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firmware: iso7816_3: fix F/D ratio for Di 8 and 9

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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
This commit is contained in:
Eric Wild
2026-07-28 17:42:32 +02:00
parent 567c03c83f
commit d55729cba7
1 changed files with 7 additions and 3 deletions
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firmware/libcommon/source/iso7816_fidi.c
+7 -3
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@@ -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;