Certificate Signing Request (CSR)

What is a Certificate Signing Request (CSR)?

A Certificate Signing Request (CSR) is a standardized file format that contains a public key and identity information about the entity requesting a digital certificate. The CSR is submitted to a Certificate Authority (CA) which uses the information to create and sign a digital certificate, binding the public key to the identity of the requester.

CSRs are a fundamental part of the Public Key Infrastructure (PKI) ecosystem and serve as the interface between certificate applicants and certificate authorities. They enable organizations and individuals to request certificates for various purposes including SSL/TLS, code signing, email security, and authentication.

CSR Structure and Format

PKCS#10 Standard

CSRs follow the PKCS#10 (Public Key Cryptography Standard #10) format, which is defined in RFC 2986. The standard specifies the syntax for certification requests.

CSR Components

classDiagram
    class CertificationRequest {
        +CertificationRequestInfo certificationRequestInfo
        +SignatureAlgorithm signatureAlgorithm
        +BIT STRING signature
    }

    class CertificationRequestInfo {
        +INTEGER version
        +Name subject
        +SubjectPublicKeyInfo subjectPKInfo
        +Attributes attributes
    }

    class SubjectPublicKeyInfo {
        +AlgorithmIdentifier algorithm
        +BIT STRING subjectPublicKey
    }

    CertificationRequest --> CertificationRequestInfo
    CertificationRequestInfo --> SubjectPublicKeyInfo

Detailed CSR Structure

1. CertificationRequestInfo:
   • Version: CSR format version (typically 0 for PKCS#10)
   • Subject: Identity information of the requester
   • Subject Public Key Info: The public key to be certified
   • Attributes: Additional attributes (optional)
2. SignatureAlgorithm: Algorithm used to sign the CSR
3. Signature: Digital signature of the CertificationRequestInfo

Subject Field Components

The subject field contains identity information in X.500 Distinguished Name (DN) format:

How CSRs Work

CSR Generation Process

sequenceDiagram
    participant User
    participant Tool
    participant CA
    User->>Tool: Generate key pair
    Tool->>User: Private key + Public key
    User->>Tool: Provide identity information
    Tool->>Tool: Create CSR with public key and identity
    Tool->>Tool: Sign CSR with private key
    Tool->>User: CSR file
    User->>CA: Submit CSR
    CA->>CA: Verify identity and CSR
    CA->>User: Issue certificate

Technical Workflow

1. Key Generation: Generate a public/private key pair
2. CSR Creation: Create CSR containing public key and identity information
3. CSR Signing: Sign the CSR with the corresponding private key
4. CSR Submission: Submit CSR to Certificate Authority
5. Identity Verification: CA verifies the applicant's identity
6. Certificate Issuance: CA issues certificate based on CSR information
7. Certificate Delivery: CA delivers certificate to applicant
8. Certificate Installation: Applicant installs certificate

Creating a CSR

Using OpenSSL

Basic CSR Creation

# Generate private key
openssl genrsa -out server.key 2048

# Create CSR
openssl req -new -key server.key -out server.csr -subj "/C=US/ST=California/L=San Francisco/O=Example Inc/CN=www.example.com"

CSR with Subject Alternative Names (SANs)

# Create configuration file with SANs
cat > csr.cnf << 'EOL'
[ req ]
default_bits        = 2048
prompt              = no
default_md          = sha256
distinguished_name  = dn
req_extensions      = req_ext

[ dn ]
C   = US
ST  = California
L   = San Francisco
O   = Example Inc
OU  = IT Department
CN  = www.example.com

[ req_ext ]
subjectAltName = @alt_names

[ alt_names ]
DNS.1 = example.com
DNS.2 = www.example.com
DNS.3 = mail.example.com
IP.1 = 192.168.1.1
EOL

# Generate private key
openssl genrsa -out server.key 2048

# Create CSR with SANs
openssl req -new -key server.key -out server.csr -config csr.cnf

Using Keytool (Java)

# Generate key pair and create CSR
keytool -genkeypair -alias mycert -keyalg RSA -keysize 2048 -keystore keystore.jks -storepass changeit -keypass changeit -dname "CN=www.example.com, OU=IT Department, O=Example Inc, L=San Francisco, ST=California, C=US"

