Digital Signatures
A cryptographic mechanism using asymmetric keys to verify authenticity and integrity of digital messages or documents. The sender signs with their private key; recipients verify with the sender's public key, providing non-repudiation.
Understanding Digital Signatures
Digital signatures are the cryptographic equivalent of handwritten signatures—but stronger. They prove that a message came from a specific sender (authenticity), hasn't been modified (integrity), and the sender cannot deny sending it (non-repudiation).
How it works: 1. Sender creates hash of the message 2. Sender encrypts the hash with their private key (this is the signature) 3. Signature is attached to the message 4. Recipient decrypts signature with sender's public key 5. Recipient hashes the message and compares
If the hashes match, the signature is valid—the message is authentic and unmodified.
Why This Matters for the Exam
Digital signatures are fundamental to internet security and heavily tested on SY0-701. They're used in code signing, email security (S/MIME), document signing, SSL/TLS certificates, and software distribution.
The exam tests the key direction (sign with private, verify with public), the security properties provided (authenticity, integrity, non-repudiation), and when digital signatures are appropriate.
Understanding digital signatures connects to PKI, certificates, and trust concepts. A signature is only trustworthy if you trust the public key actually belongs to the claimed sender—this is what certificates provide.
Deep Dive
The Digital Signature Process
Signing (Sender):
- 1.Create message to send
- 2.Hash the message (SHA-256)
- 3.Encrypt hash with sender's PRIVATE key
- 4.Attach encrypted hash (signature) to message
- 5.Send message + signature
Verification (Recipient):
- 1.Receive message + signature
- 2.Decrypt signature with sender's PUBLIC key
- 3.Independently hash the received message
- 4.Compare decrypted hash to calculated hash
- 5.If match → Signature valid
What Digital Signatures Provide
| Property | How It's Achieved |
|---|---|
| Authenticity | Only private key holder could create signature |
| Integrity | Any message change invalidates signature |
| Non-repudiation | Sender cannot deny signing (they have the only private key) |
What Signatures Do NOT Provide
- •Confidentiality — Message is not encrypted (only signed)
- •For confidentiality + authenticity: Encrypt AND sign
Why Hash First?
Efficiency: Hashing creates fixed-size data to sign
- •10KB document → 256-bit hash → sign 256 bits
- •Much faster than signing entire document
Security: Asymmetric operations are slow
- •Hash once (fast)
- •Sign hash (fast on small data)
- •Overall: fast signing of any size document
Signature Algorithms
| Algorithm | Based On | Status |
|---|---|---|
| RSA Signatures | RSA encryption | ✅ Widely used |
| DSA | Discrete logarithm | ✅ Government standard |
| ECDSA | Elliptic curves | ✅ Efficient, modern |
| EdDSA (Ed25519) | Edwards curves | ✅ Newest, very fast |
Digital Signatures Use Cases
Code Signing
- •Software developers sign executables
- •OS verifies signature before running
- •Prevents malware impersonating legitimate software
Email Signing (S/MIME, PGP)
- •Sender signs email
- •Recipient verifies sender identity
- •Detects tampering of email content
Document Signing
- •Legal documents signed digitally
- •PDF signatures
- •Contract signing platforms
Certificate Signing
- •CAs sign certificates with their private key
- •Browser verifies CA signature
- •Establishes trust chain
SSL/TLS Handshakes
- •Server proves identity via signed certificate
- •Client verifies signature against trusted CAs
Encryption vs. Signature Key Direction
| Operation | Key Used | Purpose |
|---|---|---|
| Encrypt | Recipient's PUBLIC | Confidentiality |
| Decrypt | Recipient's PRIVATE | Read encrypted data |
| Sign | Sender's PRIVATE | Non-repudiation |
| Verify | Sender's PUBLIC | Confirm authenticity |
Key insight: Encryption and signing use keys in OPPOSITE directions.
- •Encryption: Encrypt with recipient's public key
- •Signing: Sign with your own private key
How CompTIA Tests This
Example Analysis
Scenario: A software company distributes an application. Users need assurance that: 1. The software actually came from the company 2. The software hasn't been modified since release 3. The company cannot deny releasing it
Solution: Code Signing with Digital Signature
Implementation: 1. Company creates hash of the application 2. Company signs hash with their PRIVATE key 3. Signature attached to application 4. User downloads application + signature 5. User's OS verifies signature using company's PUBLIC key
What this provides: • Authenticity: Only the company has the private key • Integrity: Any modification invalidates the signature • Non-repudiation: Company cannot claim they didn't sign it
What if verification fails? • OS warns user: "Unknown publisher" or "Modified software" • User decides whether to trust and run • Malware typically fails signature verification
Key Terms to Know
Common Mistakes to Avoid
Exam Tips
Memory Trick
"SIGN-PRIVATE, VERIFY-PUBLIC"
The sender SIGNS with their PRIVATE key (only they have it) Anyone VERIFIES with the PUBLIC key (everyone has it)
- •What Signatures Give You: "AIN"
- •Authenticity — Proves who sent it
- •Integrity — Proves it wasn't changed
- •Non-repudiation — Sender can't deny it
Not Confidentiality! "AIN" not "CAIN"
Process Memory: "HASH-SIGN-SEND" 1. HASH the message 2. SIGN the hash (encrypt with private key) 3. SEND message + signature
Verification: "DECRYPT-HASH-COMPARE" 1. DECRYPT signature (with public key) 2. HASH the received message 3. COMPARE the two hashes
Test Your Knowledge
Q1.When creating a digital signature, which key does the sender use?
Q2.Which security properties does a digital signature provide?
Q3.Why is a hash created before signing a document?
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