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Reorganize security info pages
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Andrew David Wong
parent
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commit
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16 changed files with 111 additions and 48 deletions
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@ -76,7 +76,7 @@ Cons:
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other two options.)
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[verify]: /doc/verifying-signatures/
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[verify]: /security/verifying-signatures/
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[classic problem]: http://www.acm.org/classics/sep95/
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[solutions]: http://www.dwheeler.com/trusting-trust/
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[USB qube]: /doc/usb/#creating-and-using-a-usb-qube
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@ -121,7 +121,7 @@ Getting Help
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[Hardware Compatibility List]: /hcl/
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[live USB]: /doc/live-usb/
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[downloads]: /downloads/
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[verifying signatures]: /doc/verifying-signatures/
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[verifying signatures]: /security/verifying-signatures/
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[security considerations]: /doc/install-security/
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[Rufus]: http://rufus.akeo.ie/
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[documentation]: /doc/
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@ -1,316 +0,0 @@
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---
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layout: doc
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title: Verifying Signatures
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permalink: /doc/verifying-signatures/
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redirect_from:
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- /en/doc/verifying-signatures/
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- /doc/VerifyingSignatures/
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- /wiki/VerifyingSignatures/
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---
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On Digital Signatures and Key Verification
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==========================================
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What Digital Signatures Can and Cannot Prove
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--------------------------------------------
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Most people – even programmers – are confused about the basic concepts
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underlying digital signatures. Therefore, most people should read this section,
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even if it looks trivial at first sight.
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Digital signatures can prove both **authenticity** and **integrity** to a
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reasonable degree of certainty. **Authenticity** ensures that a given file was
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indeed created by the person who signed it (i.e., that it was not forged by a
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third party). **Integrity** ensures that the contents of the file have not been
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tampered with (i.e., that a third party has not undetectably altered its
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contents *en route*).
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Digital signatures **cannot** prove any other property, e.g., that the signed
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file is not malicious. In fact, there is nothing that could stop someone from
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signing a malicious program (and it happens from time to time in reality).
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The point is, of course, that people must choose who they will trust (e.g.,
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Linus Torvalds, Microsoft, the Qubes Project, etc.) and assume that if a given
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file was signed by a trusted party, then it should not be malicious or buggy in
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some horrible way. But the decision of whether to trust any given party is
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beyond the scope of digital signatures. It's more of a sociological and
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political decision.
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Once we make the decision to trust certain parties, digital signatures are
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useful, because they make it possible for us to limit our trust only to those
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few parties we choose and not to worry about all the "Bad Things That Can
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Happen In The Middle" between us and them, e.g., server compromises
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(qubes-os.org will surely be compromised one day), dishonest IT staff at the
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hosting company, dishonest staff at the ISPs, Wi-Fi attacks, etc.
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By verifying all the files we download which purport to be authored by a party
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we've chosen to trust, we eliminate concerns about the bad things discussed
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above, since we can easily detect whether any files have been tampered with
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(and subsequently choose to refrain from executing, installing, or opening
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them).
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However, for digital signatures to make any sense, we must ensure that the
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public keys we use for signature verification are indeed the original ones.
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Anybody can generate a GPG key pair that purports to belong to "The Qubes
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Project," but of course only the key pair that we (i.e., the Qubes developers)
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generated is the legitimate one. The next section explains how to verify the
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validity of the Qubes signing keys.
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Importing Qubes Signing Keys
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----------------------------
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Every file published by the Qubes Project (ISO, RPM, TGZ files and git
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repositories) is digitally signed by one of the developer or release signing
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keys. Each such key is signed by the [Qubes Master Signing Key]
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(`0xDDFA1A3E36879494`).
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The public portion of the Qubes Master Signing Key can be imported directly
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from a [keyserver] (specified on first use with `--keyserver <URI>`, keyserver
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saved in `~/.gnupg/gpg.conf`), e.g.,
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gpg --keyserver pool.sks-keyservers.net --recv-keys 0x427F11FD0FAA4B080123F01CDDFA1A3E36879494
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or downloaded [here][Qubes Master Signing Key] and imported with gpg,
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$ gpg --import ./qubes-master-signing-key.asc
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or fetched directly with gpg.
