* guix/scripts/substitute-binary.scm (<narinfo>): Add the 'signature'
and 'contents' fields.
(narinfo-signature->canonical-sexp): New function.
(narinfo-maker): Add the 'signature' argument and use it.
(assert-valid-signature): New function.
(read-narinfo): Support the Signature field.
(write-narinfo): Use 'narinfo-contents'.
(%allow-unauthenticated-substitutes?): New variable.
* guix/base64.scm, tests/base64.scm, tests/substitute-binary.scm: New files.
* Makefile.am (SCM_TESTS): Add tests/base64.scm and
tests/substitute-binary.scm.
(MODULES): Add guix/base64.scm.
* test-env.in: Set 'GUIX_ALLOW_UNAUTHENTICATED_SUBSTITUTES'.
Nikita Karetnikov