Which base62 implementations is this compatible with?

Reference Strings

For reference, I believe this is what base62 encoding and decoding should look like:

Raw   : "Hello, 世界"
Base64: SGVsbG8sIOS4lueVjA (18 chars)
Base62: 1wJfrzvdbuFbL65vcS (18 chars)

Raw   : "Hello World"
Base64: SGVsbG8gV29ybGQ (15 chars)
Base62: 73XpUgyMwkGr29M (15 chars)

Raw   : [ 0, 0, 0, 0, 255, 255, 255, 255 ]
Base64: AAAAAP____8 (11 chars)
Base62: 000004gfFC3 (11 chars)

Raw   : [ 255, 255, 255, 255, 0, 0, 0, 0 ]
Base64: _____wAAAAA (11 chars)
Base62: LygHZwPV2MC (11 chars)

Reference Implementation

I generated that using https://github.com/keybase/saltpack/encoding/basex, which seems to be correct and agree with other implementations, such as https://github.com/oconnor663/basex_gmp.

package main

import (
	"encoding/base64"
	"fmt"

	"github.com/keybase/saltpack/encoding/basex"
)

func main() {
	for _, src := range [][]byte{
		[]byte("Hello, 世界"),
		[]byte("Hello World"),
		{0, 0, 0, 0, 255, 255, 255, 255},
		{255, 255, 255, 255, 0, 0, 0, 0},
	} {
		// Uses the GMP character set "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
		b62 := basex.Base62StdEncoding.EncodeToString(src)
		b64 := base64.RawURLEncoding.EncodeToString(src)

		fmt.Printf("Base64: %s (%d chars)\n", b64, len(b64))
		fmt.Printf("Base62: %s (%d chars)\n", b62, len(b62))
		fmt.Println("")
	}
}

When trying to decode something that contains invalid base62 characters, the behavior is undefined and not always the same:

• with ascii characters, decoding works but the result is total garbage
• with more complex utf-8 characters, an exception is thrown:

java.lang.ArrayIndexOutOfBoundsException: -16

	at io.seruco.encoding.base62.Base62.translate(Base62.java:84)

Code that triggered this case:

Base62.createInstance().decode("😱".getBytes());

That might be critical when the encoded string is user/external input.

I see plenty of possible solutions here, and not sure which one you prefer. If you pick one I am willing to make a PR.

• Do as JDK Base64: throw IllegalArgumentException if the string is not valid according to the chosen alphabet
• Implement a method to check if a string is valid according to the chosen alphabet
• Publicly expose the alphabets to allow callers to make their own checks.

WDYT?