CreatingYourPersonalKeyPair
GPGusespublickeycryptographyforencryptingandsigningmessages.Publickey
cryptographyinvolvesyourpublickeywhichisdistributedtothepublicandisusedto
encryptmessagestobedeliveredyouandtodecryptsignaturesyouhavecreated,and
yourprivatekeywhichcomplementsyourpublickeybyallowingyoutodecrypt
messagesyoureceiveandtoencryptsignatures.Together,thesearereferredtoasakey
pair.
1.1AboutKeySecurity
Whenyoucreateakeypair,bothyourpublicandprivatekeysmustbestoredonyour
computer.Thiscreatesasecurityrisk,becauseanyonewhocangainaccesstoyour
privatekeycandecryptyourmessagesandimpersonateyou.Youcan'tcommityour
privatekeytomemoryanderaseitfromyourcomputer­­itisfartoolong(usuallyatleast
1024bits­­that'slikememorizinga300­digitphonenumber!)Besides,evenifyoucould
memorizeit,itwouldtaketoolongtotypeitouteachtimeyouwantedtouseit.
Thepracticalsolutiontothisproblemistogoastepfurtherandactuallyencryptyour
privatekeyusingareasonablyshortpassphraseasakeyforthekey;thisiswhatGPG
does.Eachtimeyouperformanoperationinvolvingyourprivatekey,GPGreadsthe
encryptedkeyfromthedisk,promptsyouforyourpassphrase,decryptsthekeyin
memory,andfinallyusesit.Inorderforthissystemtowork,youmusthavea
cryptographicallystrongpassphrase­­somethingthatcan'tbeguessedorbrute­force
attacked.Forexample,"orange"issoshortthatadictionaryattackwillfinditinseconds,
andyourspouse'sfullnameisapoorchoicebecauseanyonewhoknowsyoumightguess
thatyouuseditasyourpassphrase.Yourpassphraseshouldbeacombinationofletters
andnumbersandupperandlowercase.
1.2UsingtheGPGGen­keyCommand
Nowthatyou'vegotyourpassphrase,you'rereadytogenerateyourpersonalkeypair.At
thecommandprompt,type
gpg­­gen­key
GPGrespondswithamenuaskingwhatkindofkeypairyouwanttogenerate.Choose
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thedefault,"DSAandElGamal."
Next,itasksyouthesizeofthekey.Again,choosethedefaultof"1024."Ifyourkeyis
toosmall,itiseasiertocrack;ifitistoolarge,theneveryoperationthatusesitmay
executetooslowly.
Thenextpromptasksyouwhen(if)thekeyshouldexpire.Lettingthekeyexpireaftera
certainamountoftimeaddsalittlebitofsecurity,becausedocumentsencryptedafterthis
timearenotconnectedwiththeoldkeyinanyway.Thetradeoff,ofcourse,isthat
everyoneyoucorrespondwithmustfetchyournewkeywhentheoldoneexpires.Atthe
prompt,chooseareasonabletimeperiodforthelifetimeofyourkey,orselect"keydoes
notexpire."
NowyouentertheRealNameandEmailAddresswhichwillbeusedtoidentifythekey
ineveryone'skeycollections,notjustyours.Ifyoudon'tfeelcomfortableattachingyour
fullrealnametoyourkey,besuretopicksomethinguniquesothatyourkeywon'tbe
confusedwithanyoneelse's.
Finally,enterthepassphraseyouselected.GPGwillNOTgiveyouanyfeedbackasyou
typeyourpassphrase­­itwon'tprintasterisksorspacesasmostpasswordinputfunctions
do.Thisisyetanothersecuritymeasure.Afteryouenterandthenconfirmyour
passphrase,GPGwillstartdoingsomenumbercrunchingtogeneratealltherandombits
itneedsinyourkeypair.Itusesallsortsofsourcesinsidethecomputertosimulate
randomness,includingconsoleinput­­soifyouwanttospeeduptheprocess,typesome
randomcharactersonthekeyboard.
1.3PublishingYourPublicKey
Theeasiestwaytopublishyourpublickeyistosimplypostitonawebpageoremailit
directlytopeoplewhoneedit.Ofcourse,thisisalsooneoftheleastsecurewaysof
doingit­­plaintextemailandwebpagescanbe(theoretically,atleast)subjecttoman­in­
the­middleattacks.
Moreadvancedmethodsofkeyexchangeinvolvethingssuchaswebs
oftrustandkeyservers,whicharebeyondthescopeofthisguide.Ifyoudon'tbelieve
specificallythatsomeonewilltrytoattackyou,youmaybecomfortablewiththemethod
describedhere.
