Cipher is a keyword introduced in Gatecrash, wherein it is the House Dimir guild mechanic. 
From the Comprehensive Rules:
- 702.98. Cipher
- 702.98a Cipher appears on some instants and sorceries. It represents two static abilities, one that functions while the spell is on the stack and one that functions while the card with cipher is in the exile zone. "Cipher" means "If this spell is represented by a card, you may exile this card encoded on a creature you control" and "As long as this card is encoded on that creature, that creature has 'Whenever this creature deals combat damage to a player, you may copy this card and you may cast the copy without paying its mana cost.'"
- 702.98b The term "encoded" describes the relationship between the card with cipher while in the exile zone and the creature chosen when the spell represented by that card resolves.
- 702.98c The card with cipher remains encoded on the chosen creature as long as the card with cipher remains exiled and the creature remains on the battlefield. The card remains encoded on that object even if it changes controller or stops being a creature, as long as it remains on the battlefield.
- The spell with cipher is encoded on the creature as part of that spell's resolution, just after the spell's other effects. That card goes directly from the stack to exile. It never goes to the graveyard.
- You choose the creature as the spell resolves. The cipher ability doesn't target that creature, although the spell with cipher may target that creature (or a different creature) because of its other abilities.
- If the spell with cipher is countered, none of its effects will happen, including cipher. The card will go to its owner's graveyard and won't be encoded on a creature.
- If the creature leaves the battlefield, the exiled card will no longer be encoded on any creature. It will stay exiled.
- If you want to encode the card with cipher onto a noncreature permanent such as a Keyrune that can turn into a creature, that permanent has to be a creature before the spell with cipher starts resolving. You can choose only a creature to encode the card onto.
- The copy of the card with cipher is created in and cast from exile.
- You cast the copy of the card with cipher during the resolution of the triggered ability. Ignore timing restrictions based on the card's type.
- If you choose not to cast the copy, or you can't cast it (perhaps because there are no legal targets available), the copy will cease to exist the next time state-based actions are performed. You won't get a chance to cast the copy at a later time.
- The exiled card with cipher grants a triggered ability to the creature it's encoded on. If that creature loses that ability and subsequently deals combat damage to a player, the triggered ability won't trigger. However, the exiled card will continue to be encoded on that creature.
- If another player gains control of the creature, that player will control the triggered ability. That player will create a copy of the encoded card and may cast it.
- If a creature with an encoded card deals combat damage to more than one player simultaneously (perhaps because some of the combat damage was redirected), the triggered ability will trigger once for each player it deals combat damage to. Each ability will create a copy of the exiled card and allow you to cast it.
- Whispering Madness -
Each player discards his or her hand, then draws cards equal to the greatest number of cards a player discarded this way.
Cipher (Then you may exile this spell card encoded on a creature you control. Whenever that creature deals combat damage to a player, its controller may cast a copy of this card without paying its mana cost.)
There is a total of 13 cards that involve cipher, which divide by color as such:
- Blue = 46 %
- Black = 31 %
- Multicolor = 23 %