Concepts/OpenPGP Kom i gang
Introduktion
Når du opretter en nøgle er der adskillige tekniske og organisatoriske beslutninger (hvad enten du er opmærksom på dem eller ej), som vil have stor indflydelse på, hvor sikker og nyttig din nøgle er som et resultat af dens operative liv. Nogle af disse beslutninger kan ikke ændres efterfølgende. Af forskellige grunde er det ønskværdigt, at normale nøgler (i det mindste deres kerne) har et langt operativt lig, selv hvis de er ret usikre.
Denne artikel skal således gøre dig fortrolig med de vigtige aspekter af nøglegenerering og hjælpe dig med at generere en god nøgle, som kan bruges i mange år. Denne artikel omhandler kun de begreber, som du bør være fortrolig med; den indeholder ikke en skridt for skridt-vejledning i nøglegenerering, for disse bør være adskilte faser: Først forstå og planlæg hvad du vil gøre og foretag de nødvendige forberedelser. Når du er færdig med det, så kig på artiklen om OpenPGP-nøglegenerering
Hvordan du kommer i gang
Du kan let oprette en nøgle til at eksperimentere lidt med. Men hvis du lader andre verificere sådan en nøgle risikerer du at smide arbejde væk senere. Dit mål bør være at oprette en eller flere nøgler til langvarig brug. Det bedste råd er: Gør det ikke selv hvis du kan undgå det. Spørg eksperter hvis du kan, folk, som allerede selv har udskiftet en af deres egne nøgler og lært af den erfaring. Brug et sikkert system til at oprette en nøgle; brug en hovednøgle, som er off-line og giv både hovednøglen og undernøglerne en udløbsdato (ikke mere end et år). Vælg en nøglepolitik (en beskrivelse af sikkerheden og brugen af hovednøglen og undernøgler) og hold fast i den. Hvis du certificerer andre nøgler før du har en certificeringspolitik, så certificér dem ikke for offentligheden (web of trust); lav i stedet lokale signaturer (kun til eget brug). Undgå at gøre noget nyt før du gar en god forståelse af, hvad det betyder.
Og husk dette:
- "Bekvemmelighed er (næsten) altid en trussel mod din sikkerhed."
- Mere sikker er ikke altid bedre til en given opgave. Vær blot opmærksom på konsekvenserne (i begge retninger).
Welkommen til krypteringens verden!
Nøgle sikkerhed
Hvis nøgler er involveret, så er hovedspørgsmålet: Hvor sikre/pålidelige er de og brugen af dem? Den vigtigste kryptografiske regel er ikke "Gør det endnu sikrere" men "(1) Tænk over, hvor meget sikkerhed du behøver. (2) Afgør, hvilken teknisk indsats der synes nødvendig (nedre grænse) og rimelig (øvre grænse) for at opfylde disse behov. (3) Følg de regler du har bestemt slavisk (skriv dem ned). (4) Hvis andre er involveret (hvilket i reglen er tilfældet), så lad dem vide præcist og sikkert, hvilke grænser for anvendelse af og sikkerhedsniveauet for disse nøgler." Det meste af det følgende er overvejelser over nøglernes sikkerhed og gennemskueligheden af deres sikkerhed og betydning.
Ingen bryder nøgler ved direkte angreb. Indenfor det næste årti er det (selv for ret korte nøgler på fx 1024 bit) ganske enkelt umuligt for alle andre end et "rigt" lands regeringsorganer. Og det giver ikke megen mening. Det er så let bare at stjæle dem. Med stor sandsynlighed er det system, hvor du læser denne tekst (hvis det ikke er printet ud...) ikke ret sikkert. I praksis er intet system som bruges til at læse e-mails eller websider sikkert. Det kan du lige så godt acceptere. Gør du det ikke, så udsætter du sandsynligvis dig selv for fare. En nøgle er aldrig mere sikker end det mindst sikre system, hvor den er blevet anvendt (herunder selvfølgelig også: oprettet). Og den er kun mere sikker end det mindst sikre system, hvor den er blevet gemt i forhold til dens adgangskode, som ikke beskytter mod direkte angreb medmindre den er virkelig tilfældig og mindst 16 tegn lang (med brug af store og små bogstaver og tal).
