MODULE CryptoARC4;	(** AUTHOR "F.N."; PURPOSE "ALLEGED RC4 - stream cipher - keylength: from 1 to 256 bytes"; *)

IMPORT
	Ciphers := CryptoCiphers, BIT;

TYPE
	Cipher* = OBJECT (Ciphers.Cipher)
	VAR
		s: ARRAY 256 OF LONGINT; (* state array *)
		i, j: LONGINT;

		PROCEDURE &Init*;
		BEGIN
			SetNameAndBlocksize( "arc4", 1 );
			isKeyInitialized := FALSE
		END Init;

		(** initialize arc4 key. keybits = [8..2048], MUST be multiple of 8. *)
		PROCEDURE InitKey*( CONST src: ARRAY OF CHAR; keybits: LONGINT);
		VAR
			keydata: ARRAY 256 OF LONGINT;
			keybytes: LONGINT;
			temp: LONGINT;
		BEGIN
			ASSERT( keybits MOD 8 = 0 );  ASSERT( keybits >  7 );  ASSERT( keybits < 2049 );
			InitKey^( src, keybits );
			keybytes := keybits DIV 8;
			i := 0;  j := 0;
			FOR i := 0 TO 255 DO  s[i] := i  END;
			FOR i := 0 TO 255 DO  keydata[i] := ORD( src[i MOD keybytes] )  END;
			FOR i := 0 TO 255 DO
				j := (j + s[i] + keydata[i]) MOD 256;
				temp := s[i];  s[i] := s[j];  s[j] := temp
			END;
			isKeyInitialized := TRUE;
			FOR i := 0 TO 255 DO  keydata[i] := 0  END; (* sniffing protection *)
			i := 0;  j := 0;
		END InitKey;

		(** encrypt len bytes starting at position ofs. *)
		PROCEDURE Encrypt*( VAR buf: ARRAY OF CHAR; ofs, len: LONGINT );
		VAR
			k, temp: LONGINT;
			ch: CHAR;
		BEGIN
			ASSERT( isKeyInitialized );
			FOR k := 0 TO len-1 DO
				(* generation of random byte ch *)
				i := (i + 1) MOD 256;
				j := (j + s[i]) MOD 256;
				temp := s[i];  s[i] := s[j];  s[j] := temp;
				ch := CHR( s[(s[i] + s[j]) MOD 256] );

				(* XOR random byte with next plaintext byte *)
				buf[ofs + k] := BIT.CXOR( buf[ofs + k], ch );
			END
		END Encrypt;

		(** decrypt len bytes starting at position ofs. *)
		PROCEDURE Decrypt*( VAR buf: ARRAY OF CHAR; ofs, len: LONGINT );
		BEGIN
			Encrypt( buf, ofs, len )
		END Decrypt;

	END Cipher;


	(** create a new ARC4 cipher object *)
	PROCEDURE NewCipher*() : Ciphers.Cipher;
	VAR c: Cipher;
	BEGIN
		NEW( c ); RETURN c
	END NewCipher;

END CryptoARC4.