2Warm picoCTF 2019 Solution

Description

1. Step 1

   Convert decimal to binary with Python

   Observation

   I noticed the challenge asks to convert 42 from base 10 to base 2, which suggested using Python's built-in bin() function as a quick, reliable one-liner rather than performing the repeated-division-by-2 conversion by hand.

   Open a Python shell and call bin(42). Python returns '0b101010' - the '0b' prefix signals binary notation and is not part of the answer. Strip it off: the flag is 101010.

   python

   python3 -c "print(bin(42))"

   Copied!

   Expected output

   0b101010

   What didn't work first

   Tried: Submit '0b101010' as the flag, including the Python prefix.

   Python's bin() always prepends '0b' to indicate the base, but this prefix is not part of the numeric value. The flag format wraps only the raw binary digits, so picoCTF{0b101010} is rejected. Strip the prefix and submit picoCTF{101010}.

   Tried: Manually convert 42 to hex instead of binary, submitting '2a' as the answer.

   Hex (base 16) and binary (base 2) are different bases. Python's hex(42) returns '0x2a', which is the correct hexadecimal representation but wrong here. The problem explicitly asks for base 2, so only the bin() function or a manual repeated-division-by-2 process gives the right answer.

   Learn more

   Binary (base 2) is the numeral system used internally by every digital computer. Each digit, called a bit, can only be 0 or 1, corresponding to off and on states in electronic circuits. Groups of 8 bits form a byte, the fundamental unit of data storage and transmission.

   Converting from decimal to binary is done by repeatedly dividing by 2 and recording remainders. For 42: 42 ÷ 2 = 21 R0, 21 ÷ 2 = 10 R1, 10 ÷ 2 = 5 R0, 5 ÷ 2 = 2 R1, 2 ÷ 2 = 1 R0, 1 ÷ 2 = 0 R1. Reading the remainders from bottom to top gives 101010. Python's bin() automates this and prepends 0b to signal the base.

   Knowing how to convert between number bases is essential in CTF and security work. Hexadecimal (base 16) is used for memory addresses and color codes. Octal (base 8) appears in Unix file permissions. Binary shows up directly in bitwise operations, network subnetting, and binary exploitation.

   • bin(n) - decimal to binary string in Python
   • hex(n) - decimal to hex string in Python
   • oct(n) - decimal to octal string in Python
   • int('101010', 2) - binary string back to decimal

   Bitwise operations - AND, OR, XOR, NOT, left shift, and right shift - all operate directly on the binary representation of numbers. XOR (^ in Python) is particularly important in cryptography and CTF challenges: XOR-ing a value with itself gives 0, XOR-ing with 0 gives the original value, and XOR is its own inverse (applying it twice cancels out). Many simple encryption schemes use XOR with a repeating key, which is vulnerable to frequency analysis when the key is short.

   Binary in network subnetting is another essential application. An IPv4 address like 192.168.1.1 is really four binary octets. A subnet mask like 255.255.255.0 (or /24 in CIDR notation) is a sequence of 24 ones followed by 8 zeros in binary: 11111111.11111111.11111111.00000000. ANDing an IP address with its subnet mask gives the network address. Understanding subnets at the binary level makes network security concepts like ACLs, firewall rules, and CIDR ranges much more intuitive.

   Signed vs. unsigned integers is a critical distinction in binary. An 8-bit unsigned integer represents 0-255. An 8-bit signed integer (two's complement) represents -128-127 - the most significant bit signals the sign. The value 11111111 in binary is 255 as unsigned but -1 as a signed 8-bit integer. This ambiguity is the source of many real-world vulnerabilities: integer overflows, sign confusion bugs, and type confusion errors in C/C++ code often stem from the boundary between unsigned value 128 (binary 10000000) being reinterpreted as -128 in a signed context.

Flag

Binary is base-2 - each digit is a power of 2. Python's bin() function shows a '0b' prefix which is not part of the answer.