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RustCrypto: ASN.1 DER

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Pure Rust embedded-friendly implementation of the Distinguished Encoding Rules (DER) for Abstract Syntax Notation One (ASN.1) as described in ITU X.690.



This crate provides a no_std-friendly implementation of a subset of ASN.1 DER necessary for decoding/encoding the following cryptography-related formats implemented as crates maintained by the RustCrypto project:

  • pkcs1: RSA Cryptography Specifications
  • pkcs5: Password-Based Cryptography Specification
  • pkcs7: Cryptographic Message Syntax
  • pkcs8: Private-Key Information Syntax Specification
  • sec1: Elliptic Curve Cryptography
  • spki: X.509 Subject Public Key Info

The core implementation avoids any heap usage (with convenience methods that allocate gated under the off-by-default alloc feature).

The DER decoder attempts to ensure that the input document is in canonical form, and will return errors if non-canonical productions are encountered.


  • Rich support for ASN.1 types used by PKCS/PKIX documents
  • Performs DER canonicalization checks at decoding time
  • no_std friendly: supports “heapless” usage or optionally supports the alloc crate if desired
  • No hard dependencies! Self-contained implementation with optional integrations with the following crates, all of which are no_std friendly:
    • const-oid: const-friendly OID implementation
    • crypto-bigint: constant-time bignum library
    • time crate: date/time library

Minimum Supported Rust Version

This crate requires Rust 1.56 at a minimum.

We may change the MSRV in the future, but it will be accompanied by a minor version bump.


Licensed under either of:

at your option.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.


Decodable and Encodable traits

The Decodable and Encodable traits are the core abstractions on which this crate is built and control what types can be (de)serialized as ASN.1 DER.

The traits are impl’d for the following Rust core types:

  • (): ASN.1 NULL. See also Null.
  • bool: ASN.1 BOOLEAN.
  • i8, i16, i32, i64, i128: ASN.1 INTEGER.
  • u8, u16, u32, u64, u128: ASN.1 INTEGER.
  • str, String: ASN.1 UTF8String. String requires alloc feature. See also Utf8String. Requires alloc feature. See also SetOf.
  • Option: ASN.1 OPTIONAL.
  • [SystemTime][std::time::SystemTime]: ASN.1 GeneralizedTime. Requires std feature.
  • Vec: ASN.1 SEQUENCE OF. Requires alloc feature.
  • [T; N]: ASN.1 SEQUENCE OF. See also SequenceOf.

The following ASN.1 types provided by this crate also impl these traits:

Context specific fields can be modeled using these generic types:


The following example implements X.509’s AlgorithmIdentifier message type as defined in RFC 5280 Section

The ASN.1 schema for this message type is as follows:

AlgorithmIdentifier  ::=  SEQUENCE  {
     algorithm               OBJECT IDENTIFIER,
     parameters              ANY DEFINED BY algorithm OPTIONAL  }

Structured ASN.1 messages are typically encoded as a SEQUENCE, which this crate maps to a Rust struct using the Sequence trait. This trait is bounded on the Decodable trait and provides a blanket impl of the Encodable trait, so any type which impls Sequence can be used for both decoding and encoding.

The Decoder and Encoder types provide the decoding/encoding API respectively, and are designed to work in conjunction with concrete ASN.1 types which impl the Decodable and Encodable traits, including all types which impl the Sequence trait.

The following code example shows how to define a struct which maps to the above schema, as well as impl the Sequence trait for that struct:

// Note: the following example does not require the `std` feature at all.
// It does leverage the `alloc` feature, but also provides instructions for
// "heapless" usage when the `alloc` feature is disabled.
use der::{
    asn1::{Any, ObjectIdentifier},
    Decodable, Decoder, Encodable, Sequence

/// X.509 `AlgorithmIdentifier`.
#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub struct AlgorithmIdentifier<'a> {
    /// This field contains an ASN.1 `OBJECT IDENTIFIER`, a.k.a. OID.
    pub algorithm: ObjectIdentifier,