Youhavebeenwarned.Theexchangeofpublickeyswithoutatrustedintermediary
canbesubjecttoaman­in­the­middleattack.
TotransmityourpublickeyovertheInternet,thefirststepistoexportittoASCII
format.Openacommandpromptwindowandgotoafolderwhereyouwanttoplacethe
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exportedkey.Type
gpg­­armor­­output"key.txt"­­export"YOUR­NAME"
Youmaychangekey.txttosomeotherfilenameifyou'dlike.YOUR­NAMEcanbeyour
RealNameoryourEmailAddress;GPGwillfinditeitherway.The"­­armor"option
instructsGPGtoformattheoutput"armored"forplain­texttransmission.Thismakesit
easytocopyandpastethekeytoandfromwebpagesandemailmessages.The"­­
armor"optionappliestomostGPGcommandsthatproduceanykindofoutput.
Opentheoutputfile,key.txt.Youshouldseesomethingresemblingthis:
­­­­­BEGINPGPPUBLICKEYBLOCK­­­­­
Version:GnuPGv1.2.1(MingW32)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=+DfK
­­­­­ENDPGPPUBLICKEYBLOCK­­­­­
Copytheentirecontentsofthatfile,includingthelinesbeginningwith"­­­­­,"and
pastethemonawebpageorinanemailmessagetosomeoneelse.That'sallittakesto
exportyourpublickey.
Note:itispossibletoexportyourprivatekeyinthesamemanner,withthe"­­export­
secret­keys"insteadof"­­export."Thisisusefulforbackuporfortransferringit
toanothercomputer,aslongasyouknowwhatyou'redoingandyoucanbesurethenew
copywillbesecure.Makesureyoudon'teveraccidentallypublishyourprivatekey!
BeforeyoupublishakeyinASCIIform,checkthatthefirstlinehastheword
"PUBLIC,"not"PRIVATE."
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1.4BackingUpYourKeys
KeepingyourGPGkeyfilessafeisjustasimportantasrememberingyourpassphrase­­
yourpassphrasedoesyounogoodifyoudon'thaveacopyofyourfullprivatekeyon
yourcomputer.Ifyourkeyfileisdestroyed,thereisabsolutelynowaytoreconstructit,
shortofexecutingacryptanalysisonyourowndata.TobackupyourGPGkeys,
includingallyourprivatekeys,locatethefilespubring.gpg,secring.gpg,andtrustdb.gpg
inthefolderwhereyouhaveinstalledGPG,andcopythemtoasafelocation,suchasa
CD­Rdisc.Storethisdiscinasafeplacewherepotentialattackerswon'tlikelybeableto
findit.(Ofcourse,yourkeysaresafelongasyourattackerdoesn'tknowyour
passphrase.)
Now,youarefinallyreadytoactuallyencryptsomething.
2.EncryptingandDecryptingFiles
ThebasicencryptionanddecryptionprocedureinGPGisthis:Thesenderdeterminesthe
recipientofafile,acquiresthatrecipient'spublickeyifhehasn'talreadydoneso,and
thenrunstheplaintextthroughGPGalongwiththiskeytoobtainthecyphertext.
4.1
When
therecipientwantstodecryptthefile,heapplieshisprivatekeytothecyphertextto
obtaintheplaintext.
Infact,thesenderandrecipientaren'talwaysdifferentpeople.OneimportantuseofGPG
istoencryptyourowndata,storingthecyphertextanddestroyingtheplaintext.Thisisan
excellentdefenseagainstphysicalattacksonyourcomputeroryourlocalfileserver.
2.1UsingtheGPGEncryptCommand
Chooseafileyouwanttoencrypt.Forexample,let'sassumeyouhavediary,whereeach
monthisanewfile,andyou'redonewithFebruary2003,whichiscalleddiary2003­
02.txt.Supposeyouwanttoencryptthisfileandthenputitawayinanarchivefolderora
CD­Rdisc.Atthecommandprompt,type(allononeline)
gpg­­recipient"YOUR­NAME"­­output"diary2003­02.txt.gpg"
­­encrypt"diary2003­02.txt"
Don'tforgettofillinYOUR­NAMEwiththeactualnameyouattachedtoyourkey.
