der_derive: fix derive(BitString): always encode max length (#1733)

Encoded length is now data-independent

Author: dishmaker

der/src/asn1/bit_string.rs

@@ -1,6 +1,6 @@
 //! ASN.1 `BIT STRING` support.
 
-pub mod fixed_len_bit_string;
+pub mod allowed_len_bit_string;
 
 use crate::{
     BytesRef, DecodeValue, DerOrd, EncodeValue, Error, ErrorKind, FixedTag, Header, Length, Reader,

der/src/asn1/bit_string/fixed_len_bit_string.rs der/src/asn1/bit_string/allowed_len_bit_string.rs

@@ -27,15 +27,15 @@ use crate::{Error, ErrorKind, Tag};
 ///     flag4: bool,
 /// }
 /// ```
-pub trait FixedLenBitString {
+pub trait AllowedLenBitString {
     /// Implementer must specify how many bits are allowed
     const ALLOWED_LEN_RANGE: RangeInclusive<u16>;
 
     /// Returns an error if the bitstring is not in expected length range
     fn check_bit_len(bit_len: u16) -> Result<(), Error> {
         let allowed_len_range = Self::ALLOWED_LEN_RANGE;
 
-        // forces allowed range to eg. 3..=4
+        // forces allowed range to e.g. 3..=4
         if !allowed_len_range.contains(&bit_len) {
             Err(ErrorKind::Length {
                 tag: Tag::BitString,

der/src/lib.rs

@@ -365,7 +365,7 @@ mod document;
 mod str_owned;
 
 pub use crate::{
-    asn1::bit_string::fixed_len_bit_string::FixedLenBitString,
+    asn1::bit_string::allowed_len_bit_string::AllowedLenBitString,
     asn1::{AnyRef, Choice, Sequence},
     datetime::DateTime,
     decode::{Decode, DecodeOwned, DecodeValue},

der/tests/derive.rs

@@ -778,7 +778,9 @@ mod bitstring {
         assert_eq!(reencoded, BITSTRING_EXAMPLE);
     }
 
-    /// this BitString will allow only 3..=4 bits
+    /// this BitString will allow only 3..=4 bits in Decode
+    ///
+    /// but will always Encode 4 bits
     #[derive(BitString)]
     pub struct MyBitString3or4 {
         pub bit_0: bool,
@@ -844,8 +846,8 @@ mod bitstring {
         .to_der()
         .unwrap();
 
-        // 3 bits used, 5 unused
-        assert_eq!(encoded_3_zeros, hex!("03 02 05 00"));
+        // 4 bits used, 4 unused
+        assert_eq!(encoded_3_zeros, hex!("03 02 04 00"));
     }
 
     #[test]
@@ -859,8 +861,8 @@ mod bitstring {
         .to_der()
         .unwrap();
 
-        // 3 bits used, 5 unused
-        assert_eq!(encoded_3_zeros, hex!("03 02 05 E0"));
+        // 4 bits used, 4 unused
+        assert_eq!(encoded_3_zeros, hex!("03 02 04 E0"));
     }
 
     #[test]
@@ -892,6 +894,107 @@ mod bitstring {
         // 4 bits used, 4 unused
         assert_eq!(encoded_4_zeros, hex!("03 02 04 10"));
     }
+
+    /// ```asn1
+    /// PasswordFlags ::= BIT STRING {
+    ///     case-sensitive (0),
+    ///     local (1),
+    ///     change-disabled (2),
+    ///     unblock-disabled (3),
+    ///     initialized (4),
+    ///     needs-padding (5),
+    ///     unblockingPassword (6),
+    ///     soPassword (7),
+    ///     disable-allowed (8),
+    ///     integrity-protected (9),
+    ///     confidentiality-protected (10),
+    ///     exchangeRefData (11),
+    ///     resetRetryCounter1 (12),
+    ///     resetRetryCounter2 (13),
+    ///     context-dependent (14),
+    ///     multiStepProtocol (15)
+    /// }
+    ///  ```
+    #[derive(Clone, Debug, Eq, PartialEq, BitString)]
+    pub struct PasswordFlags {
+        /// case-sensitive (0)
+        pub case_sensitive: bool,
+
+        /// local (1)
+        pub local: bool,
+
+        /// change-disabled (2)
+        pub change_disabled: bool,
+
+        /// unblock-disabled (3)
+        pub unblock_disabled: bool,
+
+        /// initialized (4)
+        pub initialized: bool,
+
+        /// needs-padding (5)
+        pub needs_padding: bool,
+
+        /// unblockingPassword (6)
+        pub unblocking_password: bool,
+
+        /// soPassword (7)
+        pub so_password: bool,
+
+        /// disable-allowed (8)
+        pub disable_allowed: bool,
+
+        /// integrity-protected (9)
+        pub integrity_protected: bool,
+
+        /// confidentiality-protected (10)
+        pub confidentiality_protected: bool,
+
+        /// exchangeRefData (11)
+        pub exchange_ref_data: bool,
+
+        /// Second edition 2016-05-15
+        /// resetRetryCounter1 (12)
+        #[asn1(optional = "true")]
+        pub reset_retry_counter1: bool,
+
+        /// resetRetryCounter2 (13)
+        #[asn1(optional = "true")]
+        pub reset_retry_counter2: bool,
+
+        /// context-dependent (14)
+        #[asn1(optional = "true")]
+        pub context_dependent: bool,
+
+        /// multiStepProtocol (15)
+        #[asn1(optional = "true")]
+        pub multi_step_protocol: bool,
+
+        /// fake_bit_for_testing (16)
+        #[asn1(optional = "true")]
+        pub fake_bit_for_testing: bool,
+    }
+
+    const PASS_FLAGS_EXAMPLE_IN: &[u8] = &hex!("03 03 04 FF FF");
+    const PASS_FLAGS_EXAMPLE_OUT: &[u8] = &hex!("03 04 07 FF F0 00");
+
+    #[test]
+    fn decode_short_bitstring_2_bytes() {
+        let pass_flags = PasswordFlags::from_der(PASS_FLAGS_EXAMPLE_IN).unwrap();
+
+        // case-sensitive (0)
+        assert!(pass_flags.case_sensitive);
+
+        // exchangeRefData (11)
+        assert!(pass_flags.exchange_ref_data);
+
+        // resetRetryCounter1 (12)
+        assert!(!pass_flags.reset_retry_counter1);
+
+        let reencoded = pass_flags.to_der().unwrap();
+
+        assert_eq!(reencoded, PASS_FLAGS_EXAMPLE_OUT);
+    }
 }
 mod infer_default {
     //! When another crate might define a PartialEq for another type, the use of

