Unformatted text preview: by one-way roads, find a path from City A to City Z that passes exactly once through all other cities on the map. Each city was represented by a different random 20-base string of DNA; with conventional molecular biology techniques, Adleman synthesized 50 picomols (30 million million molecules) of the DNA string representing each city. Each road was also represented by a 20-base DNA string, but these strings were not chosen randomly: They were cleverly chosen so that the “beginning end of the DNA string representing the road from City P to City K (“Road PK”) would tend to stick to the DNA string representing City P, and the end of Road PK would tend to stick to City K. Table 7.8 Rivest’s Optimistic Key-length Recommendations (in bits) Year 1990 Low 398 Average 515 High 1289 1995 2000 2005 2010 2015 2020 405 422 439 455 472 489 542 572 602 631 661 677 1399 1512 1628 1754 1884 2017 Adleman synthesized 50 picomols of the DNA representing each road, mixed them all together with the DNA representing all the cities, and added a ligase enzyme, which links together the ends of DNA molecules. The clever relationship between the road DNA strings and the city DNA strings causes the ligase to link the road DNA strings together in a legal fashion. That is, the “exit” end of the road from P to K will always be linked to the “entrance” end of some road that originates at City K, never to the “exit” end of any road and never to the “entrance” end of a road that originates at some city other than K. After a carefully limited reaction time, the ligase has built a large number of DNA strings representing legal but otherwise random multiroad paths within the map. From this soup of random paths, Adleman can find the tiniest trace—perhaps even a single molecule—of the DNA that represents the answer to the problem. Using common techniques of molecular biology, he discards all the DNA strings representing paths that are too long or too short. (The number of roads in the desired path must equal the number of cities minus one.) Next he discards all the DNA strings that do not pass through City A, then those that miss City B, and so forth. If any DNA sur...
View Full Document
- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips