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Network Working Group                                        J. Mattsson
Internet-Draft                                                  Ericsson
Intended status: Informational                          October 27, 2014
Expires: April 30, 2015


            Overview and Analysis of Overhead Caused by TLS
                   draft-mattsson-uta-tls-overhead-01

Abstract

   A common argument against the use of TLS is that it adds overhead.
   In this document we illustrate in detail how much (or little)
   processing, latency, and traffic overhead TLS adds.  Transition to
   more secure cipher suites (TLS 1.2 with AES-GCM or ChaCha20-Poly1305)
   actually reduces both traffic and processing overhead.  AES-GCM
   combines security, low traffic overhead, and great performance on
   modern hardware.  On platforms without hardware support for AES-GCM,
   ChaCha20-Poly1305 gives the same benefits.  For everything but very
   short connections, TLS is not inducing any major traffic overhead
   (nor CPU or memory overhead).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on April 30, 2015.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  TLS Handshake . . . . . . . . . . . . . . . . . . . . . . . .   2
     2.1.  Latency Overhead  . . . . . . . . . . . . . . . . . . . .   3
   3.  TLS Record Layer  . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Ciphers in Use  . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Traffic Overhead  . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Processing Overhead . . . . . . . . . . . . . . . . . . .   6
       3.3.1.  Modern x86 Processors . . . . . . . . . . . . . . . .   7
       3.3.2.  Software  . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The overhead from TLS can be divided into several different aspects:

   o  Traffic overhead from TLS handshake

   o  Latency overhead from TLS handshake

   o  Traffic overhead from TLS record layer

   o  Processing overhead from TLS handshake

   o  Processing overhead from TLS record layer

   But in many scenarios, TLS does not add much overhead at all, and
   moving to more secure cipher suites actually reduces both traffic and
   processing overhead.

2.  TLS Handshake

   The TLS handshake typically adds 4-7 kB of traffic overhead.  TLS
   compression reduces traffic overhead, but has negative security
   implications and should be turned off [I-D.ietf-uta-tls-bcp].





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   Looking at the certificates, a move from 1024 to 2048 bit RSA keys
   increases traffic and processing overhead but is needed for security
   reasons.  Certificates with 1024 bit RSA keys should be phased out as
   they only gives 80 bit security.  NIST recommendation is to stop
   using algorithms giving 80 bit security no later that 2010
   [KeyLength].  A move from SHA-1 to SHA-256 adds processing overhead
   but is needed for security reasons.  SHA-1 should not be used anymore
   for digital signatures (e.g. in certificates) as it gives less than
   80 bit security.  To summarize, SHA-2 certificates with at least 2048
   bit RSA keys should be used.

2.1.  Latency Overhead

   In TLS 1.2 [RFC5246] and earlier versions, the initial handshake
   takes 2 round-trips and session resumption takes 1 round-trip.  In
   TLS 1.3 [I-D.ietf-tls-tls13] the target is 1 round-trip for the
   initial round-trip and 0 round-trips for session resumption.  Because
   of the emphasis on reducing latency (instead of only security), TLS
   1.3 is expected to have much faster deployment than earlier versions.

3.  TLS Record Layer

   Some of the most commonly used ciphersuites have security weaknesses.
   Encryption algorithms such as RC4 and the CBC modes (e.g.  AES and
   3DES_EDE) have security weaknesses, and the hash functions SHA-1 and
   MD5 (but not the HMAC constructions used in TLS record layer) also
   have security weaknesses.

   More recent ciphersuites using AES-GCM and CHACHA20_POLY1305 have no
   known security weaknesses, but AES-GCM, CHACHA20_POLY1305 and other
   AEAD suites requires TLS 1.2 [RFC5246].  CHACHA20_POLY1305 is
   currently only an Internet draft but is still used in practice as it
   is very fast in software [I-D.agl-tls-chacha20poly1305].  AES-GCM is
   the current IETF recommendation (Internet Draft) as part of
   TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 [I-D.ietf-uta-tls-bcp].

3.1.  Ciphers in Use

   These data is included as to motivate which algorithms to cover as
   well as showing that new secure ciphersuites are gaining signifiacant
   usage.  In data from July 2014 [ICSI], AES-CBC, RC4, and HMAC-SHA1
   dominates, these ciphersuites all have security weaknesses.  The NULL
   cipher does not provide any confidentiality at all.  The more secure
   options AES-GCM and ChaCha20-Poly1305 are starting to showing
   significant usage. 3DES_EDE_CBC_SHA is includes as it is mandatory to
   implement in TLS 1.0.





