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PDF p.55 In progress

Digital Signatures

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Summary

PDF p.55

Digital signatures combine hashing and asymmetric encryption to ensure data integrity and authenticate the sender. A digital signature is created by hashing a message and then encrypting the hash with the sender's private key. The recipient can verify the signature using the sender's public key and comparing the hash values. Standards for digital signatures include PKCS#1 (RSA), DSA, and ECDSA.

In plain words

Supplementary — not from your PDF

A digital signature proves who sent a message and that it wasn't changed. The sender hashes the message and signs the hash with their private key. Anyone can check it with the sender's public key.

Detailed explanation

PDF p.55
  • Cryptographic Primitives
    • Definition: Basic cryptographic functions like hash functions, symmetric ciphers, and asymmetric ciphers.
    • Usage: Combined in a cipher suite for different security purposes.
  • Encryption Uses
    • Confidentiality: Ensures only authorized parties can read the message.
    • Integrity and Authentication: Ensures the message is unchanged and verifies the sender's identity.
  • Digital Signature Process
    • Hashing: The sender (Alice) creates a digest of the message using a hash algorithm (e.g., SHA256).
    • Signing: Alice encrypts the digest with her private key, creating the digital signature.
    • Transmission: Alice sends the message and the digital signature to the recipient (Bob).
    • Verification: Bob uses Alice's public key to decrypt the signature and obtain the original hash.
    • Comparison: Bob hashes the received message and compares it with the decrypted hash. If they match, the message is verified and Alice's identity is confirmed.
  • Security Assurance
    • Integrity: Ensures the message has not been tampered with.
    • Authentication: Confirms the sender's identity.
    • Tampering Detection: If the message or signature is altered, the hashes will not match.
  • Standards
    • PKCS#1: Defines the use of RSA for digital signatures.
    • DSA (Digital Signature Algorithm): Uses ElGamal cipher.
    • ECDSA (Elliptic Curve DSA): More widely used, part of US government's FIPS.

Important terms

taken from the text above
Cryptographic Primitives
Basic cryptographic functions like hash functions, symmetric ciphers, and asymmetric ciphers.
Confidentiality
Ensures only authorized parties can read the message.
Integrity and Authentication
Ensures the message is unchanged and verifies the sender's identity.
Hashing
The sender (Alice) creates a digest of the message using a hash algorithm (e.g., SHA256).
Signing
Alice encrypts the digest with her private key, creating the digital signature.
Transmission
Alice sends the message and the digital signature to the recipient (Bob).
Comparison
Bob hashes the received message and compares it with the decrypted hash. If they match, the message is verified and Alice's identity is confirmed.
Integrity
Ensures the message has not been tampered with.
Authentication
Confirms the sender's identity.
Tampering Detection
If the message or signature is altered, the hashes will not match.
PKCS#1
Defines the use of RSA for digital signatures.
DSA (Digital Signature Algorithm)
Uses ElGamal cipher.
ECDSA (Elliptic Curve DSA)
More widely used, part of US government's FIPS.

Examples & real-world scenarios

Supplementary — not from your PDF
  • Signed software updates, so your computer can check the publisher.
  • Signed email with S/MIME.
  • Standards: RSA (PKCS#1), DSA and ECDSA.

Scenario

A contract PDF is signed and emailed. The recipient's software checks the signature: it matches, so the document is unchanged and came from the signer's key. The signer can't later deny signing, which gives non-repudiation.

Common mistakes

Supplementary — not from your PDF
  • Thinking a signature hides the content. It proves origin and integrity, not confidentiality.
  • Mixing up which key signs (private) and which verifies (public).

Practical skills

Supplementary — not from your PDF
  • Walk through signing and verifying step by step.

What I should remember

Key Points PDF p.55
  • Cryptographic Primitives
    • Basic Functions: Hash functions, symmetric and asymmetric ciphers.
    • Cipher Suite: Combines primitives for security.
  • Encryption Uses
    • Confidentiality: Protects message content.
    • Integrity and Authentication: Verifies message and sender.
  • Digital Signature Process
    • Hashing: Creates message digest.
    • Signing: Encrypts digest with private key.
    • Transmission: Sends message and signature.
    • Verification: Decrypts signature with public key.
    • Comparison: Matches hashes to verify integrity and identity.
  • Security Assurance
    • Integrity: Message unchanged.
    • Authentication: Sender verified.
    • Tampering Detection: Detects alterations.
  • Standards
    • PKCS#1: RSA.
    • DSA: ElGamal.
    • ECDSA: Elliptic Curve, FIPS standard.