Toggle diff (208 lines)
diff --git a/doc/guix.texi b/doc/guix.texi
index 5b854ccbd4..a4bb52bb24 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -3975,8 +3975,52 @@ 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 be @dfn{introduced} to a channel so you can start
+pulling from it and authenticate its code. The @dfn{channel
+introduction} tells Guix how to authenticate the first commit of that
+channel:
+
+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 +4194,75 @@ 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
diff --git a/guix/channels.scm b/guix/channels.scm
index 1d4b50aa48..9859bfdda8 100644
--- a/guix/channels.scm
+++ b/guix/channels.scm
@@ -69,7 +69,9 @@
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 @@
;; 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."
diff --git a/tests/channels.scm b/tests/channels.scm
index 3a2c1d429b..016c3ad9db 100644
--- a/tests/channels.scm
+++ b/tests/channels.scm
@@ -430,12 +430,11 @@
(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))))
@@ -486,12 +485,11 @@
(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))))
--
2.26.2