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$ gpg --fetch-keys https://keys.qubes-os.org/keys/qubes-master-signing-key.asc
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For additional security we also publish the fingerprint of the Qubes Master
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Signing Key here in this document:
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pub 4096R/36879494 2010-04-01
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Key fingerprint = 427F 11FD 0FAA 4B08 0123 F01C DDFA 1A3E 3687 9494
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uid Qubes Master Signing Key
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There should also be a copy of this key at the project's main website, in the
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[Qubes Security Pack], and in the archives of the project's
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[developer][devel-master-key-msg] and [user][user-master-key-msg] [mailing lists].
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Once you have obtained the Qubes Master Signing Key,
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you should verify the fingerprint of this key very carefully by obtaining
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copies of the fingerprint from trustworthy independent sources and comparing
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them to the downloaded key's fingerprint to ensure they match. Then set its
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trust level to "ultimate" (oh, well), so that it can be used to automatically
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verify all the keys signed by the Qubes Master Signing Key:
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$ gpg --edit-key 0x36879494
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gpg (GnuPG) 1.4.18; Copyright (C) 2014 Free Software Foundation, Inc.
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This is free software: you are free to change and redistribute it.
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There is NO WARRANTY, to the extent permitted by law.
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pub 4096R/36879494 created: 2010-04-01 expires: never usage: SC
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trust: unknown validity: unknown
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[ unknown] (1). Qubes Master Signing Key
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gpg> fpr
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pub 4096R/36879494 2010-04-01 Qubes Master Signing Key
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Primary key fingerprint: 427F 11FD 0FAA 4B08 0123 F01C DDFA 1A3E 3687 9494
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gpg> trust
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pub 4096R/36879494 created: 2010-04-01 expires: never usage: SC
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trust: unknown validity: unknown
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[ unknown] (1). Qubes Master Signing Key
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Please decide how far you trust this user to correctly verify other users' keys
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(by looking at passports, checking fingerprints from different sources, etc.)
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1 = I don't know or won't say
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2 = I do NOT trust
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3 = I trust marginally
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4 = I trust fully
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5 = I trust ultimately
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m = back to the main menu
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Your decision? 5
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Do you really want to set this key to ultimate trust? (y/N) y
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pub 4096R/36879494 created: 2010-04-01 expires: never usage: SC
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trust: ultimate validity: unknown
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[ unknown] (1). Qubes Master Signing Key
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Please note that the shown key validity is not necessarily correct
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unless you restart the program.
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gpg> q
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Now you can easily download any of the developer or release signing keys that
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happen to be used to sign particular ISO, RPM, TGZ files or git tags.
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For example, the Qubes OS [Release 3 Signing Key] (`0xCB11CA1D03FA5082`) is
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used for all Release 3 ISO images:
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$ gpg --recv-keys 0xC52261BE0A823221D94CA1D1CB11CA1D03FA5082
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gpg: requesting key 03FA5082 from hkp server keys.gnupg.net
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gpg: key 03FA5082: public key "Qubes OS Release 3 Signing Key" imported
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gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
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gpg: depth: 0 valid: 1 signed: 1 trust: 0-, 0q, 0n, 0m, 0f, 1u
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gpg: depth: 1 valid: 1 signed: 0 trust: 1-, 0q, 0n, 0m, 0f, 0u
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gpg: Total number processed: 1
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gpg: imported: 1 (RSA: 1)
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You can also download all the currently used developers' signing keys and
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current and older release signing keys (and also a copy of the Qubes Master
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Signing Key) from the [Qubes OS Keyserver] and from the [Qubes Security Pack].
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The developer signing keys are set to be valid for 1 year only, while the Qubes
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Master Signing Key has no expiration date. This latter key was generated and is
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kept only within a dedicated, air-gapped "vault" machine, and the private
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portion will (hopefully) never leave this isolated machine.
