X.509 certificates use ASN.1 (Abstract Syntax Notation One) encoding for their structure. The DER (Distinguished Encoding Rules) format provides the binary representation of this data. When working with certificate parsing or cryptographic operations, you often need direct access to the raw ASN.1 components.


Here are the most effective methods to extract raw ASN.1 data:

Using OpenSSL Command Line
openssl asn1parse -in certificate.pem -dump
This outputs the entire ASN.1 structure with offsets. For specific components:
openssl x509 -in certificate.pem -subject -issuer -noout

Python Implementation
Using the pyasn1 and pyasn1-modules libraries:

from pyasn1_modules import rfc2459
from pyasn1.codec.der import decoder, encoder
import binascii

with open('certificate.der', 'rb') as f:
    cert = f.read()

cert_asn1 = decoder.decode(cert, asn1Spec=rfc2459.Certificate())[0]

# Get raw hex of subject
subject_hex = binascii.hexlify(encoder.encode(cert_asn1['tbsCertificate']['subject']))
print(f"Subject HEX: {subject_hex.decode('ascii')}")

# Get raw hex of issuer  
issuer_hex = binascii.hexlify(encoder.encode(cert_asn1['tbsCertificate']['issuer']))
print(f"Issuer HEX: {issuer_hex.decode('ascii')}")



For low-level parsing without libraries:

import binascii

def parse_asn1_length(data):
    # Implementation of ASN.1 length parsing
    pass

def extract_component(cert_bytes, component_tag):
    # Search for tag and return component
    pass

# Example usage:
with open('certificate.der', 'rb') as f:
    cert = f.read()

subject = extract_component(cert, b'\x30')  # SEQUENCE tag for subject
issuer = extract_component(cert, b'\x30')   # SEQUENCE tag for issuer

print(binascii.hexlify(subject).decode('ascii'))
print(binascii.hexlify(issuer).decode('ascii'))



For better understanding, use these tools:

Online ASN.1 decoder: https://lapo.it/asn1js/
Wireshark's ASN.1 dissector
dumpasn1 command line tool



Raw ASN.1 extraction is particularly useful for:

Certificate fingerprinting and comparison
Custom certificate validation logic
Debugging certificate parsing issues
Security research and analysis



When processing large numbers of certificates:

Prefer streaming parsers over loading entire certs
Cache frequently accessed components
Consider compiled implementations for performance-critical applications

X.509 certificates use ASN.1 (Abstract Syntax Notation One) for data encoding, typically in DER (Distinguished Encoding Rules) format. Each component like subject and issuer fields are encoded as ASN.1 SEQUENCE objects with specific tags.

For command-line operations, OpenSSL provides the most direct approach:

openssl x509 -in cert.pem -inform PEM -outform DER -out cert.der
xxd -p cert.der | tr -d '\n' > cert.hex

To locate specific components programmatically:

from cryptography import x509
from cryptography.hazmat.primitives import serialization

with open("cert.pem", "rb") as f:
    cert = x509.load_pem_x509_certificate(f.read())

# Get raw DER bytes of entire subject field
subject_der = cert.subject.public_bytes(serialization.Encoding.DER)

import binascii
print(binascii.hexlify(subject_der).decode('ascii'))

For more advanced parsing of the ASN.1 structure:

from pyasn1_modules import rfc5280
from pyasn1.codec.der import decoder

der_bytes = open('cert.der', 'rb').read()
cert = decoder.decode(der_bytes, asn1Spec=rfc5280.Certificate())[0]

# Extract raw issuer bytes
issuer_der = bytes(cert['tbsCertificate']['issuer'])
print(issuer_der.hex())

When parsing manually, remember that ASN.1 uses TLV (Tag-Length-Value) format:

30 82 01 2F  - SEQUENCE (tag 0x30) of length 0x012F
  31 0B      - SET (tag 0x31) of length 0x0B
    30 09    - SEQUENCE of length 0x09
      06 03  - OID tag (0x06) of length 0x03
        55 04 06 - Country OID (2.5.4.6)
      13 02  - PrintableString tag (0x13) of length 0x02
        55 53 - "US"

For quick verification, use OpenSSL's ASN.1 parser:

openssl asn1parse -in cert.der -inform DER -i

This will show the complete structure with byte offsets, helping you identify the exact location of subject/issuer components.