The Clipper Chip is a NSA designed tamperproof chip for encrypting data and it uses the SkipJack algorithm. Each Clipper Chip has a unique serial number and a copy of the unit key is stored in the database under this serial number. The sending Clipper Chip generates and sends a Law Enforcement Access Field (LEAF) value included in the transmitted message. It is based on a 80-bit key and a 16-bit checksum. Source: WALLHOFF, John, CBK#5 Cryptography (CISSP Study Guide), April 2002 (page 1).