From c4d5cb52f5255989c80c0e512a240c67d37b3217 Mon Sep 17 00:00:00 2001 From: David Cooper Date: Wed, 2 Jan 2019 10:41:07 +0100 Subject: [PATCH] Bug 1167857, Thunderbird should use AES for S/MIME based on correspondent's key sizes, for compliance with RFC 5751, r=rrelyea --- lib/smime/smimeutil.c | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) diff --git a/lib/smime/smimeutil.c b/lib/smime/smimeutil.c index 0e6bd32fdc..a7df96e917 100644 --- a/lib/smime/smimeutil.c +++ b/lib/smime/smimeutil.c @@ -457,6 +457,25 @@ smime_choose_cipher(CERTCertificate *scert, CERTCertificate **rcerts) cipher_votes[strong_mapi] += pref; pref--; } else { + if (pklen_bits > 3072) { + /* While support for AES 256 is a SHOULD+ in RFC 5751 + * rather than a MUST, RSA and DSA keys longer than 3072 + * bits provide more than 128 bits of security strength. + * So, AES 256 should be used to provide comparable + * security. */ + cipher_abilities[aes256_mapi]++; + cipher_votes[aes256_mapi] += pref; + pref--; + } + if (pklen_bits > 1023) { + /* RFC 5751 mandates support for AES 128, but also says + * that RSA and DSA signature keys SHOULD NOT be less than + * 1024 bits. So, cast vote for AES 128 if key length + * is at least 1024 bits. */ + cipher_abilities[aes128_mapi]++; + cipher_votes[aes128_mapi] += pref; + pref--; + } if (pklen_bits > 512) { /* cast votes for the strong algorithm */ cipher_abilities[strong_mapi]++;