X-Git-Url: https://git.exim.org/users/heiko/exim.git/blobdiff_plain/a799883d8ad340d935db4d729a31c02cb8a1d977..5d96613b0938fe5447159cc1e3971a633c1c2d65:/doc/doc-txt/GnuTLS-FAQ.txt diff --git a/doc/doc-txt/GnuTLS-FAQ.txt b/doc/doc-txt/GnuTLS-FAQ.txt index 8d5887bac..ab4e5aaa6 100644 --- a/doc/doc-txt/GnuTLS-FAQ.txt +++ b/doc/doc-txt/GnuTLS-FAQ.txt @@ -6,7 +6,7 @@ Using Exim 4.80+ with GnuTLS (3) I'm seeing: "(gnutls_handshake): A TLS packet with unexpected length was received" Why? -(4) What's the deal with MD5? +(4) What's the deal with MD5? (And SHA-1?) (5) What happened to gnutls_require_kx / gnutls_require_mac / gnutls_require_protocols? (6) What's the deal with tls_dh_max_bits? What's DH? @@ -89,8 +89,8 @@ option fixes the problem, this was the cause. See Q6. -(4): What's the deal with MD5? ------------------------------- +(4): What's the deal with MD5? (And SHA-1?) +-------------------------------------------- MD5 is a hash algorithm. Hash algorithms are used to reduce a lot of data down to a fairly short value, which is supposed to be extremely hard to @@ -103,9 +103,9 @@ signed. MD5 was once very popular. It still is far too popular. Real world attacks have been proven possible against MD5. Including an attack against PKI (Public Key Infrastructure) certificates used for SSL/TLS. In that attack, -the attackers got a certificate for one identity but we able to then public a -certificate with the same signature but a different identity. This undermines -the whole purpose of having certificates. +the attackers got a certificate for one identity but were able to then publish +a certificate with the same signature but a different identity. This +undermines the whole purpose of having certificates. So GnuTLS stopped trusting any certificate with an MD5-based hash used in it. The world has been hurriedly moving away from MD5 in certificates for a while. @@ -119,6 +119,10 @@ the ongoing costs of proving a trust relationship, such as providing revocation protocols. This is just another of those ongoing costs you have already paid for. +The same has happened to SHA-1: there are real-world collision attacks against +SHA-1, so SHA-1 is mostly defunct in certificates. GnuTLS no longer supports +its use in TLS certificates. + (5): ... gnutls_require_kx / gnutls_require_mac / gnutls_require_protocols? @@ -150,7 +154,7 @@ unset, so that you get one of the standard built-in primes used for DH. DH, Diffie-Hellman (or Diffie-Hellman-Merkle, or something naming Williamson) is the common name for a way for two parties to a communication stream to exchange some private random data so that both end up with a shared secret -which no evesdropper can get. It does not provide for proof of the identity +which no eavesdropper can get. It does not provide for proof of the identity of either party, so on its own is subject to man-in-the-middle attacks, but is often combined with systems which do provide such proof, improving them by separating the session key (the shared secret) from the long-term identity, @@ -159,7 +163,7 @@ and so protecting past communications from a break of the long-term identity. To do this, the server sends to the client a very large prime number; this is in the clear, an attacker can see it. This is not a problem; it's so not a problem, that there are standard named primes which applications can use, and -which a future release of Exim will probably support. +which Exim now supports. The size of the prime number affects how difficult it is to break apart the shared secret and decrypt the data. As time passes, the size required to @@ -177,13 +181,14 @@ such matters than the Exim folks, we just say "er, what they said". One of the new pieces of the GnuTLS API is a means for an application to ask it for guidance and advice on how large some numbers should be. This is not -entirely internal to GnuTLS since generating the numbers is slow, an +entirely internal to GnuTLS, since generating the numbers is slow, an application might want to use a standard prime, etc. So, in an attempt to get away from being involved in cryptographic policy, and to get rid of a hard-coded "1024" in Exim's source-code, we switched to asking GnuTLS how many -bits should be in the prime number generated for use for Diffie-Hellman. To -give back to GnuTLS for use We can ask for various sizes, and did not expose -this to the administrator but instead just asked for "NORMAL" protection. +bits should be in the prime number generated for use for Diffie-Hellman. We +then give this number straight back to GnuTLS when generating a DH prime. +We can ask for various sizes, and did not expose this to the administrator but +instead just asked for "NORMAL" protection. Literally: dh_bits = gnutls_sec_param_to_pk_bits(GNUTLS_PK_DH, GNUTLS_SEC_PARAM_NORMAL); @@ -299,7 +304,7 @@ NORMAL.) See Q8. The current documentation, for the most recent release of GnuTLS, is available online at: - http://www.gnu.org/software/gnutls/manual/html_node/Priority-Strings.html + http://www.gnutls.org/manual/html_node/Priority-Strings.html Beware that if you are not using the most recent GnuTLS release then this documentation will be wrong for you! You should find the "info" documentation