der_derive/src/bitstring.rs

@@ -35,12 +35,26 @@ impl DeriveBitString {
 
         let type_attrs = TypeAttrs::parse(&input.attrs)?;
 
-        let fields = data
+        let fields: Vec<_> = data
             .fields
             .iter()
             .map(|field| BitStringField::new(field, &type_attrs))
             .collect::<syn::Result<_>>()?;
 
+        let mut started_optionals = false;
+        for field in &fields {
+            if !field.attrs.optional {
+                if started_optionals {
+                    abort!(
+                        input.ident,
+                        "derive `BitString` only supports trailing optional fields one after another",
+                    )
+                }
+            } else {
+                started_optionals = true;
+            }
+        }
+
         Ok(Self {
             ident: input.ident,
             generics: input.generics.clone(),
@@ -75,14 +89,18 @@ impl DeriveBitString {
 
         let mut min_expected_fields: u16 = 0;
         let mut max_expected_fields: u16 = 0;
+        let mut started_optionals = false;
         for field in &self.fields {
             max_expected_fields += 1;
 
-            if !field.attrs.optional {
+            if field.attrs.optional {
+                started_optionals = true;
+            }
+            if !started_optionals {
                 min_expected_fields += 1;
             }
         }
-        let min_expected_bytes = (min_expected_fields + 7) / 8;
+        let max_expected_bytes = (max_expected_fields + 7) / 8;
 
         for (i, field) in self.fields.iter().enumerate().rev() {
             let field_name = &field.ident;
@@ -115,7 +133,7 @@ impl DeriveBitString {
             impl ::der::FixedTag for #ident #ty_generics #where_clause {
                 const TAG: der::Tag = ::der::Tag::BitString;
             }
-            impl ::der::FixedLenBitString for #ident #ty_generics #where_clause {
+            impl ::der::AllowedLenBitString for #ident #ty_generics #where_clause {
                 const ALLOWED_LEN_RANGE: ::core::ops::RangeInclusive<u16> = #min_expected_fields..=#max_expected_fields;
             }
 
@@ -127,7 +145,7 @@ impl DeriveBitString {
                     header: ::der::Header,
                 ) -> ::core::result::Result<Self, ::der::Error> {
                     use ::der::{Decode as _, DecodeValue as _, Reader as _};
-                    use ::der::FixedLenBitString as _;
+                    use ::der::AllowedLenBitString as _;
 
 
                     let bs = ::der::asn1::BitStringRef::decode_value(reader, header)?;
@@ -147,33 +165,15 @@ impl DeriveBitString {
 
             impl #impl_generics ::der::EncodeValue for #ident #ty_generics #where_clause {
                 fn value_len(&self) -> der::Result<der::Length> {
-                    Ok(der::Length::new(#min_expected_bytes + 1))
+                    Ok(der::Length::new(#max_expected_bytes + 1))
                 }
 
                 fn encode_value(&self, writer: &mut impl ::der::Writer) -> ::der::Result<()> {
                     use ::der::Encode as _;
-                    use der::FixedLenBitString as _;
+                    use ::der::AllowedLenBitString as _;
 
                     let arr = [#(#encode_bytes),*];
-
-                    let min_bits = {
-                        let max_bits = *Self::ALLOWED_LEN_RANGE.end();
-                        let last_byte_bits = (max_bits % 8) as u8;
-                        let bs = ::der::asn1::BitStringRef::new(8 - last_byte_bits, &arr)?;
-
-                        let mut min_bits = *Self::ALLOWED_LEN_RANGE.start();
-
-                        // find last lit bit
-                        for bit_index in Self::ALLOWED_LEN_RANGE.rev() {
-                            if bs.get(bit_index as usize).unwrap_or_default() {
-                                min_bits = bit_index + 1;
-                                break;
-                            }
-                        }
-                        min_bits
-                    };
-
-                    let last_byte_bits = (min_bits % 8) as u8;
+                    let last_byte_bits = (#max_expected_fields % 8) as u8;
                     let bs = ::der::asn1::BitStringRef::new(8 - last_byte_bits, &arr)?;
                     bs.encode_value(writer)
                 }