# Generate CSR from keystore
keytool -certreq -alias mycert -keystore keystore.jks -file server.csr -storepass changeit

Using PowerShell (Windows)

# Generate private key and create CSR
$cert = New-SelfSignedCertificate -DnsName "www.example.com", "example.com" -CertStoreLocation "cert:\LocalMachine\My" -KeyAlgorithm RSA -KeyLength 2048 -KeyExportPolicy Exportable

# Export CSR
$csr = @{
    Subject = "CN=www.example.com, OU=IT Department, O=Example Inc, L=San Francisco, ST=California, C=US"
    KeyAlgorithm = "RSA"
    KeyLength = 2048
    CertStoreLocation = "cert:\LocalMachine\My"
    NotAfter = (Get-Date).AddYears(1)
}
$newCert = New-SelfSignedCertificate @csr
Export-Certificate -Cert $newCert -FilePath "server.csr" -Type CERT

Using CFSSL

# Create CSR configuration
cat > csr.json << 'EOL'
{
  "CN": "www.example.com",
  "hosts": [
    "example.com",
    "www.example.com",
    "mail.example.com"
  ],
  "key": {
    "algo": "rsa",
    "size": 2048
  },
  "names": [
    {
      "C": "US",
      "L": "San Francisco",
      "O": "Example Inc",
      "OU": "IT Department",
      "ST": "California"
    }
  ]
}
EOL

# Generate private key and CSR
cfssl genkey csr.json | cfssljson -bare server

CSR Formats

CSRs can be created and stored in various formats:

PEM Format

• Extension: .csr, .pem
• Content: Base64 encoded with -----BEGIN CERTIFICATE REQUEST----- and -----END CERTIFICATE REQUEST----- delimiters
• Usage: Most common format for CSRs

Example PEM CSR:

-----BEGIN CERTIFICATE REQUEST-----
MIIByjCCATMCAQAwgYkxCzAJBgNVBAYTAlVTMRMwEQYDVQQIDApDYWxpZm9ybmlh
MRYwFAYDVQQHDA1TYW4gRnJhbmNpc2NvMRgwFgYDVQQKDA9FeGFtcGxlIEluYy4x
FTATBgNVBAsMDElUIERlcGFydG1lbnQxGTAXBgNVBAMMEHd3dy5leGFtcGxlLmNv
bTCBnzANBgkqhkiG9w0BAQEFAAOBjQAwgYkCgYEAx5X... (truncated)
-----END CERTIFICATE REQUEST-----

DER Format

• Extension: .der
• Content: Binary encoded CSR
• Usage: Less common, used in some applications

PKCS#10 Format

• Description: The underlying standard format for CSRs
• Encoding: Can be PEM or DER encoded
• Usage: Standard format recognized by all CAs

CSR Validation and Verification

What CAs Validate in a CSR

1. Signature Verification: Verify the CSR signature using the public key
2. Public Key Validity: Check that the public key is valid and appropriate
3. Identity Information: Verify the identity information matches the applicant
4. Key Strength: Check that the key meets minimum strength requirements
5. CSR Format: Verify the CSR is properly formatted
6. Duplicate Prevention: Check for duplicate requests

CSR Verification Commands

# View CSR information
openssl req -in server.csr -noout -text

# Verify CSR signature
openssl req -in server.csr -noout -verify

# Check CSR subject
openssl req -in server.csr -noout -subject

# Check public key
openssl req -in server.csr -noout -pubkey

# Check CSR in PEM format
openssl req -in server.csr -text -noout

# Check CSR in DER format
openssl req -inform der -in server.der -text -noout

Common CSR Issues

1. Weak Key Size: Using RSA keys smaller than 2048 bits
2. Invalid Signature: CSR not properly signed
3. Missing Information: Incomplete subject information
4. Incorrect Common Name: CN doesn't match intended domain
5. Format Issues: Improperly formatted CSR
6. Duplicate Requests: Submitting the same CSR multiple times
7. Expired CSR: CSR too old (typically valid for 30-90 days)
8. Algorithm Issues: Using weak or deprecated algorithms

CSR Use Cases

SSL/TLS Certificates

• Domain Validation (DV): Basic SSL certificates
• Organization Validation (OV): Business-validated certificates
• Extended Validation (EV): High-assurance certificates
• Wildcard Certificates: Certificates for multiple subdomains
• Multi-Domain Certificates: Certificates for multiple domains