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You can now verify the ISO image (`Qubes-R3.2-x86_64.iso`) matches its
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signature (`Qubes-R3.2-x86_64.iso.asc`):
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$ gpg -v --verify Qubes-R3.2-x86_64.iso.asc Qubes-R3.2-x86_64.iso
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gpg: armor header: Version: GnuPG v1
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gpg: Signature made Tue 08 Mar 2016 07:40:56 PM PST using RSA key ID 03FA5082
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gpg: using PGP trust model
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gpg: Good signature from "Qubes OS Release 3 Signing Key"
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gpg: binary signature, digest algorithm SHA256
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The Release 3 Signing Key used to sign this ISO image should be signed by the
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Qubes Master Signing Key:
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$ gpg --list-sig 03FA5082
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pub 4096R/03FA5082 2014-11-19
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uid Qubes OS Release 3 Signing Key
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sig 3 03FA5082 2014-11-19 Qubes OS Release 3 Signing Key
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sig 36879494 2014-11-19 Qubes Master Signing Key
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Verifying Digests
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-----------------
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Each ISO is also accompanied by a plain text file ending in `.DIGESTS`. This
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file contains the output of running several different crytographic hash
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functions on the ISO in order to obtain alphanumeric outputs known as "digests"
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or "hash values." These hash values are provided as an alternative verification
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method to PGP signatures (though the `.DIGESTS` file is itself also PGP-signed
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--- see below). If you've already verified the signatures on the ISO directly,
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then verifying digests is not necessary. You can always find all the `.DIGESTS`
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files for every Qubes ISO in the [Qubes Security Pack].
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As an example, `Qubes-R3.2-x86_64.iso` is accompanied by
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`Qubes-R3.2-x86_64.iso.DIGESTS` which has the following content:
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-----BEGIN PGP SIGNED MESSAGE-----
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Hash: SHA256
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3c951138b8b9867d8657f173c1b58b82 *Qubes-R3.2-x86_64.iso
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1fc9508160d7c4cba6cacc3025165b0f996c843f *Qubes-R3.2-x86_64.iso
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6b998045a513dcdd45c1c6e61ace4f1b4e7eff799f381dccb9eb0170c80f678a *Qubes-R3.2-x86_64.iso
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de1eb2e76bdb48559906f6fe344027ece20658d4a7f04ba00d4e40c63723171c62bdcc869375e7a4a4499d7bff484d7a621c3acfe9c2b221baee497d13cd02fe *Qubes-R3.2-x86_64.iso
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-----BEGIN PGP SIGNATURE-----
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Version: GnuPG v2
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iQIcBAEBCAAGBQJX4XO/AAoJEMsRyh0D+lCCL9sP/jlZ26zhvlDEX/eaA/ANa/6b
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Dpsh/sqZEpz1SWoUxdm0gS+anc8nSDoCQSMBxnafuBbmwTChdHI/P7NvNirCULma
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9nw+EYCsCiNZ9+WCeroR8XDFSiDjvfkve0R8nwfma1XDqu1bN2ed4n/zNoGgQ8w0
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t5LEVDKCVJ+65pI7RzOSMbWaw+uWfGehbgumD7a6rfEOqOTONoZOjJJTnM0+NFJF
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Qz5yBg+0FQYc7FmfX+tY801AwSyevj3LKGqZN1GVcU9hhoHH7f2BcbdNk9I5WHHq
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doKMnZtcdyadQGwMNB68Wu9+0CWsXvk6E00QfW69M4d6w0gbyoJyUL1uzxgixb5O
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qodxrqeitXQSZZvU4kom5zlSjqZs4dGK+Ueplpkr8voT8TSWer0Nbh/VMfrNSt1z
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0/j+e/KMjor7XxehR+XhNWa2YLjA5l5H9rP+Ct/LAfVFp4uhsAnYf0rUskhCStxf
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Zmtqz4FOw/iSz0Os+IVcnRcyTYWh3e9XaW56b9J/ou0wlwmJ7oJuEikOHBDjrUph
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2a8AM+QzNmnc0tDBWTtT2frXcotqL+Evp/kQr5G5pJM/mTR5EQm7+LKSl7yCPoCj
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g8JqGYYptgkxjQdX3YAy9VDsCJ/6EkFc2lkQHbgZxjXqyrEMbgeSXtMltZ7cCqw1
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3N/6YZw1gSuvBlTquP27
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=e9oD
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-----END PGP SIGNATURE-----
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Four digests have been computed for this ISO. The hash functions used, in order
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from top to bottom, are MD5, SHA1, SHA256, and SHA512. One way to verify that
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the ISO you downloaded matches any of these hash values is by using the
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respective `*sum` programs:
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$ md5sum -c Qubes-R3.2-x86_64.iso.DIGESTS
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Qubes-R3.2-x86_64.iso: OK
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md5sum: WARNING: 23 lines are improperly formatted
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$ sha1sum -c Qubes-R3.2-x86_64.iso.DIGESTS
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Qubes-R3.2-x86_64.iso: OK
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sha1sum: WARNING: 23 lines are improperly formatted
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$ sha256sum -c Qubes-R3.2-x86_64.iso.DIGESTS
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Qubes-R3.2-x86_64.iso: OK
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sha256sum: WARNING: 23 lines are improperly formatted
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$ sha512sum -c Qubes-R3.2-x86_64.iso.DIGESTS
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Qubes-R3.2-x86_64.iso: OK
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sha512sum: WARNING: 23 lines are improperly formatted
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The `OK` response tells us that the hash value for that particular hash
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function matches. The program also warns us that there are 23 improperly
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formatted lines, but this is to be expected. This is because each file contains
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lines for several different hash values (as mentioned above), but each `*sum`
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program verifies only the line for its own hash function. In addition, there
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are lines for the PGP signature which the `*sum` programs do not know how to
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read.