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   Algorithm                                            Usage
   ----------------------------------------------------------
   AES_128_CBC_SHA                                     29.1 %
   RC4_128_SHA                                         17.4 %
   AES_128_GCM                                         14.7 %
   AES_256_CBC_SHA                                     14.0 %
   NULL_SHA                                             9.8 %
   RC4_128_MD5                                          8.3 %
   CHACHA20_POLY1305                                    1.4 %
   3DES_EDE_CBC_SHA                                   < 5.6 %
   ----------------------------------------------------------

                 Table 1: Ciphers in Use (ICSI, July 2014)

3.2.  Traffic Overhead

   The traffic overhead comes in different forms: the TLS record layer
   header, Explicit IV/Nonce, MAC tag, and encryption algorithm padding.
   Figure 1 illustrates the packet format for a TLS protected package
   where [ ] indicates fields where usage depends on the TLS version and
   the ciphersuite used.

 +----+-----+------------+------------+--------------+-----+-----------+
 | IP | TCP | TLS Header | [IV/Nonce] | Enc. Content | MAC | [Padding] |
 +----+-----+------------+------------+--------------+-----+-----------+

                 Figure 1: Format of TLS protected packet

   The size of the TLS header is fixed (5 bytes).  The size of the IV/
   Nonce depends on the TLS version and the ciphersuite used.  Explicit
   IV is used by CBC ciphersuites in TLS 1.1 and TLS 1.2, but not TLS
   1.0.  Explicit Nonce is used by AEAD algorithms.  The size of the MAC
   tag depends on the ciphersuite used, it is either a separate field
   (non-AEAD algorithms like SHA-1, MD5) or included in the ciphertext
   (AEAD algorithms like GCM, Poly1305).  Padding is used by CBC
   ciphersuites.

   The per-packet overhead for the most important ciphersuites are shown
   below (the values are all theoretical and the averages are calculates
   over a uniform distribution).  For comparision, the TCP/IP overhead
   for IPv4 and IPv6 are 52 and 72 bytes, respectively.










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   -----------------------------------------------------------------
   AES_128_CBC_SHA, AES_256_CBC_SHA
   -----------------------------------------------------------------
   Per-packet overhead (TLS 1.0)             26-41 bytes (avg. 33.5)
      TLS header                                             5 bytes
      HMAC-SHA-1                                            20 bytes
      CBC padding                                         1-16 bytes

   Per-packet overhead (TLS 1.1, 1.2)        42-57 bytes (avg. 49.5)
      TLS header                                             5 bytes
      Explicit IV                                           16 bytes
      HMAC-SHA-1                                            20 bytes
      CBC padding                                         1-16 bytes
   -----------------------------------------------------------------


   -----------------------------------------------------------------
   3DES_EDE_CBC_SHA
   -----------------------------------------------------------------
   Per-packet overhead (TLS 1.0)             26-33 bytes (avg. 29.5)
      TLS header                                             5 bytes
      HMAC-SHA-1                                            20 bytes
      CBC padding                                          1-8 bytes

   Per-packet overhead (TLS 1.1, 1.2)        34-41 bytes (avg. 37.5)
      TLS header                                             5 bytes
      Explicit IV                                            8 bytes
      HMAC-SHA-1                                            20 bytes
      CBC padding                                          1-8 bytes
   -----------------------------------------------------------------


   -----------------------------------------------------------------
   RC4_128_SHA, NULL_SHA
   -----------------------------------------------------------------
   Per-packet overhead (TLS 1.0, 1.1, 1.2)                  25 bytes
      TLS header                                             5 bytes
      HMAC-SHA-1                                            20 bytes
   -----------------------------------------------------------------












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   -----------------------------------------------------------------
   RC4_128_MD5
   -----------------------------------------------------------------
   Per-packet overhead (TLS 1.0, 1.1, 1.2)                  21 bytes
      TLS header                                             5 bytes
      HMAC-MD5                                              16 bytes
   -----------------------------------------------------------------


   -----------------------------------------------------------------
   AES_128_GCM, AES_256_GCM
   -----------------------------------------------------------------
   Per-packet overhead (TLS 1.0, 1.1, 1.2)                  29 bytes
      TLS header                                             5 bytes
      Explicit Nonce                                         8 bytes
      GMAC                                                  16 bytes
   -----------------------------------------------------------------


   -----------------------------------------------------------------
   CHACHA20_POLY1305
   -----------------------------------------------------------------
   Per-packet overhead (TLS 1.0, 1.1, 1.2)                  29 bytes
      TLS header                                             5 bytes
      Explicit Nonce                                         8 bytes
      Poly1305                                              16 bytes
   -----------------------------------------------------------------

   As can be seen from the tables above, there is a correlation between
   better security and low traffic overhead.  Going from TLS 1.1
   [RFC4346] with AES_CBC_SHA (mandatory to implement in TLS 1.1) to TLS
   1.2 [RFC5246] with one of the more secure options AES_GCM (current
   IETF recommendation) or CHACHA20_POLY1305 reduces record layer
   traffic overhead with 41 %.  Going from TLS 1.0 [RFC2246] with
   AES_CBC_SHA to TLS 1.2 with AES_GCM or CHACHA20_POLY1305 reduces
   record layer traffic overhead with 14 %.

