diff --git a/src/letter_frequencies.zig b/src/letter_frequencies.zig
new file mode 100644
index 0000000..e87014d
--- /dev/null
+++ b/src/letter_frequencies.zig
@@ -0,0 +1,71 @@
+const std = @import("std");
+const Freq = struct { letter: u8, frequency: f32 };
+
+// relative frequencies a-z, with lowest scaled to 1.0
+// from some random website
+const en = [_]f32{
+    43.3,
+    10.5,
+    23.1,
+    17.2,
+    56.8,
+    9.2,
+    12.5,
+    15.3,
+    38.4,
+    1.0,
+    5.6,
+    27.9,
+    15.3,
+    33.9,
+    36.5,
+    16.1,
+    1.0,
+    38.6,
+    29.2,
+    35.4,
+    18.5,
+    5.1,
+    6.5,
+    1.4,
+    9.0,
+    1.3,
+};
+
+pub fn score_en(ascii: u8) f32 {
+    const ascii_lower = std.ascii.toLower(ascii);
+    const index: i32 = @as(i32, ascii_lower) - @as(i32, 'a');
+
+    if (index >= 0 and index < en.len) {
+        if (ascii_lower != ascii) {
+            // don't score uppercase chars as they shouldn't be that
+            // frequent, except someone screams. Retrospectively
+            // pulled that one out of my ass but works.
+            return 0.0;
+        }
+        return en[@intCast(index)] / 2;
+    }
+
+    if (std.ascii.isPrint(ascii)) {
+        // we don't know that char, but it is still printable ascii,
+        // so give essentially a score of 0. Pulled that one out of my
+        // ass too.
+        return 0.0;
+    }
+
+    if (std.ascii.isWhitespace(ascii)) {
+        // we don't know that char, but it is some printable
+        // whitespace, so give essentially a score of 0. Same, I have
+        // a very roomy ass.
+        return 0.0;
+    }
+
+    return -penalty(); // unscoreable
+}
+
+fn penalty() comptime_float {
+    var sum: comptime_float = 0.0;
+    inline for (en) |f| sum += f;
+
+    return sum / @as(f32, @floatFromInt(en.len));
+}
diff --git a/src/main.zig b/src/main.zig
index a830a36..f029fbc 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -2,6 +2,7 @@ const std = @import("std");
 const b64 = @import("base64.zig");
 const hex = @import("hex.zig");
 const xor = @import("xor.zig");
+const xor_crack = @import("xor_crack.zig");
 
 pub fn main() !void {
     var gpa = std.heap.GeneralPurposeAllocator(.{}){};
@@ -48,4 +49,27 @@ pub fn main() !void {
 
         try stdout.print("{s}", .{out_hex});
     }
+
+    if (std.mem.eql(u8, args[1], "crack-xor")) {
+        const in_a = args[2];
+
+        const buf_a = try hex.decode(allocator, in_a);
+        defer allocator.free(buf_a);
+
+        const oracle = xor_crack.Oracle{
+            .decrypt = xor.xor_byte_noalloc,
+        };
+
+        const res = try xor_crack.single(allocator, buf_a, oracle) orelse {
+            try stdout.print("Did not find a solution", .{});
+            return;
+        };
+
+        try stdout.print("Found solution key={c}, score={d}\n", .{ res[0], res[1] });
+
+        const out = try xor.xor_byte(allocator, buf_a, res[0]);
+        defer allocator.free(out);
+
+        try stdout.print("{s}\n", .{out});
+    }
 }
diff --git a/src/xor.zig b/src/xor.zig
index 762a095..d52c89a 100644
--- a/src/xor.zig
+++ b/src/xor.zig
@@ -12,3 +12,22 @@ pub fn xor_buffers(allocator: std.mem.Allocator, buf_a: []u8, buf_b: []u8) ![]u8
 
     return out;
 }
+
+/// buf_a ^ byte_b. Caller must free result.
+pub fn xor_byte(allocator: std.mem.Allocator, buf_a: []u8, byte_b: u8) ![]u8 {
+    var out: []u8 = try allocator.alloc(u8, buf_a.len);
+
+    for (0..buf_a.len) |i| {
+        out[i] = buf_a[i] ^ byte_b;
+    }
+
+    return out;
+}
+
+/// buf_a ^ byte_b. Caller must provide out buffer long enough to hold
+/// result (buf_a.len == out.len).
+pub fn xor_byte_noalloc(buf_a: []const u8, byte_b: u8, out: []u8) void {
+    for (0..buf_a.len) |i| {
+        out[i] = buf_a[i] ^ byte_b;
+    }
+}
diff --git a/src/xor_crack.zig b/src/xor_crack.zig
new file mode 100644
index 0000000..f757327
--- /dev/null
+++ b/src/xor_crack.zig
@@ -0,0 +1,58 @@
+const std = @import("std");
+
+const score_en = @import("letter_frequencies.zig").score_en;
+const penalty = @import("letter_frequencies.zig").penalty();
+
+pub const Oracle = struct {
+    decrypt: fn (in_buf: []u8, key: u8, out_buf: []u8) void,
+};
+
+/// Crack a buffer encrypted with single-byte XOR
+pub fn single(allocator: std.mem.Allocator, cipher: []u8, oracle: Oracle) !?struct { u8, f32 } {
+    var key_scores = std.AutoHashMap(u8, f32).init(allocator);
+    defer key_scores.deinit();
+
+    const out = try allocator.alloc(u8, cipher.len);
+    defer allocator.free(out);
+
+    for (32..127) |c| {
+        oracle.decrypt(cipher, @intCast(c), out);
+
+        var buf_score: f32 = 0.0;
+        for (out) |o| buf_score += score_en(o);
+
+        try key_scores.put(@intCast(c), buf_score);
+    }
+
+    var init_search_it = key_scores.iterator();
+    var max_key: u8 = undefined;
+    var max_score: f32 = undefined;
+    if (init_search_it.next()) |entry| {
+        max_key = entry.key_ptr.*;
+        max_score = entry.value_ptr.*;
+    } else {
+        return null;
+    }
+
+    var it = key_scores.iterator();
+    var is_init = true;
+    while (it.next()) |entry| {
+        if (entry.value_ptr.* == max_score and !is_init) {
+            std.log.warn("Found equal max scores ({c}, {d}), ({c}, {d})", .{
+                max_key,
+                max_score,
+                entry.key_ptr.*,
+                entry.value_ptr.*,
+            });
+        }
+
+        if (entry.value_ptr.* > max_score) {
+            max_key = entry.key_ptr.*;
+            max_score = entry.value_ptr.*;
+        }
+
+        is_init = false;
+    }
+
+    return .{ max_key, max_score };
+}