Follow these steps to generate and verify digital signatures using RSA and SHA-1 in the simulation.

⚙️ Step 1: Select RSA Key Pair

Start by selecting one key preset in Step 1 before signing.

• Choose one of the four key buttons:
  • Load 1024-bit Key (e=F4)
  • Load 1024-bit Key (e=3)
  • Load 512-bit Key (e=F4)
  • Load 512-bit Key (e=3)
• After selection, the simulation displays all active key parameters:
  • Public exponent e (hex)
  • Modulus n (hex)
  • Private exponent d (hex)
• The Apply RSA Signature button remains disabled until a key is selected.
• If you change the key after signing, previous signature output is invalidated and cleared automatically.

📝 Step 2: Enter Plaintext and Generate Hash

• Enter your message in Plaintext (default: test).
• Click Generate SHA-1 Hash.
• Hash output (hex) is computed and displayed.
• The same hash is auto-copied to the RSA input field used for signing.

🔑 Step 3: Input Hash to RSA and Sign

• Confirm the auto-populated Input to RSA (hex) value.
• Click Apply RSA Signature.
• Signature generation uses the active key selected in Step 1.

✅ Step 4: View Digital Signature Results

Step 4 shows the signature and computation details:

• Digital Signature (hex)
• Digital Signature (base64)
• Status message
• Signature Computation panel showing the exact operation:
  • Signature = HashAsInteger^d mod n
  • With current numeric values substituted from the selected key and hash

🔍 Step 5: Verify Signature

Use Step 5 to verify and experiment with tampering.

• Message for verification is pre-filled from the original message.
• Signature to verify (hex) is pre-filled from the generated signature.
• Click Verify Signature to compute and compare:
  • Computed H(m)
  • Recovered s^e mod n
  • Verification Result (Valid signature or Invalid signature)

To observe failure behavior:

• Edit either the verification message or the signature hex.
• Click Verify Signature again.
• The result should become Invalid signature when values no longer match.

📊 Summary Section

The Digital Signature Summary updates automatically with:

• Original Message
• SHA-1 Hash
• Key Size (active preset)
• Signature Status

🔍 What This Demonstrates

1. Hashing binds the signature to message content.
2. Signing uses the private exponent d of the active key pair.
3. Verification uses the public key values (n, e).
4. Any message or signature tampering causes verification failure.

🔧 Troubleshooting

Issue	What to check
Apply RSA Signature is disabled	Select a key preset in Step 1 first
Signature cleared unexpectedly	Key/message/hash was changed; regenerate signature
Verification shows Invalid signature	Ensure message and signature are the same pair from Step 4
Empty/old summary values	Regenerate hash/signature and verify again