Alwaysrememberthe"­output"optionwhenyouuseanencryptioncommandinGPG;if
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youomitthisoption,theoutputwillbedumpedtothecommandpromptwindowinstead
oftoafile.Finally,noticethatthecommand(usuallyanactionverb)alwaysgoesinthe
lastpositionontheGPGcommandline,afteranyoptions.Nowdiary2003­02.txt.gpg
willcontainaseeminglyrandomstringofbytes.YoucanlookatitwithNotepadifyou'd
like.
Thereisasimilarcommand,"­encrypt­files,"whichwillautomaticallychooseandname
anoutputfileforyou.Butthefilenameitchooseswillbemissingtheextensionofthe
plaintextfilename(.txt,.jpg,.zip,etc.)soIdon'tuseit,myself.
2.2UsingtheGPGDecrypt­Files
Command
Now,supposeayearfromnowyou'refeelingnostalgicandyouwanttoreadFebruary
2003'sdiary.Youwouldcopythecyphertextbacktoyourworkspaceonyourcomputer,
andtypethefollowingatthecommandprompt:
gpg­­decrypt­files"diary2003­02.txt.gpg"
GPGwilllookupyourprivatekeyandpromptyouforthepassphrase.Providedyour
privatekeyisstillinstalledonyourcomputer,andyoustillrememberyourpassphrase
(youdidn'twriteitonaPost­Itandstickitonyourmonitor,didyou?)youwillgetback
theoriginalplaintextexactlyasitwasbeforeyouencryptedit.Ifyouwanttodecrypta
shortfileanddisplayitimmediatelyintheconsole,youcanusethe"­­decrypt"
commandinsteadofthe"­­decrypt­files"command.
2.3SendinganEncryptedFilebyEmail
Encryptingyourownfilesisuseful,butamorecommonuseofGPGistosendencrypted
datatosomeoneelse.BeforeyoucanuseGPGtoencryptafileforsomeoneelse,you
needtogettheirpublickey.
2.3.1Importingthekey
AsIsaidbefore,twoconvenientwaysofgettingsomeone'spublickeyareemailand
personalwebpages.Asanexample,youcandownloadmypublickeyfrommyweb
server;tryitrightnow.Gotowww.glump.net/signature
.Ifyouscrolldownapageorso,
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youwillseeafamiliar­lookingblockoftextrepresentingmypublickey.Savetheentire
pagetoatextfile.(InInternetExplorer,choosethetype"TextFile"inthe"SaveAs"
dialogbox.)Supposeyounamedthesavedfilebrendan.txt.Atthecommandprompt,
type
gpg­­import"brendan.txt"
GPGshouldsay
gpg:keyA3CA0378:publickey"BrendanKidwell<brendan@glump.net>"
imported
gpg:Totalnumberprocessed:1
gpg:imported:1
NoticethatGPGwasn'tdistractedbyalltheextratextonthepage.Itlooksforthetelltale
"BEGINPGPPUBLICKEYBLOCK"lineandignoreseverythingoutsidethatblock
oftext.
Onemorestepyouneedtoperformafteryou'veimportedakeyfromanexternalsource
issetthetrustlevelonit.GPGisparanoid,andifyouusethekeyrightnowasitis,you
willgetawarningmessagesayingthatyouhaven'testablishedtheauthenticityofthekey.
Tomakethiswarningmessagegoaway,usetheGPG"­­edit­key"commandtoset
thetrustlevel:
gpg­­edit­key"BrendanKidwell"
GPGwillentertheinteractivekeyeditingmode.Enterthecommand"trust"andselect
level"5)Itrustultimately."Thenenter"quit"tosaveyourchange.
2.3.2Encryptingthemessage
Nowyou'rereadytoencryptthefile.Let'sassumeyouhaveafileyouwanttosendtome
calledmessagetobrendan.txt.Atthecommandprompt,type
gpg­­armor­­recipient"BrendanKidwell"
­­output"messagetobrendan.txt.asc"
­­encrypt"messagetobrendan.txt"
GPGwillproduceafilecalledmessagetobrendan.txt.asc,whosecontentyoucancopy
andpasteintoanemail.
Alternatively,ifyouneedtosendaparticularlylargefile,youshouldusetheencrypt
commandwithoutthe"­­armor"option:
gpg­­recipient"RECIPIENT"­­output"FILE.gpg"­­encrypt"FILE"
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andinsteadofpastingFILE.gpgintothebodyoftheemail,includeitasanattachment.
Makesurethenameofthefiledoesn'trevealanythingthatshouldbesecret.
Ifyou'dlike,youcangoaheadandtrytosendanencryptedmessagetome.I'llletyou
knowifIreceiveitcorrectly.Ifyouwantanencryptedreply,makesureyouincludeyour
publickeyortellmewhereIcangetit.
