The key 0xBB572E0E2D182910 is listed at https://www.gentoo.org/proj/en/releng/index.xml with expiration date 2015/08/24 but it actually expired in August 24th 2013. So there are two issues: incorrect documentation and no good key (since August 24th) for signing automated stuff like stage3 builds.
$ gpg --recv-keys BB572E0E2D182910 gpg: requesting key 2D182910 from hkp server pgp.mit.edu gpg: key 2D182910: public key "Gentoo Linux Release Engineering (Automated Weekly Release Key) <releng@gentoo.org>" imported gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model gpg: depth: 0 valid: 1 signed: 0 trust: 0-, 0q, 0n, 0m, 0f, 1u gpg: next trustdb check due at 2014-04-14 gpg: Total number processed: 1 gpg: imported: 1 (RSA: 1) $ gpg -k BB572E0E2D182910 pub 4096R/2D182910 2009-08-25 [expires: 2015-08-24] uid Gentoo Linux Release Engineering (Automated Weekly Release Key) <releng@gentoo.org> lgtm.
Now this is strange: [Sun Sep 15 01:02:34] nedko@mobix32 /tmp/gentoo # gpg --fin BB572E0E2D182910 pub 4096R/2D182910 2009-08-25 [expired: 2013-08-24] Key fingerprint = 13EB BDBE DE7A 1277 5DFD B1BA BB57 2E0E 2D18 2910 uid Gentoo Linux Release Engineering (Automated Weekly Release Key) <releng@gentoo.org> [Sun Sep 15 01:02:36] nedko@mobix32 /tmp/gentoo # date -u Sat Sep 14 22:02:41 UTC 2013 Also note that this matches what is recorded in https://bugs.gentoo.org/show_bug.cgi?id=277319#c6
Ah, I got it. There is key renewal.
*** Bug 523084 has been marked as a duplicate of this bug. ***
*** Bug 550472 has been marked as a duplicate of this bug. ***