< draft-ietf-uta-tls-bcp-08.txt   draft-ietf-uta-tls-bcp-09.txt >
UTA Y. Sheffer UTA Y. Sheffer
Internet-Draft Porticor Internet-Draft Porticor
Intended status: Best Current Practice R. Holz Intended status: Best Current Practice R. Holz
Expires: June 10, 2015 TUM Expires: August 15, 2015 TUM
P. Saint-Andre P. Saint-Andre
&yet &yet
December 7, 2014 February 11, 2015
Recommendations for Secure Use of TLS and DTLS Recommendations for Secure Use of TLS and DTLS
draft-ietf-uta-tls-bcp-08 draft-ietf-uta-tls-bcp-09
Abstract Abstract
Transport Layer Security (TLS) and Datagram Transport Layer Security Transport Layer Security (TLS) and Datagram Transport Layer Security
(DTLS) are widely used to protect data exchanged over application (DTLS) are widely used to protect data exchanged over application
protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the
last few years, several serious attacks on TLS have emerged, last few years, several serious attacks on TLS have emerged,
including attacks on its most commonly used cipher suites and modes including attacks on its most commonly used cipher suites and modes
of operation. This document provides recommendations for improving of operation. This document provides recommendations for improving
the security of deployed services that use TLS and DTLS. The the security of deployed services that use TLS and DTLS. The
skipping to change at page 1, line 40 skipping to change at page 1, line 40
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 10, 2015. This Internet-Draft will expire on August 15, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
skipping to change at page 2, line 27 skipping to change at page 2, line 27
3.1.1. SSL/TLS Protocol Versions . . . . . . . . . . . . . . 4 3.1.1. SSL/TLS Protocol Versions . . . . . . . . . . . . . . 4
3.1.2. DTLS Protocol Versions . . . . . . . . . . . . . . . 5 3.1.2. DTLS Protocol Versions . . . . . . . . . . . . . . . 5
3.1.3. Fallback to Lower Versions . . . . . . . . . . . . . 6 3.1.3. Fallback to Lower Versions . . . . . . . . . . . . . 6
3.2. Strict TLS . . . . . . . . . . . . . . . . . . . . . . . 6 3.2. Strict TLS . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Compression . . . . . . . . . . . . . . . . . . . . . . . 7 3.3. Compression . . . . . . . . . . . . . . . . . . . . . . . 7
3.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 7 3.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 7
3.5. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . 7 3.5. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . 7
3.6. Server Name Indication . . . . . . . . . . . . . . . . . 8 3.6. Server Name Indication . . . . . . . . . . . . . . . . . 8
4. Recommendations: Cipher Suites . . . . . . . . . . . . . . . 8 4. Recommendations: Cipher Suites . . . . . . . . . . . . . . . 8
4.1. General Guidelines . . . . . . . . . . . . . . . . . . . 8 4.1. General Guidelines . . . . . . . . . . . . . . . . . . . 8
4.2. Recommended Cipher Suites . . . . . . . . . . . . . . . . 9 4.2. Recommended Cipher Suites . . . . . . . . . . . . . . . . 10
4.2.1. Implementation Details . . . . . . . . . . . . . . . 10 4.2.1. Implementation Details . . . . . . . . . . . . . . . 10
4.3. Public Key Length . . . . . . . . . . . . . . . . . . . . 11 4.3. Public Key Length . . . . . . . . . . . . . . . . . . . . 11
4.4. Modular vs. Elliptic Curve DH Cipher Suites . . . . . . . 11 4.4. Modular vs. Elliptic Curve DH Cipher Suites . . . . . . . 12
4.5. Truncated HMAC . . . . . . . . . . . . . . . . . . . . . 12 4.5. Truncated HMAC . . . . . . . . . . . . . . . . . . . . . 13
5. Applicability Statement . . . . . . . . . . . . . . . . . . . 13 5. Applicability Statement . . . . . . . . . . . . . . . . . . . 13
5.1. Security Services . . . . . . . . . . . . . . . . . . . . 13 5.1. Security Services . . . . . . . . . . . . . . . . . . . . 13
