Day 14 Complete

day14/main.go

@@ -0,0 +1,82 @@
+package main
+
+import (
+	"crypto/md5"
+	"fmt"
+	"log"
+	"os"
+	"strconv"
+	"strings"
+)
+
+var hashStore []string
+var salt string
+
+func main() {
+	if len(os.Args) < 3 {
+		fmt.Println("Usage: ./day14 <salt> <key-num>")
+		fmt.Println("  My input: ./day14 cuanljph 64")
+		os.Exit(1)
+	}
+	salt = os.Args[1]
+	keyNum := atoi(os.Args[2])
+	var foundKeys []string
+	tryIdx := 0
+	for len(foundKeys) < keyNum {
+		tst := getHash(tryIdx)
+		for i := range tst {
+			if len(tst) > i+3 {
+				if tst[i:i+3] == strings.Repeat(string(tst[i]), 3) {
+					// Found a triple, search the next 1000 for a quintuple
+					if searchForQuint(salt, tryIdx+1, string(tst[i])) {
+						foundKeys = append(foundKeys, tst)
+					}
+					// Once we test the first triplet, we're done with this one
+					break
+				}
+			}
+		}
+		tryIdx++
+	}
+	fmt.Println("Key:", getHash(tryIdx-1), "Idx:", tryIdx)
+}
+
+func getHash(idx int) string {
+	for len(hashStore)-1 < idx {
+		hashStore = append(
+			hashStore,
+			getStretchedHash(fmt.Sprintf("%x", md5.Sum([]byte(salt+itoa(len(hashStore)+1))))),
+		)
+	}
+	return hashStore[idx]
+}
+
+func getStretchedHash(s string) string {
+	for i := 0; i < 2016; i++ {
+		s = fmt.Sprintf("%x", md5.Sum([]byte(s)))
+	}
+	return s
+}
+
+func searchForQuint(salt string, startIdx int, srch string) bool {
+	for i := startIdx; i <= startIdx+1000; i++ {
+		tst := getHash(i)
+		if strings.Contains(tst, strings.Repeat(srch, 5)) {
+			return true
+		}
+	}
+	return false
+}
+
+func atoi(i string) int {
+	var ret int
+	var err error
+	if ret, err = strconv.Atoi(i); err != nil {
+		log.Fatal("Invalid Atoi")
+	}
+	return ret
+}
+
+func itoa(i int) string {
+	return strconv.Itoa(i)
+}

day14/problem

@@ -0,0 +1,89 @@
+Advent of Code
+
+--- Day 14: One-Time Pad ---
+
+   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.
+   Unfortunately, you've run out of keys in your one-time pad, and so you need to generate some more.
+
+   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,
+   and represented in decimal); the resulting MD5 hash should be represented as a string of lowercase hexadecimal digits.
+
+   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:
+
+     • It contains three of the same character in a row, like 777. Only consider the first such triplet in a hash.
+     • One of the next 1000 hashes in the stream contains that same character five times in a row, like 77777.
+
+   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
+   resume testing for keys starting with the very next hash.
+
+   For example, if the pre-arranged salt is abc:
+
+     • 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
+       pad, because none of the next thousand hashes (index 19 through index 1018) contain 88888.
+     • 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)
+       contains eeeee.
+     • 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.
+     • Eventually, index 22728 meets all of the criteria to generate the 64th key.
+
+   So, using our example salt of abc, index 22728 produces the 64th key.
+
+   Given the actual salt in your puzzle input, what index produces your 64th one-time pad key?
+
+   Your puzzle input was cuanljph.
+
+   Your puzzle answer was ________.
+
+--- Part Two ---
+
+   Of course, in order to make this process even more secure, you've also implemented key stretching.
+
+   Key stretching forces attackers to spend more time generating hashes. Unfortunately, it forces everyone else to spend more time, too.
+
+   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
+   total of 2016 additional hashings. Always use lowercase hexadecimal representations of hashes.
+
+   For example, to find the stretched hash for index 0 and salt abc:
+
+     • Find the MD5 hash of abc0: 577571be4de9dcce85a041ba0410f29f.
+     • Then, find the MD5 hash of that hash: eec80a0c92dc8a0777c619d9bb51e910.
+     • Then, find the MD5 hash of that hash: 16062ce768787384c81fe17a7a60c7e3.
+     • ...repeat many times...
+     • Then, find the MD5 hash of that hash: a107ff634856bb300138cac6568c0f24.
+
+   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
+   times, for a total of 2017 uses of MD5.
+
+   The rest of the process remains the same, but now the keys are entirely different. Again for salt abc:
+
+     • The first triple (222, at index 5) has no matching 22222 in the next thousand hashes.
+     • The second triple (eee, at index 10) hash a matching eeeee at index 89, and so it is the first key.
+     • Eventually, index 22551 produces the 64th key (triple fff with matching fffff at index 22859.
+
+   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?
+
+   Your puzzle answer was ________.
+
+   Your puzzle input was cuanljph.
+
+References
+
+   Visible links
+   . http://adventofcode.com/
+   . http://adventofcode.com/2016/about
+   . http://adventofcode.com/2016/support
+   . http://adventofcode.com/2016/events
+   . http://adventofcode.com/2016/settings
+   . http://adventofcode.com/2016/auth/logout
+   . http://adventofcode.com/2016
+   . http://adventofcode.com/2016
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+   . http://adventofcode.com/2016/stats
+   . http://adventofcode.com/2016/sponsors
+   . http://adventofcode.com/2016/sponsors
+   . https://en.wikipedia.org/wiki/One-time_pad
+   . https://en.wikipedia.org/wiki/Security_through_obscurity
+   . https://en.wikipedia.org/wiki/MD5
+   . https://en.wikipedia.org/wiki/Salt_(cryptography)
+   . https://en.wikipedia.org/wiki/MD5#Security
+   . https://en.wikipedia.org/wiki/Key_stretching
+   . http://adventofcode.com/2016