question: public static byte [ ] keyGenPrivate ( byte [ ] randomBytes ) throws NoSuchAlgorithmException , DigestException { if ( randomBytes . length < 32 ) { throw new RuntimeException ( "Random bytes too small" ) ; } MessageDigest digest = MessageDigest . getInstance ( "SHA-256" ) ; digest . digest ( randomBytes , 0 , randomBytes . length ) ; byte [ ] privateKey = digest . digest ( ) ; privateKey [ 0 ] &= 248 ; privateKey [ 31 ] &= 127 ; privateKey [ 31 ] |= 64 ; return privateKey ; }
target:  Generating private key. Source: https://cr.yp.to/ecdh.html