5.2. Unauthenticated TLS and Opportunistic Security . . . . . 14 5.2. Unauthenticated TLS and Opportunistic Security . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15 7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
7.1. Host Name Validation . . . . . . . . . . . . . . . . . . 15 7.1. Host Name Validation . . . . . . . . . . . . . . . . . . 15
7.2. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 16 7.2. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.3. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 16 7.3. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 16
7.4. Diffie-Hellman Exponent Reuse . . . . . . . . . . . . . . 17 7.4. Diffie-Hellman Exponent Reuse . . . . . . . . . . . . . . 17
7.5. Certificate Revocation . . . . . . . . . . . . . . . . . 17 7.5. Certificate Revocation . . . . . . . . . . . . . . . . . 18
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . 19 9.1. Normative References . . . . . . . . . . . . . . . . . . 19
9.2. Informative References . . . . . . . . . . . . . . . . . 20 9.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 23 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 24
A.1. draft-ietf-uta-tls-bcp-08 . . . . . . . . . . . . . . . . 23 A.1. draft-ietf-uta-tls-bcp-08 . . . . . . . . . . . . . . . . 24
A.2. draft-ietf-uta-tls-bcp-07 . . . . . . . . . . . . . . . . 23 A.2. draft-ietf-uta-tls-bcp-07 . . . . . . . . . . . . . . . . 24
A.3. draft-ietf-uta-tls-bcp-06 . . . . . . . . . . . . . . . . 23 A.3. draft-ietf-uta-tls-bcp-06 . . . . . . . . . . . . . . . . 24
A.4. draft-ietf-uta-tls-bcp-05 . . . . . . . . . . . . . . . . 23 A.4. draft-ietf-uta-tls-bcp-05 . . . . . . . . . . . . . . . . 24
A.5. draft-ietf-uta-tls-bcp-04 . . . . . . . . . . . . . . . . 23 A.5. draft-ietf-uta-tls-bcp-04 . . . . . . . . . . . . . . . . 24
A.6. draft-ietf-uta-tls-bcp-03 . . . . . . . . . . . . . . . . 23 A.6. draft-ietf-uta-tls-bcp-03 . . . . . . . . . . . . . . . . 24
A.7. draft-ietf-uta-tls-bcp-02 . . . . . . . . . . . . . . . . 24 A.7. draft-ietf-uta-tls-bcp-02 . . . . . . . . . . . . . . . . 25
A.8. draft-ietf-tls-bcp-01 . . . . . . . . . . . . . . . . . . 24 A.8. draft-ietf-tls-bcp-01 . . . . . . . . . . . . . . . . . . 25
A.9. draft-ietf-tls-bcp-00 . . . . . . . . . . . . . . . . . . 24 A.9. draft-ietf-tls-bcp-00 . . . . . . . . . . . . . . . . . . 25
A.10. draft-sheffer-tls-bcp-02 . . . . . . . . . . . . . . . . 25 A.10. draft-sheffer-tls-bcp-02 . . . . . . . . . . . . . . . . 25
A.11. draft-sheffer-tls-bcp-01 . . . . . . . . . . . . . . . . 25 A.11. draft-sheffer-tls-bcp-01 . . . . . . . . . . . . . . . . 26
A.12. draft-sheffer-tls-bcp-00 . . . . . . . . . . . . . . . . 25 A.12. draft-sheffer-tls-bcp-00 . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction 1. Introduction
Transport Layer Security (TLS) [RFC5246] and Datagram Transport Transport Layer Security (TLS) [RFC5246] and Datagram Transport
Security Layer (DTLS) [RFC6347] are widely used to protect data Security Layer (DTLS) [RFC6347] are widely used to protect data
exchanged over application protocols such as HTTP, SMTP, IMAP, POP, exchanged over application protocols such as HTTP, SMTP, IMAP, POP,
SIP, and XMPP. Over the last few years, several serious attacks on SIP, and XMPP. Over the last few years, several serious attacks on
TLS have emerged, including attacks on its most commonly used cipher TLS have emerged, including attacks on its most commonly used cipher
suites and modes of operation. For instance, both the AES-CBC suites and modes of operation. For instance, both the AES-CBC
[RFC3602] and RC4 [I-D.ietf-tls-prohibiting-rc4] encryption [RFC3602] and RC4 [I-D.ietf-tls-prohibiting-rc4] encryption
algorithms, which together are the most widely deployed ciphers, have algorithms, which together are the most widely deployed ciphers, have
been attacked in the context of TLS. A companion document been attacked in the context of TLS. A companion document [RFC7457]
[I-D.ietf-uta-tls-attacks] provides detailed information about these provides detailed information about these attacks.
attacks.
