channels: Make channel introductions public.

* guix/channels.scm (<channel-introduction>): Rename constructor to
'%make-channel-introduction'.
(make-channel-introduction): New procedure.
* tests/channels.scm ("authenticate-channel, wrong first commit signer")
("authenticate-channel, .guix-authorizations"): Use
'make-channel-introduction' without '@@' and without third argument.
* doc/guix.texi (Channels)[Channel Authentication, Specifying Channel
Authorizations]: New subsections.

doc/guix.texi

@@ -3975,8 +3975,47 @@ deploys Guix itself from the official GNU@tie{}Guix repository.  This can be
 customized by defining @dfn{channels} in the
 @file{~/.config/guix/channels.scm} file.  A channel specifies a URL and branch
 of a Git repository to be deployed, and @command{guix pull} can be instructed
-to pull from one or more channels.  In other words, channels can be used to
-@emph{customize} and to @emph{extend} Guix, as we will see below.
+to pull from one or more channels.  In other words, channels can be used
+to @emph{customize} and to @emph{extend} Guix, as we will see below.
+Before that, some security considerations.
+
+@subsection Channel Authentication
+
+@cindex authentication, of channel code
+The @command{guix pull} and @command{guix time-machine} commands
+@dfn{authenticate} the code retrieved from channels: they make sure each
+commit that is fetched is signed by an authorized developer.  The goal
+is to protect from unauthorized modifications to the channel that would
+lead users to run malicious code.
+
+As a user, you must provide a @dfn{channel introduction} in your
+channels file so that Guix knows how to authenticate its first commit.
+A channel specification, including its introduction, looks something
+along these lines:
+
+@lisp
+(channel
+  (name 'my-channel)
+  (url "https://example.org/my-channel.git")
+  (introduction
+   (make-channel-introduction
+    "6f0d8cc0d88abb59c324b2990bfee2876016bb86"
+    (openpgp-fingerprint
+     "CABB A931 C0FF EEC6 900D  0CFB 090B 1199 3D9A EBB5"))))
+@end lisp
+
+The specification above shows the name and URL of the channel.  The call
+to @code{make-channel-introduction} above specifies that authentication
+of this channel starts at commit @code{6f0d8cc@dots{}}, which is signed
+by the OpenPGP key with fingerprint @code{CABB A931@dots{}}.
+
+For the main channel, called @code{guix}, you automatically get that
+information from your Guix installation.  For other channels, include
+the channel introduction provided by the channel authors in your
+@file{channels.scm} file.  Make sure you retrieve the channel
+introduction from a trusted source since that is the root of your trust.
+
+If you're curious about the authentication mechanics, read on!
 
 @subsection Using a Custom Guix Channel
 
@@ -4150,6 +4189,75 @@ add a meta-data file @file{.guix-channel} that contains:
   (directory "guix"))
 @end lisp
 
+@cindex channel authorizations
+@subsection Specifying Channel Authorizations
+
+As we saw above, Guix ensures the source code it pulls from channels
+comes from authorized developers.  As a channel author, you need to
+specify the list of authorized developers in the
+@file{.guix-authorizations} file in the channel's Git repository.  The
+authentication rule is simple: each commit must be signed by a key
+listed in the @file{.guix-authorizations} file of its parent
+commit(s)@footnote{Git commits form a @dfn{directed acyclic graph}
+(DAG).  Each commit can have zero or more parents; ``regular'' commits
+have one parent and merge commits have two parent commits.  Read
+@uref{https://eagain.net/articles/git-for-computer-scientists/, @i{Git
+for Computer Scientists}} for a great overview.}  The
+@file{.guix-authorizations} file looks like this:
+
+@lisp
+;; Example '.guix-authorizations' file.
+
+(authorizations
+ (version 0)               ;current file format version
+
+ (("AD17 A21E F8AE D8F1 CC02  DBD9 F8AE D8F1 765C 61E3"
+   (name "alice"))
+  ("2A39 3FFF 68F4 EF7A 3D29  12AF 68F4 EF7A 22FB B2D5"
+   (name "bob"))
+  ("CABB A931 C0FF EEC6 900D  0CFB 090B 1199 3D9A EBB5"
+   (name "charlie"))))
+@end lisp
+
+Each fingerprint is followed by optional key/value pairs, as in the
+example above.  Currently these key/value pairs are ignored.
+
+This authentication rule creates a chicken-and-egg issue: how do we
+authenticate the first commit?  Related to that: how do we deal with
+channels whose repository history contains unsigned commits and lack
+@file{.guix-authorizations}?  And how do we fork existing channels?
+
+@cindex channel introduction
+Channel introductions answer these questions by describing the first
+commit of a channel that should be authenticated.  The first time a
+channel is fetched with @command{guix pull} or @command{guix
+time-machine}, the command looks up the introductory commit and verifies
+that it is signed by the specified OpenPGP key.  From then on, it
+authenticates commits according to the rule above.
+
+To summarize, as the author of a channel, there are two things you have
+to do to allow users to authenticate your code:
+
+@enumerate
+@item
+Introduce an initial @file{.guix-authorizations} in the channel's
+repository.  Do that in a signed commit (@pxref{Commit Access}, for
+information on how to sign Git commits.)
+
+@item
+Advertise the channel introduction, for instance on your channel's web
+page.  The channel introduction, as we saw above, is the commit/key
+pair---i.e., the commit that introduced @file{.guix-authorizations}, and
+the fingerprint of the OpenPGP used to sign it.
+@end enumerate
+
+Publishing a signed channel requires discipline: any mistake, such as an
+unsigned commit or a commit signed by an unauthorized key, will prevent
+users from pulling from your channel---well, that's the whole point of
+authentication!  Pay attention to merges in particular: merge commits
+are considered authentic if and only if they are signed by a key present
+in the @file{.guix-authorizations} file of @emph{both} branches.
+
 @cindex primary URL, channels
 @subsection Primary URL
 

