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sudo nvram security-password

security-password %fa%cb%d9%d9%dd%c5%d8%ce

In the password string, count the number of percent symbols, which are separators for the hex codes that represent a character of your password, where two hex code characters together represent one ASCII text character. Since the Calculator can only handle words up to 8 characters (16 hex characters), if there are more than 8 symbols, then you will have to split the password up and convert in sections.

Therefore, copy the security password output from the Terminal to a text editor and delete the percent symbols in it, followed by splitting the password string at every 16th character. After this, perform the following steps on each 16-character section:

Open the Calculator and set it to Programmer mode in the View menu or by pressing Command-3.
Copy one 16-character section of your password and paste it into the calculator. You should see its binary equivalent shown below the yellow-green display, and also see its ASCII-text representation at the bottom-left of the display (you may have to click the “ASCII” button to reveal this).
Starting with the first bit in the binary output (the one furthest from the blue zero at the right), reverse every other bit by clicking its corresponding 1 or 0. For example, if you see “1010 0101” then change it to “0000 1111.”Each ASCII character of the password will be a group of eight bits (a “byte”). Each of the two hex values that represents one of these characters is a group of four bits (a “nibble”), giving 16 possible combinations for a nibble. Hexadecimal numbering goes from 0 through 9 and then continues with A through F, giving 16 possible values to represent the combinations of a nibble.

http://reviews.cnet.com/8301-13727_7-57521667-263/use-the-calculator-to-reveal-a-macs-firmware-password/