Solution for 'Implement repeating-key XOR' challenge

http://cryptopals.com/sets/1/challenges/5/

Problem Statement:

Here is the opening stanza of an important work of the English language:

Burning 'em, if you ain't quick and nimble
I go crazy when I hear a cymbal

Encrypt it, under the key "ICE", using repeating-key XOR.

In repeating-key XOR, you'll sequentially apply each byte of the key; the first
byte of plaintext will be XOR'd against I, the next C, the next E, then I again
for the 4th byte, and so on.

It should come out to:
0b3637272a2b2e63622c2e69692a23693a2a3c6324202d623d63343c2a26226324272765272
a282b2f20430a652e2c652a3124333a653e2b2027630c692b20283165286326302e27282f

Encrypt a bunch of stuff using your repeating-key XOR function. Encrypt your
mail. Encrypt your password file. Your .sig file. Get a feel for it. I promise,
we aren't wasting your time with this.

Usage:

> cat in
Burning 'em, if you ain't quick and nimble
I go crazy when I hear a cymbal

> ./target/debug/xor "ICE" < in

Cargo.toml

@@ -2,3 +2,7 @@
 name = "cryptopals"
 version = "0.1.0"
 authors = ["Jake Kerr <[email protected]>"]
+
+[[bin]]
+name = "xor"
+path = "src/challenges/05_repeating_key_xor.rs"

src/challenges/05_repeating_key_xor.rs

@@ -0,0 +1,21 @@
+extern crate cryptopals;
+
+use std::env;
+use std::io::{stdin, stdout, Read, Write};
+use cryptopals::conversions::hex_to_string;
+
+fn main() {
+    let args: Vec<_> = env::args().collect();
+    if args.len() != 2 {
+        panic!("Requires one argument which is the key with which to do repeating key xor \
+                over stdin's bytes");
+    }
+    let key = args.get(1).unwrap().bytes().cycle();
+    let stdin = stdin().bytes().map(|b| b.unwrap());
+    let mut stdout = stdout();
+    for (a, b) in stdin.zip(key) {
+        if a == 10 { continue } // Line feed
+        write!(&mut stdout, "{}", hex_to_string(&[a ^ b]));
+    }
+    write!(&mut stdout, "\n");
+}