Code Signing Certificates

• Software Distribution: Signing software installers
• Driver Signing: Signing device drivers
• Script Signing: Signing scripts and macros
• Application Signing: Signing desktop and mobile applications
• Update Signing: Signing software updates

Email Certificates

• S/MIME Certificates: Secure email certificates
• Email Encryption: Certificates for encrypting emails
• Email Signing: Certificates for digitally signing emails
• Client Authentication: Certificates for email client authentication

Client Authentication Certificates

• User Authentication: Certificates for user authentication
• Device Authentication: Certificates for device authentication
• VPN Access: Certificates for remote access
• Wi-Fi Authentication: Certificates for wireless network access
• 802.1X Authentication: Certificates for network access control

Document Signing Certificates

• PDF Signing: Certificates for signing PDF documents
• Office Document Signing: Certificates for signing Office documents
• Legal Documents: Certificates for signing contracts
• Digital Signatures: Certificates for legally binding signatures

IoT and Device Certificates

• Device Identity: Certificates for IoT devices
• Firmware Signing: Certificates for signing firmware updates
• Secure Communication: Certificates for device-to-cloud communication
• Device Management: Certificates for secure device management

CSR Best Practices

Key Generation Best Practices

1. Key Strength:
   • Use RSA 2048-bit or ECC 256-bit minimum
   • For long-term certificates, consider RSA 3072-bit or ECC 384-bit
   • Avoid RSA keys smaller than 2048 bits
2. Key Algorithms:
   • RSA: Widely supported, good for compatibility
   • ECC: More efficient, better performance
   • EdDSA: Modern, efficient elliptic curve algorithm
3. Key Protection:
   • Protect private keys with strong passwords
   • Store private keys securely
   • Limit access to private keys
   • Never share private keys
4. Key Generation:
   • Use cryptographically secure random number generators
   • Generate keys on secure systems
   • Avoid generating keys on shared or untrusted systems

CSR Content Best Practices

1. Common Name (CN):
   • For SSL certificates, use the Fully Qualified Domain Name (FQDN)
   • For wildcard certificates, use *.example.com
   • For internal certificates, use the internal domain name
2. Subject Alternative Names (SANs):
   • Include all domains that will use the certificate
   • Include both example.com and www.example.com
   • Include IP addresses if needed
   • Include internal domain names for internal certificates
3. Organization Information:
   • Use the legal organization name
   • Use accurate location information
   • Use appropriate organizational unit
   • Ensure information matches business registration
4. Key Usage:
   • Specify appropriate key usage extensions
   • For SSL: digitalSignature, keyEncipherment
   • For code signing: digitalSignature
   • For CA certificates: keyCertSign, cRLSign

CSR Submission Best Practices

1. CA Selection:
   • Choose a reputable CA
   • Consider validation level needed
   • Consider certificate features needed
   • Consider pricing and support
2. Validation Level:
   • Domain Validation (DV): Basic validation, fastest issuance
   • Organization Validation (OV): Business validation, moderate issuance time
   • Extended Validation (EV): Highest validation, longest issuance time
3. Certificate Features:
   • Wildcard: Certificates for multiple subdomains
   • Multi-Domain: Certificates for multiple domains
   • SANs: Subject Alternative Names
   • Key Type: RSA or ECC
4. Automation:
   • Use ACME protocol for automated certificate management
   • Consider Let's Encrypt for free, automated certificates
   • Implement automated renewal processes

CSR in Different Environments

Web Servers

• Apache:

  # Generate CSR for Apache
  openssl req -new -newkey rsa:2048 -nodes -keyout server.key -out server.csr
  

• Nginx:

  # Generate CSR for Nginx
  openssl req -new -newkey rsa:2048 -nodes -keyout server.key -out server.csr -config csr.cnf
  

• IIS:
  • Use IIS Manager to generate CSR
  • Or use PowerShell commands
  • Or use certreq utility
• Node.js:

  // Generate CSR programmatically
  const { generateKeyPairSync } = require('crypto');
  const { CSR } = require('pkijs');
  
  const { privateKey, publicKey } = generateKeyPairSync('rsa', {
    modulusLength: 2048,
  });
  
  const csr = new CSR({
    subject: {
      commonName: 'www.example.com',
      organizationName: 'Example Inc',
      organizationalUnitName: 'IT Department',
      localityName: 'San Francisco',
      stateOrProvinceName: 'California',
      countryName: 'US'
    },
    keys: { privateKey, publicKey }
  });
  
  const csrPem = csr.toPEM();
  

