ASN1.jl

ASN1.jl is a small Julia package for working with ASN.1 BER-encoded data. It focuses on turning BER/DER/CER byte buffers into a tree of ASNTag values and serializing those trees back to bytes.

Features

• Deserialize BER-encoded buffers into an ASNTag tree.
• Serialize ASNTag values back into BER form.
• Support primitive and constructed tags.
• Preserve tag metadata such as class, encoding, tag number, and content length form.
• Integrate with AbstractTrees.jl, so parsed values can be traversed like a tree.

Installation

Install from the Julia package manager:

using Pkg
Pkg.add(url="https://github.com/NegaScout/ASN1.jl")

Then load the package:

using ASN1

Quick start

using ASN1

# A primitive universal tag with a one-byte payload.
bytes = UInt8[0x01, 0x01, 0xFF]
tag = deserialize_ber(bytes)

println(tag)
# ASNTag(universal, primitive, false, 0, u_boolean, 1, definite_short, UInt8[...], ASNTag[])

roundtrip = serialize_ber(tag)
@assert roundtrip == bytes

Data model

The package represents BER data with the ASNTag struct. Each node stores:

• the tag class,
• whether the tag is primitive or constructed,
• the tag number and whether it uses long-form encoding,
• the encoded content length form,
• the raw content bytes, and
• child tags for constructed values.

Constructed tags are parsed recursively. Because ASNTag implements AbstractTrees.children, tree utilities from AbstractTrees.jl can be used directly.

Core API

deserialize_ber(buff::Vector{UInt8})

Parses an entire BER document recursively and returns an ASNTag. If the buffer does not contain a valid tag structure, it returns nothing.

serialize_ber(tag::ASNTag)

Serializes an ASNTag tree back into BER bytes.

deserialize_tag(buff::Vector{UInt8})

Parses a single BER tag without recursively deserializing child content.

serialized_length(tag::ASNTag)

Returns the number of bytes the tag occupies in serialized form.

Example: parsing a long-form length

using ASN1

bytes = UInt8[0x01, 0x82, 0x00, 0x01, 0x00]
tag = deserialize_ber(bytes)

@assert tag.tag_length_length == 3
@assert tag.content_length_type == 0x01
@assert tag.content == UInt8[0x00]

Tree traversal

Because the package integrates with AbstractTrees.jl, constructed values can be walked naturally:

using ASN1
using AbstractTrees

bytes = UInt8[
    0x30, 0x06,       # SEQUENCE, length 6
    0x01, 0x01, 0x00, # BOOLEAN false
    0x01, 0x01, 0xFF  # BOOLEAN true
]

root = deserialize_ber(bytes)
for node in PreOrderDFS(root)
    println(node)
end

Development

Run the test suite with:

julia --project=. -e 'using Pkg; Pkg.test()'

Project documentation sources live under docs/, and the main package code lives under src/.