Print better error if identity file is not valid UTF-8?

[tfg13]

Hey, I just banged my head against this for a bit, I was wondering if a better error message might be helpful :)

Basically this happens with the default (modern) Windows terminal:

> age-plugin-yubikey.exe --identity --slot 1 > yubikey-identity.txt
> age -d -i yubikey-identity.txt somefile.age
age: error: reading "yubikey-identity.txt": failed to read "yubikey-identity.txt": error at line 1: unknown identity type
age: report unexpected or unhelpful errors at https://filippo.io/age/report

It seems the > redirect in Windows terminal creates the text file with UTF-16 LE encoding. The error comes from here:

age/cmd/age/parse.go

Line 205 in 15153e6

return nil, fmt.Errorf("unknown identity type")

Opening the file in notepad and saving it again as UTF8 fixes this and makes everything work. I only figured it out because rage produces a more specific error: Error: stream did not contain valid UTF-8

I was wondering if it is worth doing something sililar in age? Actually handling other encodings seems very annoying, but maybe something like https://pkg.go.dev/unicode/utf8#Valid can be run somewhere here:

age/cmd/age/parse.go

Line 213 in 15153e6

scanner := bufio.NewScanner(io.LimitReader(f, privateKeySizeLimit))