Cloud Environments

• AWS Certificate Manager (ACM):

  # Request certificate using AWS CLI
  aws acm request-certificate --domain-name example.com --validation-method DNS
  

• Azure Key Vault:

  # Create CSR in Azure Key Vault
  $policy = New-AzKeyVaultCertificatePolicy -SubjectName "CN=www.example.com" -IssuerName "Self" -ValidityInMonths 12
  Add-AzKeyVaultCertificate -VaultName "MyVault" -Name "MyCert" -CertificatePolicy $policy
  

• Google Cloud:

  # Create CSR for Google Cloud
  gcloud compute ssl-certificates create my-cert --domains www.example.com --global
  

Development Environments

• Docker:

  # Generate CSR in Dockerfile
  RUN openssl req -new -newkey rsa:2048 -nodes -keyout server.key -out server.csr \
      -subj "/C=US/ST=California/L=San Francisco/O=Example Inc/CN=localhost"
  

• Kubernetes:

  # CertificateSigningRequest resource in Kubernetes
  apiVersion: certificates.k8s.io/v1
  kind: CertificateSigningRequest
  metadata:
    name: my-svc.my-namespace
  spec:
    request: $(cat server.csr | base64 | tr -d '\n')
    signerName: kubernetes.io/kube-apiserver-client
    expirationSeconds: 86400
    usages:
    - client auth
  

• CI/CD Pipelines:

  # Generate CSR in GitHub Actions
  - name: Generate CSR
    run: |
      openssl req -new -newkey rsa:2048 -nodes -keyout server.key -out server.csr \
        -subj "/C=US/ST=California/L=San Francisco/O=Example Inc/CN=${{ secrets.DOMAIN }}"
  

Enterprise Environments

• Microsoft Active Directory Certificate Services:

  # Submit CSR to AD CS
  certreq -submit -attrib "CertificateTemplate:WebServer" server.csr server.crt
  

• OpenSSL CA:

  # Sign CSR with OpenSSL CA
  openssl ca -in server.csr -out server.crt -policy policy_anything -days 365
  

• EJBCA:

  # Submit CSR to EJBCA
  ejbca.sh ra addendentity --username myuser --password mypassword --dn "CN=www.example.com" --caname "MyCA" --type 1
  ejbca.sh ra setclearpwd --username myuser --password mypassword
  ejbca.sh batch --username myuser --password mypassword
  

CSR Security Considerations

Security Risks

1. Private Key Exposure: Private key used to sign CSR could be compromised
2. Weak Cryptography: Using weak algorithms or small key sizes
3. Information Leakage: CSR contains public key and identity information
4. CSR Tampering: CSR could be modified before submission to CA
5. Duplicate CSRs: Multiple CSRs with the same key could cause issues
6. Expired CSRs: CSRs have limited validity period
7. Social Engineering: Attackers could submit CSRs for domains they don't own

Security Best Practices

1. Key Protection:
   • Protect private keys with strong passwords
   • Store private keys securely (HSMs, encrypted storage)
   • Limit access to private keys
   • Never store private keys in version control
2. CSR Generation:
   • Generate CSRs on secure systems
   • Use strong cryptographic algorithms
   • Use appropriate key sizes
   • Verify CSR content before submission
3. CSR Submission:
   • Submit CSRs through secure channels
   • Verify CA identity before submission
   • Use secure methods for CSR transmission
   • Monitor certificate issuance process
4. Certificate Management:
   • Track issued certificates
   • Monitor certificate expiration
   • Implement certificate lifecycle management
   • Use certificate management tools
5. Monitoring:
   • Monitor for rogue CSRs
   • Monitor for unauthorized certificate issuance
   • Implement certificate transparency monitoring
   • Set up alerts for certificate-related events

