Secret code

"I see, I see. A flying saucer has arrived for you. Here you go. Thank you much."

- Tom Nook

A secret code is a 28-character password used in to give and receive in-game items. The codes are alphanumeric and are usually displayed as two rows of 14 characters each, that being the format that they are received from and input into the game. All uppercase and lowercase letters of the English alphabet, the numbers 0, 2, 3, 4, 5, 6, 7, 8, 9, as well as the pound (#) and at sign (@) are valid code characters. The number zero and uppercase "o" are interchangeable. When generated by Nook, a capital "i" can be distinguished from a lowercase "L" by the serif drawn at both the top and bottom of its stem. Notice also that the number "1" is never generated in a secret code.

Types of codes
Animal Crossing makes use of a variety of code types, each of which serves a different purpose and has a different origin. All code types follow the same description as outlined in the introduction and thus appear very similar to the untrained eye.

Player-to-player codes
As an alternative to online play, the player-to-player code system allows players to send items to anyone in the world so long as they know the receiving player's character and town name. These codes can be both received and redeemed at Nook's shop via the "Other things" menu. Unlike other code types which can be redeemed by any player in any town, player-to-player codes are specific to the character and town name combination they were generated for. This was done to mimic the experience of a personalized delivery service. Nook will only generate passwords for items that are orderable from the catalog (plus Mushrooms and Candy).

Prior to the discovery of universal codes which work for any player regardless of name/town combination, a group named Project Hyrule was formed whose sole goal was to amass a list of player-to-player codes for every item in the game. To take advantage of such a list, each member used the credentials Link in Hyrule when creating their town. After the advent and proliferation of code generators the group eventually became redundant, however their dedicated cryptologists did manage to generate many universal codes, which when redeemed at Nook's provide the player with a gift from "Project Hyrule."

Universal codes
Codes of this type will work for any player, making them much more popular among the Animal Crossing community. Additionally, they can be used to acquire almost any item (including some beta items), whereas player-to-player codes may only be generated for orderable items. Universal codes were originally discovered by accident as players (sometimes intentionally) input the wrong code to Nook. They were also used by Nintendo to distribute the Mario Theme, with codes for individual pieces listed in various issues of Nintendo Power magazine.

NES contest codes
Of the 19 NES games located in the game's code, only 15 can have NES contest codes generated for them (the remaining games being known as the Forbidden Four). Soccer, Donkey Kong Jr., Donkey Kong 3, Punch Out!! and Clu Clu Land D were exclusively given out on Animal Crossing's official website using NES contest codes and are otherwise unobtainable; universal codes cannot be generated for these titles. As the game's official site has since been removed, these five games are now solely available via code generators capable of creating this specialized code type. Like universal and player-to-player codes, NES contest codes are redeemed through Nook and are unique to a specific player and town combination.

Contest codes
These are distinct from their NES counterparts in that they are not limited to NES titles. Furthermore, they do not always guarantee an item; sometimes the player is told they haven't won the contest. These codes may have been included because Nintendo was planning to run contests for Animal Crossing items, however these plans never came to fruition. Contest codes are similar to universal codes in that they can be entered by any player regardless of character and town name.

Villager codes
Found on the back of Animal Crossing E-Reader cards, these codes include an extra key icon in their first line (for a total of three lines of code), and must be sent to villagers via letter. In their reply, the villager will include a present with an item inside. If the player sends a code to the villager pictured on the front of the e-card (e.g. sends the code on the back of Bob's card to Bob), that villager will provide the player with a random NES game instead of a regular item from the catalog. These codes can be particularly hazardous if mistyped, so if using or generating codes it is recommended to use universal codes instead.

Cross-compatibility
The secret codes used in Animal Crossing are not compatible with any Japanese titles, nor can English code generators create codes that will work for Japanese games. Japanese universal codes do exist, however, and Japanese code generators may have been developed independently of their English counterparts, however their current status is unknown.

Code messages
Different code types will result in different messages from Nook. You can use these messages to determine a code's type as well as to diagnose potential issues when a code is not working.

Wrong password: This message is received after entering a player-to-player code that has a few typographical errors, but that is otherwise correct and can be redeemed by the player.
 * "Now that's odd... I think there's something the matter with your password. Do you want to tell it to me again?"

Someone else's password: This message is received after entering a player-to-player code that was not sent to your specific character and/or town name. Remember that these names must be entered exactly as they appear when generating a code for it to work.
 * "Oh?! Whats this? You can't use that password, [player name]! Don't try to trick me by using someone else's password! Do you have a password of your own that you'd like to try?"

Send to animal: This message is received after telling Nook a villager code. This code type can only be redeemed by sending a letter to a villager in town. When mailing a villager code, make sure the key symbol appears by itself on the first line, the first line of code is placed on the second line, and second line of code sits on the third line of the letter. The villager will respond in a few days. If there is not a present attached to the letter then the code was typed incorrectly or is invalid.
 * "Eh? I'm sorry, but that password isn't valid here. Try sending it to one of the animals living in [player's town]."

Contest winner: There are two alternate dialogs for contest codes, one which mentions the sender's name and one which mentions the contest's name.
 * "I see, I see. A [item] has arrived for you from [sender's name]. Please take your present. Thank you much."
 * "I see, I see. This [item] is the prize for winning [contest name]. Congratulations! Thank you much."

Contest loser: This message is received after entering a losing contest code. Note that if the player attempts to reenter the code they may eventually 'win' the item.
 * "I see, I see... ...Now, this IS regrettable! Based on the notification Ive received directly from [name]... it seems you are NOT a winner. But thank you much for trying!"

Code redemption limit
Nook will only distribute three items via password per game load. For this reason players will often trade items in groups divisible by three. To redeem more items, the player must save and quit and then reload the game.