Det er helt O.k. at bruge OpenPGP på sådanne usikre systemer (dvs. normale computere). Du og dem du kommunikerer med skal blot være opmærksomme på sikkerhedsniveauet. Det næste sikkerhedsniveau er smartcards. Du kan ikke stjæle en nøgle fra et smartcard (du kan dog stadig misbruge det, hvis du kontrollerer systemet, som smatrcardet er forbundet til). Niveauet over smartcards er sikre systemer: Kobl din harddisk og netværket fra, fjern USB-sticks (og lignende), start op fra et sikkert medium som en Linux live DVD (fra en kilde, du har tillid til, selvfølgelig!). Brug kun nøgler med høj sikkerhed i sådanne sikre omgivelser (og måske endda kun med sikre dokumentformater som ren tekst og HTML).
Hovednøglen og undernøgler
De fleste OpenPGP-nøgler har mindst en undernøgle (de har alle præcis én hovednøgle). Som regel behøver du ikke at bekymre dig om forskellen: dit program (eller rettere det underliggende program, som regel "GnuPG") vælger automatisk den rigtige. Hovednøglen der den, som nøglens fingeraftryk refererer til, og kun hovednøglen kan certificere dine egne undernøgler og bruger-ID'er og andre nøglers bruger-ID'er. Undernøglerne kan gøre alt andet (hovedsaligt kryptering og signering) hvis du konfigurerer dem således. Årsagen til at nævne forskellen imellem disse nøgletyper er, at den har stor betydning under generering af nøgler. Med GnuPG kan du separere hovednøglen fra undernøglerne (dette er ikke en del af OpenPGP-standarden!) Undernøglerne kan senere erstattes, men det kan hovednøglen ikke (det ville være en ny nøgle, ikke blot en modificeret nøgle). Ved nøglegenereringen kan du således oprette en hovednøgle off-line, som du kan beskytte med en meget stærk adgangskode, gemme i det mindste adgangskoden sikkert og så bruge hovednøglen (og dens adgangskode) i sikre omgivelser; kun således kan du beholde denne nøgle "altid" (lad os sige 20 år). Dette er vigtigt for dagligdagsnøgler. Nøgler med høj sikkerhed her egentlig ikke brug for denne adskillelse (som regel behøver du slet ikke undernøgler). Du bør oprette en undernøgle til hver af de funktioner, du har brug for: kryptering, signering og måske .autentifikation (til SSH)
Sikkert omgivelser
Nøglen skal genereres i sikre omgivelser og den hemmelige hovednøglen må aldrig bruges i usikre omgivelser. Men hvad er sikre omgivelser? Det afhænger af nøglens sikkerhedsniveau. Hvis du vil sikre kernevåbens affyringskoder, så vil du stille større krav end hvis du blot vil beskytte dit eget privatliv.
At any rate you should not boot from your harddisk (ideally disconnect it) but from some trustworthy read-only medium. This is usually a Linux CD/DVD. You should pay attention to getting this from a safe source. Just downloading some image from the Internet and burning it to a blank doesn't make a secure environment up. In general a pressed CD/DVD (e.g. from a magazine or bought on its own) seems more trustworthy.
Hardware is important, too. If you use a system which was known in advance to be used for the generation of valuable keys somebody might like the idea to add a hardware keylogger. Pay attention to rather trivial attacks: Prevent everybody else from seeing you typing your passphrase (even through windows) and from seeing your slip of paper with the passphrase. Reboot the system after key creation or (better) power it off and keep it so for three minutes.
