Internet Security Professional Reference:Encryption Overview

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Caesar Cipher

Julius Caesar was one of the first to use substitution encryption to send messages to troops during war. The substitution method he is credited with involves advancing each character three spaces in the alphabet. Thus:

DIAL THE NUMBER OF THE NEW PARTY AND WAIT FOR AN ANSWER

becomes

GLDO WKH QXPEHU RI WKH QHZ SDUWB DQG ZDLW IRU DQ DQVZHU

Note that at the end of the alphabet, characters wrap back around to the beginning; thus, Z becomes C, Y becomes B, and so on.

This encrypting method serves its purpose well, but once one figures out that they need only subtract three characters from each given to obtain the correct answer, the code loses all security. A modification of this is allowing the number of characters shifted to differ for each message. In so doing, the number of possibilities changes from 1 to 26 and becomes increasingly more difficult.

Figure 10.8 shows an example of the ENCRYPT utility included on the CD with this book, and uses a modified version of the Caesar Cipher to change a message 10 spaces. Figure 10.9 shows how the same message is decrypted.

Figure 10.8 Encrypting using a modified version of the Caesar Cipher.

Figure 10.9 Decrypting using a modified version of the Caesar Cipher.

Although this method is stronger than always shifting three characters, it is still relatively easy to break. The only unknown that must be determined is the number of places in the alphabet to switch each character. Because there are only 25 possibilities, a routine can be written to quickly run through all feasible combinations and print the results, as follows:

DBEMUC	KXN	FORSMVOC	GSDR	DBKSVOBC	ECO	BSQRD	VKXO
ECFNVD	LYO	GPSTNWPD	HTES	ECLTWPCD	FDP	CTRSE	WLYP
FDGOWE	MZP	HQTUOXQE	IUFT	FDMUXQDE	GEQ	DUSTF	XMZQ
GEHPXF	NAQ	IRUVPYRF	JVGU	GENVYREF	HFR	EVTUG	YNAR
HFIQYG	OBR	JSVWQZSG	KWHV	HFOWZSFG	IGS	FWUVH	ZOBS
IGJRZH	PCS	KTWXRATH	LXIW	IGPXATGH	JHT	GXVWI	APCT
JHKSAI	QDT	LUXYSBUI	MYJX	JHQYBUHI	KIU	HYWXJ	BQDU
KILTBJ	REU	MVYZTCVJ	NZKY	KIRZCVIJ	LJV	IZXYK	CREV
LJMUCK	SFV	NWZAUDWK	OALZ	LJSADWJK	MKW	JAYZL	DSFW
MKNVDL	TGW	OXABVEXL	PBMA	MKTBEXKL	NLX	KBZAM	ETGX
NLOWEM	UHX	PYBCWFYM	QCNB	NLUCFYLM	OMY	LCABN	FUHY
OMPXFN	VIY	QZCDXGZN	RDOC	OMVDGZMN	PNZ	MDBCO	GVIZ
PNQYGO	WJZ	RADEYHAO	SEPD	PNWEHANO	QOA	NECDP	HWJA
QORZHP	XKA	SBEFZIBP	TFQE	QOXFIBOP	RPB	OFDEQ	IXKB
RPSAIQ	YLB	TCFGAJCQ	UGRF	RPYGJCPQ	SQC	PGEFR	JYLC
SQTBJR	ZMC	UDGHBKDR	VHSG	SQZHKDQR	TRD	QHFGS	KZMD
TRUCKS	AND	VEHICLES	WITH	TRAILERS	USE	RIGHT	LANE
USVDLT	BOE	WFIJDMFT	XJUI	USBJMFST	VTF	SJHIU	MBOF
VTWEMU	CPF	XGJKENGU	YKVJ	VTCKNGTU	WUG	TKIJV	NCPG
WUXFNV	DQG	YHKLFOHV	ZLWK	WUDLOHUV	XVH	ULJKW	ODQH
XVYGOW	ERH	ZILMGPIW	AMXL	XVEMPIVW	YWI	VMKLX	PERI
YWZHPX	FSI	AJMNHQJX	BNYM	YWFNQJWX	ZXJ	WNLMY	QFSJ
ZXAIQY	GTJ	BKNOIRKY	COZN	ZXGORKXY	AYK	XOMNZ	RGTK
AYBJRZ	HUK	CLOPJSLZ	DPAO	AYHPSLYZ	BZL	YPNOA	SHUL
BZCKSA	IVL	DMPQKTMA	EQBP	BZIQTMZA	CAM	ZQOPB	TIVM
CADLTB	JWM	ENQRLUNB	FRCQ	CAJRUNAB	DBN	ARPQC	UJWN

Running such a routine quickly reveals the correct translation of the code, as it is the only sentence that is readable of the lot.

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