90 lines
4.7 KiB
Plaintext
90 lines
4.7 KiB
Plaintext
Advent of Code
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--- Day 14: One-Time Pad ---
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In order to communicate securely with Santa while you're on this mission, you've been using a one-time pad that you generate using a pre-agreed algorithm.
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Unfortunately, you've run out of keys in your one-time pad, and so you need to generate some more.
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To generate keys, you first get a stream of random data by taking the MD5 of a pre-arranged salt (your puzzle input) and an increasing integer index (starting with 0,
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and represented in decimal); the resulting MD5 hash should be represented as a string of lowercase hexadecimal digits.
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However, not all of these MD5 hashes are keys, and you need 64 new keys for your one-time pad. A hash is a key only if:
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• It contains three of the same character in a row, like 777. Only consider the first such triplet in a hash.
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• One of the next 1000 hashes in the stream contains that same character five times in a row, like 77777.
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Considering future hashes for five-of-a-kind sequences does not cause those hashes to be skipped; instead, regardless of whether the current hash is a key, always
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resume testing for keys starting with the very next hash.
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For example, if the pre-arranged salt is abc:
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• The first index which produces a triple is 18, because the MD5 hash of abc18 contains ...cc38887a5.... However, index 18 does not count as a key for your one-time
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pad, because none of the next thousand hashes (index 19 through index 1018) contain 88888.
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• The next index which produces a triple is 39; the hash of abc39 contains eee. It is also the first key: one of the next thousand hashes (the one at index 816)
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contains eeeee.
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• None of the next six triples are keys, but the one after that, at index 92, is: it contains 999 and index 200 contains 99999.
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• Eventually, index 22728 meets all of the criteria to generate the 64th key.
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So, using our example salt of abc, index 22728 produces the 64th key.
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Given the actual salt in your puzzle input, what index produces your 64th one-time pad key?
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Your puzzle input was cuanljph.
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Your puzzle answer was ________.
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--- Part Two ---
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Of course, in order to make this process even more secure, you've also implemented key stretching.
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Key stretching forces attackers to spend more time generating hashes. Unfortunately, it forces everyone else to spend more time, too.
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To implement key stretching, whenever you generate a hash, before you use it, you first find the MD5 hash of that hash, then the MD5 hash of that hash, and so on, a
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total of 2016 additional hashings. Always use lowercase hexadecimal representations of hashes.
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For example, to find the stretched hash for index 0 and salt abc:
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• Find the MD5 hash of abc0: 577571be4de9dcce85a041ba0410f29f.
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• Then, find the MD5 hash of that hash: eec80a0c92dc8a0777c619d9bb51e910.
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• Then, find the MD5 hash of that hash: 16062ce768787384c81fe17a7a60c7e3.
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• ...repeat many times...
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• Then, find the MD5 hash of that hash: a107ff634856bb300138cac6568c0f24.
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So, the stretched hash for index 0 in this situation is a107ff.... In the end, you find the original hash (one use of MD5), then find the hash-of-the-previous-hash 2016
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times, for a total of 2017 uses of MD5.
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The rest of the process remains the same, but now the keys are entirely different. Again for salt abc:
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• The first triple (222, at index 5) has no matching 22222 in the next thousand hashes.
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• The second triple (eee, at index 10) hash a matching eeeee at index 89, and so it is the first key.
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• Eventually, index 22551 produces the 64th key (triple fff with matching fffff at index 22859.
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Given the actual salt in your puzzle input and using 2016 extra MD5 calls of key stretching, what index now produces your 64th one-time pad key?
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Your puzzle answer was ________.
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Your puzzle input was cuanljph.
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References
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Visible links
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. http://adventofcode.com/
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. http://adventofcode.com/2016/about
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. http://adventofcode.com/2016/support
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. http://adventofcode.com/2016/events
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. http://adventofcode.com/2016
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. http://adventofcode.com/2016
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. http://adventofcode.com/2016/stats
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. http://adventofcode.com/2016/sponsors
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. http://adventofcode.com/2016/sponsors
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. https://en.wikipedia.org/wiki/One-time_pad
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. https://en.wikipedia.org/wiki/Security_through_obscurity
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. https://en.wikipedia.org/wiki/MD5
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. https://en.wikipedia.org/wiki/Salt_(cryptography)
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. https://en.wikipedia.org/wiki/MD5#Security
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. https://en.wikipedia.org/wiki/Key_stretching
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. http://adventofcode.com/2016
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