Binary Digits picoCTF 2026 Solution

Description

1. Step 1Inspect how the digits are framed
   Look at the first ~200 bytes with od -c. You need to know whether '0' and '1' are space-separated, newline-separated, or contiguous before you slice into 8-bit groups.
   bash

   head -c 200 digits.bin | od -c

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   bash

   wc -c digits.bin

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   od -c prints each byte as a printable character (or escape), so you immediately see whether the file is one long run of 0/1, or whether there are spaces, tabs, or newlines between groups. wc -c divided by the bit-grouping (usually 8) tells you how many bytes the decoded flag should be, which catches off-by-one mistakes early.

2. Step 2Inspect the file
   Open digits.bin and confirm it contains a sequence of '1' and '0' ASCII characters representing binary data.
   bash

   cat digits.bin

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   The distinction between binary data and ASCII text representing binary is fundamental. The file here contains the literal characters '0' (ASCII 48) and '1' (ASCII 49) - not actual binary values 0 and 1. The file is valid text that you read with your eyes and then interpret mathematically.

   This encoding is common in low-level education and CTF introductory challenges because it makes binary arithmetic visible. Every 8 characters form one byte (octet), and each character position represents a power of two from 2⁷ (128) down to 2⁰ (1). For example, 01100101 = 0+64+32+0+0+4+0+1 = 101 = ASCII 'e'.

   In real-world applications, binary-to-text representations are used whenever binary data must traverse a text-only channel: Base64 is the most common (used in email MIME, JSON web tokens, and PEM certificates), while raw binary-as-ASCII is mostly pedagogical or used in signal-level protocols like old-school modems.

3. Step 3Convert binary to text
   Split the string into 8-bit chunks and convert each to its ASCII byte. Strip trailing null padding. See CTF encodings and Python for CTF for the broader toolkit.
   python

   python3 -c "
   data = open('digits.bin').read().strip().replace('\n','').replace(' ','')
   flag = bytes(int(data[i:i+8],2) for i in range(0,len(data),8))
   print(flag.rstrip(b'\x00').decode())
   "

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   Python's int(bits, 2) converts a binary string to an integer using base 2. Slicing with data[i:i+8] steps through the string in 8-character windows, one byte at a time. Building a bytes object first lets you call .rstrip(b'\x00') to strip trailing null padding before decoding - many encoders zero-pad to a block boundary, and a stray \x00 in the decoded string trips up downstream tooling.

   The same logic applies to other bases: int(s, 8) for octal, int(s, 16) for hexadecimal. bytes.fromhex() and base64.b64decode() handle the most common CTF encodings.

   A useful mental shortcut: printable ASCII (32-126) fits in 7 bits, so the MSB is always 0 for those characters. If the flag is all printable ASCII you can sanity-check by confirming every 8th bit is 0, but always slice the full 8 bits when decoding so you don't mangle non-ASCII bytes.

Alternate Solution

Flag

The flag is encoded as a plain binary string - convert each 8-bit group to ASCII.