TLS Session Resumption
----------------------
-TLS Session Resumption for TLS 1.2 and TLS1.3 connections can be used (defined
+TLS Session Resumption for TLS 1.2 and TLS 1.3 connections can be used (defined
in RFC 5077 for 1.2). The support for this can be included by building with
-EXPERIMENTAL_TLS_RESUME defined.
+EXPERIMENTAL_TLS_RESUME defined. This requires GnuTLS 3.6.3 or OpenSSL 1.1.1
+(or later).
Session resumption (this is the "stateless" variant) involves the server sending
a "session ticket" to the client on one connection, which can be stored by the
Operational cost/benefit:
The extra data being transmitted costs a minor amount, and the client has
-extra costs in storing and retrieving the data.
+ extra costs in storing and retrieving the data.
-In the Exim/Gnutls implementation the extra cost on an initial connection
-which is TLS1.2 over a loopback path is about 6ms on 2017-laptop class hardware.
-The saved cost on a subsequent connection is about 4ms; three or more
-connections become a net win. On longer network paths, two or more
-connections will have an average lower startup time thanks to the one
-saved packet roundtrip. TLS1.3 will save the crypto cpu costs but not any
-packet roundtrips.
+ In the Exim/Gnutls implementation the extra cost on an initial connection
+ which is TLS1.2 over a loopback path is about 6ms on 2017-laptop class hardware.
+ The saved cost on a subsequent connection is about 4ms; three or more
+ connections become a net win. On longer network paths, two or more
+ connections will have an average lower startup time thanks to the one
+ saved packet roundtrip. TLS1.3 will save the crypto cpu costs but not any
+ packet roundtrips.
+
+ Since a new hints DB is used, the hints DB maintenance should be updated
+ to additionally handle "tls".
Security aspects:
The session ticket is encrypted, but is obviously an additional security
-vulnarability surface. An attacker able to decrypt it would have access
-all connections using the resumed session.
-The session ticket encryption key is not committed to storage by the server
-and is rotated regularly. Tickets have limited lifetime.
+ vulnarability surface. An attacker able to decrypt it would have access
+ all connections using the resumed session.
+ The session ticket encryption key is not committed to storage by the server
+ and is rotated regularly. Tickets have limited lifetime.
-There is a question-mark over the security of the Diffie-Helman parameters
-used for session negotiation. TBD. q-value; cf bug 1895
+ There is a question-mark over the security of the Diffie-Helman parameters
+ used for session negotiation. TBD. q-value; cf bug 1895
Observability:
New log_selector "tls_resumption", appends an asterisk to the tls_cipher "X="
-element.
-
-Variables $tls_{in,out}_resumption have bit 0-4 indicating respectively
-support built, client requested ticket, client offered session,
-server issued ticket, resume used. A suitable decode list is provided
-in the builtin macro _RESUME_DECODE for ${listextract {}{}}.
+ element.
+
+ Variables $tls_{in,out}_resumption have bits 0-4 indicating respectively
+ support built, client requested ticket, client offered session,
+ server issued ticket, resume used. A suitable decode list is provided
+ in the builtin macro _RESUME_DECODE for ${listextract {}{}}.
+
+Issues:
+ In a resumed session:
+ $tls_{in,out}_certificate_verified will be set, and verify = certificate
+ will be true, when verify failed but tls_try_verify_hosts allowed the
+ connection (under OpenSSL)
+ $tls_{in,out}_cipher will have values different to the original (under GnuTLS)
+ $tls_{in,out}_ocsp will be "not requested" or "no response"
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git://git.exim.org / exim.git / blobdiff