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USD encoding replaces each letter by its upside-down representation in the alphabet. For example, Logan becomes ue6o7 (or, decrypting, ue6o7 becomes Logan again).

USD ("UpSide-Down") is a simple substitution cipher used for obscuring text by replacing each letter by its upside-down representation. a becomes e, b becomes q, and so on. The algorithm was created by Jan Albartus to be used in online forums as a means of hiding joke punchlines, puzzle solutions, movie and story spoilers from the casual glance. USD can be described as the "ASCII equivalent of a magazine printing the answer to a quiz upside down". With a little efford the text is readable without the need to use a decoder.

As a substitution cipher (a method of encryption thousands of years old), USD provides no cryptographic security as text can is readable without encoding, and is not used for such. An additional feature of the cipher is that it is symmetrical; that is, to undo USD, the same algorithm is applied, so the same code can be used for encoding and decoding.

Contents 1 Description 2 USD as encryption 3 See also 4 References 5 External links

Applying USD to a piece of text merely requires examining its alphabetic characters and replacing each one by the letter indicated in a lookup table, preserving case: a becomes e, A becomes V, and so forth, down to Z, which becomes Z. The USD function is its own inverse:

In other words, two successive applications of USD restore the original text (in mathematics, this is sometimes called an involution; in cryptography, a reciprocal cipher).

The transformation can be done using a lookup table.

For example, in the following joke, the answer (punchline) has been obscured by USD:

How can you tell an extrovert from an introvert person?
'saoys s.hn6 R3HTO ayt te skool tJa^oJtxa ayt  `sJote^ala ayt uI

Transforming the entire text via USD form, the answer to the joke is revealed:

In the elevators,  the extrovert looks at the OTHER guy's shoes.
¿uosJad tJa^oJtu! ue woJf tJa^oJtxa ue llat noh uec moH

A second application of USD would restore the original.

Since it replaces alphabetic letters with other letters, USD does not to cause problems for any newsgroup software which might have been incompatible with unusual kinds of characters.

USD is one of a set of encryption algorithms known as substitution ciphers. USD is not intended to be used where secrecy is of any concern—the use of a constant lookup table means that the encryption effectively has no key, and decryption requires no more knowledge than the fact that USD is in use. Even without this knowledge, like any substitution cipher the algorithm is easily broken through frequency analysis or the exploitation of pattern words and can even be decoded without the help of a decoder tool by simply trying to read the upside-down text.

The real effect of USD is merely to make sure that the viewer of a message must consciously choose to decipher it, which typically means invoking the relevant USD tool or reading it directly. Rather than protecting a confidential message from unauthorized readers, USD safeguards authorized readers from material they may not wish to inadvertently read, such as spoilers in book or movie reviews.

USD also provides an opportunity for letter games. Some words will, when transformed with USD, produce another word.

• Cryptanalysis

• Online Converter for USD, ROT13, Morse Code, Base8, Base64 and more (Flash)

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