import java.security.*;
import javax.crypto.*;


KeyPairGenerator kpg = KeyPairGenerator.getInstance( "RSA")
KeyPair kp = kpg.generateKeyPair()

Cipher c = Cipher.getInstance( "RSA");
c.init( Cipher.ENCRYPT_MODE, kp.getPublic()) 

s = "ahoj"

byte[] b = s.getBytes( "UTF-8")

1.
byte[] zasifrovano = c.doFinal( b)

2.
for( ...)
{
  byte[] aktualni_cast_dat = ...
  byte[] zasifrovana_cast = c.update( aktualni_cast_dat)
}
  byte[] zbytek_dat = ...
  byte[] zasifrovana_posledni_cast =
                              c.doFinal( zbytek_dat)

// vypis
for( byte b: zasifrovano)
{
  System.out.println( b);
}

//opatrne, neni korektni
System.out.println(  new String( pole_bytu, "UTF-8")   )

-----
Cipher d = Cipher.getInstance( "RSA");
d.init( Cipher.DECRYPT_MODE, kp.getPrivate()) 

byte[] rozsifrovano = d.doFinal( zasifrovano) 
String xxx = new String( rozsifrovano, "UTF-8") 

Arrays.toString( pole_neceho)  .... [ a1, a2, ...an]


-----
MessageDigest
...m = MessageDigest.getInstance "MD5"   "SHA-1"...
// update
byte[] digest = m.digest( data)

-----

// podpis
Signature
  sign = Signature.getInstance( "SHA1withRSA")
  sign.initSign( ??? )
  // sign.update( ...)
  byte[] documentSignature = sign.sign(...)

// overeni podpisu
  byte[] documentSignature = ...
Signature
  vsign = Signature.getInstance( "SHA1withRSA")
  vsign.initVerify( ??? )
  // vsign.update( ...)
  boolean result = vsign.verify( documentSignature)