add two = at the end. When there are two bytes remaining and the last 6 base64 bits contain two values of 0, you need to add one = at the end.

Refer to the table below: Base64 with Zero Padding

Because the zero paddings are not used in operation, hidden information can be stored there.

Similar to base64, base32 uses 32 characters to encode since 2 to the fifth power is 32, each 5 bit is an element that corresponds to a printable character. - If there are less than 5 bit, fill it with 0 at the end. - Fill the remaining with = until there are 5 bytes. - Base32 can have up to six = at the end.

Refer to the table below:

base32

Features

  • Base64 can have = at the end, but two maximum.
  • Base64 can have = at the end, but six maximum.
  • Characters set differ from different bases.
  • It may be necessary to manually add = sign
  • = is also 3d (in hex)
  • For more details, see Base RFC

Tools

Example

The challenge details can be found in the data.txt file on here

Use a script to read steganographic information:

import base64

def deStego (stegoFile):
    b64table = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"

    with open(stegoFile,'r') as stegoText:
        message = ""

        for line in stegoText:
            try:
                text = line[line.index("=") - 1:-1]
                message += "".join([ bin( 0 if i == '=' else b64table.find(i))[2:].zfill(6) for i in text])[2 if text.count('=') ==2 else 4:6]  
            except:
                pass

    return "".join([chr(int(message[i:i+8],2)) for i in range(0,len(message),8)])

print(deStego("text.txt"))

Output: flag{BASE64_i5_amaz1ng}

Huffman Coding

See Hoffman Coding - Wikipedia

XXencoding

XXencode encodes the input text in units of three bytes. If the final remaining data is less than three bytes, the missing parts are filled out with zeros. These three bytes have 24 bits in total, divided into four groups of 6 bits, each of which is represented in decimal terms with values that only fall between 0 and 63. Replace the position character of the corresponding value.

           1         2         3         4         5         6

 0123456789012345678901234567890123456789012345678901234567890123

 |         |         |         |         |         |         |

 +-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz

See Xxencoding - Wikipedia

Features

  • only contains numbers and uppercase/lowercase letters
  • - sign, - sign

Tools

URL Encoding

See URL Encoding - Wikipedia

Features

  • Many % sign

Unicode Encoding

See Unicode - Wikipedia

Note that it can take four different forms

Example

Source text: The

&#x [Hex]: The

&# [Decimal]: The

\U [Hex]: \U0054\U0068\U0065

\U+ [Hex]: \U+0054\U+0068\U+0065

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