guix/channels.scm

@@ -69,7 +69,9 @@ (define-module (guix channels)
             channel-location
 
             channel-introduction?
-            ;; <channel-introduction> accessors purposefully omitted for now.
+            make-channel-introduction
+            channel-introduction-first-signed-commit
+            channel-introduction-first-commit-signer
 
             openpgp-fingerprint->bytevector
             openpgp-fingerprint
@@ -130,13 +132,19 @@ (define-record-type* <channel> channel make-channel
 ;; commit so that only them may emit this introduction.  Introductions are
 ;; used to bootstrap trust in a channel.
 (define-record-type <channel-introduction>
-  (make-channel-introduction first-signed-commit first-commit-signer
-                             signature)
+  (%make-channel-introduction first-signed-commit first-commit-signer
+                              signature)
   channel-introduction?
   (first-signed-commit  channel-introduction-first-signed-commit) ;hex string
   (first-commit-signer  channel-introduction-first-commit-signer) ;bytevector
   (signature            channel-introduction-signature))          ;string
 
+(define (make-channel-introduction commit signer)
+  "Return a new channel introduction: COMMIT is the introductory where
+authentication starts, and SIGNER is the OpenPGP fingerprint (a bytevector) of
+the signer of that commit."
+  (%make-channel-introduction commit signer #f))
+
 (define (openpgp-fingerprint->bytevector str)
   "Convert STR, an OpenPGP fingerprint (hexadecimal string with whitespace),
 to the corresponding bytevector."

tests/channels.scm

@@ -451,12 +451,11 @@ (define (find-commit* message)
       (with-repository directory repository
         (let* ((commit1 (find-commit repository "first"))
                (commit2 (find-commit repository "second"))
-               (intro   ((@@ (guix channels) make-channel-introduction)
+               (intro   (make-channel-introduction
                          (commit-id-string commit1)
                          (openpgp-public-key-fingerprint
                           (read-openpgp-packet
-                           %ed25519bis-public-key-file)) ;different key
-                         #f))                     ;no signature
+                           %ed25519bis-public-key-file)))) ;different key
                (channel (channel (name 'example)
                                  (url (string-append "file://" directory))
                                  (introduction intro))))
@@ -507,12 +506,11 @@ (define (find-commit* message)
         (let* ((commit1 (find-commit repository "first"))
                (commit2 (find-commit repository "second"))
                (commit3 (find-commit repository "third"))
-               (intro   ((@@ (guix channels) make-channel-introduction)
+               (intro   (make-channel-introduction
                          (commit-id-string commit1)
                          (openpgp-public-key-fingerprint
                           (read-openpgp-packet
-                           %ed25519-public-key-file))
-                         #f))                     ;no signature
+                           %ed25519-public-key-file))))
                (channel (channel (name 'example)
                                  (url (string-append "file://" directory))
                                  (introduction intro))))