Q: Java String to SHA1

Message Digest (hash) is byte[] in byte[] out #

A message digest is defined as a function that takes a raw byte array and returns a raw byte array (aka byte[]). For example SHA-1 (Secure Hash Algorithm 1) has a digest size of 160 bit or 20 bytes. Raw byte arrays cannot usually be interpreted as character encodings like UTF-8, because not every byte in every order is a legal that encoding. So converting them to a String with:

new String(md.digest(subject), StandardCharsets.UTF_8)

might create some illegal sequences or has code-pointers to undefined Unicode mappings:

[�a�ɹ??�%l�3~��.

Binary-to-text Encoding #

For that binary-to-text encoding is used. With hashes, the one that is used most is the HEX encoding or Base16. Basically a byte can have the value from 0 to 255 (or -128 to 127 signed) which is equivalent to the HEX representation of 0x00-0xFF. Therefore, hex will double the required length of the output, that means a 20 byte output will create a 40 character long hex string, e.g.:

2fd4e1c67a2d28fced849ee1bb76e7391b93eb12

Note that it is not required to use hex encoding. You could also use something like base64. Hex is often preferred because it is easier readable by humans and has a defined output length without the need for padding.

You can convert a byte array to hex with JDK functionality alone:

new BigInteger(1, token).toString(16)

Note however that BigInteger will interpret given byte array as number and not as a byte string. That means leading zeros will not be outputted, and the resulting string may be shorter than 40 chars.

Using Libraries to Encode to HEX #

You could now copy and paste an untested byte-to-hex method from Stack Overflow or use massive dependencies like Guava.

To have a go-to solution for most byte related issues I implemented a utility to handle these cases: bytes-java (GitHub)

To convert your message digest byte array you could just do

String hex = Bytes.wrap(md.digest(subject)).encodeHex();

or you could just use the built-in hash feature

String hex =  Bytes.from(subject).hashSha1().encodeHex();