What does update method of MessageDigest do and what is BASE64Encoder meant for?

Have a look at Apache Commons Codec: https://commons.apache.org/codec/

E.g.: https://commons.apache.org/codec/api-release/org/apache/commons/codec/digest/DigestUtils.html

1. First of all, you're not performing any encryption. You're computing a one-way hash or digest of your input. This hash can be later used to verify the integrity of the message. See Hashing, SHA1 and MessageDigest.

2. Base64 encoding is a method of representing binary data in ASCII. This is often desirable because not all data storage and transmission mechanisms support raw binary. For example, if you want to transfer your computed digest via an http query string parameter, you'll want to encode it as Base64. Also, saving or printing raw binary to the console will produce a stream of funky characters which may be outside of the printable range, and may also produce beeps from your PC speaker!

3. The Base64Encoder you're using comes from the sun.misc package and should NEVER be used. This is internal Sun JVM code which may or may not be available in the future. That also explains why you're weren't able to find any javadoc.

4. Fortunately, several free and open Base64 encoders and decoders exist. Apache Commons Codec is a widely used and stable library which contains several codecs include Base64.

5. md.update(plainText.getBytes("UTF-8")) updates the input to the digest. Calling digest performs a final update and computes the digest of the input. See javadoc of md.digest and md.update