User IDs
User IDs are formally just arbitrary strings but expected to consist of a name, an optional comment, and an email address. This structure allows email software to find the suitable key for a recipient address (which most programs require you to confirm for obvious reasons). An OpenPGP public key (more precise: certificate) must have a user ID but can have as many as you like. Thus you can use the same key for several email addresses (which makes sense only if these addresses shall be used at the same security level). Combining addresses has advantages and disadvantages. The main advantage is that you need less keys and thus you and others have less work with certifications. The main disadvantages are that you can revoke user IDs but they keep visible forever and that such a combined key allows to make a trivial connections between different roles of you which you may not want connected: private person, business, other organizations (associations, political parties, whatever). You may want to keep these groups separate. And there are even reasons to keep addresses from the same group separate: reputable addresses ([email protected]), anonymous addresses ([email protected]), fun addresses ([email protected]). It also may be that you have no reason to ever use OpenPGP with certain addresses; such addresses should not be part of a user ID, of course. The decision to add an address to a key and publish that key is forever. But it is easy to add an address later. Thus consider this well before you generate the key. In case of doubt start with fewer addresses.
It makes sense to have a special user ID without an email address. With most email addresses you cannot be sure to have them forever. If you revoke a User ID because you don't use the address any more then you lose all the certifications for this user ID. But you will never lose your name (even if you marry or the like there is usually little reason to revoke that user ID). So you keep the certifications for this ID at any rate. And you can use the comment for this ID for a statement about the key itself: "everyday key with secure offline main key and key policy"
Key type and length
The well supported key types are DSA (signatures only), ElGamal (encryption only) and RSA (both); ECDSA (elliptic curves) is going to be added soon. GnuPG offers the creation of keys with a length from 1024 to 4096 bit. This is the current situation (2013).
The differences in security, execution time, signature size, and being required or just considered optional by the standard (rfc4880) are irrelevant for most scenarios. The one relevant difference is: The g10 smartcard supports RSA only. Thus generate an RSA key unless you have concrete and good reason for a different decision. 1024 bit keys are considered breakable by certain well known government agencies today or in the near future. 2048 bit keys are considered safe for decades. But remember: These well known agencies would not waste their time and money by computing your key. They would steal your key. Thus if you want a larger key in order to be safe against this kind of opponent then make sure that they cannot steal it. This obviously requires profound knowledge, discipline and probably some money. The practical reasons against very long keys are: Commonly used versions of GnuPG do not support keys with more than 2048 or 3072 bit respectively length. Operations with asymmetric keys are costly in general. To make it worse: Operation with twice the key size take eight times as long. And it takes long to generate huge keys. For mobile devices this CPU load can become a problem.
Thus: Unless you have concrete and good reason for a different decision stick to 2048 bit (at least for everyday keys).
Key expiration
The main key can set (and change) the expiration date for itself and its subkeys. The only "disadvantage" of an expiration date for others is that they have to update the key to keep it usable. But keys shall be updated regularly anyway so you may consider that an advantage as well. The main advantage of an expiration date (for the main key, too) is that keys which are not used any more can easily be recognized as such. The "official" way is another, of course. If a key is abandoned then a revocation certificate should be published. But this may be impossible (key or passphrase lost and no certificate created before (or lost, too)) or you may simply forget to do so (or to publish it everywhere). The "right" validity period is a compromise between reducing disturbance by expired keys (on both sides; remember that you need a secure environment to change the expiration date with an offline main key) and the time which a key still appears valid. One year may be a good choice.
Revocation certificate
A revocation certificate is a file (or print-out) which you may create preventively to later revoke a whole key in case you have no access to the secret main key any more. This is a huge advantage if you have not just lost access to the key but someone else has! The disadvantage is: You should protect this file / print-out similarly well as the secret main key itself. It can be serious damage if you have only one (suitable) key, need it urgently and someone else destroys it right then. Obviously a main key revocation is forever (cannot be superseded by a newer self-signature).
