From d55729cba7ff5e2287c2db3ad25934e63ed60a45 Mon Sep 17 00:00:00 2001 From: Eric Wild Date: Tue, 28 Jul 2026 17:42:32 +0200 Subject: [PATCH] firmware: iso7816_3: fix F/D ratio for Di 8 and 9 iso7816_3_compute_fd_ratio() multiplied F by D for every d_index >= 8, presumably because the upper half of ISO 7816-3 Table 8 encodes 1/D. But 7816-3 2006 and 1997 differ! That assumption is only true for the range 1010..1111, which in the 2006 version is RFU. Indices 1000 and 1001 are Di = 12 and Di = 20, see iso7816_3_di_table[]. So right now Fi=372/Di=12 -> 372 * 12 = 4464 instead of 372 / 12 = 31. In the cemu value is rejected in emu_update_fidi() and the old baud rate is silently kept. In the sniffer update_fidi() programs US_FIDI as 4464 & 0x7ff = 368, which is garbage. Use F/D for indices 1..9 and keep the legacy 1/D reading only for the RFU range, where we cant really do anything useful anyway. Change-Id: I44d6451d8b04aea2b0db7291b06a812afe84e52f --- firmware/libcommon/source/iso7816_fidi.c | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-) diff --git a/firmware/libcommon/source/iso7816_fidi.c b/firmware/libcommon/source/iso7816_fidi.c index 024663b6..4e87dbd8 100644 --- a/firmware/libcommon/source/iso7816_fidi.c +++ b/firmware/libcommon/source/iso7816_fidi.c @@ -48,9 +48,13 @@ int iso7816_3_compute_fd_ratio(uint8_t f_index, uint8_t d_index) if (d == 0) return -EINVAL; - /* See table 7 of ISO 7816-3: From 1000 on we divide by 1/d, - * which equals a multiplication by d */ - if (d_index < 8) + /* DI defined in Table 8 of ISO/IEC 7816-3:2006 + * has values 0001..1001 as div 1, 2, 4, 8, 16, 32, 64, 12, 20 + * so indices 1..9 are all divisors and the ratio is F/D. + * But Indices 1010..1111 are RFU in the 2006 edition! + * 1997 used those for 1/2 .. 1/64, where dividing by 1/d equals multiplying by d. + * Keep that legacy interpretation for the RFU range only. */ + if (d_index < 10) ret = f / d; else ret = f * d;