Introduce Padding for Payment and Message Blinded Tlvs

[shaavan]

Description

This PR introduces padding for Payment and Message Blinded TLVs to ensure that the size of each packet in the path is uniform.

[shaavan]

Notes:

1. Padding Implementation:
   • The padding field is now expanded to contain Option<Vec>. This change allows the inclusion of padding within the packet while retaining the flexibility to leave the padding as None.
2. Impact on Path Length:
   • With the added padding, the length of each individual packet increases, which results in a reduction of the maximum path length. Consequently, the max_path_length in forward_check_failures has been adjusted from 18 to 17.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.48485% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.53%. Comparing base (1434e9c) to head (758d06d).

Files with missing lines	Patch %	Lines
lightning/src/blinded_path/utils.rs	94.44%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3177      +/-   ##
==========================================
- Coverage   88.54%   88.53%   -0.01%     
==========================================
  Files         149      149              
  Lines      114579   114704     +125     
  Branches   114579   114704     +125     
==========================================
+ Hits       101457   101558     +101     
- Misses      10633    10650      +17     
- Partials     2489     2496       +7     

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[shaavan]

Updated from pr3177.01 to pr3177.02 (diff):
Addressed @jkczyz's comments.

Changes:

1. Reordered commits and separated the update to the Padding struct into a separate commit.
2. Updated the Padding struct to contain a usize instead of a Vec to save on heap allocation, and made the Writable implementation more efficient.
3. Introduced two f: commits to use iter clone instead of Vec allocation, keeping the code efficient.
4. Introduced a test and a test utility to verify that blinded message and payment paths are properly padded, ensuring a consistent payload size.

[shaavan]

Updated from pr3177.02 to pr3177.03 (diff):

Changes:

1. Rebase on main to fix ci.

Diff post rebase:

--- a/lightning/src/ln/offers_tests.rs
+++ b/lightning/src/ln/offers_tests.rs
@@ -1789,7 +1789,7 @@ fn test_blinded_path_padding() {
        let onion_message = charlie.onion_messenger.next_onion_message_for_peer(david_id).unwrap();
        david.onion_messenger.handle_onion_message(&charlie_id, &onion_message);
 
-       let invoice = extract_invoice(david, &onion_message);
+       let (invoice, _) = extract_invoice(david, &onion_message);
        assert_eq!(invoice, expected_invoice);

[shaavan]

Updated from pr3177.03 to pr3177.04 (diff):
Addressed @jkczyz comments

1. Refactor max_length calculation to be cleaner.
2. Introduce wrapper struct instead of tuple to make the code clearer to understand.
3. Move the padding testing to onion_message/functional_tests.rs to simplify the setup, and test the code at a more appropriate place.

[shaavan]

Updated from pr3177.04 to pr3177.05 (diff):

Updates:

1. Rebase on main.

[shaavan]

Updated from pr3177.05 to pr3177.06 (diff):
Addressed @jkczyz comments

Changes:

1. Squash commits.
2. Introduce a generic struct WithPadding to keep the code DRY.
3. Introduce a test for Blinded Payment Paths.
4. Use saturating sub in padding length calculation to avoid overflows.

[shaavan]

Updated from pr3177.06 to pr3177.07 (diff):
Addressed @jkczyz comments

1. Update comments.
2. Introduce debug_assert!() in Writeable for WithPadding.
3. Properly assert!() the condition in test.

[shaavan]

Updated from pr3177.07 to pr3177.08 (diff):
Addressed @jkczyz comments

Changes:

1. Introduce an f: commit with an alternative approach to WithPadding.
2. The new approach introduces padding as an optional field within the ForwardTlvs and the ReceiveTlvs struct.

[shaavan]

Update: From pr3177.08 to pr3177.09 (diff):

Changes based on @jkczyz's feedback:

1. Fixed the imports.
2. Removed BlindedPaymentTlvsRef and replaced it with BlindedPaymentTlvs. The former contained an immutable reference to the underlying TLVs, but the new implementation required mutability. Therefore, I transitioned to using BlindedPaymentTlvs directly, making BlindedPaymentTlvsRef redundant.

[jkczyz]

@TheBlueMatt One thing that I'm unsure of is whether we only need to pad the blinded path or if we should also pad the unblinded portion of the onion.

[shaavan]

Update: From pr3177.09 to pr3177.10 (diff):

Changes based on @jkczyz's feedback:

1. Rename pad_tlvs -> pad_to_length.

[shaavan]

Update: From pr3177.28 to pr3177.29 (diff):
Addressed @jkczyz, @TheBlueMatt comments

Changes:

1. Reverted individual padding for offer, and refund payment tlvs, and updated the value 35 -> 30.
2. Update padding logic so that we also account for TLV Overhead introduced by padding TLV type and encryption data.
3. Remove Padding Readable, as the value is never read after use.
4. Update struct name, Padding -> BlindedPathPadding & documentation to be specific, and clarifying the TLV type 1 usage.

[shaavan]

Update: From pr3177.29 to pr3177.30 (diff):
Addressed @jkczyz comments

Changes:

1. Updated the comment on the reasoning behind not reading padding.
2. Update TLV_OVERHEAD to only account for padding type and length bytes.
3. Reverted changes to the is_padded function.

[shaavan]

Update: From pr3177.30 to pr3177.31 (diff):

Changes:

1. Rebase on main to resolve to merge conflicts.

[shaavan]

Update: From pr3177.31 to pr3177.32 (diff):

Changes:

1. Squash commits, and expands commit message to make their purpose clearer.

[shaavan]

Update: From pr3177.32 to pr3177.33 (diff):

Changes:

1. Rebase on main, to resolve merge conflicts.

[TheBlueMatt]

LGTM, @jkczyz you wanna take another look?

[jkczyz]

Looks like rustfmt CI checks are failing. Otherwise, just one nit and LGTM.

[shaavan]

Update: From pr3177.33 to pr3177.34 (diff):
Addressed @jkczyz comments:

1. Fix rust formatting
2. Addressed nit suggestion

[shaavan]

Update: From pr3177.34 to pr3177.35 (diff):

Changes:

1. Rebase on main, resolving merge conflicts.