(WIP) X.509 certificate parser

Cargo.toml

@@ -10,3 +10,5 @@ log = "*"
 rand = "*"
 num = "*"
 enum_primitive = "*"
+rustc-serialize = "*"
+chrono = "*"

src/der/bit_string.rs

@@ -0,0 +1,63 @@
+use der::{Tag, FromValue, DerResult};
+use super::DerErrorKind::InvalidVal;
+
+// there are *two* BIT STRING types.
+// the first one is, well, bit vector.
+// all bits except for unused ones are meaningful:
+// for example, `0000_0000` != `0000` since the first one has 8 bits
+// but the second one has 4 bits.
+//
+// the second one is bitfield structure. this works in a different way!
+// for example, in the following structure:
+// ```
+// example ::= BIT STRING {
+//     first (0),
+//     second (1),
+//     third (2)
+// }
+// ```
+//
+// `{ first = false, second = true, third = false }` is encoded to `01`,
+// not `010`. that is, all trailing zeros are removed.
+// it is specified in x.680 (22.7) and x.690 (11.2.2) in a horrible way.
+//
+// this module only defines the first bit vector type. for the second one,
+// use `bit_string_fields` macro.
+
+#[derive(Clone, PartialEq, Debug)]
+pub struct BitString {
+    pub unused_bits: u8,
+    pub data: Vec<u8>,
+}
+
+from_value!(BitString: Tag::BitString);
+
+/// Parse DER value and return (unused bits, bitstring value).
+pub fn from_der<'a>(value: &'a [u8]) -> DerResult<(u8, &'a [u8])> {
+    let len = value.len();
+    if len == 0 {
+        return der_err!(InvalidVal, "found zero-length BitString");
+    }
+
+    let unused_bits = value[0];
+    if unused_bits >= 8 {
+        return der_err!(InvalidVal, "unused bits >= 8 found");
+    }
+    if unused_bits > 0 && len == 1 {
+        return der_err!(InvalidVal, "unused bits != 0 but no bits found");
+    }
+
+    Ok((unused_bits, &value[1..]))
+}
+
+impl FromValue for BitString {
+    fn from_value(value: &[u8]) -> DerResult<BitString> {
+        let (unused_bits, data) = try!(from_der(value));
+        let data = data.to_vec();
+
+        Ok(BitString {
+            unused_bits: unused_bits,
+            data: data,
+        })
+    }
+}