sifr (صفر)

sifr (Arabic for zero or cipher) is a fast, lightweight, POSIX-compliant Caesar cipher solver written in Go, specifically optimized for picoCTF and general CTF challenges.

Designed to be highly compatible (compiled for macOS and Linux), dependency-free, and shell-pipeline friendly.

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Features

• ⚡ Zero-dependency, Single Binary: Built entirely using the Go standard library for instant startup.
• 🤖 Zero-Config Intelligent Auto-Solve: Run sifr <ciphertext> without any flags, and it will automatically search all 25 shifts for a standard flag prefix (default: picoCTF). If no matches are found, it gracefully falls back to brute-forcing all shifts.
• 🎨 Beautiful ANSI Color Terminal Output: Highlights matching substrings in bold green and formats shifts clearly in cyan. Automatically disables color codes when output is piped or redirected to non-TTY endpoints.
• 🧼 Clean Output Mode (-c, --clean): Emits only the raw decrypted text—perfect for Unix pipeline automation (e.g. sifr -a -c | pbcopy).
• 📁 Flexible Input Targets: Seamlessly reads ciphertext directly from arguments, files, or standard input (pipes).
• 🧭 Short & Long Flag Configurations: Supports standard POSIX single-dash and GNU double-dash style parameters.

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Installation

Ensure you have Go installed, then run:

# Build locally
go build -o sifr .

# Or install it to your $GOPATH/bin
go install .

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Usage Guide

sifr - A fast, POSIX-compliant Caesar cipher solver for picoCTF

Usage:
  sifr [flags] [ciphertext_or_filepath]

Flags:
  -s, --shift <int>     Shift the input by a specific number of positions (1-25)
  -b, --brute           Brute-force and print all 25 possible shifts
  -a, --auto            Auto-solve mode: check all shifts for pattern (default pattern: picoCTF)
  -p, --pattern <str>   Custom pattern to look for in auto-solve mode (default: picoCTF)
  -c, --clean           Only output the raw plaintext, omitting metadata and color codes
  -h, --help            Show this help message

Examples

1. Zero-Config Flag Hunting (Automatic Auto-Solve)

Pass the ciphertext as an argument. sifr automatically scans all shifts for picoCTF and highlights the match:

$ sifr "cvpbPGS{guvf_vf_n_grfg}"
Shift 13 (MATCH): picoCTF{this_is_a_test}

2. Brute-Forcing Unfamiliar Ciphertext

If no flag matches are found, it shows all 25 possibilities:

$ sifr "uau"
No shifts matched pattern. Brute-forcing all shifts:
Shift  1: vbv
...
Shift 12: gmg
...
Shift 25: tzt

3. Custom Flag Format Patterns

For other CTFs, use the -p / --pattern flag:

$ sifr -p "flag" "synt{guvf_vf_n_flzcyr_flag}"
Shift 13 (MATCH): flag{this_is_a_symple_synt}

4. Clean Output & UNIX Piping

Extract and copy the decrypted flag straight to your clipboard:

$ echo "cvpbPGS{guvf_vf_n_grfg}" | sifr -c | pbcopy

5. File Inputs

Read from an encrypted text file directly:

$ sifr flag.enc

License

This software is released into the public domain under The Unlicense. Feel free to copy, modify, publish, sell, or distribute it however you wish. See the LICENSE file for more details.