From Halopedia, the Halo wiki
An encryption code, otherwise known as an encryption key or an encryption scheme, is a functional parameter of cryptological encoding used for security purposes.
Encryption only enables senders with the appropriate decryption devices to read secretive transmissions or documents. Persons or organizations that do not possess the appropriate decryption key to decrypt an encrypted file must result to a battery of cryptological techniques to decrypt files, even using artificial cognitive computation sources as powerful as "smart" AIs.
The sender will encrypt a piece and send it to the receiver, and will list both the public key and private key of the file. The public key is widely available, and was used to encrypt the file. The private key is available only to the receiver, and is used to decrypt the file.
The Covenant occasionally place advanced encryption on high-importance locked doors within their warships, such as the CCS-class battlecruiser Truth and Reconciliation. During the UNSC raid on the battlecruiser, the Covenant locked the UNSC assault squad in the hangar bays with 128,000-bit modulating encryption until Cortana was able to hack the doors to open and let the UNSC forces venture further into the ship.
Encryption is placed on UNSC documentation or transmissions in order to maintain the level of secrecy demanded by the Transmission Classification. Encryption codes, like classifications, come in varying types, based on the importance of the information in the file.
Below is a list of transmissions sent that were encrypted by encryption codes:
To help advertise the then upcoming "Gears 5", Jeff Easterling wrote a transmission called UNSC PRIORITY TRANSMISSION 00106C-54, in this transmission had the "PINION" encryption code. This transmission is not considered "Core Canon".
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