Abstract: A secure electronic monetary transaction system (SEMTS) provides absolute security for electronic financial transactions. These transactions can be of any kind provided they are numeric in content and of known length. The SEMTS encrypts and decrypts source numeric data using a private, numeric key known only by both parties in the transaction. The secure distribution of these keys will be under the same methods that the financial institutions use to distribute the original source data such as credit cards, account numbers, etc. The system uses nine simple, open formulas for translating source numbers into encrypted cipher numbers. These formulas return every possible value, except the input value, and are completely dependent on the key. Because there are no hidden parts, the architecture of the SEMTS is completely available to anyone in the public sector. This open architecture makes stealing the cipher numbers worthless. The only way to break a number is to know the key.