2.4DecryptingFilesSentbyEmail
Howyoudealwithanencryptedemailmessagewhichyouhavereceiveddependson
howitwassenttoyou.Whenyoureceiveanencryptedmessage,itsbodymightcontain
­­­­­BEGINPGPMESSAGE­­­­­
followedbyastringofrandom­lookingcharacters.Orthemessagemightsimplyhavean
attachedfilewhosenameendswith".gpg"or".pgp."
2.4.1Encrypteddataisinthemessagebody
Iftheencrypteddataisinthemessagebody,savetheentiremessagetoafile,andendthe
filenamewith.asc.Ifyouknowthattheencrypteddataissomebinaryformat,include
thefileextensionbeforethe.asc.Forexample,ifyouknowthemessagecontainsan
encryptedMicrosoftWordfile,youwouldnamethefilemessage.doc.asc.Atthe
commandprompt,type
gpg­­decrypt­files"FILE.asc"
whereFILE.ascisthefilenameyouusedtosavethemessage.GPGwilltellyouwhothe
filewasencryptedforandpromptyouforthepassphrase.Ifthefilewasn'tencrypted
usingyourpublickey,GPGgiveupandtellyouthatitdoesn'thavetheprivatekey
neededtodecryptthisfile.Ifthedecryptionsucceeded,youshouldgettheoriginalfile
back,withthenameyougaveit,minusthe.ascextension.
Remember,ifyouknowtheencrypteddataisjustashorttextmessage,youcandisplayit
ontheconsoleinsteadofstoringitinafilewiththe"­­decrypt"command:
gpg­­decrypt"FILE.asc"
2.4.2Encrypteddataisinanattachedfile
Iftheencrypteddataisinanattachedfile,savethatfiletoyourcomputer.Atthe
commandprompt,type
gpg­­decrypt­files"FILE.gpg"
whereFILE.gpgisthenameofthefileyousaved.(Ifthemessagewascreatedusing
PGP,
4.2
thenameoftheattachedfilewillprobablyendwith".pgp"instead.)Again,GPG
willonlyworkifyouhavetheprivatekeyneededtodecryptthefile.
2.5EncryptingforMultipleRecipients
Sometimes,you'llwanttosendanencryptedfiletomorethanoneperson.Thiscould
createaproblem,though,becausenooneshouldbesharingaprivatekeywithanyone
else.Youcouldalwaysmakeaseparateencryptedfileforeachrecipient,butthiscanget
tiringifyouneedtosendafiletoasmanyasfivepeople.
Thereisabetterway:GPGallowsyoutospecifyalistofpeoplewhomaybeableto
decryptafile.GPGwillthenuseallofthoseindividuals'publickeystoencryptthedata
insuchawaythatanyoneoftheirprivatekeys(andnooneelse's)candecryptthedata.
Thesyntaxisstraightforward.Justaddmore"­­recipient"optionstothecommand
line.SupposeyouwantedtoencryptthesamemessageasinSubsection4.3.2above,but
wantedtosendthemessagesothatbothyouandIcoulddecryptitlater.Youwouldtype
thefollowingatthecommandprompt:
gpg­­armor­­recipient"BrendanKidwell"
­­recipient"YOUR­NAME"
­­output"messagetobrendan.txt.asc"
­­encrypt"messagetobrendan.txt"
andthencopytheoutputfileintoanemailmessageasbefore.
Sendingencryptedemailthiswaycanmakeiteasiertomanageyoursaved
correspondence.Normallywhenyousendplaintextemail,acopyofthesentmessageis
savedsomewhereinyouremailsoftware(unlessyouspecifiedthatyoudon'twanttosave
copies.)Youcanalwaysgobackandreviewyoursentemailtorecallwhatwassaid.If
youspecifyyourtargetandyourselfasrecipientswhenyouprepareanencrypted
message,thenyoucangobackandreviewitinyoursentemailcollectionwheneveryou
needto,withonlytheaddedstepthatyouneedtodecryptitbeforeyouviewit.You
needn'tsaveaseparateplaintextcopyofthemessage,nordoyouneedtomakeanother
copyencryptedforyourself.
Specifyingseveralrecipientsdoesnotadverselyaffectthesizeoftheencrypteddata.I
triedencryptingalarge(~8MB)compressedbinaryfileforoneandthentworecipients.
Thedifferenceinthesizeoftheoutputwasonlyafewhundredbytes.