    /// This field is `OPTIONAL` and contains the ASN.1 `ANY` type, which
    /// in this example allows arbitrary algorithm-defined parameters.
    pub parameters: Option<Any<'a>>

impl<'a> Decodable<'a> for AlgorithmIdentifier<'a> {
    fn decode(decoder: &mut Decoder<'a>) -> der::Result<Self> {
        // The `Decoder::sequence` method decodes an ASN.1 `SEQUENCE` tag
        // and length then calls the provided `FnOnce` with a nested
        // `der::Decoder` which can be used to decode it.
        decoder.sequence(|decoder| {
            // The `der::Decoder::Decode` method can be used to decode any
            // type which impls the `Decodable` trait, which is impl'd for
            // all of the ASN.1 built-in types in the `der` crate.
            // Note that if your struct's fields don't contain an ASN.1
            // built-in type specifically, there are also helper methods
            // for all of the built-in types supported by this library
            // which can be used to select a specific type.
            // For example, another way of decoding this particular field,
            // which contains an ASN.1 `OBJECT IDENTIFIER`, is by calling
            // `decoder.oid()`. Similar methods are defined for other
            // ASN.1 built-in types.
            let algorithm = decoder.decode()?;

            // This field contains an ASN.1 `OPTIONAL` type. The `der` crate
            // maps this directly to Rust's `Option` type and provides
            // impls of the `Decodable` and `Encodable` traits for `Option`.
            // To explicitly request an `OPTIONAL` type be decoded, use the
            // `decoder.optional()` method.
            let parameters = decoder.decode()?;

            // The value returned from the provided `FnOnce` will be
            // returned from the `any.sequence(...)` call above.
            // Note that the entire sequence body *MUST* be consumed
            // or an error will be returned.
            Ok(Self { algorithm, parameters })

impl<'a> Sequence<'a> for AlgorithmIdentifier<'a> {
    // The `Sequence::fields` method is used for encoding and functions as
    // a visitor for all of the fields in a message.
    // To implement it, you must define a slice containing `Encodable`
    // trait objects, then pass it to the provided `field_encoder`
    // function, which is implemented by the `der` crate and handles
    // message serialization.
    // Trait objects are used because they allow for slices containing
    // heterogeneous field types, and a callback is used to allow for the
    // construction of temporary field encoder types. The latter means
    // that the fields of your Rust struct don't necessarily need to
    // impl the `Encodable` trait, but if they don't you must construct
    // a temporary wrapper value which does.
    // Types which impl the `Sequence` trait receive blanket impls of both
    // the `Encodable` and `Tagged` traits (where the latter is impl'd as
    // `Tagged::TAG = der::Tag::Sequence`.
    fn fields<F, T>(&self, field_encoder: F) -> der::Result<T>
        F: FnOnce(&[&dyn Encodable]) -> der::Result<T>,
        field_encoder(&[&self.algorithm, &self.parameters])

// Example parameters value: OID for the NIST P-256 elliptic curve.
let parameters = "1.2.840.10045.3.1.7".parse::<ObjectIdentifier>().unwrap();

// We need to convert `parameters` into an `Any<'a>` type, which wraps a
// `&'a [u8]` byte slice.
// To do that, we need owned DER-encoded data so that we can have
// `Any` borrow a reference to it, so we have to serialize the OID.
// When the `alloc` feature of this crate is enabled, any type that impls
// the `Encodable` trait including all ASN.1 built-in types and any type
// which impls `Sequence` can be serialized by calling `Encodable::to_vec()`.
// If you would prefer to avoid allocations, you can create a byte array
// as backing storage instead, pass that to `der::Encoder::new`, and then
// encode the `parameters` value using `encoder.encode(parameters)`.
let der_encoded_parameters = parameters.to_vec().unwrap();

let algorithm_identifier = AlgorithmIdentifier {
    // OID for `id-ecPublicKey`, if you're curious
    algorithm: "1.2.840.10045.2.1".parse().unwrap(),

    // `Any<'a>` impls `TryFrom<&'a [u8]>`, which parses the provided
    // slice as an ASN.1 DER-encoded message.
    parameters: Some(der_encoded_parameters.as_slice().try_into().unwrap())