CSR Automation and Management

Automated CSR Generation

1. ACME Protocol: Automated Certificate Management Environment
   • Used by Let's Encrypt
   • Automates CSR generation and submission
   • Supports automated validation and issuance
2. Certificate Management Tools:
   • Certbot: Let's Encrypt client
   • CFSSL: Cloudflare's PKI toolkit
   • OpenSSL: Command-line tool
   • Keyfactor: Enterprise certificate management
   • Venafi: Enterprise certificate management
3. Infrastructure as Code:
   • Terraform: Certificate management with providers
   • Ansible: Certificate management modules
   • Puppet: Certificate management modules
   • Chef: Certificate management cookbooks

CSR Management Workflows

1. Manual Workflow:
   • Generate CSR manually
   • Submit CSR to CA manually
   • Receive certificate manually
   • Install certificate manually
2. Semi-Automated Workflow:
   • Generate CSR manually
   • Submit CSR through CA portal
   • Receive certificate automatically
   • Install certificate manually
3. Fully Automated Workflow:
   • Generate CSR automatically
   • Submit CSR through API
   • Automated validation
   • Automated certificate installation
   • Automated renewal

CSR Tracking and Inventory

1. Certificate Inventory:
   • Track all CSRs and certificates
   • Maintain database of certificate information
   • Track certificate expiration dates
   • Monitor certificate usage
2. CSR Metadata:
   • Requester information
   • Request date
   • Intended use
   • Approval status
   • Certificate status
3. Audit Logging:
   • Log CSR generation events
   • Log CSR submission events
   • Log certificate issuance events
   • Log certificate installation events

CSR in Certificate Lifecycle Management

CSR Role in Certificate Lifecycle

graph TD
    A[Key Generation] --> B[CSR Creation]
    B --> C[CSR Submission]
    C --> D[Identity Verification]
    D --> E[Certificate Issuance]
    E --> F[Certificate Installation]
    F --> G[Certificate Usage]
    G --> H[Certificate Renewal]
    H --> B
    G --> I[Certificate Revocation]
    G --> J[Certificate Expiration]

CSR in Renewal Process

1. Renewal Trigger: Certificate approaching expiration
2. CSR Generation: Generate new CSR (may reuse existing key or generate new one)
3. CSR Submission: Submit CSR to CA
4. Identity Verification: CA verifies identity (may be streamlined for renewals)
5. Certificate Issuance: CA issues new certificate
6. Certificate Installation: Install new certificate
7. Old Certificate Replacement: Replace old certificate with new one

CSR in Key Rollover

1. Key Rollover Trigger: Security policy or key compromise
2. New Key Generation: Generate new key pair
3. New CSR Creation: Create CSR with new public key
4. CSR Submission: Submit new CSR to CA
5. Certificate Issuance: CA issues certificate with new key
6. Certificate Installation: Install new certificate
7. Old Key Retirement: Securely retire old key

CSR Troubleshooting

Common CSR Problems and Solutions

CSR Debugging Tools

1. OpenSSL:

   # View CSR details
   openssl req -in server.csr -noout -text
   
   # Verify CSR signature
   openssl req -in server.csr -noout -verify
   
   # Check CSR subject
   openssl req -in server.csr -noout -subject
   
   # Check public key
   openssl req -in server.csr -noout -pubkey
   

2. Certificate Transparency Logs:
   • Monitor for issued certificates
   • Verify certificate issuance
   • Check for rogue certificates
3. CSR Validators:
   • Online CSR validators
   • CA-provided validation tools
   • Certificate management tools
4. Network Tools:
   • Check CSR submission endpoints
   • Verify network connectivity to CA
   • Check firewall and proxy settings

CSR Case Studies

Case Study 1: E-Commerce Website SSL Certificate

Scenario: An e-commerce company needs to secure their website with an SSL certificate

Solution:

1. CSR Generation:

   openssl req -new -newkey rsa:2048 -nodes -keyout www_example_com.key \
     -out www_example_com.csr \
     -subj "/C=US/ST=California/L=San Francisco/O=Example E-Commerce Inc/CN=www.example.com"
   

2. SAN Configuration: Added example.com, www.example.com, shop.example.com to SANs
3. CA Selection: Chose DigiCert for OV certificate with good reputation
4. Validation: Provided business documents for organization validation
5. Certificate Installation: Installed certificate on web servers and load balancers

Benefits:

• Secure customer transactions
• Build customer trust
• Improve search engine ranking
• Meet PCI DSS compliance requirements

Challenges:

• Coordinating domain validation
• Managing certificate across multiple servers
• Ensuring proper SAN configuration

