Bitcoin Forum

    public static ECKey decodeBase58PK(String base58Priv) throws Exception {
        byte[] privBytes = Base58.decode(base58Priv);

        // Prepend a zero byte to make the biginteger unsigned
        byte[] appendZeroByte = concat(new byte[1], privBytes);

        ECKey ecKey = new ECKey(new BigInteger(appendZeroByte));

        return ecKey;
    }

    /**
     * Try to produce an ECKey Object from the given arguments.
     * BCI has a very uncommon way of encoding the private key, its not the
     * usual dumped private key format of the Satoshi client, its just base58 of
     * the key bytes. Most importantly it is also lacking the information whether
     * it is meant to produce a compressed or uncompressed public key. For this
     * we try both and compare with the supplied bitcoin address, if none of
     * them match (which should never happen) then this will throw an exception.
     *
     * @param base58Priv String containing the BCI encoded private key
     * @param addr String containing the bitcoin address
     * @return a new ECKey object representing this key
     * @throws Exception if the input can not be interpreted in any meaningful way
     */
    private def ECKey decodeBase58PK(String base58Priv, String addr) throws Exception {
        val privBytes = Base58.decode(base58Priv);
        var ecKey = new ECKey(new BigInteger(1, privBytes), null, false);
        if (ecKey.toAddress(new MainNetParams).toString.equals(addr)){
            log.debug("{} has uncompressed key", addr)
            return ecKey;
        } else {
            ecKey = new ECKey(new BigInteger(1, privBytes), null, true);
            if (ecKey.toAddress(new MainNetParams).toString.equals(addr)){
                log.debug("{} has compressed key", addr)
                return ecKey;
            } else {
                val err = addr + " and private key don't match, neither compressed nor uncompressed"
                log.error(err)
                throw new Exception(err)
            }
        }
    }

    public static ECKey decodeBase58PK(String base58Priv) throws Exception {
        byte[] privBytes = Base58.decode(base58Priv);

        // Prepend a zero byte to make the biginteger unsigned
        byte[] appendZeroByte = concat(new byte[1], privBytes);

        ECKey ecKey = new ECKey(new BigInteger(appendZeroByte));

        return ecKey;
    }