Description

An encrypted file enc is provided. A comment in the source reveals the encoding scheme. The task is to reverse it.

Overview

• The flag is encoded by packing each pair of ASCII characters into a single Unicode code point.
• The encoding shifts the first character left by 8 bits and adds the second character, producing a 16-bit value stored as one Unicode character.
• Decoding unpacks each Unicode character back into two ASCII bytes.

Background: Bit Packing

• ASCII characters occupy one byte (8 bits), with values in the range 0-127.
• A Unicode code point can represent values up to 0x10FFFF, well above 16 bits.
• Shifting the first byte left by 8 and OR-ing in the second byte creates a single 16-bit integer that stores both characters: chr((ord(a) << 8) | ord(b)).
• This halves the character count of the encoded string, making it visually unrecognizable.

Solution

Step 1: Read the Encoding Comment

• The source file contains an explicit comment describing the encoding:

# code : ''.join([chr((ord(flag[i]) << 8) + ord(flag[i + 1])) for i in range(0, len(flag), 2)])

• Reading this tells us that pairs of characters are packed into single Unicode code points.

Step 2: Reverse the Encoding

• For each character d in the encoded file, extract the high byte with a right shift of 8 bits, and the low byte from the UTF-16 big-endian encoding.

result = ""
for d in data:
    result += chr(ord(d) >> 8)           # high byte: first original char
    result += chr(d.encode("utf-16be")[-1])  # low byte: second original char

• ord(d) >> 8 extracts the top 8 bits (the first original character).
• d.encode("utf-16be")[-1] extracts the bottom 8 bits as a byte (the second original character).

Flag

picoCTF{16_bits_inst34d_of_8_26684c20}