Because of these attacks, those who implement and deploy TLS and DTLS Because of these attacks, those who implement and deploy TLS and DTLS
need updated guidance on how TLS can be used securely. This document need updated guidance on how TLS can be used securely. This document
provides guidance for deployed services as well as for software provides guidance for deployed services as well as for software
implementations, assuming the implementer expects his or her code to implementations, assuming the implementer expects his or her code to
be deployed in environments defined in the following section. In be deployed in environments defined in the following section. In
fact, this document calls for the deployment of algorithms that are fact, this document calls for the deployment of algorithms that are
widely implemented but not yet widely deployed. Concerning widely implemented but not yet widely deployed. Concerning
deployment, this document targets a wide audience, namely all deployment, this document targets a wide audience, namely all
deployers who wish to add authentication (be it one-way only or deployers who wish to add authentication (be it one-way only or
skipping to change at page 5, line 33 skipping to change at page 5, line 33
suites. suites.
o Implementations MUST support TLS 1.2 [RFC5246] and MUST prefer to o Implementations MUST support TLS 1.2 [RFC5246] and MUST prefer to
negotiate TLS version 1.2 over earlier versions of TLS. negotiate TLS version 1.2 over earlier versions of TLS.
Rationale: Several stronger cipher suites are available only with Rationale: Several stronger cipher suites are available only with
TLS 1.2 (published in 2008). In fact, the cipher suites TLS 1.2 (published in 2008). In fact, the cipher suites
recommended by this document (Section 4.2 below) are only recommended by this document (Section 4.2 below) are only
available in TLS 1.2. available in TLS 1.2.
This BCP applies to TLS 1.2. It is not safe for readers to assume This BCP applies to TLS 1.2, and also to earlier versions. It is not
that the recommendations in this BCP apply to any future version of safe for readers to assume that the recommendations in this BCP apply
TLS. to any future version of TLS.
3.1.2. DTLS Protocol Versions 3.1.2. DTLS Protocol Versions
DTLS, an adaptation of TLS for UDP datagrams, was introduced when TLS DTLS, an adaptation of TLS for UDP datagrams, was introduced when TLS
1.1 was published. The following are the recommendations with 1.1 was published. The following are the recommendations with
respect to DTLS: respect to DTLS:
o Implementations MAY negotiate DTLS version 1.0 [RFC4347]. o Implementations SHOULD NOT negotiate DTLS version 1.0 [RFC4347].
Version 1.0 of DTLS correlates to version 1.1 of TLS (see above). Version 1.0 of DTLS correlates to version 1.1 of TLS (see above).
o Implementations MUST support, and prefer to negotiate, DTLS o Implementations MUST support, and prefer to negotiate, DTLS
version 1.2 [RFC6347]. version 1.2 [RFC6347].
Version 1.2 of DTLS correlates to Version 1.2 of TLS 1.2 (see Version 1.2 of DTLS correlates to Version 1.2 of TLS 1.2 (see
above). (There is no Version 1.1 of DTLS.) above). (There is no Version 1.1 of DTLS.)
3.1.3. Fallback to Lower Versions 3.1.3. Fallback to Lower Versions
Clients that "fall back" to lower versions of the protocol after the Clients that "fall back" to lower versions of the protocol after the
server rejects higher versions of the protocol MUST NOT fall back to server rejects higher versions of the protocol MUST NOT fall back to
SSLv3. SSLv3 or earlier.
Rationale: Some client implementations revert to lower versions of Rationale: Some client implementations revert to lower versions of
TLS or even to SSLv3 if the server rejected higher versions of the TLS or even to SSLv3 if the server rejected higher versions of the
protocol. This fallback can be forced by a man in the middle (MITM) protocol. This fallback can be forced by a man in the middle (MITM)
attacker. TLS 1.0 and SSLv3 are significantly less secure than TLS attacker. TLS 1.0 and SSLv3 are significantly less secure than TLS
1.2, the version recommended by this document. While TLS 1.0-only 1.2, the version recommended by this document. While TLS 1.0-only
servers are still quite common, IP scans show that SSLv3-only servers servers are still quite common, IP scans show that SSLv3-only servers
amount to only about 3% of the current Web server population. (At amount to only about 3% of the current Web server population. (At
the time of this writing, an explicit method for preventing downgrade the time of this writing, an explicit method for preventing downgrade
attacks is being defined in [I-D.ietf-tls-downgrade-scsv].) attacks is being defined in [I-D.ietf-tls-downgrade-scsv].)
