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SNMP AES Decryption Debug Session

Troubleshooting corrupted first 16 bytes in AES CFB mode

User

I am writing an implementation of snmpwalk with authentication and encryption. Everything works (and I can see the conversations decrypted in Wireshark in-between the client and server), however in the AES decryption, sometimes the first 16 bytes of the decrypted data are corrupted.

like this:

Decrypted data: [c3, e8, 35, c6, 03, 91, 26, 8f, 55, b3, 26, b8, e2, 03, f8, 06, 68, 00, 00, 00, 00, 04, 00, a2, 26, 02, 04, 42, 50, 15, 77, 02, 01, 00, 02, 01, 00, 30, 18, 30, 16, 06, 08, 2b, 06, 01, 02, 01, 01, 02, 00, 06, 0a, 2b, 06, 01, 04, 01, bf, 08, 03, 02, 0a]

You can see the first 16 bytes do not make sense, however, after that the bytes look like normal.

Attached is the full log. Could you help debug this? Please first verify your logical conclusions by posing a theory and confirming it based on the log file that I have uploaded, only after my confirmation that I agree you can show the fixes.

Edit
Assistant

Looking at your log file, I can see the issue with AES decryption where the first 16 bytes are corrupted. Let me analyze this step by step.

Theory:

The problem appears to be with the AES CFB (Cipher Feedback) mode implementation. Looking at your code, I see you're using AES-128 in CFB mode for privacy, but there's a mismatch in how the IV is being calculated and used between encryption and decryption.

Here are the key observations from your logs:

Successful first decryption:
Salt: [00, 00, 00, 00, 00, 00, 00, 01]
IV: [00, 00, 00, 08, 00, 01, 72, db, 00, 00, 00, 00, 00, 00, 00, 01]
Result: Clean decryption starting with [30, 81, 8a, 04, 11, 80, 00, 1f, 88...] (valid ASN.1 SEQUENCE)
Failed second decryption:
Salt: [11, 57, b1, 11, 81, 13, 88, 71] (different salt from server)
IV: [00, 00, 00, 08, 00, 01, 72, db, 11, 57, b1, 11, 81, 13, 88, 71]
Result: Corrupted first 16 bytes [c3, e8, 35, c6, 03, 91, 26, 8f, 55, b3, 26, b8, e2, 03, f8, 06...]

My theory is: The issue is that you're generating your own salt for encryption ([00, 00, 00, 00, 00, 00, 00, 01]) but the server is responding with a different salt ([11, 57, b1, 11, 81, 13, 88, 71]). When you try to decrypt the server's response using your client-generated IV instead of the server's IV, the first block decrypts incorrectly.

Root cause: In AES CFB mode, the IV must match exactly between encryption and decryption. The server is using a different salt/IV than what your client is calculating for decryption.

Questions to confirm my theory:

  1. Can you confirm that the salt values I identified above are correct - that your client is generating [00, 00, 00, 00, 00, 00, 00, 01] but the server is responding with [11, 57, b1, 11, 81, 13, 88, 71]?
  2. Looking at your code, do you see that the priv_counter is being reset or reused between requests? I notice both requests show "Privacy counter: 1" in the logs.
  3. In your encryption process, are you incrementing the privacy counter properly for each new request, or is it being reset?

Please confirm if this analysis matches what you're seeing, and then I can show you the specific fixes needed.

User

I have another log, with a successful run - without any code changes! should I upload it, so you can compare?

Edit
Assistant

Yes, absolutely! Please upload the successful run log. Comparing a successful run with the failed run will be extremely valuable for understanding what's different between the working and non-working cases.

This will help me:

  • See what the salt/IV values look like when it works vs when it fails
  • Understand if there's a timing or state issue
  • Identify any patterns in the encryption/decryption parameters
  • Confirm or refine my theory about the root cause

Please upload the successful log - this comparison will likely make the issue much clearer!

User
Starting SNMP walk on 10.2.2.118:161 Starting OID: 1.3.6.1.2.1.1.1 Walking from OID: 1.3.6.1.2.1.1.1 ---------------------------------------- Setting up authentication: HmacSha1 with password length 24 USM config - has_auth: true, has_priv: true Doing discovery Starting SNMPv3 engine discovery... IN PASTED

Here it goes. The command line to request the data is exactly the same as in the non-working case.

Edit
Assistant

Perfect! Now I can see the key difference. Looking at both logs, I've identified the exact issue:

Confirmed Theory:

The problem is NOT with your encryption/decryption logic - that's working correctly. The issue is with salt reuse and the privacy counter not being properly incremented.

Key Evidence from the logs:

Failed case (second request):
• Client generates salt: [00, 00, 00, 00, 00, 00, 00, 01]
• Server responds with salt: [11, 57, b1, 11, 81, 13, 88, 71]
• Privacy counter: 1 (same as first request)
• Engine time: 94939 → 94940 (incremented)
Successful case (second request):
• Client generates salt: [00, 00, 00, 00, 00, 00, 00, 01]
• Server responds with salt: [11, 57, b1, 11, 81, 13, 88, 77] (note last byte is 77 not 71)
• Privacy counter: 1 (same as first request)
• Engine time: 95373 (same as first request)

Root Cause:

  • Your privacy counter is being reset to 1 for each new request instead of incrementing
  • In the failed case, the engine time changed between requests, causing the server to use a different salt
  • Since you're using the same privacy counter (1) with different engine time, the server and client calculate different IVs
  • In the successful case, the engine time stayed the same, so the salt calculation aligned

The Fix: You need to ensure the privacy counter increments properly across requests within the same session. The issue is in your UsmConfig::next_priv_counter() method being called on a fresh config each time.

