0011663: Prevent replay attacks on totp

ID	Project	Category	View Status	Date Submitted	Last Update
0011663	Taler	other	public	2026-07-22 13:38	2026-08-04 23:06

Reporter	MarcS	Assigned To		Resolution deadline
Implementer		Reviewer
Release signed by		Deployed by
Verifier		Communicator
Data protection authority notification required	No
CSIRT notification required	No
Estimated effort (hours)		Actual effort (hours)	0
Fast-track	No
Work package	none
Priority	normal	Severity	minor	Reproducibility	N/A
Status	acknowledged	Resolution	open
Product Version	git (master)
Target Version	post-1.0

Summary	0011663: Prevent replay attacks on totp
Description	Currently it is possible to use the same totp codes for the offline coffee machine to pay for 1 coffee and receive multiple coffees.
Additional Information	Solution: Instead of treating "nfc=1" as Bool, we will interpret 1..255 as session counter. Each time the device registers a smartphone reading NFC it will increment this session counter (reverting to 1 after 255). The merchant backend will include this counter in the input data for the totp generator, and thus can generate 255 different sets of 5 totp codes for the same timestamp. The wallet will add this session counter byte after the start byte 0x42 ("B") and before the five 4-byte totp codes when writing back the totp data to the device. The device uses the same algorithm as the merchant backend to compute the totp codes from the current timestamp, the secret and the passed on session counter byte.
Tags	No tags attached.

Date Modified	Username	Field	Change
2026-07-22 13:38	MarcS	New Issue
2026-08-04 23:06	Christian Grothoff	Status	new => acknowledged
2026-08-04 23:06	Christian Grothoff	Product Version	post-1.0 => git (master)