skipping to change at page 6, line 31 skipping to change at page 6, line 31
3.2. Strict TLS 3.2. Strict TLS
To prevent SSL Stripping: To prevent SSL Stripping:
o In cases where an application protocol allows implementations or o In cases where an application protocol allows implementations or
deployments a choice between strict TLS configuration and dynamic deployments a choice between strict TLS configuration and dynamic
upgrade from unencrypted to TLS-protected traffic (such as upgrade from unencrypted to TLS-protected traffic (such as
STARTTLS), clients and servers SHOULD prefer strict TLS STARTTLS), clients and servers SHOULD prefer strict TLS
configuration. configuration.
o In many application protocols, clients can be configured to use o Application protocols typically provide a way for the server to
TLS no matter whether the server offers TLS during a protocol offer TLS during an initial protocol exchange, and sometimes also
exchange or advertises support for TLS (e.g., through a flag provide a way for the server to advertise support for TLS (e.g.,
indicating that TLS is required). Application clients SHOULD use through a flag indicating that TLS is required); unfortunately,
TLS by default, and disable this default only through explicit these indications are sent before the communication channel is
configuration by the user. encrypted. A client SHOULD attempt to negotiate TLS even if these
indications are not communicated by the server.
o HTTP client and server implementations MUST support the HTTP o HTTP client and server implementations MUST support the HTTP
Strict Transport Security (HSTS) header [RFC6797], in order to Strict Transport Security (HSTS) header [RFC6797], in order to
allow Web servers to advertise that they are willing to accept allow Web servers to advertise that they are willing to accept
TLS-only clients. TLS-only clients.
o When applicable, Web servers SHOULD use HSTS to indicate that they o When applicable, Web servers SHOULD use HSTS to indicate that they
are willing to accept TLS-only clients. are willing to accept TLS-only clients.
Rationale: Combining unprotected and TLS-protected communication Rationale: Combining unprotected and TLS-protected communication
skipping to change at page 7, line 17 skipping to change at page 7, line 17
Implementations and deployments SHOULD disable TLS-level compression Implementations and deployments SHOULD disable TLS-level compression
([RFC5246], Section 6.2.2). ([RFC5246], Section 6.2.2).
Rationale: TLS compression has been subject to security attacks, such Rationale: TLS compression has been subject to security attacks, such
as the CRIME attack. as the CRIME attack.
Implementers should note that compression at higher protocol levels Implementers should note that compression at higher protocol levels
can allow an active attacker to extract cleartext information from can allow an active attacker to extract cleartext information from
the connection. The BREACH attack is one such case. These issues the connection. The BREACH attack is one such case. These issues
can only be mitigated outside of TLS and are thus out of scope of the can only be mitigated outside of TLS and are thus out of scope of the
current document. See Section 2.6 of [I-D.ietf-uta-tls-attacks] for current document. See Section 2.6 of [RFC7457] for further details.
further details.
3.4. TLS Session Resumption 3.4. TLS Session Resumption
If TLS session resumption is used, care ought to be taken to do so If TLS session resumption is used, care ought to be taken to do so
safely. In particular, when using session tickets [RFC5077], the safely. In particular, when using session tickets [RFC5077], the
resumption information MUST be authenticated and encrypted to prevent resumption information MUST be authenticated and encrypted to prevent
modification or eavesdropping by an attacker. Further modification or eavesdropping by an attacker. Further
recommendations apply to session tickets: recommendations apply to session tickets:
o A strong cipher suite MUST be used when encrypting the ticket (as o A strong cipher suite MUST be used when encrypting the ticket (as
skipping to change at page 8, line 5 skipping to change at page 7, line 51
the TLS endpoint (either client or server) and its secrets from the TLS endpoint (either client or server) and its secrets from
reading either past or future communication. The tickets must be reading either past or future communication. The tickets must be
managed so as not to negate this security property. managed so as not to negate this security property.
3.5. TLS Renegotiation 3.5. TLS Renegotiation
Where handshake renegotiation is implemented, both clients and Where handshake renegotiation is implemented, both clients and
servers MUST implement the renegotiation_info extension, as defined servers MUST implement the renegotiation_info extension, as defined
in [RFC5746]. in [RFC5746].
To counter the Triple Handshake attack, we adopt the recommended To counter the Triple Handshake attack, the recommended
countermeasures from [triple-handshake]: TLS clients SHOULD apply the countermeasures from [triple-handshake] are adopted: TLS clients
same validation policy for all certificates received over a SHOULD apply the same validation policy for all certificates received
connection, bind the master secret to the full handshake, and bind over a connection, bind the master secret to the full handshake, and
the abbreviated session resumption handshake to the original full bind the abbreviated session resumption handshake to the original
handshake. In some usages, it may be simplest to refuse any change full handshake. In some usages, the most secure option might be to
of certificates during renegotiation. refuse any change of certificates during renegotiation.
