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In game description - "A pair of universal, metal keys that can be used to unlock any set of Handcuffs. Can only be used one time. Cannot unlock non-standard handcuffs such as maximum security handcuffs."
It is still possible to escape from handcuffs without the use of keys, to do this, press the space bar to struggle free. The amount of time it takes to escape from handcuffs depends on their durability but if you own a set of keys, it is instant. You can still access your inventory to collect your keys even when handcuffed.
As mentioned in the item description, it seems likely that "Maximum Security Handcuffs" may be implemented into the game in the future. Whether you are still able to break out of these handcuffs is unknown, but you will not be able to use regular handcuff keys to do so.