Task 084: .CER File Format
Task 084: .CER File Format
File Format Specifications for .CER
The .CER file format is used for X.509 public key certificates, as defined in RFC 5280. These files typically contain a digital certificate encoded in binary DER (Distinguished Encoding Rules) format, which is an ASN.1 (Abstract Syntax Notation One) structure. Alternatively, .CER files may use PEM (Privacy-Enhanced Mail) encoding, which is base64-encoded DER with header and footer lines. The X.509 standard specifies the certificate's structure, including fields for identification, validity, and cryptographic details.
- List of Properties Intrinsic to the File Format
The properties are derived from the X.509 certificate structure and are intrinsic to the format's ASN.1 definition. They include:
- Version: The certificate version number (e.g., v1, v2, or v3).
- Serial Number: A unique integer assigned by the issuing certificate authority.
- Signature Algorithm: The algorithm used for the certificate's signature (e.g., sha256WithRSAEncryption).
- Issuer Name: The distinguished name of the issuing certificate authority.
- Validity Not Before: The date and time from which the certificate is valid.
- Validity Not After: The date and time after which the certificate expires.
- Subject Name: The distinguished name of the entity to which the certificate is issued.
- Subject Public Key Info: Includes the public key algorithm and the public key itself.
- Issuer Unique Identifier: An optional unique identifier for the issuer (present in v2 or v3).
- Subject Unique Identifier: An optional unique identifier for the subject (present in v2 or v3).
- Extensions: A sequence of optional extensions (present in v3), each with an object identifier (OID), criticality flag, and value (e.g., Key Usage, Basic Constraints, Subject Alternative Name).
- Signature Value: The digital signature over the TBSCertificate structure.
These properties are encoded in ASN.1 DER format, ensuring interoperability across systems.
- Two Direct Download Links for .CER Files
Note that .CER and .CRT extensions are often used interchangeably for X.509 certificates. The following links provide DER-encoded X.509 certificates, which can be renamed to .cer if required:
- https://cacerts.digicert.com/DigiCertGlobalRootCA.crt
- https://cacerts.digicert.com/DigiCertGlobalRootG2.crt
- HTML/JavaScript for Drag-and-Drop .CER File Analysis
The following is an embeddable HTML snippet with JavaScript for a Ghost blog post (or similar platform). It allows users to drag and drop a .CER file (assuming DER encoding) and displays the properties on screen. For parsing, it uses the open-source 'asn1js' and 'pkijs' libraries (loaded via CDN for simplicity). Include this in a blog post's HTML embed section.
Drag and drop a .CER file here
- Python Class for .CER File Handling
The following Python class uses the cryptography library to open, decode, read, write, and print properties of a .CER file (DER-encoded). Install via pip install cryptography.
from cryptography.x509 import load_der_x509_certificate
from cryptography.x509.oid import NameOID
from cryptography.hazmat.primitives.serialization import Encoding
import os
class CerHandler:
def __init__(self, file_path):
self.file_path = file_path
self.cert = None
def read(self):
with open(self.file_path, 'rb') as f:
der_data = f.read()
self.cert = load_der_x509_certificate(der_data)
return self.cert
def print_properties(self):
if not self.cert:
self.read()
print(f"Version: {self.cert.version.value + 1}")
print(f"Serial Number: {self.cert.serial_number}")
print(f"Signature Algorithm: {self.cert.signature_hash_algorithm.name}")
print(f"Issuer: {self.cert.issuer.rfc4514_string()}")
print(f"Validity Not Before: {self.cert.not_valid_before_utc}")
print(f"Validity Not After: {self.cert.not_valid_after_utc}")
print(f"Subject: {self.cert.subject.rfc4514_string()}")
print(f"Public Key Algorithm: {self.cert.public_key().__class__.__name__}")
print(f"Public Key: {self.cert.public_bytes(Encoding.DER).hex()}")
extensions = self.cert.extensions
print("Extensions:")
for ext in extensions:
print(f" {ext.oid.dotted_string} (Critical: {ext.critical})")
def write(self, new_file_path):
if not self.cert:
self.read()
with open(new_file_path, 'wb') as f:
f.write(self.cert.public_bytes(Encoding.DER))
# Example usage:
# handler = CerHandler('example.cer')
# handler.print_properties()
# handler.write('copy.cer')
- Java Class for .CER File Handling
The following Java class uses built-in java.security.cert to handle .CER files.
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.security.cert.CertificateFactory;
import java.security.cert.X509Certificate;
import java.util.Collection;
import java.util.Iterator;
public class CerHandler {
private String filePath;
private X509Certificate cert;
public CerHandler(String filePath) {
this.filePath = filePath;
}
public void read() throws Exception {
FileInputStream fis = new FileInputStream(filePath);
CertificateFactory cf = CertificateFactory.getInstance("X.509");
cert = (X509Certificate) cf.generateCertificate(fis);
fis.close();
}
public void printProperties() throws Exception {
if (cert == null) {
read();
}
System.out.println("Version: " + cert.getVersion());
System.out.println("Serial Number: " + cert.getSerialNumber());
System.out.println("Signature Algorithm: " + cert.getSigAlgName());
System.out.println("Issuer: " + cert.getIssuerX500Principal());
System.out.println("Validity Not Before: " + cert.getNotBefore());
System.out.println("Validity Not After: " + cert.getNotAfter());
System.out.println("Subject: " + cert.getSubjectX500Principal());
System.out.println("Public Key Algorithm: " + cert.getPublicKey().getAlgorithm());
System.out.println("Public Key: " + cert.getPublicKey());
System.out.println("Extensions:");
Collection<?> extensions = cert.getCriticalExtensionOIDs();
if (extensions != null) {
Iterator<?> it = extensions.iterator();
while (it.hasNext()) {
System.out.println(" " + it.next() + " (Critical)");
}
}
extensions = cert.getNonCriticalExtensionOIDs();
if (extensions != null) {
Iterator<?> it = extensions.iterator();
while (it.hasNext()) {
System.out.println(" " + it.next() + " (Non-Critical)");
}
}
}
public void write(String newFilePath) throws Exception {
if (cert == null) {
read();
}
FileOutputStream fos = new FileOutputStream(newFilePath);
fos.write(cert.getEncoded());
fos.close();
}
// Example usage:
// public static void main(String[] args) throws Exception {
// CerHandler handler = new CerHandler("example.cer");
// handler.printProperties();
// handler.write("copy.cer");
// }
}
- JavaScript Class for .CER File Handling
The following JavaScript class uses 'pkijs' and 'asn1js' for Node.js (install via npm install pkijs asn1js). It handles reading, parsing, and writing.