Do you confirm this analysis matches what you're seeing? The key insight is that engine time changes between some requests but not others, and your privacy counter resets instead of incrementing properly.

User

so, it looks like the response when it fails has the engine time incrementing, which causes the IV to be calculated wrongly?

Edit
Assistant

Exactly! You've identified the core issue perfectly.

What's happening:

  1. Your client always generates salt using privacy counter 1: [00, 00, 00, 00, 00, 00, 00, 01]
  2. Your client calculates IV using the engine time from discovery (which stays constant in your client):
    • Failed case: [00, 00, 00, 08, 00, 01, 72, db, 00, 00, 00, 00, 00, 00, 00, 01] (engine time 94939 = 0x172db)
    • Success case: [00, 00, 00, 08, 00, 01, 74, 8d, 00, 00, 00, 00, 00, 00, 00, 01] (engine time 95373 = 0x1748d)
  3. The server calculates its own salt/IV using its CURRENT engine time (which increments):
    • Failed case response salt: [11, 57, b1, 11, 81, 13, 88, 71] (server used engine time 94940)
    • Success case response salt: [11, 57, b1, 11, 81, 13, 88, 77] (server used engine time 95373)

During decryption, you use the server's salt but YOUR engine time from discovery, creating a mismatched IV that corrupts the first 16 bytes in CFB mode.

The fix: You need to update your local engine time before each request and use the CURRENT engine time (not the discovery-time engine time) when calculating IVs for decryption.

Do you want me to show you the specific code changes needed?

User

yeah, please do!

Edit
Assistant

Here are the specific code changes needed:

Fix 1: Update Engine Time Before Each Request

In your snmpwalk.rs, modify the send_getnext_request and send_getbulk_request methods to update the engine time:

fn send_getnext_request(&mut self, oid: &str) -> Result> { let request = match &self.config.version { SnmpVersion::V3 { .. } => { if let Some(mut usm_config) = self.create_usm_config() { // If engine ID is empty, do discovery first if usm_config.engine_id.is_empty() { let engine_id = self.snmpv3_discovery(&mut usm_config)?; usm_config.engine_id = engine_id; } // ADD THIS: Update engine time before each request self.update_engine_time(&mut usm_config); // ... rest of your existing code

Fix 2: Add Engine Time Update Method

Add this method to your SnmpWalker implementation:

impl SnmpWalker { fn update_engine_time(&mut self, usm_config: &mut UsmConfig) { // Get current time elapsed since discovery let time_elapsed = std::time::SystemTime::now() .duration_since(std::time::UNIX_EPOCH) .unwrap_or_default() .as_secs() as u32; // Update engine time (simplified - you might want to track discovery time more precisely) usm_config.engine_time = usm_config.engine_time.saturating_add(1); // Update in your config as well if let SnmpVersion::V3 { engine_time: ref mut stored_time, .. } = &mut self.config.version { *stored_time = usm_config.engine_time; } }

Fix 3: Fix Privacy Counter Management

In your snmp.rs, modify the UsmConfig to properly track the privacy counter:

impl UsmConfig { /// Get and increment the privacy counter pub fn next_priv_counter(&mut self) -> u64 { let current = self.priv_counter; self.priv_counter = self.priv_counter.wrapping_add(1); current } }

Fix 4: Use Server's Engine Time for Decryption

Most importantly, in your decrypt_scoped_pdu method in snmpwalk.rs, extract and use the server's engine time from the response:

fn decrypt_scoped_pdu( &self, encrypted_data: &[u8], privacy_params: &[u8], usm_config: &UsmConfig, server_engine_time: u32, // ADD THIS PARAMETER ) -> Result> { println!("=== DECRYPTION WITH SERVER ENGINE TIME ==="); println!("Server engine time: {}", server_engine_time); // ... existing code for deriving priv_key and salt ... // Calculate IV using SERVER's engine time, not client's cached time let iv = match usm_config.priv_algorithm { usm_crypto::PrivAlgorithm::Aes128 => { // Use server's engine time here usm_config.priv_algorithm.calculate_iv( salt, &priv_key, usm_config.engine_boots, server_engine_time, // Use server time, not usm_config.engine_time ).map_err(|e| format!("Failed to calculate AES IV: {}", e))? } // ... handle other algorithms }; // ... rest of decryption logic }

Fix 5: Extract Server Engine Time from Response

Modify your extract_bindings_from_snmpv3 method to extract and pass the server's engine time:

fn extract_bindings_from_snmpv3( &self, response: &oside::LayerStack, usm_config: &UsmConfig, ) -> Result, Box> { if let Some(snmpv3) = response.get_layer(SnmpV3::new()) { // Extract server's current engine time from response let server_engine_time = if let Value::Set(SnmpV3SecurityParameters::Usm(ref usm)) = &snmpv3.msg_security_parameters { usm.msg_authoritative_engine_time.value() } else { usm_config.engine_time // fallback }; println!("Server engine time from response: {}", server_engine_time); // ... existing auth verification code ... if usm_config.has_priv() { if let Some(encrypted_data) = self.extract_encrypted_data(response)? { let privacy_params = self.extract_privacy_params(response)?; let decrypted_scoped_pdu = self.decrypt_scoped_pdu( &encrypted_data, &privacy_params, usm_config, server_engine_time // Pass server time )?; return self.extract_bindings_from_scoped_pdu(&decrypted_scoped_pdu); } } } // ... rest of method }

Key Insight

The key insight is that the server uses its current engine time for encryption, but you were using your cached discovery-time engine time for decryption. This mismatch corrupts the first 16 bytes in AES CFB mode because the IV doesn't match.

(c) Andrew Yourtchenko