From f56c03bf65dabf67ca35951e62e0f7b0929bf05c Mon Sep 17 00:00:00 2001
From: Thomas Lindner <tom@dl6tom.de>
Date: Sun, 6 Feb 2022 21:31:00 +0100
Subject: [PATCH] solve Challenge 3 of Set 1

---
 set1/challenge3_xor_cipher/build.zig    | 27 ++++++++++
 set1/challenge3_xor_cipher/src/main.zig | 72 +++++++++++++++++++++++++
 2 files changed, 99 insertions(+)
 create mode 100644 set1/challenge3_xor_cipher/build.zig
 create mode 100644 set1/challenge3_xor_cipher/src/main.zig

diff --git a/set1/challenge3_xor_cipher/build.zig b/set1/challenge3_xor_cipher/build.zig
new file mode 100644
index 0000000..a79c717
--- /dev/null
+++ b/set1/challenge3_xor_cipher/build.zig
@@ -0,0 +1,27 @@
+const std = @import("std");
+
+pub fn build(b: *std.build.Builder) void {
+    // Standard target options allows the person running `zig build` to choose
+    // what target to build for. Here we do not override the defaults, which
+    // means any target is allowed, and the default is native. Other options
+    // for restricting supported target set are available.
+    const target = b.standardTargetOptions(.{});
+
+    // Standard release options allow the person running `zig build` to select
+    // between Debug, ReleaseSafe, ReleaseFast, and ReleaseSmall.
+    const mode = b.standardReleaseOptions();
+
+    const exe = b.addExecutable("challenge3_xor_cipher", "src/main.zig");
+    exe.setTarget(target);
+    exe.setBuildMode(mode);
+    exe.install();
+
+    const run_cmd = exe.run();
+    run_cmd.step.dependOn(b.getInstallStep());
+    if (b.args) |args| {
+        run_cmd.addArgs(args);
+    }
+
+    const run_step = b.step("run", "Run the app");
+    run_step.dependOn(&run_cmd.step);
+}
diff --git a/set1/challenge3_xor_cipher/src/main.zig b/set1/challenge3_xor_cipher/src/main.zig
new file mode 100644
index 0000000..a9193a4
--- /dev/null
+++ b/set1/challenge3_xor_cipher/src/main.zig
@@ -0,0 +1,72 @@
+const std = @import("std");
+
+fn readHex(allocator: std.mem.Allocator, max_size: usize) ![]u8 {
+    const stdin = std.io.bufferedReader(std.io.getStdIn().reader()).reader();
+
+    var inputbuffer = std.ArrayList(u8).init(allocator);
+    defer inputbuffer.deinit();
+    var resultbuffer = std.ArrayList(u8).init(allocator);
+    defer resultbuffer.deinit();
+
+    while (true) {
+        try stdin.readUntilDelimiterArrayList(&inputbuffer, '\n', max_size * 2);
+        try resultbuffer.resize(inputbuffer.items.len / 2);
+        _ = std.fmt.hexToBytes(resultbuffer.items, inputbuffer.items) catch {
+            std.log.err("not a valid hexadecimal", .{});
+            continue;
+        };
+        return resultbuffer.toOwnedSlice();
+    }
+}
+
+fn decrypt(plaintext: []u8, ciphertext: []const u8, key: u8) void {
+    std.debug.assert(plaintext.len == ciphertext.len);
+    var i: usize = 0;
+    while (i < ciphertext.len) : (i += 1) {
+        plaintext[i] = ciphertext[i] ^ key;
+    }
+}
+
+fn score(plaintext: []const u8) u32 {
+    const ranking = [_]u8{ 24, 7, 15, 17, 26, 11, 10, 19, 22, 4, 5, 16, 13, 21, 23, 8, 2, 18, 20, 25, 14, 6, 12, 3, 9, 1 };
+    var s: u32 = 0;
+    for (plaintext) |char| {
+        if (char >= 'a' and char <= 'z') {
+            s += ranking[char - 'a'];
+        }
+    }
+    return s;
+}
+
+pub fn main() anyerror!void {
+    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
+    defer _ = gpa.deinit();
+    const allocator = gpa.allocator();
+    const stdout = std.io.getStdOut().writer();
+
+    while (true) {
+        const ciphertext = try readHex(allocator, 4096);
+        defer allocator.free(ciphertext);
+
+        var plaintext = try allocator.alloc(u8, ciphertext.len);
+        defer allocator.free(plaintext);
+        var bestkey: u8 = 0;
+        var bestscore: u32 = 0;
+        var key: u8 = 0;
+        while (true) {
+            decrypt(plaintext, ciphertext, key);
+            const s = score(plaintext);
+            if (s > bestscore) {
+                bestkey = key;
+                bestscore = s;
+            }
+            key +%= 1;
+            if (key == 0) {
+                break;
+            }
+        }
+
+        decrypt(plaintext, ciphertext, bestkey);
+        try stdout.print("key: {x}\n{s}\n", .{ bestkey, plaintext });
+    }
+}