const fs = require('fs');
const asn1js = require('asn1js');
const pkijs = require('pkijs');
class CerHandler {
constructor(filePath) {
this.filePath = filePath;
this.cert = null;
}
read() {
const derBuffer = fs.readFileSync(this.filePath);
const asn1 = asn1js.fromBER(derBuffer.buffer);
this.cert = new pkijs.Certificate({ schema: asn1.result });
}
printProperties() {
if (!this.cert) {
this.read();
}
console.log(`Version: ${this.cert.version + 1}`);
console.log(`Serial Number: ${this.cert.serialNumber.valueBlock.toString()}`);
console.log(`Signature Algorithm: ${this.cert.signatureAlgorithm.algorithmId}`);
console.log(`Issuer: ${this.cert.issuer.typesAndValues.map(tv => `${tv.type}=${tv.value.valueBlock.value}`).join(', ')}`);
console.log(`Validity Not Before: ${this.cert.notBefore.value.toString()}`);
console.log(`Validity Not After: ${this.cert.notAfter.value.toString()}`);
console.log(`Subject: ${this.cert.subject.typesAndValues.map(tv => `${tv.type}=${tv.value.valueBlock.value}`).join(', ')}`);
console.log(`Public Key Algorithm: ${this.cert.subjectPublicKeyInfo.algorithm.algorithmId}`);
console.log(`Public Key: ${this.cert.subjectPublicKeyInfo.subjectPublicKey.valueBlock.toString()}`);
console.log('Extensions:');
if (this.cert.extensions) {
this.cert.extensions.forEach(ext => {
console.log(` ${ext.extnID} (Critical: ${ext.critical})`);
});
}
}
write(newFilePath) {
if (!this.cert) {
this.read();
}
const der = this.cert.toSchema().toBER(false);
fs.writeFileSync(newFilePath, Buffer.from(der));
}
}
// Example usage:
// const handler = new CerHandler('example.cer');
// handler.printProperties();
// handler.write('copy.cer');
- C++ Class for .CER File Handling
The following C++ class uses OpenSSL for handling .CER files. Compile with -lssl -lcrypto.
#include <iostream>
#include <fstream>
#include <openssl/x509.h>
#include <openssl/bio.h>
#include <openssl/pem.h>
class CerHandler {
private:
std::string filePath;
X509* cert;
public:
CerHandler(const std::string& path) : filePath(path), cert(nullptr) {}
~CerHandler() {
if (cert) X509_free(cert);
}
void read() {
BIO* bio = BIO_new_file(filePath.c_str(), "rb");
if (!bio) throw std::runtime_error("Failed to open file");
cert = d2i_X509_bio(bio, nullptr);
BIO_free(bio);
if (!cert) throw std::runtime_error("Failed to load certificate");
}
void printProperties() {
if (!cert) read();
char buffer[1024];
std::cout << "Version: " << X509_get_version(cert) + 1 << std::endl;
const ASN1_INTEGER* serial = X509_get0_serialNumber(cert);
std::cout << "Serial Number: " << ASN1_INTEGER_get(serial) << std::endl;
X509_signature_print(nullptr, X509_get0_signature_algorithm(cert), nullptr);
X509_NAME_oneline(X509_get_issuer_name(cert), buffer, sizeof(buffer));
std::cout << "Issuer: " << buffer << std::endl;
ASN1_TIME_print(nullptr, X509_get0_notBefore(cert));
std::cout << std::endl;
ASN1_TIME_print(nullptr, X509_get0_notAfter(cert));
std::cout << std::endl;
X509_NAME_oneline(X509_get_subject_name(cert), buffer, sizeof(buffer));
std::cout << "Subject: " << buffer << std::endl;
std::cout << "Public Key Algorithm: " << OBJ_nid2sn(X509_get0_pubkey_algor(cert)->algorithm->nid) << std::endl;
i2d_PUBKEY_bio(nullptr, X509_get_pubkey(cert));
std::cout << "Extensions:" << std::endl;
int extCount = X509_get_ext_count(cert);
for (int i = 0; i < extCount; ++i) {
X509_EXTENSION* ext = X509_get_ext(cert, i);
std::cout << " " << OBJ_nid2sn(OBJ_obj2nid(X509_EXTENSION_get_object(ext)))
<< " (Critical: " << X509_EXTENSION_get_critical(ext) << ")" << std::endl;
}
}
void write(const std::string& newFilePath) {
if (!cert) read();
BIO* bio = BIO_new_file(newFilePath.c_str(), "wb");
if (!bio) throw std::runtime_error("Failed to open output file");
i2d_X509_bio(bio, cert);
BIO_free(bio);
}
};
// Example usage:
// int main() {
// try {
// CerHandler handler("example.cer");
// handler.printProperties();
// handler.write("copy.cer");
// } catch (const std::exception& e) {
// std::cerr << e.what() << std::endl;
// }
// return 0;
// }