Digitale Signature

Digital Signature

Was ist authentication und integrity?

Empfänger wird und dem Glauben gelassen, dass die Nachricht von einem bekannten Sender kommt (authentication) und auf der Durchreise nicht verändert (integrity) wurde.

What are the applications of digital signature?

[Wikipedia]

As organizations move away from paper documents with ink signatures or authenticity stamps, digital signatures can provide added assurances of the evidence to provenance, identity, and status of an electronic document as well as acknowledging informed consent and approval by a signatory. 

How do they work?

[Wikipedia]

To create RSA signature keys, generate an RSA key pair containing a modulus N that is the product of two large primes, along with integers e and d such that e d ≡ 1 (mod φ(N)), where φ is the Euler phi-function. The signer's public key consists of N and e, and the signer's secret key contains d.

What is the hash of a message? Which hash functions are the most popular ones?

[Wikipedia]

To prevent attacks, one can first apply a cryptographic hash function to the message m and then apply the RSA algorithm described above to the result.

In practice, however, this type of signature is not used directly, but rather, the message to be signed is first hashed to produce a short digest that is then signed. 

• For efficiency: The signature will be much shorter and thus save time since hashing is generally much faster than signing in practice.
• For compatibility: Messages are typically bit strings, but some signature schemes operate on other domains (such as, in the case of RSA, numbers modulo a composite number N). A hash function can be used to convert an arbitrary input into the proper format.
• For integrity: Without the hash function, the text "to be signed" may have to be split (separated) in blocks small enough for the signature scheme to act on them directly. However, the receiver of the signed blocks is not able to recognize if all the blocks are present and in the appropriate order.

Name some digital signature algorithms

• Pairing-based schemes such as BLS
• RSA-based signature schemes, such as RSA-PSS
• DSA and its elliptic curve variant ECDSA
• ElGamal signature scheme as the predecessor to DSA, and variants Schnorr signature and Pointcheval–Stern signature algorithm
• Undeniable signatures
• Rabin signature algorithm

Which additional security precautions can be used?

• Using smart card readers with a separate keyboard
• Other smart card designs
• Putting the private key on a smart card
• Digital signatures vs. ink on paper signatures
• Using digital signatures only with trusted applications
• Using a network attached hardware security module