Private Key Checker
Validate that your private key matches a certificate or CSR before installing it on your server. Paste both PEM files and the tool compares RSA modulus or ECDSA parameters to confirm they are a cryptographic pair. Supports PKCS#8 private keys, X.509 certificates, and PKCS#10 CSRs. Fully private - all processing happens in your browser, and your private key never leaves your device. 100% free, no signup required.
Paste a private key and its corresponding certificate, CSR, or public key above, then click Check Match to verify they belong together.
All processing happens locally in your browser - your private key never leaves your device.
Supported PEM formats:
Why Use Our Private Key Checker?
Real Private Key Verification
Our private key checker validates that your private key corresponds to your certificate or CSR by comparing the underlying cryptographic parameters. For RSA keys, it compares the modulus. For ECDSA keys, it validates the curve and key pairing. This ensures your certificate and private key belong together before you install them on your server.
100% Private & Client-Side
All processing happens entirely in your browser using the Web Crypto API. Your private key PEM content is never sent to any server, stored in any database, or logged. The key content is hidden by default with a blur overlay - you choose when to reveal it. No signup, no uploads, complete privacy.
Instant Results with Key Details
Get immediate match/mismatch results along with detailed information about both keys: key type (RSA, ECDSA), key size in bits, and format detection. Clear explanations show exactly why keys match or mismatch, including modulus comparison details and any parsing errors.
Safe by Design
The tool NEVER exposes your private key content. Key details shown are limited to type, size, and match status - the actual key material, modulus values, and cryptographic parameters remain hidden. An additional blur overlay keeps your private key obscured by default for screen privacy.
Common Use Cases for Private Key Checker
SSL/TLS Certificate Installation
Before installing an SSL/TLS certificate on your web server, verify that the private key matches the certificate. Installing a mismatched key-certificate pair will cause SSL handshake failures and browser security warnings that can damage user trust and SEO rankings.
Certificate Renewal Verification
When renewing a certificate, ensure your existing private key still matches the renewed certificate. Some Certificate Authorities issue renewed certificates with new key pairs, while others reuse the existing public key. Our private key checker confirms compatibility before deployment.
CSR Generation Validation
After generating a Certificate Signing Request (CSR), verify that the private key you generated it with matches the CSR before submitting it to your Certificate Authority. This catches mismatches early, saving you from having to re-issue the certificate.
DevOps & CI/CD Pipeline Checks
Integrate private key matching into your deployment pipelines. Before pushing certificate updates to production, verify that the private key and certificate in your secrets management system are a valid pair to prevent deployment failures.
Server Migration & Key Transfers
When migrating servers or transferring SSL certificates between environments (staging to production, old server to new server), confirm that the private key you copied matches the certificate to avoid certificate errors after migration.
Learning & Certificate Management Training
Use the private key checker as an educational tool to understand how public-key cryptography works. See how RSA moduli are compared, learn to identify key types and sizes, and understand the relationship between private keys, certificates, and CSRs.
Understanding Private Key & Certificate Matching
What is Private Key Matching?
Private key matching is the process of verifying that a private key corresponds to a specific certificate or Certificate Signing Request (CSR). In public-key cryptography, each certificate contains a public key that is mathematically paired with a private key. The private key is used to prove ownership of the certificate during SSL/TLS handshakes - if they don't match, the handshake fails and browsers show security errors.
For RSA keys, the match is verified by comparing the modulus (n) - a large integer that forms part of both the public and private key. For ECDSA keys, the match is verified by comparing the curve parameters and key derivation. Our private key checker performs these comparisons entirely in your browser using ASN.1 DER parsing and the Web Crypto API.
How Our Private Key Checker Works
Our private key checker uses a multi-step process to validate the relationship between your private key and certificate or CSR:
- PEM parsing - The tool parses the PEM (Privacy-Enhanced Mail) format to extract the raw DER-encoded binary data. It validates the PEM headers, footers, and base64 content to ensure the input is well-formed.
- ASN.1 DER decoding - A built-in ASN.1 parser reads the DER structure to identify the key type (RSA or ECDSA), key format (PKCS#8 private key, SubjectPublicKeyInfo from certificates), and extract cryptographic parameters.
- Parameter comparison - For RSA keys, the tool extracts the modulus (n) from both the private key and the certificate's public key and compares them. For ECDSA keys, it validates the curve and key relationship.
- Result presentation - Results are displayed with a clear match/mismatch status, detailed key information (type, size, format, validity), and human-readable explanations of the comparison.
Key Parameters Extracted & Compared
The private key checker extracts and compares the following cryptographic parameters:
- Key type - Identifies whether the key is RSA, ECDSA, DSA, or another algorithm. A type mismatch (e.g., RSA private key with an ECDSA certificate) is immediately flagged.
