Digital Signature

Sign and verify messages with RSA
This free online digital signature runs in your browser and is part of our Cryptography collection.

✍️ Digital Signature

Sign and verify messages using RSA digital signatures for authentication and non-repudiation

About Digital Signatures:

  • Authentication: Proves the message came from the holder of the private key
  • Integrity: Ensures the message hasn't been modified
  • Non-repudiation: Signer cannot deny signing the message
  • Process: Sign with private key, verify with public key
  • Use Cases: Document signing, code signing, email signatures, blockchain transactions

⚠️ Important:

  • Keep your private key secure - never share it
  • Public keys can be freely shared
  • Both parties must use the same hash algorithm
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Instant In-Browser Execution Runs locally in JavaScript without wait times or server delays.
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100% Privacy & Security Your inputs and files are never stored or logged on remote servers.
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Cross-Device Responsive Fully optimized for mobile smartphones, tablets, and desktop displays.
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100% Free Forever No subscription fees, no signups, and no software installations.

How to Use the Online Digital Signature

1 Select Action (Sign or Verify)

Choose "Sign Message" to create a signature with your private key, or "Verify Signature" to validate with a public key.

2 Input Message Text

Paste the document, contract terms, or message string to be signed or verified.

3 Load RSA PEM Key

Paste your PEM private key to sign, or paste the sender's PEM public key to verify.

4 Generate or Check Signature

Click "Sign" to produce a Base64 signature, or click "Verify" to validate whether the signature matches the message without tampering.

Frequently Asked Questions

Common questions about using this free online digital signature.

How does an RSA digital signature prove authenticity? ▾
A digital signature hashes the message and encrypts that hash using the sender's private key. Anyone with the sender's public key can decrypt the signature and verify it matches the message hash, proving the author's identity.
What happens if even a single character in the message is changed? ▾
The cryptographic hash will change completely (the avalanche effect). Verification will immediately fail, signaling that the message was altered in transit.
What signing algorithm and hash function are used? ▾
The tool uses standard RSASSA-PKCS1-v1_5 or RSA-PSS with SHA-256 digest, compatible with standard cryptographic libraries in Java, Python, Node.js, and Go.
Can someone forge my signature if they have my public key? ▾
No. Generating a valid signature requires knowing the private key. Deriving the private key from the public key would require factoring large composite semiprimes, which is currently computationally infeasible.

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