btcutil/base58: fix bug with range

[egonelbre]

The decoding was using a range over the string instead of iterating
over the bytes.

[coveralls]

Pull Request Test Coverage Report for Build 1771996489

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 79.676%

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Totals
Change from base Build 1753768655:	0.0%
Covered Lines:	1231
Relevant Lines:	1545

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💛 - Coveralls

[schjonhaug]

Is this PR fixing the crash that occurs if you try to decode something with an emoji? What is the allowed character set here?

For example, using example-Decode, if you try to decode a string with 🤡, it’ll crash:

package main

import (
	"fmt"

	"github.com/btcsuite/btcd/btcutil/base58"
)

func main() {
	// Decode example modified base58 encoded data.
	encoded := "25JnwSn7XKfNQ🤡"
	decoded := base58.Decode(encoded)

	// Show the decoded data.
	fmt.Println("Decoded Data:", string(decoded))

}

Result:

Output:

panic: runtime error: index out of range [10095] with length 256

goroutine 1 [running]:
github.com/btcsuite/btcd/btcutil/base58.Decode({0x49fff5, 0x10})
	/tmp/gopath2191683068/pkg/mod/github.com/btcsuite/btcd/btcutil@v1.1.0/base58/base58.go:58 +0x395
main.main()
	/tmp/sandbox2173415571/prog.go:12 +0x29

[egonelbre]

Yes, it's a fix for runes that are larger 0xFF, which does include emoji. The valid alphabet is 123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz -- i.e. base58 alphabet used by bitcoin.

[ssnickolay]

Why this is not merged yet? I'm using the lib to check BTC address is valid and the issue could crash the app ;[

[chappjc]

Similar fix downstream: https://github.com/decred/base58/blob/5b84bad1ebfd283724c012ec3e0e02f6f80b0ee2/base58.go#L25
Worth merging this PR or a direct port of that solution.