rev/idk

idk/Dockerfile

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+FROM gcc:4.9
+COPY idk.c .
+RUN gcc -o chall idk.c

idk/chall.yaml

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+name: idk
+categories:
+  - rev
+value: 100
+flag: bcactf{H1gH_QUAlITy_coDE_ccec60e091ba2}
+description: |-
+  i threw some random stuff at a wall
+hints:
+  - i didn't invent any of these
+files:
+  - src: /chall
+    dest: idk
+    container: static
+deploy:
+  static:
+    build: .
+authors:
+  - Marvin
+visible: true
+# TODO: verify deployment

idk/idk.c

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+#include <stdio.h>
+
+int main() {
+    // read input in as char array
+    char flag[40];
+    printf("what: ");
+    scanf("%s", flag);
+    unsigned int cd[20];
+    // Interleave
+    for (int i = 0; i < 20; i ++) {
+        cd[i] = ((flag[2*i] * 0x0101010101010101ULL & 0x8040201008040201ULL) *
+                     0x0102040810204081ULL >>
+                 49) &
+                    0x5555 |
+                ((flag[2*i + 1] * 0x0101010101010101ULL & 0x8040201008040201ULL) *
+                     0x0102040810204081ULL >>
+                 48) &
+                    0xAAAA;
+    }
+    // XOR Swap
+    int ind1[20] = {11, 19, 14, 1, 3, 5, 18, 13, 0, 17, 6, 7, 8, 16, 12, 10, 4, 9, 15, 2};
+    for (int i = 0; i < 19; i += 2) {
+        cd[ind1[i]] = cd[ind1[i]] ^ cd[ind1[i + 1]];
+        cd[ind1[i + 1]] = cd[ind1[i + 1]] ^ cd[ind1[i]];
+        cd[ind1[i]] = cd[ind1[i]] ^ cd[ind1[i + 1]];
+    }
+    // Max and min
+    int ind2[20] = {18, 6, 17, 4, 13, 12, 10, 5, 0, 14, 8, 11, 16, 7, 15, 1, 2, 19, 9, 3};
+    unsigned int check = 0;
+    for (int i = 0; i < 19; i += 2) {
+        unsigned int c = (cd[ind2[i]] < cd[ind2[i + 1]]);
+        check += c << i;
+        unsigned int temp = cd[ind2[i + 1]] ^ ((cd[ind2[i]] ^ cd[ind2[i + 1]]) & -c);
+        cd[ind2[i]] ^= ((cd[ind2[i]] ^ cd[ind2[i + 1]]) & -c);
+        cd[ind2[i + 1]] = temp;
+    }
+    // Get bit count and remove last bit
+    for (int i = 0; i < 20; i++) {
+        unsigned int t = cd[i];
+        t = t - ((t >> 1) & 0x55555555);
+        t = (t & 0x33333333) + ((t >> 2) & 0x33333333);
+        t = (((t + (t >> 4)) & 0x0F0F0F0F) * 0x01010101) >> 24;
+        cd[i] >>= 1;
+        cd[i] += t << 16;
+    }
+    // Advance bit permutations
+    for (int i = 0; i < 20; i++) {
+        for (int j = 0; j < i; j++) {
+            unsigned int temp = cd[i] | (cd[i]-1);
+            cd[i] = (temp + 1) | (((~temp & -~temp) - 1) >> (__builtin_ctz(cd[i]) + 1));  
+        }
+    }
+
+    unsigned int goal_check = 333072;
+    unsigned int goal[20] = {466437, 528153, 333852, 530074, 728060, 531211, 400528, 399745, 396035, 530846, 662759, 395326, 397355, 663164, 399371, 532170, 465419, 466482, 532038, 399114};
+    if (check == goal_check) {
+        for (int i = 0; i < 20; i++) {
+            if (cd[i] != goal[i]) {
+                printf("no\n");
+                return 0;
+            }
+        }
+        printf("yes\n");
+    } else {
+        printf("no\n");
+    }
+    return 0;
+}

idk/solve.py

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+# most things stolen from https://graphics.stanford.edu/~seander/bithacks.html
+
+goal_check = 333072
+goal = [466437, 528153, 333852, 530074, 728060, 531211, 400528, 399745, 396035, 530846, 662759, 395326, 397355, 663164, 399371, 532170, 465419, 466482, 532038, 399114]
+
+def bits(n):
+    c = 0
+    while n:
+        c += 1
+        n &= n - 1
+    return c
+
+
+# previous bit permutation
+for i in range(20):
+    for j in range(i):
+        bit_count = bits(goal[i])
+        goal[i] -= 1
+        while bits(goal[i]) != bit_count:
+            goal[i] -= 1
+
+print(goal)
+
+# reverse this thing "Get bit count and remove last bit"
+for i in range(20):
+    bit_count = goal[i] >> 16
+    goal[i] = goal[i] & ((1 << 16) - 1)
+    goal[i] <<= 1
+    t = bits(goal[i])
+    if (t == bit_count): continue
+    if (t == bit_count - 1): goal[i] += 1; continue
+    raise Exception("???", t, bit_count)
+print(goal)
+# reverse Max and min with help of check
+ind2 = [18, 6, 17, 4, 13, 12, 10, 5, 0, 14, 8, 11, 16, 7, 15, 1, 2, 19, 9, 3]
+for i in range(0, 20, 2):
+    if not (goal_check & (1 << i)): # if 1st > 2nd in orig
+        # already correct order (this part sets the 2nd one to the min)
+        continue
+    goal[ind2[i]], goal[ind2[i + 1]] = goal[ind2[i + 1]], goal[ind2[i]]
+print(goal)
+# reverse XOR swap
+ind = [11, 19, 14, 1, 3, 5, 18, 13, 0, 17, 6, 7, 8, 16, 12, 10, 4, 9, 15, 2]
+for i in range(0, 20, 2):
+    goal[ind[i]], goal[ind[i + 1]] = goal[ind[i + 1]], goal[ind[i]]
+
+flag = []
+print(goal)
+# uninterleave
+for i in range(20):
+    temp_x = 0
+    temp_y = 0
+    for j in range(0, 16, 2):
+        temp_x |= ((goal[i] >> j) & 1) << (j//2)
+        temp_y |= ((goal[i] >> (j+1)) & 1) << (j//2)
+    flag.append(temp_x)
+    flag.append(temp_y)
+
+print("".join([chr(x) for x in flag]))