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git-authenticate: Disallow SHA1 (and MD5) signatures.

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* guix/git-authenticate.scm (commit-signing-key): Add
 #:disallowed-hash-algorithms and honor it.
(authenticate-commit)[recent-commit?]: New variable.
Pass #:disallowed-hash-algorithms to 'commit-signing-key'.
* tests/git-authenticate.scm ("signed commits, SHA1 signature"): New test.
This commit is contained in:
Ludovic Courtès 2020-06-10 14:54:13 +02:00
parent 7def5056b6
commit 52c529ff20
No known key found for this signature in database
GPG key ID: 090B11993D9AEBB5
2 changed files with 55 additions and 3 deletions
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29
guix/git-authenticate.scm
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@ -85,9 +85,11 @@
(signature missing-key-error-signature))
(define (commit-signing-key repo commit-id keyring)
(define* (commit-signing-key repo commit-id keyring
#:key (disallowed-hash-algorithms '(sha1)))
"Return the OpenPGP key that signed COMMIT-ID (an OID). Raise an exception
if the commit is unsigned, has an invalid signature, or if its signing key is
if the commit is unsigned, has an invalid signature, has a signature using one
of the hash algorithms in DISALLOWED-HASH-ALGORITHMS, or if its signing key is
not in KEYRING."
(let-values (((signature signed-data)
(catch 'git-error
@ -103,6 +105,17 @@ not in KEYRING."
(oid->string commit-id)))))))
(let ((signature (string->openpgp-packet signature)))
(when (memq (openpgp-signature-hash-algorithm signature)
`(,@disallowed-hash-algorithms md5))
(raise (condition
(&unsigned-commit-error (commit commit-id))
(&message
(message (format #f (G_ "commit ~a has a ~a signature, \
which is not permitted")
(oid->string commit-id)
(openpgp-signature-hash-algorithm
signature)))))))
(with-fluids ((%default-port-encoding "UTF-8"))
(let-values (((status data)
(verify-openpgp-signature signature keyring
@ -198,8 +211,18 @@ not specify anything, fall back to DEFAULT-AUTHORIZATIONS."
(define id
(commit-id commit))
(define recent-commit?
(false-if-git-not-found
(tree-entry-bypath (commit-tree commit) ".guix-authorizations")))
(define signing-key
(commit-signing-key repository id keyring))
(commit-signing-key repository id keyring
;; Reject SHA1 signatures unconditionally as suggested
;; by the authors of "SHA-1 is a Shambles" (2019).
;; Accept it for "historical" commits (there are such
;; signatures from April 2020 in the repository).
#:disallowed-hash-algorithms
(if recent-commit? '(sha1) '())))
(unless (member (openpgp-public-key-fingerprint signing-key)
(commit-authorized-keys repository commit