Case Study 2: Enterprise Internal PKI

Scenario: A large enterprise needs to implement an internal PKI for device authentication

Solution:

1. CSR Generation: Automated CSR generation for thousands of devices
2. CSR Submission: Automated submission to internal CA
3. Certificate Issuance: Automated issuance and distribution
4. Certificate Installation: Automated installation on devices
5. Certificate Management: Centralized certificate inventory and monitoring

Implementation:

• Used Microsoft Active Directory Certificate Services
• Implemented auto-enrollment for domain-joined devices
• Created certificate templates for different device types
• Set up automated renewal processes

Benefits:

• Secure device authentication
• Centralized certificate management
• Automated certificate lifecycle
• Reduced operational overhead

Challenges:

• Scaling CSR generation and submission
• Managing certificate templates
• Ensuring proper key protection
• Monitoring certificate usage

Case Study 3: IoT Device Security

Scenario: An IoT manufacturer needs to secure device communication

Solution:

1. CSR Generation: Each device generates its own CSR during manufacturing
2. CSR Submission: Devices submit CSRs to manufacturer's CA
3. Certificate Issuance: CA issues device-specific certificates
4. Certificate Installation: Certificates embedded in device firmware
5. Secure Communication: Devices use certificates for TLS communication

Implementation:

• Used lightweight cryptography for constrained devices
• Implemented automated CSR processing pipeline
• Created device-specific certificate profiles
• Implemented certificate pinning in device software

Benefits:

• Secure device-to-cloud communication
• Unique identity for each device
• Protection against device spoofing
• Secure firmware updates

Challenges:

• Managing CSRs at scale
• Secure key generation on devices
• Certificate lifecycle management
• Revocation for compromised devices

Future of CSRs

Automation Trends

• ACME Protocol: Increased adoption of automated certificate management
• Let's Encrypt: Continued growth of free, automated certificates
• Short-Lived Certificates: Certificates with very short validity periods
• Automated Validation: Automated domain and organization validation
• Certificate Lifecycle Automation: End-to-end automation of certificate lifecycle

Integration Trends

• DevOps Integration: CSR generation in CI/CD pipelines
• Cloud Integration: Native CSR support in cloud platforms
• Container Integration: CSR generation for containerized applications
• Serverless Integration: CSR support for serverless architectures
• Infrastructure as Code: CSR management in IaC tools

Security Enhancements

• Hardware Security Modules: Increased use of HSMs for key generation
• Post-Quantum Cryptography: CSRs with quantum-resistant algorithms
• Multi-Party Computation: Distributed key generation and CSR signing
• Threshold Cryptography: Distributed control over CSR generation
• Improved Validation: Enhanced CSR validation mechanisms

Standard Evolution

• New CSR Formats: More flexible and extensible formats
• Improved Attributes: Better support for modern use cases
• Enhanced Security: Stronger security requirements for CSRs
• Simplified Processes: Easier CSR generation and submission
• Better Integration: Improved integration with certificate authorities

Emerging Use Cases

• Zero Trust Architecture: CSRs for identity-based access control
• Blockchain: CSRs for blockchain-based identity
• Decentralized Identity: CSRs for self-sovereign identity
• 5G Security: CSRs for 5G network security
• Edge Computing: CSRs for edge device security

Conclusion

Certificate Signing Requests (CSRs) are a fundamental component of the Public Key Infrastructure ecosystem, serving as the bridge between certificate applicants and certificate authorities. They enable organizations and individuals to request digital certificates for a wide range of security applications, from securing websites to authenticating devices and signing code.

Understanding the CSR generation process, proper formatting, and best practices is essential for implementing effective PKI solutions. Whether for public-facing websites, internal enterprise systems, IoT devices, or development environments, CSRs play a crucial role in establishing secure digital identities.

As the digital landscape continues to evolve, CSRs will remain a critical part of certificate management. The ongoing trends toward automation, improved security, and better integration with modern development and deployment workflows will continue to enhance the utility and efficiency of CSR processes.

Organizations should implement proper CSR management practices, including secure key generation, proper identity information, appropriate validation levels, and automated lifecycle management. By following best practices and staying informed about emerging trends, businesses can leverage CSRs to enhance security, build trust, and enable secure digital interactions in an increasingly connected world.