3.SigningFiles
Oftenitisdesirabletoverifytheoriginofdata,whetheritisencryptedornot.GPG's
signaturefunctionsprovideameansofverifyingauthenticity.
Thetheoryissimple.PublicandprivateGPGkeysworkeitherway.Onceyouhave
encrypteddatawithoneofthekeysinapair,itcanonlybedecryptedwithits
complementinathesamekeypair.NormallyGPGoperatesbyencryptingwiththe
publickeysothatonlytherecipientcandecryptthedatausinghisprivatekey.
Digitalsignaturesworktheotherwayaround;dataisencryptedusingthesigner'sprivate
key.Ifsomeonereceivesthefileandsucceedsindecryptingthedatawiththesigner's
publickey,thenpresumably,thedatamusthavebeenencryptedbythatsigner.Therefore,
thesignermusthavecreatedthedatahimself,oratleastapproveofitscontentsinsome
way(dependingonthenatureoftheactualdata.)Adigitalsignatureisjustasusefulasa
physicalonemadewithapen,andarguably,itismoresecure.
WhenGPGcreatesadigitalsignature,itdoesn'tencrypttheentirefilewiththesigner's
privatekey.Instead,itcomputesahashvalue,
5.1
encryptsthat,andappendsittothe
originaldataasthesignature.Thismakesitpossibletocreatesignedfilesthatare
readablewithoutanyencryptionsoftware,andaren'tsignificantlylarger;GPGisneeded
onlytoverifytheauthenticityofthefile.
Toverifyasignature,GPGreadsthedatathatwassignedandcomputesitshashvalue.
Thenitdecryptsthesignature,usingthesigner'spublickey,toobtainthetruehashvalue.
Ifthetwohashvaluesmatch,thesignatureisvalidandthedatayouhaveisexactlythe
datathesignerhadwhenhecreatedthesignature.
3.1UsingtheGPGClearsignCommand
Supposeyouwanttosendamessagetosomeoneinsuchawaythattheycanproveitwas
youwhoauthoredthemessage.First,composethemessageinatexteditorandsaveitas
message.txtinaconvenientfolder.Then,atthecommandprompt,type
gpg­­local­user"YOUR­NAME"­­clearsign"message.txt"
Sincethisoperationinvolvesyourprivatekey,GPGwillpromptyouforyourpassphrase.
Afterthat,GPGwillcomputeasignatureandwriteanewfilecalledmessage.asc
containingtheplaintextandthesignature.Thecontentsofthisfilecanbecopiedintoan
emailandsenttotheintendedrecipient.
Asanexample,hereismessagethatIhavesigned:
­­­­­BEGINPGPSIGNEDMESSAGE­­­­­
Hash:SHA1
ThisisatestmessagesignedbyBrendanKidwell.
­­­­­BEGINPGPSIGNATURE­­­­­
Version:GnuPGv1.2.1(MingW32)
iD8DBQE+fnwc4lxlBKPKA3gRAq13AJ4557Md6xF15OoEDyIIB+UvDQKwmwCfcrCY
na12Ng9W4K5mP1ZWEueNjCo=
=73hB
­­­­­ENDPGPSIGNATURE­­­­­
3.2VerifyingaClearsignedMessage
Supposeyoureceiveamessageliketheoneproducedintheprevioussection.Oryou
mightfindsuchamessagepostedonapublicwebsiteorelectronicmessageboard.
Beforeyoucanverifyitssignature,youneedtoobtainthesigner'spublickeyandinstall
itonyourcomputer.Thisprocedureisdescribedinthepreviouschapter,underthe
heading"Importingthekey."
Ifthemessagetobeverifiediscontainedinanemail,exportittoatextfile.Ifthe
messageisdisplayedonawebpageorsomeotheronlinemedium,saveitasatextfile
(named,forexample,message.txt.)Thentypethefollowingatthecommandprompt:
gpg­­verify"message.txt.asc"
GPGwilllocatethesigner'skeyifyouhaveit,anduseittocheckthesignatureand
reportwhetherornotitisvalid.
Ifyou'rereadingtheonlineversionofthisdocumentandyou'vealreadyinstalledmykey,
youcantrycopyingthetestmessagedisplayedintheprevioussectionintoatextfileand
verifyingthesignaturewiththisprocedure.
3.3SigningandVerifyingBinaryFiles
Textmessagescanhavesignaturesappendedtothemwithoutdisruptingthecontentsof
themessagetoomuch,butbinaryfilessuchasMicrosoftWorddocumentsandZip
archivescan'thavearbitrarydataattachedtothem.Tosignbinaryfiles,itiscostumaryto
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