pki: Factorize signature manipulation procedures.

* guix/pki.scm (signature-subject, signature-signed-data,
  valid-signature?): New procedures.
* guix/scripts/authenticate.scm (guix-authenticate): Adjust to use
  them.

guix/pki.scm

@@ -29,8 +29,12 @@
             current-acl
             public-keys->acl
             acl->public-keys
+            authorized-key?
+
             signature-sexp
-            authorized-key?))
+            signature-subject
+            signature-signed-data
+            valid-signature?))
 
 ;;; Commentary:
 ;;;
@@ -136,4 +140,21 @@ PUBLIC-KEY (see <http://theworld.com/~cme/spki.txt> for examples.)"
            (canonical-sexp->string (sign data secret-key))
            (canonical-sexp->string public-key))))
 
+(define (signature-subject sig)
+  "Return the signer's public key for SIG."
+  (find-sexp-token sig 'public-key))
+
+(define (signature-signed-data sig)
+  "Return the signed data from SIG, typically an sexp such as
+  (hash \"sha256\" #...#)."
+  (find-sexp-token sig 'data))
+
+(define (valid-signature? sig)
+  "Return #t if SIG is valid."
+  (let* ((data       (signature-signed-data sig))
+         (signature  (find-sexp-token sig 'sig-val))
+         (public-key (signature-subject sig)))
+    (and data signature
+         (verify signature data public-key))))
+
 ;;; pki.scm ends here

guix/scripts/authenticate.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -72,23 +72,21 @@
      ;; Read the signature as produced above, check whether its public key is
      ;; authorized, and verify the signature, and print the signed data to
      ;; stdout upon success.
-     (let* ((sig+data   (read-canonical-sexp signature-file))
-            (public-key (find-sexp-token sig+data 'public-key))
-            (data       (find-sexp-token sig+data 'data))
-            (signature  (find-sexp-token sig+data 'sig-val)))
-       (if (and data signature)
-           (if (authorized-key? public-key)
-               (if (verify signature data public-key)
-                   (begin
-                     (display (bytevector->base16-string
-                               (hash-data->bytevector data)))
+     (let* ((signature (read-canonical-sexp signature-file))
+            (subject   (signature-subject signature))
+            (data      (signature-signed-data signature)))
+       (if (and data subject)
+           (if (authorized-key? subject)
+               (if (valid-signature? signature)
+                   (let ((hash (hash-data->bytevector data)))
+                     (display (bytevector->base16-string hash))
                      #t)                          ; success
                    (leave (_ "error: invalid signature: ~a~%")
                           (canonical-sexp->string signature)))
                (leave (_ "error: unauthorized public key: ~a~%")
-                      (canonical-sexp->string public-key)))
+                      (canonical-sexp->string subject)))
            (leave (_ "error: corrupt signature data: ~a~%")
-                  (canonical-sexp->string sig+data)))))
+                  (canonical-sexp->string signature)))))
     (("--help")
      (display (_ "Usage: guix authenticate OPTION...
 Sign or verify the signature on the given file.  This tool is meant to