// Serialize the `AlgorithmIdentifier` created above as ASN.1 DER,
// allocating a `Vec<u8>` for storage.
// As mentioned earlier, if you don't have the `alloc` feature enabled you
// can create a fix-sized array instead, then call `Encoder::new` with a
// reference to it, then encode the message using
// `encoder.encode(algorithm_identifier)`, then finally `encoder.finish()`
// to obtain a byte slice containing the encoded message.
let der_encoded_algorithm_identifier = algorithm_identifier.to_vec().unwrap();

// Deserialize the `AlgorithmIdentifier` we just serialized from ASN.1 DER
// using `der::Decodable::from_bytes`.
let decoded_algorithm_identifier = AlgorithmIdentifier::from_der(

// Ensure the original `AlgorithmIdentifier` is the same as the one we just
// decoded from ASN.1 DER.
assert_eq!(algorithm_identifier, decoded_algorithm_identifier);

Custom derive support

When the derive feature of this crate is enabled, the following custom derive macros are available:

  • Choice: derive for CHOICE enum (see [der_derive::Choice])
  • [Enumerated]: derive for ENUMERATED enum (see [der_derive::Enumerated])
  • Sequence: derive for SEQUENCE struct (see [der_derive::Sequence])

Derive Sequence for struct

The following is a code example of how to use the Sequence custom derive:

use der::{asn1::{Any, ObjectIdentifier}, Encodable, Decodable, Sequence};

/// X.509 `AlgorithmIdentifier` (same as above)
#[derive(Copy, Clone, Debug, Eq, PartialEq, Sequence)] // NOTE: added `Sequence`
pub struct AlgorithmIdentifier<'a> {
    /// This field contains an ASN.1 `OBJECT IDENTIFIER`, a.k.a. OID.
    pub algorithm: ObjectIdentifier,

    /// This field is `OPTIONAL` and contains the ASN.1 `ANY` type, which
    /// in this example allows arbitrary algorithm-defined parameters.
    pub parameters: Option<Any<'a>>

// Example parameters value: OID for the NIST P-256 elliptic curve.
let parameters_oid = "1.2.840.10045.3.1.7".parse::<ObjectIdentifier>().unwrap();

let algorithm_identifier = AlgorithmIdentifier {
    // OID for `id-ecPublicKey`, if you're curious
    algorithm: "1.2.840.10045.2.1".parse().unwrap(),

    // `Any<'a>` impls `From<&'a ObjectIdentifier>`, allowing OID constants to
    // be directly converted to an `Any` type for this use case.
    parameters: Some(Any::from(&parameters_oid))

// Encode
let der_encoded_algorithm_identifier = algorithm_identifier.to_vec().unwrap();

// Decode
let decoded_algorithm_identifier = AlgorithmIdentifier::from_der(

assert_eq!(algorithm_identifier, decoded_algorithm_identifier);

For fields which don’t directly impl Decodable and Encodable, you can add annotations to convert to an intermediate ASN.1 type first, so long as that type impls TryFrom and Into for the ASN.1 type.

For example, structs containing &'a [u8] fields may want them encoded as either a BIT STRING or OCTET STRING. By using the #[asn1(type = "BIT STRING")] annotation it’s possible to select which ASN.1 type should be used.

Building off the above example:

/// X.509 `SubjectPublicKeyInfo` (SPKI)
#[derive(Copy, Clone, Debug, Eq, PartialEq, Sequence)]
pub struct SubjectPublicKeyInfo<'a> {
    /// X.509 `AlgorithmIdentifier`
    pub algorithm: AlgorithmIdentifier<'a>,

    /// Public key data
    pub subject_public_key: BitString<'a>,

See also

For more information about ASN.1 DER we recommend the following guides:



  • Module containing all of the various ASN.1 built-in types supported by this library.


  • Date-and-time type shared by multiple ASN.1 types (e.g. GeneralizedTime, UTCTime).
  • DER decoder.
  • DER encoder.
  • Error type.
  • ASN.1 DER headers: tag + length component of TLV-encoded values
  • ASN.1-encoded length.
  • ASN.1 tag numbers (i.e. lower 5 bits of a Tag).



Type Definitions