presto-main/src/main/java/io/prestosql/server/security/PasswordAuthenticator.java/PasswordAuthenticator/decodeCredentials
class PasswordAuthenticator:    private static String decodeCredentials(String credentials)
            throws AuthenticationException
    {
        // The original basic auth RFC 2617 did not specify a character set.
        // Many clients, including the Presto CLI and JDBC driver, use ISO-8859-1.
        // RFC 7617 allows the server to specify UTF-8 as the character set during
        // the challenge, but this doesn't help as most clients pre-authenticate.
        try {
            return new String(Base64.getDecoder().decode(credentials), ISO_8859_1);
        }
        catch (IllegalArgumentException e) {
            throw new AuthenticationException("Invalid base64 encoded credentials");
        }
    }