▁prepar es ▁a ▁header ▁and ▁app ends ▁a ▁counter ▁value ▁initialized ▁to ▁a ▁random ▁number . ▁It ▁then ▁comput es ▁the ▁ 1 6 0 - bit ▁S HA - 1 ▁hash ▁of ▁the ▁header . ▁If ▁the ▁first ▁ 2 0 ▁bits ▁( i . e . ▁the ▁ 5 ▁most ▁significant ▁hex ▁digits ) ▁of ▁the ▁hash ▁are ▁all ▁zeros , ▁then ▁this ▁is ▁an ▁acceptable ▁header . ▁If ▁not , ▁then ▁the ▁sender ▁incre ments ▁the ▁counter ▁and ▁tries ▁the ▁hash ▁again . ▁Out ▁of ▁ 2 1 6 0 ▁possible ▁hash ▁values , ▁there ▁are ▁ 2 1 4 0 ▁hash ▁values ▁that ▁satisfy ▁this ▁criter ion . ▁Thus ▁the ▁chance ▁of ▁randomly ▁selecting ▁a ▁header ▁that ▁will ▁have ▁ 2 0 ▁zeros ▁as ▁the ▁beginning ▁of ▁the ▁hash ▁is ▁ 1 ▁in ▁ 2 2 0 ▁( approx . ▁ 1 0 6 , ▁or ▁about ▁one ▁in ▁a ▁million ). ▁The ▁number ▁of ▁times ▁that ▁the ▁sender ▁needs ▁to ▁try ▁to ▁get ▁a ▁valid ▁hash ▁value ▁is ▁mode led ▁by ▁geometric ▁distribution . ▁Hence ▁the ▁sender ▁will ▁on ▁average ▁have ▁to ▁try ▁ 2 2 0 ▁values ▁to ▁find ▁a ▁valid ▁header . ▁Given ▁reasonable ▁estimates ▁of ▁the ▁time ▁needed ▁to ▁compute ▁the ▁hash , ▁this ▁would ▁take ▁about ▁one ▁second ▁to ▁find . ▁No ▁more ▁efficient ▁method ▁than ▁this ▁br ute ▁force ▁approach ▁is ▁known ▁to ▁find ▁a ▁valid ▁header . ▁ ▁A ▁normal ▁user ▁on ▁a ▁desktop ▁PC ▁would ▁not ▁be ▁significantly ▁incon ven ien