Digital Signatures

Also Known As

TL;DR

Explanation

Signing: hash the data, encrypt the hash with the private key. Verification: decrypt the signature with the public key, hash the data independently, compare. If they match, the data is authentic and unmodified. Ed25519 is the modern recommended algorithm — faster and more secure than RSA signing. Used in: TLS certificates, JWT signing, Git signed commits, code signing, and API request authentication. Non-repudiation means the signer cannot deny having signed.

Diagram

sequenceDiagram
    participant SIGNER as Signer
    participant MSG as Message
    participant VERIFIER as Verifier
    SIGNER->>MSG: hash the message
    SIGNER->>SIGNER: encrypt hash with private key
    SIGNER->>VERIFIER: send message + signature
    VERIFIER->>VERIFIER: hash received message
    VERIFIER->>VERIFIER: decrypt signature with public key
    VERIFIER->>VERIFIER: compare hashes
    Note over VERIFIER: Hashes match = authentic + untampered
    Note over VERIFIER: Hashes differ = tampered or wrong key

Common Misconception

Why It Matters

Common Mistakes

• Using weak hash algorithms (MD5, SHA1) for signing — collision attacks allow forged signatures.
• Signing the wrong data — sign the canonical form of the data, not a developer-friendly representation.
• Not verifying the certificate chain when verifying a signature — the public key must be trusted.
• Confusing HMAC (symmetric shared secret) with digital signatures (asymmetric) — HMAC requires both parties to have the secret.

Code Examples

// Weak signing — SHA1 hash, vulnerable to collisions:
$signature = openssl_sign($data, $sig, $privateKey, OPENSSL_ALGO_SHA1);

// JWT signature verification skipped:
$payload = json_decode(base64_decode(explode('.', $jwt)[1]));
// Using payload without verifying signature — unsigned claims trusted

// Ed25519 signature with PHP:
openssl_sign($data, $signature, $privateKey, OPENSSL_ALGO_SHA256);

// Verify:
$valid = openssl_verify($data, $signature, $publicKey, OPENSSL_ALGO_SHA256) === 1;
if (!$valid) throw new SecurityException('Invalid signature');

// HMAC for webhook verification (symmetric):
$expected = hash_hmac('sha256', $payload, $secret);
if (!hash_equals($expected, $receivedSig)) throw new SecurityException('Bad signature');

References

• ↗ https://en.wikipedia.org/wiki/Digital_signature

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