X25519 Key: ASN.1 Encoding Guide

TL;DR

This guide shows you how to encode an X25519 private key using ASN.1 (Abstract Syntax Notation One). This is often needed when storing keys in standard formats like PKCS#8 or for compatibility with certain crypto libraries and protocols.

Encoding X25519 Private Keys with ASN.1

1. Understand the Basics

• ASN.1: A way to describe data structures in a platform-independent manner. It doesn’t define *how* the data is stored, just what it looks like.
• DER (Distinguished Encoding Rules): A specific rule set for encoding ASN.1 data. We’ll be using DER here because it’s common and well-defined.
• X25519 Private Key: A 32-byte random number.

ASN.1 Structure

We’ll encode the X25519 private key as a sequence containing an integer.

• Sequence: Represents a collection of data elements.
• Integer: The 32-byte X25519 private key itself. It’s important to use the correct encoding for integers – typically, we want it to be positive and not have leading zeros.

Using OpenSSL (Recommended)

OpenSSL is a powerful toolkit for crypto operations. It’s available on most Linux distributions and macOS, and pre-built binaries are available for Windows.

• Generate a Private Key (if you don’t have one): This step isn’t about encoding but shows how to get the key we need.

  openssl genpkey -algorithm x25519 | openssl pkey -text -noout

  Note down the ‘Private Key’ section. You will need the raw bytes from this output.

• Encode with OpenSSL: Use the following command to encode the private key.

  echo -n "YOUR_RAW_PRIVATE_KEY_BYTES" | openssl asn1parse -i -dump -octal

  Replace YOUR_RAW_PRIVATE_KEY_BYTES with the actual 32-byte hexadecimal representation of your private key. The -octal flag ensures that the output is in a standard octal format.

Using Python (with pyasn1)

If you prefer a programmatic approach, use the pyasn1 library. Install it with: pip install pyasn1

• Python Code Example:

  from pyasn1 import encodable, decode, encode
  from pyasn1.type import Integer
  
  private_key_bytes = b'YOUR_RAW_PRIVATE_KEY_BYTES'
  
  # Create an ASN.1 integer object
  key_int = Integer(private_key_bytes)
  
  # Encode the key as a sequence containing the integer
  encoded_key = encode([key_int])
  
  print(encoded_key.hex())

  Replace YOUR_RAW_PRIVATE_KEY_BYTES with your actual private key bytes.

Verification

• The encoded output should start with 30... (sequence tag) followed by the length of the sequence, and then the integer tag (02...) and its length. The rest is the key data in hexadecimal format.
• You can use an online ASN.1 decoder to verify the structure if needed: https://lapo.it/asn1/

Common Issues

• Incorrect Integer Encoding: Ensure your key is encoded as a positive integer without leading zeros. OpenSSL usually handles this correctly, but double-check if using custom code.
• Byte Order: X25519 keys are generally represented in big-endian byte order. Ensure your encoding tools use the correct byte order.