Anti-Circumvention law has made the circumvention of some technological barriers to copying intellectual property illegal.

For example, Sec. 1201 (a)(1)(A) of the DMCA states that (among other things):

No person shall circumvent a technological measure that effectively controls access to a work protected under this title.

Specifically, if there is some "technological measure that effectively controls access to a work", it is now illegal to circumvent that measure. Before the DMCA, purchasing a music compact disc and copying it to a hard drive as mp3s (so the music could be listened to without the need for the physical disc) could be strongly argued as Fair Use. But under this section, if someone purchased a music compact disc that had some technological measure on it that kept them from copying it to their hard drive as mp3s, it would now be illegal for them to circumvent that technological measure.

Distribution of Circumvention Tools

Sec. 1201 (a)(2)(A) of the DMCA states that

No person shall manufacture, import, offer to the public, provide, or otherwise traffic in any technology, product, service, device, component, or part thereof, that is primarily designed or produced for the purpose of circumventing a technological measure that effectively controls access to a work protected under this title.

This section says that you cannot create or distribute tools or software that allows circumvention of technological anti-copying measures. To continue the above example, it is illegal for someone else to create a method to turn a protected purchased disc into mp3s, or to give that method to others.

Reverse Engineering and Circumvention

Sec. 1201 (f)(1) of the DMCA states that

Notwithstanding the provisions of subsection (a)(1)(A), a person who has lawfully obtained the right to use a copy of a computer program may circumvent a technological measure that effectively controls access to a particular portion of that program for the sole purpose of identifying and analyzing those elements of the program that are necessary to achieve interoperability of an independently created computer program with other programs, and that have not previously been readily available to the person engaging in the circumvention, to the extent any such acts of identification and analysis do not constitute infringement under this title.

This section says that if you legally obtain a program that is protected, you are allowed to reverse-engineer and circumvent the protection for the sole purpose of creating an interoperable and independent computer program. This interoperation must not already be readily available.