ASCII Numbers picoGym Exclusive Solution

Description

1. Step 1

   Pipe into xxd

   Observation

   I noticed the challenge provided a sequence of 0x-prefixed hex values, each corresponding to one ASCII character, which suggested using xxd -p -r to reverse plain hex bytes back into readable text in a single pipeline.

   xxd -p -r treats the input as plain hex bytes (-p) and reverses them to ASCII (-r). The decoded output is the flag.

   bash

   echo "0x70 0x69 0x63 0x6f 0x43 0x54 0x46 0x7b 0x34 0x35 0x63 0x31 0x31 0x5f 0x6e 0x30 0x5f 0x71 0x75 0x33 0x35 0x37 0x31 0x30 0x6e 0x35 0x5f 0x31 0x6c 0x6c 0x5f 0x74 0x33 0x31 0x31 0x5f 0x79 0x33 0x5f 0x6e 0x30 0x5f 0x6c 0x31 0x33 0x35 0x5f 0x34 0x34 0x35 0x64 0x34 0x31 0x38 0x30 0x7d" | xxd -p -r

   Copied!

   Expected output

   picoCTF{45c11_n0_qu35710n5_1ll_t311_y3_n0_l135_...}

   What didn't work first

   Tried: Use Python's bytes.fromhex() with the raw 0x-prefixed values: bytes.fromhex('0x70 0x69 0x63'.replace(' ', ''))

   Python's bytes.fromhex() is strict and only accepts pairs of bare hex digits with no '0x' prefix and no spaces. Passing a string that includes '0x' or extra spaces raises a ValueError. Use a loop with int(v, 16).to_bytes(1, 'big') or strip the prefix first: bytes.fromhex(''.join(v[2:] for v in values)).

   Tried: Decode the values as decimal ASCII codes instead of hex - treating 0x70 as the number 70 decimal

   Interpreting 0x70 as decimal 70 gives the character 'F' instead of 'p', so every character in the output shifts to a wrong letter and the string never forms a recognizable flag. The 0x prefix is the explicit signal that each value is hexadecimal; decimal 112 and hex 0x70 both encode 'p', but decimal 70 encodes something else entirely.

   Learn more

   ASCII (American Standard Code for Information Interchange) is a character encoding standard that assigns a unique 7-bit integer to each printable character and control code. For example, the letter 'p' maps to decimal 112, which is 0x70 in hexadecimal. Every character in the flag corresponds exactly to one of these values.

   The xxd tool is the standard Unix hex-dump utility. Running it with -r -p (reverse, plain) reads whitespace-separated hex values and writes their byte equivalents - essentially undoing the hex encoding. You could accomplish the same thing in Python with bytes.fromhex(), in CyberChef with the "From Hex" recipe, or even manually using an ASCII table.

   Understanding the relationship between characters and their numeric representations is foundational to binary exploitation, cryptography, and network protocol analysis. Any time data is serialized or transmitted, it ultimately moves as numbers, and being able to convert fluently between representations (hex, decimal, binary, ASCII) is an essential skill for any security researcher.

Flag

Any hex → ASCII conversion yields the same output; xxd is just a quick CLI option.