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Another way is to use `openssl` to compute each hash value, then compare them
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to the contents of the `.DIGESTS` file.:
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$ openssl dgst -md5 Qubes-R3.2-x86_64.iso
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MD5(Qubes-R3.2-x86_64.iso)= 3c951138b8b9867d8657f173c1b58b82
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$ openssl dgst -sha1 Qubes-R3.2-x86_64.iso
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SHA1(Qubes-R3.2-x86_64.iso)= 1fc9508160d7c4cba6cacc3025165b0f996c843f
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$ openssl dgst -sha256 Qubes-R3.2-x86_64.iso
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SHA256(Qubes-R3.2-x86_64.iso)= 6b998045a513dcdd45c1c6e61ace4f1b4e7eff799f381dccb9eb0170c80f678a
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$ openssl dgst -sha512 Qubes-R3.2-x86_64.iso
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SHA512(Qubes-R3.2-x86_64.iso)= de1eb2e76bdb48559906f6fe344027ece20658d4a7f04ba00d4e40c63723171c62bdcc869375e7a4a4499d7bff484d7a621c3acfe9c2b221baee497d13cd02fe
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(Notice that the outputs match the values from the `.DIGESTS` file.)
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However, it is possible that an attacker replaced `Qubes-R3.2-x86_64.iso` with
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a malicious ISO, computed the hash values for that ISO, and replaced the values
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in `Qubes-R3.2-x86_64.iso.DIGESTS` with his own set of values. Therefore,
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ideally, we should also verify the authenticity of the listed hash values.
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Since `Qubes-R3.2-x86_64.iso.DIGESTS` is a clearsigned PGP file, we can use
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`gpg` to verify it from the command line:
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$ gpg -v --verify Qubes-R3.2-x86_64.iso.DIGESTS
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gpg: armor header: Hash: SHA256
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gpg: armor header: Version: GnuPG v2
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gpg: original file name=''
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gpg: Signature made Tue 20 Sep 2016 10:37:03 AM PDT using RSA key ID 03FA5082
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gpg: using PGP trust model
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gpg: Good signature from "Qubes OS Release 3 Signing Key"
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gpg: textmode signature, digest algorithm SHA256
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The signature is good. Assuming our copy of the `Qubes OS Release 3 Signing
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Key` is also authentic (see above), we can be confident that these hash values
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came from the Qubes devs.
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Verifying Qubes Code
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--------------------
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Developers who fetch code from our Git server should always verify the PGP signature of the tag on the latest commit.
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In some cases, commits themselves may also be signed.
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Any unsigned commit that is not followed by a signed tag should not be trusted!
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To verify a signature on a git tag:
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$ git tag -v <tag name>
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or
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$ git verify-tag <tag name>
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To verify a signature on a git commit:
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$ git log --show-signature <commit ID>
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or
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$ git verify-commit <commit ID>
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[Qubes Master Signing Key]: https://keys.qubes-os.org/keys/qubes-master-signing-key.asc
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[keyserver]: https://en.wikipedia.org/wiki/Key_server_%28cryptographic%29#Keyserver_examples
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[Qubes Security Pack]: /doc/security-pack/
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[devel-master-key-msg]: https://groups.google.com/d/msg/qubes-devel/RqR9WPxICwg/kaQwknZPDHkJ
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[user-master-key-msg]: https://groups.google.com/d/msg/qubes-users/CLnB5uFu_YQ/ZjObBpz0S9UJ
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[mailing lists]: /mailing-lists/
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[Release 3 Signing Key]: https://keys.qubes-os.org/keys/qubes-release-3-signing-key.asc
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[Qubes OS Keyserver]: https://keys.qubes-os.org/keys/
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