You may find the following tools useful in challenges. While you are not required to use any of these tools, you will need a similar tool for a few challenges.
http://rumkin.com/tools/cipher/ (Many good web-based cipher tools)
http://cyber.meme.tips/xlate (JXlate - multibase data translator)
http://cyber.meme.tips/xlate/shifts.html (Shift/Xor report - see all 256 Byte shifts or 1-byte xor key outputs on one screen)
http://cyber.meme.tips/jpdump (See JPEG header information, file abnormalities)
http://cyber.meme.tips/xorjs/ (XorJS - in-browser equivalent of XorJ)
https://github.com/crashdemons/XorJ (XorJ - Java GUI app for XORing two or more files together at given offsets)
http://www.w1hkj.com/ (fldigi - Digital modem program with many different audio encodings for the radio enthusiast)
http://dialabc.com/sound/detect/ (DTMF decoder)
http://www.teworks.com/dtmf.htm (DTMF decoder)
https://www.djsoft.net/smf/index.php?topic=3512.0 (DTMF decoder)
http://exif.regex.info/exif.cgi (Image EXIF+metadata viewer)
http://fotoforensics.com/ (Image analysis and ELA [Error Level Analysis])
https://addons.mozilla.org/en-GB/firefox/addon/referer-modifier/ (An HTTP Referer extension - multiple exist but note these have not been vetted)
http://mh-nexus.de/en/hxd/ (Hex-Editor / binary file viewer)