KEY BLANK

A 'key blank' is a key that has not been cut to a specific bitting. The blank has specific cross-sectional profile to match the keyway in a cylinder. Key blanks can be stamped with a manufacturer or end-user logo or with phrases such as 'Do Not Duplicate'. Blanks are typically stocked by locksmiths for duplicating keys. Often the shape of the key bow (the part you turn) will reference individual manufacturers.
Regarding duplication law in the US:
Despite common belief and what locksmiths may tell you, keys coined "Do not duplicate" or even "IT IS UNLAWFUL TO DUPLICATE THIS KEY" are perfectly legal to duplicate them with a few exceptions and for the most part, it is a matter of individual ethics. Some manufacturers, such as Best Access Systems strictly restricts the sale of its own blanks, however it simply means they won't sell them to you and you have the every right to obtain them from a third party manufacturer. There are some blanks only made in a small volume that may not be available, yet not legally restricted. It is within your rights to fabricate such a blank on a CNC milling machine.
Keyblanks pre-coined with powerless demands. Every one of these keys are not protected under patent and anyone is free to make them.


ALOA, an internationally recognized association for locksmiths also considers such stamps an empty threat.

link ALOA TECHNICAL STANDARDS
Federal law regulates the duplication/fabrication of keys for use with locks owned by the US Postal Service and the US Military.
There are a few states that impose some restriction at the state level. California prohibits the duplication of keys for properties owned/operated/leased/under the control of any state agency, not limited to CA. Arizona has a similar prohibition, but the limitation only applies to keys to State of Arizona facilities.
'State of California, penal code 469'
Any person who knowingly makes, duplicates, causes to be
duplicated, or uses, or attempts to make, duplicate, cause to be
duplicated, or use, or has in his possession any key to a building or
other area owned, operated, or controlled by the State of
California, any state agency, board, or commission, a county, city,
or any public school or community college district without
authorization from the person in charge of such building or area or
his designated representative and with knowledge of the lack of such
authorization is guilty of a misdemeanor.
'State of Arizona'
13-3715. Unauthorized manufacture, duplication, use or possession of key to a public building classification.
A. A person who knowingly causes to be manufactured or duplicated or who possesses or uses a key to any building or other area owned, operated or controlled by this state or any agency, board, commission, institution or political subdivision of this state without authorization from the person, or his designated representative, in charge of such building or area is guilty of a class 3 misdemeanor.
B. A person who manufactures or duplicates a key for himself or another to any building or other area owned, operated or controlled by this state or any agency, board, commission, institution or political subdivision of this state, with knowledge that he or the person requesting the manufacturing or duplication of such key does not have authorization from the person or his designated representative in charge of such building or area, is guilty of a class 3 misdemeanor.
State of Kentucky:
KRS 164.337. Duplication of university keys prohibited.
Some localities may have even more stringent regulations. Los Angeles county has an ordinance prohibiting the duplication of any keys coined with an effect of "do not copy".
There is no implication of legal penalty, unless the key is restricted under one of the criteria above.
If one duplicates a key not legally protected under the above criteria, but is protected by patents valid at the time of duplication, it is a civil matter. Patented key blanks normally reference a patent number and are protected by patent laws from unauthorized duplication of the patented features, but if the patent is expired, you're free to duplicate it.
There are organizations/universities that claims "it is illegal to duplicate our keys". It is NOT illegal in the absence of a law having jurisdiction over the matter such as in CA and AZ. Although, if you have a duplicate made of a key issued to you for which you signed a non-duplcation agreement, you maybe held civilly liable for a breach of contract.