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Tripcodes are a method of authentication that does not require registration. They are most often used in 2channel-style message boards or Futaba Channel-style imageboards. A tripcode is a hashed password by which a person can be identified by others.
A tripcode is the result of input to a cryptographic hash function on the message board server, usually entered in the same field as the name. Using the common 2channel format, name#tripcode when entered as a username becomes name!3GqYIJ3Obs when displayed in the post. The ! is the separator between name and tripcode; on some boards it is replaced with ◆.
Readers of the board can identify postings made by the same user by comparing tripcodes. If two people use the same user name, they can be told apart because they, presumably, don't know each other's passwords that generate the different tripcodes. This way, the names and passwords don't have to be stored in a database.
Tripcodes and their separators are usually not displayed in bold text, unlike the username, making it more difficult to fake them. As many boards use the same algorithm tripcodes are usually consistent.
Secure tripcodes
Tripcodes are not a very secure authentication method. Since the keyspace of 2channel-style tripcodes is not very large (254) some boards implement a secure tripcode along with normal tripcodes. In their case another hash is used that takes a second input (typically in the form of
name#tripcode#securetripcode) and uses a secret salt stored on the server. As this salt is secret and site specific one cannot use a pre-computed preimage attack such as Rainbow tables.
External links
"Tripcode"