- Key size - Determines the key length in bits (e.g., 2048-bit, 4096-bit RSA, or 256-bit, 384-bit ECDSA). Size mismatches between private key and certificate indicate incompatible keys.
- RSA modulus (n) - For RSA keys, the modulus is the core mathematical component shared between the public and private key. Identical moduli confirm the keys are a matching pair.
- Key format - Detects the PEM format (PKCS#8 private key, X.509 certificate, PKCS#10 CSR, or standalone SubjectPublicKeyInfo).
Privacy & Security Guarantees
Your private key never leaves your device.All parsing, decoding, and comparison operations are performed client-side using JavaScript's Web Crypto API and a built-in ASN.1 DER parser. No data is sent to any server, stored in any database, or transmitted over the network. The tool works entirely offline once the page is loaded.
Key content is visually protected. The private key textarea is hidden by default behind a blur overlay. You must explicitly click to reveal the key content. The tool never displays the actual cryptographic parameters (modulus values, private exponents) in the results - only key type, size, and match status are shown. This ensures that even if someone looks at your screen, the sensitive key material remains confidential.
Related Tools
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Decode and validate a Certificate Signing Request. Shows subject, SANs, key algorithm, key size, and signature information.
SSL Certificate Expiry Date Parser
Paste a PEM certificate and decode its validity dates. Shows not-before, not-after, days remaining, and visual expiry status.
Certificate SANs Extractor
Paste a PEM certificate and extract all Subject Alternative Names (SANs). Shows DNS names, IP addresses, and email addresses with counts.
Certificate Fingerprint Generator
Compute SHA-1 and SHA-256 fingerprints of any PEM certificate. Shows the familiar colon-separated fingerprint format.
Frequently Asked Questions About Private Key Checker
A private key checker is a tool that validates whether a private key corresponds to a certificate or Certificate Signing Request (CSR). In SSL/TLS security, the private key and certificate must be a cryptographic pair - if they don't match, web servers will fail to establish secure connections, causing browser security warnings. Our private key checker compares the internal cryptographic parameters of both files to confirm whether they belong together.
For RSA keys, the tool extracts the modulus (a large integer that is a core component of both the public and private key) from both the private key and the certificate's public key. If the modulus values are identical, the keys are a matching pair. For ECDSA keys, the tool validates the curve parameters and key relationship. All processing uses ASN.1 DER parsing combined with the browser's built-in Web Crypto API for accurate cryptographic validation.
Yes. All processing happens entirely in your browser - your private key PEM content is never sent to any server, stored in any database, or logged anywhere. The tool uses JavaScript's Web Crypto API and a built-in ASN.1 parser, both of which operate completely client-side. Additionally, the private key textarea is hidden behind a blur overlay by default, and the tool never displays the actual cryptographic key material in its results.
The private key checker supports multiple PEM formats: "-----BEGIN PRIVATE KEY-----" (PKCS#8 private keys), "-----BEGIN RSA PRIVATE KEY-----" (PKCS#1 RSA private keys), "-----BEGIN EC PRIVATE KEY-----" (PKCS#1 EC private keys), "-----BEGIN CERTIFICATE-----" (X.509 certificates), "-----BEGIN CERTIFICATE REQUEST-----" (PKCS#10 CSRs), and "-----BEGIN PUBLIC KEY-----" (SubjectPublicKeyInfo). The tool automatically detects and parses each format.
When keys do not match, it means the private key and certificate were generated independently and are not a cryptographic pair. This typically happens when you accidentally mix up certificate files from different installations, use a private key from a different server, or generate a new key pair without updating the certificate. You will need to either obtain the correct matching private key or generate a new key pair and re-issue the certificate.
Yes, our private key checker is 100% free to use with no signup required, no usage limits, and no hidden charges. Check as many key-certificate pairs as you need. All processing happens locally in your browser, so there are no server costs or rate limits to worry about.
The private key checker supports RSA keys (commonly 2048-bit, 4096-bit) and ECDSA keys (P-256, P-384, P-521 curves). It can detect DSA and Ed25519 keys but may have limited comparison capabilities for those types. The tool reports the detected key type and size in bits for both the private key and certificate, and flags any type or size mismatches.
Yes. The tool supports both X.509 certificates ("-----BEGIN CERTIFICATE-----") and PKCS#10 Certificate Signing Requests ("-----BEGIN CERTIFICATE REQUEST-----"). Paste your private key in the top textarea and your CSR in the bottom textarea, then click "Check Match" to verify they were generated together. This is especially useful before submitting a CSR to a Certificate Authority.