Crypto
Caesar Cipher
Caesar Cipher is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a left shift of 3, D would be replaced by A, E would become B, and so on. The method is named after Julius Caesar, who used it in his private correspondence. And here is a good online decoder for that. There is also a tool in linux for caesar cipher called caesar.
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Rot13
ROT13 ("rotate by 13 places", sometimes hyphenated ROT-13) is a simple letter substitution cipher that replaces a letter with the 13th letter after it in the alphabet. This is same as doing caesar cipher with 13 as the key. There are multiple variants for this like: rot47, rot5.
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Rot47
ROT47 is a derivative of ROT13 which, in addition to scrambling the basic letters, treats numbers and common symbols. Instead of using the sequence A–Z as the alphabet, ROT47 uses a larger set of characters from the common character encoding known as ASCII. Specifically, the 7-bit printable characters, excluding space, from decimal 33 '!' through 126 '~', 94 in total, taken in the order of the numerical values of their ASCII codes, are rotated by 47 positions, without special consideration of case. For example, the character A is mapped to p, while a is mapped to 2. The use of a larger alphabet produces a more thorough obfuscation than that of ROT13.
For example: The Quick Brown Fox Jumps Over The Lazy Dog. enciphers to:%96 "F:4< qC@H? u@I yF>AD ~G6C %96 {2KJ s@8]
Here is a decoder for Rot47
Maritime Flags
A maritime flag is a flag designated for use on ships, boats, and other watercraft. But it also comes often in puzzles and CTF challenges. Here is a decoder for Mary time flags.
Birds on wire
The encryption composed of birds represented as perched on an electric wire is in fact an alphabet of substitution by drawings (of the birds). Each bird represents a letter of the Latin alphabet (26 letters from A to Z) according to the correspondence. Here is a decoder.
Twitter Secret Message
Sometimes you will get messages looking like this for CTFs:
Ｉ dｏn't ｔｈｉnk ｓο ｅⅰtｈeｒ. Τｈey ｓhｏuｌｄ worｋ ｈａrdeｒ foｒ ｔｈese ｆlａｇｓ！ Ｗｈｅｎ I wａs tｈeіr age, Ｉ had tｏ tｒaνeｌ
tｈοusａnｄｓ οｆ milｅｓ to get them!
If you don't know what these are, these are Twitter secret messages. You could use an online tool like https://injecti0n.github.io/tweet-hidden-message/ to solve these kind of challenges.