If you (or someone else you really trust) have another key which is secure enough then you may add this key as a designated revoker to yours (you need not publish this). A designated revoker can revoke another key. Of course, the signature for the designated revoker must be available when this shall be used. If you have not given it to the other person and lost it together with your key then this is useless. If you prefer security over availability then you may make friend A your designated revoker, encrypt this signature for friend B and ask friend C to store it.
Passphrase, safe storage, and backup
You should think about where you will store the passphrase for the main key and the revocation certificate file or print-out. Secure passphrases are hard to remember. You will most probably have to write it down. You really should not use a passphrase you have ever typed on an insecure system (and, of course, you really should not ever do so in the future). You should choose something like rsbBwNl137LcWP33RI: 18 chars consisting of lower and upper case letters and digits. Don't use special chars or umlauts. You gain little security (if you cannot remember 18 random chars then you probably cannot remember 15, too) but may get problems if you are ever forced to use the key on a rescue system (text mode Linux) with "wrong" keyboard settings. You improve security if you memorize a part of the passphrase and write down just the rest or if you write down both halves of it separately and store them in different places (one in your wallet, the other one at home). But if you store an 18 chars passphrase in two parts and an attacker gets one of them then the remaining 9 chars are not a secure protection any more. If you have created a revocation certificate then you have to store that in a sage place, too.
And, of course, you should have reliable backups of your key. It is nice if you need not be afraid that somebody has stolen your key but it is probably very unpleasant if you cannot decrypt your data any more. If you have a secure passphrase then you may even put a backup of your secret main key on your web site.
Key policy and policy URL
You should write a document (plain text or HTML) which describes the intended usage and security of your key and (maybe added later) your criteria for certifying the keys of other people. You can write one or more URLs at which this document can (later) be found into the key and in every signature you make. This key component is called a policy URL. It is a good idea to publish only user ID signatures which contain this policy URL(s). It is important that the users of your key can check whether a certain document belongs to a policy URL (the web server download is not safe, not even over HTTPS). Thus you should change the policy URL every time you change the document and mention the URL well visible in the document. You may use this pattern: http://yourdomain.example.org/openpgp/0x12345678__policy.1.html This document should have a detached signature (or a cleartext signature if it is plain text) by the offline main key. You should link the detached signature from the document.
Preferred key server
Like the policy URL a key server URL can be written into the key. You should decide which key server shall be authoritative for your key so that the users of your key know where they have to search for the "officially current version" of your key. This should be the key server which you upload key updates to first. This should be the one you configure in the GnuPG config file (--keyserver) for key searches and uploads. This address should mainly be available for a long time and have only few and short outages. You may even use your own web site: http://yourdomain.example.org/openpgp/0x12345678.asc
Availability is improved if a server pool (with the same DNS address) is used. If you have no better idea then you may use hkp://pool.sks-keyservers.net or one of the more local pools hkp://eu.pool.sks-keyservers.net (Europe), hkp://na.pool.sks-keyservers.net (North America) or hkp://sa.pool.sks-keyservers.net (South America). See http://sks-keyservers.net/overview-of-pools.php
Algorithm preferences
You can also write into your key (for others) and into your config file (for your actions) in which order you prefer the cipher and digest algorithms. You should do this before you generate the key (this is easier than changing it afterwards). It makes sense to avoid SHA-1. Please be aware that you cannot prevent that GnuPG creates SHA-1 signatures with your key (because that is the only digest required by the standard). You may want to put lines like these in your gpg.conf (due to design flaws AES256 and AES192 are less secure than AES(128) is currently believed to be...):
personal-cipher-preferences AES,AES256,AES192,CAST5,3DES personal-digest-preferences SHA512,SHA384,SHA256,SHA224,RIPEMD160,SHA1 cert-digest-algo SHA512 default-preference-list AES,AES256,AES192,CAST5,3DES,SHA512,SHA384,SHA256,SHA224,RIPEMD160,SHA1,ZLIB,BZIP2,ZIP