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diff --git a/doc/gnutls.texi b/doc/gnutls.texi index 817806923b..0cfdd44cb1 100644 --- a/doc/gnutls.texi +++ b/doc/gnutls.texi @@ -566,6 +566,7 @@ differences of these protocols are minor. Older protocols such as * TLS Extensions:: * Selecting cryptographic key sizes:: * On SSL 2 and older protocols:: +* On Record Padding:: @end menu @node TLS layers @@ -1115,6 +1116,55 @@ Other protocols such as Microsoft's @acronym{PCT} 1 and @acronym{PCT} 2 were not implemented because they were also abandoned and deprecated by @acronym{SSL} 3.0 and later @acronym{TLS} 1.0. +@node On Record Padding +@section On Record Padding +@cindex Record padding +@cindex Bad record MAC + +The TLS protocol allows for random padding of records, to make it more +difficult to perform analysis on the length of exchanged messages. +(In RFC 4346 this is specified in section 6.2.3.2.) GnuTLS appears to +be one of few implementation that take advantage of this text, and pad +records by a random length. + +The TLS implementation in the Symbian operating system, frequently +used by Nokia and Sony-Ericsson mobile phones, cannot handle +non-minimal record padding. What happens when one of these clients +handshake with a GnuTLS server is that the client will fail to compute +the correct MAC for the record. The client sends a TLS alert +(@code{bad_record_mac}) and disconnects. Typically this will result +in error messages such as 'A TLS fatal alert has been received', 'Bad +record MAC', or both, on the GnuTLS server side. + +GnuTLS implements a work around for this problem. However, it has to +be enabled specifically. It can be enabled by using +@ref{gnutls_record_disable_padding}, or @ref{gnutls_priority_set} with +the @code{%COMPAT} priority string. + +If you implement an application that have a configuration file, we +recommend that you make it possible for users or administrators to +specify a GnuTLS protocol priority string, which is used by your +application via @ref{gnutls_priority_set}. To allow the best +flexibility, make it possible to have a different priority string for +different incoming IP addresses. + +To enable the workaround in the @code{gnutls-cli} client or the +@code{gnutls-serv} server, for testing of other implementations, use +the following parameter: @code{--priority "%COMPAT"}. + +This problem has been discussed on mailing lists and in bug reports. +This section tries to collect all pieces of information that we know +about the problem. If you wish to go back to the old discussions, +here are some links: + +@url{http://bugs.debian.org/390712} + +@url{http://bugs.debian.org/402861} + +@url{http://bugs.debian.org/438137} + +@url{http://thread.gmane.org/gmane.ietf.tls/3079} + @node Authentication methods @chapter Authentication Methods |