Introduce hard limit on transaction fee (core)

[tsusanka]

We currently have soft warning when transaction fee seems too high. For a usual small segwit transaction the limit is around 0.003 BTC which is ~30 USD at the current price (~9000 USD).

When there is a really big fee (by accident or malware) we are just a user's click away from spending a huge fee. As requested by Product, let's introduce a hard limit which is 10x more than the soft limit and may be turned of using advanced setting introduced in #1064. By hard limit I mean we refuse to sign and raise an error.

[tsusanka]

@mmilata would you be interested in a small research? There are two things we would like to look at.

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First let's have a look on the current fee limits. That means create a table for top10 Bitcoin-like coins that we support (or simpler way: use the ones that we currently support in wallet.trezor.io) and list for each one of them:

1. The current maxfee_kb.
2. The current fee_threshold for a "traditional" transaction (let's say something the user commonly deals with, 2 inputs, 2 outputs, doesn't have to be exact).
3. The current fee_threshold in USD. Basically (2) but in USD.
4. The hard limit we suggest (that is 10x (3) but it would be nice to see it in one table).

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And second, let's investigate how we deal with fee thresholds in the other non-Bitcoin-like coins because I am not sure we do.

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Do you think you could look at it at some point in future? The Port has definitely a priority.

[mmilata]

@tsusanka sure, can do that

[mmilata]

@tsusanka For some of the bitcoin-based coins, the threshold for 250B transaction is following:

Coin	maxfee_kb	fee_threshold	USD rate	fee_threshold (USD)	hard limit (USD)
BTC	2000000	500000 sats = 0.005	11500.0	57.5	575
BCH	500000	125000 sats = 0.00125	280.0	0.4	4.0
BTG	500000	125000 sats = 0.00125	10.0	0.01	0.1
DASH	100000	25000 sats = 0.00025	90.0	0.02	0.2
DGB	500000	125000 sats = 0.00125	0.03	0.00004	0.0004
DOGE	1000000000	250000000 sats = 2.5	0.003	0.008	0.08
LTC	40000000	10000000 sats = 0.1	55.0	5.5	55.0
NMC	10000000	2500000 sats = 0.025	0.4	0.01	0.1
VTC	40000000	10000000 sats = 0.1	0.3	0.03	0.3
ZEC	1000000	250000 sats = 0.0025	85.0	0.2	2.0

Other coins such as ETH, ETC, NEM, XLM, ADA, XTZ don't have any warning or hard limit implemented.

[tsusanka]

Closed by #1155 with two follow-up issues #1191 and #1192.

[tsusanka]

QA

• Please test BTC transaction signing (both Wallet and Suite).
  • With normal fee (as suggested by Wallet/Suite).
  • With fee around 100 USD. Warning should be shown. (I am not sure you can test that?)
  • WIth ridiculous fee around 1000 USD. Error should be raised. (I am not sure you can test that?)

[tsusanka]

@bosomt please include a picture of Suite/Wallet when you are testing it so support can have a look.

[bosomt]

maximum fee in suite is 2000 sat/b
i will check for another ways to test hard limit

[bosomt]

QA OK

tested 2.3.3, no warning in older FW
@tsusanka is this feature core specific or is this legacy T1 included ?

FW version 2.3.3 df5421e
Wallet Git Commit: ec6b4d36cbae2ec1c3f479eb3c1891090790b7c0

wallet log :

["log","Fri, 28 Aug 2020 07:40:33 GMT","[trezor.js] [call] Sending","ButtonAck",{}]
["log","Fri, 28 Aug 2020 07:40:37 GMT","[trezor.js] [call] Received","Failure",{"code":"Failure_DataError","message":"The fee is unexpectedly large"}]
["log","Fri, 28 Aug 2020 07:40:37 GMT","[trezor.js] [session] releasing"]
["error","Fri, 28 Aug 2020 07:40:37 GMT","[AccountSendCtrl] Failed to send transaction","Error: The fee is unexpectedly large"]

[tsusanka]

TT only for now. We will introduce it to T1 via Port.

[bosomt]

tried Fee: $211.06

[bosomt]

test results for Electrum 4.0.2:

1. treshold: same results, warning on device and in Electrum
2. hard limit:

maybe we will need fix for Electrum to properly handle FW error returned by device when you hit hard limit ? @matejcik

[tsusanka]

This is a huge edge case let's not worry about that. (Unless this happens for every wire.DataErorr then it might be worth the fix.)

[prusnak]

Let's implement this for Legacy too.