Introduce ReceiveAuthKey verification for Blinded Payment Paths #4126

shaavan · 2025-09-25T11:52:08Z

Building on the goals set forth in #3917, this PR introduces ReceiveAuthKey-based verification for Blinded Payment Paths.

Key Outcomes

Uniform verification mechanism: Both payment and message blinded paths now share the same authentication logic.
Smaller payloads: The final ReceiveTlvs are noticeably reduced in size, making blinded paths lighter.

Follow-Up Preparation

This PR also lays the groundwork for introducing dummy payment hops in a follow-up PR.
By minimizing per-hop authentication data, we keep dummy hops compact — preserving overall path size and ensuring that forward and dummy TLVs are padded to the same length, improving hop indistinguishability and privacy.

ldk-reviews-bot · 2025-09-25T11:52:11Z

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codecov · 2025-09-25T12:21:07Z

Codecov Report

❌ Patch coverage is 89.83051% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.33%. Comparing base (95c4eaf) to head (a6719b1).
⚠️ Report is 24 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/blinded_path/payment.rs	70.37%	3 Missing and 5 partials ⚠️
lightning/src/routing/router.rs	70.00%	3 Missing ⚠️
lightning/src/ln/msgs.rs	94.11%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4126      +/-   ##
==========================================
+ Coverage   89.28%   89.33%   +0.04%     
==========================================
  Files         180      180              
  Lines      137913   138086     +173     
  Branches   137913   138086     +173     
==========================================
+ Hits       123142   123354     +212     
+ Misses      12167    12136      -31     
+ Partials     2604     2596       -8

Flag	Coverage Δ
fuzzing	`35.86% <5.19%> (+2.30%)`	⬆️
tests	`88.70% <89.83%> (+0.01%)`	⬆️

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ldk-reviews-bot · 2025-09-27T12:02:56Z

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shaavan · 2025-10-06T18:13:40Z

Updated from pr4126.01 to pr4126.02 (diff):

Changes:

Various cleanups
Expanded Documentation
Restructured the fails_receive_tlvs_authentication test so that it can properly test the new ReceiveAuthKey based authentication

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lightning/src/ln/channelmanager.rs

lightning/src/ln/onion_utils.rs

lightning/src/blinded_path/payment.rs

lightning/src/offers/signer.rs

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shaavan · 2025-11-04T11:07:02Z

Updated .02 → .03

Thanks @jkczyz - changes:

Test cleanups
Various documentation changes.

shaavan · 2025-11-04T15:21:52Z

Rebased .03 → .04

shaavan · 2025-11-08T13:14:39Z

Rebased .04 → .05

TheBlueMatt

Thanks!

lightning/src/blinded_path/payment.rs

lightning/src/ln/msgs.rs

lightning/src/offers/signer.rs

shaavan · 2025-11-12T15:15:39Z

Updated .05 → .06

Thanks @TheBlueMatt — changes:

Added used_aad check in decrypt_intro_payload
Restored HMAC variable names for clarity on reuse risk
Moved used_aad check to read-time to simplify downstream logic

jkczyz

LGTM. Mostly some nits but one is needed to be addressed to fix CI.

lightning/src/ln/onion_payment.rs

lightning/src/ln/channelmanager.rs

lightning/src/blinded_path/payment.rs

Extends the work started in [PR#3917](lightningdevkit#3917) by adding ReceiveAuthKey-based verification for Blinded Payment Paths. This reduces space previously taken by individual ReceiveTlvs and aligns the verification logic with that used for Blinded Message Paths.

Now that we have introduced an alternate mechanism for authentication in the codebase, we can safely remove the now redundant (hmac, nonce) fields from the Payment ReceiveTlvs's while maintaining the security of the onion messages.

shaavan · 2025-11-12T18:23:43Z

Updated: .06 → .07

Thanks @jkczyz — changes:

Removed unrelated edits from onion_payment.rs
Fix lint errors
Rename receive_auth_key → local_node_receive_key (match message.rs)
Doc nits

jkczyz · 2025-11-12T21:45:01Z

lightning/src/routing/router.rs

 	/// Creates [`BlindedPaymentPath`]s for payment to the `recipient` node. The channels in `first_hops`
 	/// are assumed to be with the `recipient`'s peers. The payment secret and any constraints are
-	/// given in `tlvs`.
+	/// given in `tlvs`. The `receive_auth_key` is required to authenticate the blinded payment paths.


s/receive_auth_key/local_node_receive_key

jkczyz · 2025-11-12T21:50:05Z

lightning/src/ln/onion_utils.rs

+	/// for the payer, resulting in a payment that cannot be authenticated.
+	///
+	/// [`BlindedPaymentPath`]: crate::blinded_path::payment::BlindedPaymentPath
+	UnauthenticatedPayload,


Is there an easy way to add a test checking for this failure?

shaavan force-pushed the aad-payment branch from 69c57f9 to 359267e Compare September 25, 2025 11:58

ldk-reviews-bot requested a review from jkczyz September 25, 2025 12:02

shaavan force-pushed the aad-payment branch from 359267e to 13c4041 Compare September 25, 2025 12:07

shaavan force-pushed the aad-payment branch from 13c4041 to a93113c Compare October 6, 2025 18:13

shaavan mentioned this pull request Oct 9, 2025

Introduce Dummy Hop support for Blinded Payment Path #4152

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jkczyz reviewed Oct 27, 2025

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shaavan force-pushed the aad-payment branch from a93113c to e60f085 Compare November 4, 2025 11:06

shaavan force-pushed the aad-payment branch 2 times, most recently from acbadec to 18e6b33 Compare November 4, 2025 15:21

shaavan force-pushed the aad-payment branch from 18e6b33 to ee46e45 Compare November 8, 2025 13:14

TheBlueMatt reviewed Nov 11, 2025

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shaavan force-pushed the aad-payment branch from ee46e45 to 516c6f5 Compare November 12, 2025 15:15

jkczyz reviewed Nov 12, 2025

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shaavan added 2 commits November 12, 2025 23:41

Cleanup: Remove redundant (hmac, nonce) from codebase

a6719b1

Now that we have introduced an alternate mechanism for authentication in the codebase, we can safely remove the now redundant (hmac, nonce) fields from the Payment ReceiveTlvs's while maintaining the security of the onion messages.

shaavan force-pushed the aad-payment branch from 516c6f5 to a6719b1 Compare November 12, 2025 18:23

jkczyz reviewed Nov 12, 2025

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Introduce ReceiveAuthKey verification for Blinded Payment Paths #4126

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shaavan commented Sep 25, 2025

Key Outcomes

Follow-Up Preparation

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