fix(issue-88): Fix decoding asn1 code with empty bitstring (#93)

* fix(issue-88): Fix decoding asn1 code with empty bitstring

* Fix formatting

* Remove octets validation

Author: grok-rs

crates/asn1-parser/src/string/bit_string.rs

@@ -32,7 +32,10 @@ impl BitString<'_> {
     }
 
     pub fn bits_amount(&self) -> usize {
-        (self.octets.as_ref().len() - 1) * 8 - usize::from(self.octets.as_ref()[0])
+        let data_len = self.octets.len() - 1;
+        let padding = usize::from(self.octets[0]);
+
+        (data_len * 8).saturating_sub(padding)
     }
 
     /// Creates a new [BitString] from amount of bits and actual bits buffer
@@ -100,7 +103,12 @@ impl<'data> Asn1ValueDecoder<'data> for BitString<'data> {
     fn decode(_: Tag, reader: &mut Reader<'data>) -> Asn1Result<Self> {
         let data = reader.read_remaining();
 
-        let inner = if !data.is_empty() {
+        if data.is_empty() {
+            return Err(Error::from("BitString must have at least one byte (unused bits)"));
+        }
+
+        let inner = if data.len() > 1 {
+            // Check len > 1 since first byte is unused bits
             let mut inner_reader = Reader::new(&data[1..]);
             inner_reader.set_next_id(reader.next_id());
             inner_reader.set_offset(reader.full_offset() - data.len());

crates/asn1-parser/tests/decode_encode.rs

@@ -1,6 +1,6 @@
 use std::sync::Once;
 
-use asn1_parser::{Asn1, Asn1Decoder, Asn1Encoder, Asn1Type, MetaInfo, ObjectIdentifier, Taggable};
+use asn1_parser::{Asn1, Asn1Decoder, Asn1Encoder, Asn1Type, BitString, MetaInfo, ObjectIdentifier, Tag, Taggable};
 use prop_strategies::any_asn1_type;
 use proptest::proptest;
 
@@ -305,3 +305,52 @@ fn test_2() {
     let asn1 = Asn1::decode_buff(raw).unwrap();
     println!("{:?}", asn1);
 }
+
+#[test]
+fn empty_bitstring() {
+    init_logging();
+
+    let raw = [0x03, 0x01, 0x00];
+    let asn1 = Asn1::decode_buff(&raw).expect("Failed to decode empty BitString");
+
+    assert_eq!(asn1.inner_asn1().tag(), Tag::from(3), "Tag should be 0x03");
+
+    if let Asn1Type::BitString(bitstring) = asn1.inner_asn1() {
+        assert_eq!(bitstring.raw_bits(), &[0], "Raw bits should be [0]");
+        assert_eq!(bitstring.bits_amount(), 0, "Bits amount should be 0");
+        assert!(bitstring.inner().is_none(), "Inner should be None");
+    } else {
+        panic!("Expected BitString type");
+    }
+
+    // Test encoding back
+    let mut encoded = vec![0; asn1.needed_buf_size()];
+    asn1.encode_buff(&mut encoded)
+        .expect("Failed to encode empty BitString");
+    assert_eq!(encoded, raw, "Encoded bytes should match original");
+
+    // Test creating an empty BitString directly
+    let empty_bits = BitString::from_raw_vec(0, vec![]).expect("Failed to create empty BitString");
+    assert_eq!(empty_bits.raw_bits(), &[0], "Created raw bits should be [0]");
+    assert_eq!(empty_bits.bits_amount(), 0, "Created bits amount should be 0");
+    assert!(empty_bits.inner().is_none(), "Created inner should be None");
+
+    // Test invalid decoding: BitString with length 0 (invalid in DER)
+    let invalid_raw = [0x03, 0x00];
+    assert!(
+        Asn1::decode_buff(&invalid_raw).is_err(),
+        "Decoding length 0 should fail"
+    );
+
+    // Test invalid creation: too many bits
+    assert!(
+        BitString::from_raw_vec(1, vec![]).is_err(),
+        "Creating with too many bits should fail"
+    );
+
+    // Test invalid creation: too many unused bits
+    assert!(
+        BitString::from_raw_vec(0, vec![0]).is_err(),
+        "Creating with excess unused bits should fail"
+    );
+}

src/asn1/scheme/strings.rs

@@ -1,3 +1,5 @@
+use std::fmt::Write;
+
 use asn1_parser::{
     OwnedBitString, OwnedBmpString, OwnedGeneralString, OwnedIA5String, OwnedNumericString, OwnedOctetString,
     OwnedPrintableString, OwnedRawAsn1EntityData, OwnedUtf8String, OwnedVisibleString,
@@ -68,15 +70,20 @@ pub struct BitStringNodeProps {
 
 #[function_component(BitStringNode)]
 pub fn bit_string(props: &BitStringNodeProps) -> Html {
-    let bits = props.node.raw_bits()[1..]
-        .iter()
-        .map(|byte| format!("{:08b}", byte))
-        .fold(String::new(), |mut ac, new| {
-            ac.push_str(&new);
-            ac
-        });
+    let raw_bits = props.node.raw_bits();
     let bits_amount = props.node.bits_amount();
-    let bits = &bits[0..bits_amount];
+
+    let bits = if raw_bits.len() > 1 {
+        let mut bits = String::with_capacity((raw_bits.len() - 1) * 8);
+        for byte in &raw_bits[1..] {
+            write!(bits, "{:08b}", byte).unwrap();
+        }
+        bits
+    } else {
+        String::new()
+    };
+
+    let display_bits = &bits[0..bits_amount.min(bits.len())];
 
     let offset = props.meta.tag_position();
     let length_len = props.meta.length_range().len();
@@ -99,7 +106,7 @@ pub fn bit_string(props: &BitStringNodeProps) -> Html {
                 <div class="terminal-asn1-node">
                     <NodeOptions node_bytes={RcSlice::from(props.meta.raw_bytes())} {offset} {length_len} {data_len} name={String::from("BitString")} />
                     <span class="asn1-node-info-label">{format!("({} bits)", bits_amount)}</span>
-                    <span class="asn-simple-value">{bits}</span>
+                    <span class="asn-simple-value">{display_bits}</span>
                 </div>
             }
         }