3.6. Server Name Indication 3.6. Server Name Indication
TLS implementations MUST support the Server Name Indication (SNI) TLS implementations MUST support the Server Name Indication (SNI)
extension for those higher level protocols which would benefit from extension for those higher level protocols which would benefit from
it, including HTTPS. However, unlike implementation, the use of SNI it, including HTTPS. However, unlike implementation, the use of SNI
in particular circumstances is a matter of local policy. in particular circumstances is a matter of local policy.
Rationale: SNI supports deployment of multiple TLS-protected virtual Rationale: SNI supports deployment of multiple TLS-protected virtual
servers on a single address, and therefore enables fine-grained servers on a single address, and therefore enables fine-grained
skipping to change at page 8, line 37 skipping to change at page 8, line 34
TLS and its implementations provide considerable flexibility in the TLS and its implementations provide considerable flexibility in the
selection of cipher suites. Unfortunately, some available cipher selection of cipher suites. Unfortunately, some available cipher
suites are insecure, some do not provide the targeted security suites are insecure, some do not provide the targeted security
services, and some no longer provide enough security. Incorrectly services, and some no longer provide enough security. Incorrectly
configuring a server leads to no or reduced security. This section configuring a server leads to no or reduced security. This section
includes recommendations on the selection and negotiation of cipher includes recommendations on the selection and negotiation of cipher
suites. suites.
4.1. General Guidelines 4.1. General Guidelines
Cryptographic algorithms weaken over time as cryptanalysis improves. Cryptographic algorithms weaken over time as cryptanalysis improves:
In other words, as time progresses, algorithms that were once algorithms that were once considered strong become weak. Such
considered strong but are now weak, need to be phased out over time algorithms need to be phased out over time and replaced with more
and replaced with more secure cipher suites to ensure that desired secure cipher suites. This helps to ensure that the desired security
security properties still hold. SSL/TLS has been in existence for properties still hold. SSL/TLS has been in existence for almost 20
almost 20 years at this point and this section provides some much years and many of the cipher suites that have been recommended in
needed recommendations concerning cipher suite selection: various versions of SSL/TLS are now considered weak or at least not
as strong as desired. Therefore this section modernizes the
recommendations concerning cipher suite selection:
o Implementations MUST NOT negotiate the cipher suites with NULL o Implementations MUST NOT negotiate the cipher suites with NULL
encryption. encryption.
Rationale: The NULL cipher suites do not encrypt traffic and so Rationale: The NULL cipher suites do not encrypt traffic and so
provide no confidentiality services. Any entity in the network provide no confidentiality services. Any entity in the network
with access to the connection can view the plaintext of contents with access to the connection can view the plaintext of contents
being exchanged by the client and server. being exchanged by the client and server. (Nevertheless, this
document does not discourage software from implementing NULL
cipher suites, since they can be useful for testing and
debugging.)
o Implementations MUST NOT negotiate RC4 cipher suites. o Implementations MUST NOT negotiate RC4 cipher suites.
Rationale: The RC4 stream cipher has a variety of cryptographic Rationale: The RC4 stream cipher has a variety of cryptographic
weaknesses, as documented in [I-D.ietf-tls-prohibiting-rc4]. We weaknesses, as documented in [I-D.ietf-tls-prohibiting-rc4]. Note
note that this guideline does not apply to DTLS, which that DTLS specifically forbids the use of RC4 already.
specifically forbids the use of RC4.
o Implementations MUST NOT negotiate cipher suites offering less o Implementations MUST NOT negotiate cipher suites offering less
than 112 bits of security, including the so-called "export-level" than 112 bits of security, including the so-called "export-level"
encryption (which provide 40 or 56 bits of security). encryption (which provide 40 or 56 bits of security).
Rationale: Based on [RFC3766], at least 112 bits of security is Rationale: Based on [RFC3766], at least 112 bits of security is
needed. 40-bit and 56-bit security are considered insecure today. needed. 40-bit and 56-bit security are considered insecure today.
