trial: Encrypt and decrypt timestamp

Add functions to encrypt and decrypt a timestamp. We'll be using those
functions to write the encrypted timestamp of the first trial launch
date to a file.

Decided to go for some light encryption instead of just storing a plain
or base64-encoded value. Still need to come up with a key, but that will
be stored as a plain string in the binary.

Need a way to capture the results so we don't end up panicking. Trouble
is, 'magic-crypt' doesn't implement the required `Error` traits, so I
can't just slot it into 'error-chain'. Still trying to work out how to
deal with that.

Also add a function to get the days remaining from now.

2 files changed, 49 insertions, 3 deletions

diff --git a/src/lib.rs b/src/lib.rs
index 8e23367..27fc7a7 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -16,6 +16,7 @@ extern crate libc;
 
 #[macro_use]
 extern crate log;
+extern crate magic_crypt;
 
 #[macro_use]
 extern crate objc;
diff --git a/src/trial.rs b/src/trial.rs
index 9604f62..e716559 100644
--- a/src/trial.rs
+++ b/src/trial.rs
@@ -1,18 +1,22 @@
-use chrono::{DateTime, Local, TimeZone};
+use std::result;
 
-use errors::DurationError;
+use chrono::{DateTime, FixedOffset, Local, TimeZone};
+use magic_crypt::{self, MagicCrypt};
+
+use errors::*;
 
 // Start timestamp on October 1 at 23h
 // Trial should be valid until November 1 00h
 
 
 const DAYS_REMAINING: u8 = 30;
+const KEY: &'static str = "TODO SECRET";
 
 fn days_remaining(
     start: DateTime<Local>,
     now: DateTime<Local>,
     days_available: u8,
-) -> Result<u8, DurationError> {
+) -> result::Result<u8, DurationError> {
     let duration = (now.date() - start.date()).num_days() as u8;
 
     if duration > days_available {
@@ -24,6 +28,47 @@ fn days_remaining(
     }
 }
 
+fn days_remaining_from_now(
+    start: DateTime<Local>
+) -> result::Result<u8, DurationError> {
+    days_remaining(start, Local::now(), DAYS_REMAINING)
+}
+
+fn encode_datetime(d: DateTime<Local>) -> String {
+    let iv = initialization_vector();
+
+    let mut mc = MagicCrypt::new(KEY, magic_crypt::SecureBit::Bit64, Some(&iv));
+
+    let timestamp = mc.encrypt_str_to_base64(&d.to_rfc3339());
+
+    format!("{}//{}", timestamp, iv)
+}
+
+fn decode_datetime(s: &str) -> DateTime<FixedOffset> {
+    let encrypted: Vec<_> = s.rsplitn(2, "//").collect();
+    let timestamp = encrypted[0];
+    let iv = encrypted[1];
+
+    let mut mc = MagicCrypt::new(KEY, magic_crypt::SecureBit::Bit64, Some(&iv));
+
+    let timestamp = mc.decrypt_base64_to_string(&timestamp)
+        .expect("unable to read trial key");
+
+    DateTime::parse_from_rfc3339(&timestamp)
+        .expect("unable to parse timestamp")
+}
+
+fn initialization_vector() -> String {
+    // Multiplied by 2 for no good reason other than to make the value
+    // different from the actual timestamp.
+    (Local::now().timestamp_millis() * 2).to_string()
+}
+
+// initialize_trial_start
+// encode_datetime
+// check_days_remaining
+// days_remaining_from_now
+
 
 #[cfg(test)]
 mod tests {