3.3.  Processing Overhead

   Just as with traffic overhead, there is a correlation between better
   security and low processing overhead.  Going from AES_CBC_SHA
   (mandatory to implement in TLS 1.1. and 1.2) to the more secure
   option AES-GCM reduces processing overhead on a Core-i7-3770
   processor with 57 %. Another fact is that the overhead for
   AES_128_GCM and CHACHA20_POLY1305 is so low, there is no overhead
   reasons to not use encryption (i.e.  NULL_SHA).





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3.3.1.  Modern x86 Processors

   On modern x86 processors with hardware support for AES (AES-NI) and
   carry-less multplication (CLMUL), AES_GCM is much faster that
   RC4_SHA, AES_CBC_SHA, or CHACHA20_POLY1305.  Another performance
   advantage with AES-GCM is that is designed for parallelization.

   Algorithm                                            Speed
   ----------------------------------------------------------
   AES_128_GCM                                    1909.1 MB/s
   CHACHA20_POLY1305                               625.2 MB/s
   AES_128_CBC_SHA                                 573.7 MB/s
   AES_256_CBC_SHA                                 486.6 MB/s
   RC4_128_MD5                                     233.9 MB/s
   ----------------------------------------------------------

                   Table 2: Speed on 2 GHz Intel Core i7

   These measuments are not fair to Chacha20-Poly1305, but this does not
   matter, the important thing is who fast does the algorithms run on
   current hardwware.

3.3.2.  Software

   Without hardware support for AES-GCM, ChaCha20-Poly1305 is much
   faster that AES-GCM (and AES-CBC).  Data from [Software].

   Algorithm                                            Speed
   ----------------------------------------------------------
   CHACHA20_POLY1305                               130.9 MB/s
   AES_128_GCM                                      41.5 MB/s
   ----------------------------------------------------------

                    Table 3: Speed on Snapdragon S4 Pro

   Several companies have deployed ChaCha20-Poly1305 to get better
   performance (and security) on platforms without AES and CLMUL
   hardware support.  This may have less significance in the future if
   mobile CPUs implement hardware support for AES-GCM.

4.  Conclusions

   Transition to more secure cipher suites (TLS 1.2 with AES-GCM or
   ChaCha20-Poly1305) actually reduces both traffic and processing
   overhead.  Going from TLS 1.1 with AES_CBC_SHA (mandatory to
   implement in TLS 1.1) to TLS 1.2 with AES_GCM (current IETF
   recommendation) or CHACHA20_POLY1305 reduces record layer traffic
   overhead with 41 %, and record layer processing overhead with even



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   more.  AES-GCM combines security, low traffic overhead, and great
   performance on modern x86 hardware.  On platforms without hardware
   support for AES-GCM, ChaCha20-Poly1305 gives the same benefits.

   Looking at the certificates, a transition to SHA-2 certificates with
   RSA-2048 keys increases TLS handshake traffic and processing overhead
   but is needed for security reasons.

   For everything but very short connections, TLS is not inducing any
   major traffic overhead (nor CPU or memory overhead).  Server people
   from Google Gmail has stated that "TLS accounts for less than 1% of
   the CPU load, less than 10 KB of memory per connection and less than
   2% of network overhead".  Main impact of TLS is increased latency,
   this can by reduced by using session resumption, cache information
   closer to end users, or waiting for TLS 1.3.

5.  Security Considerations

   The whole document is about increasing the use of TLS and secure
   ciphersuites by showing that TLS is many cases does not add much
   overhead, and that there for many types of overhead is a correlation
   between better security and low overhead.

6.  Acknowledgements

   The authors would like to thank Stephen Farrell and Ivan Ristic for
   their valuable comments and feedback.

7.  References

   [Gueron]   Shay Gueron, "AES-GCM for Efficient Authenticated
              Encryption - Ending the Reign of HMAC-SHA-1?",
              <https://crypto.stanford.edu/RealWorldCrypto/slides/
              gueron.pdf>.

   [I-D.agl-tls-chacha20poly1305]
              Langley, A. and W. Chang, "ChaCha20 and Poly1305 based
              Cipher Suites for TLS", draft-agl-tls-chacha20poly1305-04
              (work in progress), November 2013.

   [I-D.ietf-tls-tls13]
              Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.3", draft-ietf-tls-tls13-02 (work
              in progress), July 2014.







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   [I-D.ietf-uta-tls-bcp]
              Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of TLS and DTLS", draft-
              ietf-uta-tls-bcp-06 (work in progress), October 2014.

   [ICSI]     ICSI, "The ICSI Certificate Notary",
              <http://notary.icsi.berkeley.edu/#statistics>.

   [KeyLength]
              BlueKrypt, "Cryptographic Key Length Recommendation",
              <http://www.keylength.com/>.

   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [Software]
              ImperialViolet, "TLS Symmetric Crypto",
              <https://www.imperialviolet.org/2014/02/27/
              tlssymmetriccrypto.html>.

Author's Address

   John Mattsson
   Ericsson AB
   SE-164 80 Stockholm
   Sweden

   Phone: +46 10 71 43 501
   Email: john.mattsson@ericsson.com
















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