Digital Signature

Validates the identity of the signer by binding their private key to the signature. The recipient uses the sender's public key to verify that the signature was created by the corresponding private key, proving the message originated from the claimed source.

• Relies on public-key infrastructure (PKI) to associate keys with identities
• Prevents impersonation attacks where an adversary pretends to be a trusted sender
• Commonly implemented via X.509 certificates issued by Certificate Authorities

Guarantees that the message has not been altered in transit. The signing process generates a cryptographic hash of the message, which is then encrypted with the signer's private key. Any modification to the message—even a single bit—produces a completely different hash, causing verification to fail.

• Uses collision-resistant hash functions like SHA-256 or SHA-3
• Detects both malicious tampering and accidental corruption
• Forms the basis for code signing in software distribution

Prevents the signer from credibly denying they signed the document. Because the private key is uniquely controlled by the signer, a valid signature constitutes legally binding proof of origin. This property distinguishes digital signatures from simpler message authentication codes (MACs).

• Critical for e-commerce transactions and legal contracts
• Requires secure key management to prevent key compromise claims
• Supported by regulations like eIDAS in the EU and the ESIGN Act in the US

Digital signatures operate through a three-phase process: key generation, signing, and verification. The signer generates a key pair—a private key kept secret and a public key distributed openly. Signing produces a signature by encrypting the message hash with the private key. Verification decrypts the signature with the public key and compares hashes.

• ECDSA (Elliptic Curve Digital Signature Algorithm) offers smaller keys with equivalent security
• EdDSA (Edwards-curve Digital Signature Algorithm) provides deterministic signing
• RSA-PSS remains widely deployed in legacy systems

The asymmetric cryptographic framework that underpins digital signatures. A key pair—consisting of a public key (widely shared) and a private key (kept secret)—enables two critical operations: encryption with the public key that only the private key can decrypt, and signing with the private key that anyone can verify using the corresponding public key. Algorithms include RSA (based on integer factorization), ECDSA (elliptic curve variant offering smaller key sizes), and EdDSA (Edwards-curve scheme designed for speed and security).

A deterministic algorithm that maps arbitrary-sized input data to a fixed-size output called a digest. Essential properties for digital signatures include: pre-image resistance (cannot reverse the hash), second pre-image resistance (cannot find different input with same hash), and collision resistance (cannot find any two inputs with same hash). Digital signatures typically sign the hash of a message rather than the message itself for efficiency. Common algorithms: SHA-256, SHA-3, and BLAKE2.

A trusted third-party entity that issues digital certificates binding a public key to an identity. The CA digitally signs the certificate using its own private key, creating a chain of trust. This solves the key distribution problem: instead of trusting every public key directly, parties trust a root CA, which vouches for subordinate certificates. The X.509 standard defines certificate format. Major CAs include Let's Encrypt, DigiCert, and Sectigo. The CA model is central to TLS/SSL and code signing.

The assurance that a signer cannot credibly deny having signed a document. Digital signatures achieve this because: the private key is assumed to be under the sole control of the signer, and the signature is computationally infeasible to forge without that key. This distinguishes digital signatures from simpler integrity checks like MACs (Message Authentication Codes), which use shared symmetric keys and therefore cannot prove who generated the authentication tag—both parties hold the same secret.

The complete framework of hardware, software, policies, and procedures required to create, manage, distribute, use, store, and revoke digital certificates. PKI includes: Registration Authorities (RAs) that verify identity before certificate issuance, Certificate Revocation Lists (CRLs) and OCSP responders for checking certificate validity, and HSMs (Hardware Security Modules) for secure key storage. PKI is the operational backbone that makes digital signatures trustworthy at scale.

A trusted service that issues cryptographic timestamps proving that specific data existed at a particular point in time. The TSA receives a hash of the data, appends the current time, and signs the combined structure. This is critical for long-term signature validation: even if a signing certificate later expires or is revoked, a timestamp from before the revocation proves the signature was valid when created. Defined in RFC 3161.