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A bit for which an attacker has exactly a 50% probability of success of guessing the value of the bit as either zero or one.
Source(s):
NIST SP 800-106
A bit for which an attacker has exactly a 50% probability of success of guessing the value of the bit as either a zero or one. It is also called an unbiased bit.
Source(s):
NIST SP 800-107 Rev. 1