TLS 1.1 and 1.2 never negotiate 40-bit or 56-bit export ciphers. TLS 1.1 and 1.2 never negotiate 40-bit or 56-bit export ciphers.
o Implementations SHOULD NOT negotiate cipher suites that use o Implementations SHOULD NOT negotiate cipher suites that use
algorithms offering less than 128 bits of security. algorithms offering less than 128 bits of security.
Rationale: Cipher suites that offer between 112-bits and 128-bits Rationale: Cipher suites that offer between 112-bits and 128-bits
of security are not considered weak at this time, however it is of security are not considered weak at this time, however it is
expected that their useful lifespan is short enough to justify expected that their useful lifespan is short enough to justify
supporting stronger cipher suites at this time. 128-bit ciphers supporting stronger cipher suites at this time. 128-bit ciphers
are expected to remain secure for at least several years, and are expected to remain secure for at least several years, and
256-bit ciphers "until the next fundamental technology 256-bit ciphers "until the next fundamental technology
breakthrough". Note that some legacy cipher suites (e.g., 168-bit breakthrough". Note that, because of so-called "meet-in-the-
3DES) have an effective key length which is smaller than their middle" attacks [Multiple-Encryption] some legacy cipher suites
nominal key length (112 bits in the case of 3DES). Such cipher (e.g., 168-bit 3DES) have an effective key length which is smaller
suites should be evaluated according to their effective key than their nominal key length (112 bits in the case of 3DES).
length. Such cipher suites should be evaluated according to their
effective key length.
o Implementations MUST support, and SHOULD prefer to negotiate, o Implementations MUST support, and SHOULD prefer to negotiate,
cipher suites offering forward secrecy, such as those in the cipher suites offering forward secrecy, such as those in the
Ephemeral Diffie-Hellman and Elliptic Curve Ephemeral Diffie- Ephemeral Diffie-Hellman and Elliptic Curve Ephemeral Diffie-
Hellman ("DHE" and "ECDHE") families. Hellman ("DHE" and "ECDHE") families.
Rationale: Forward secrecy (sometimes called "perfect forward Rationale: Forward secrecy (sometimes called "perfect forward
secrecy") prevents the recovery of information that was encrypted secrecy") prevents the recovery of information that was encrypted
with older session keys, thus limiting the amount of time during with older session keys, thus limiting the amount of time during
which attacks can be successful. See Section 7.3 for a detailed which attacks can be successful. See Section 7.3 for a detailed
skipping to change at page 10, line 4 skipping to change at page 10, line 11
with older session keys, thus limiting the amount of time during with older session keys, thus limiting the amount of time during
which attacks can be successful. See Section 7.3 for a detailed which attacks can be successful. See Section 7.3 for a detailed
discussion. discussion.
4.2. Recommended Cipher Suites 4.2. Recommended Cipher Suites
Given the foregoing considerations, implementation and deployment of Given the foregoing considerations, implementation and deployment of
the following cipher suites is RECOMMENDED: the following cipher suites is RECOMMENDED:
o TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 o TLS_DHE_RSA_WITH_AES_128_GCM_SHA256
o TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 o TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
o TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 o TLS_DHE_RSA_WITH_AES_256_GCM_SHA384
o TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 o TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
These cipher suites are supported only in TLS 1.2 because they are These cipher suites are supported only in TLS 1.2 because they are
authenticated encryption (AEAD) algorithms [RFC5116]. authenticated encryption (AEAD) algorithms [RFC5116].
Typically, in order to prefer these suites, the order of suites needs Typically, in order to prefer these suites, the order of suites needs
to be explicitly configured in server software. to be explicitly configured in server software. It would be ideal if
server software implementations were to prefer these suites by
default.
Some devices have hardware support for AES-CCM but not AES-GCM. Some devices have hardware support for AES-CCM but not AES-GCM, so
There are even devices that do not support public key cryptography at they are unable to follow the foregoing recommendations regarding
all. This BCP does not cover such devices. cipher suites. There are even devices that do not support public key
cryptography at all, but they are out of scope entirely.
4.2.1. Implementation Details 4.2.1. Implementation Details
Clients SHOULD include TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 as the Clients SHOULD include TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 as the
first proposal to any server, unless they have prior knowledge that first proposal to any server, unless they have prior knowledge that
the server cannot respond to a TLS 1.2 client_hello message. the server cannot respond to a TLS 1.2 client_hello message.
Servers SHOULD prefer this cipher suite whenever it is proposed, even Servers SHOULD prefer this cipher suite over weaker cipher suites
if it is not the first proposal. whenever it is proposed, even if it is not the first proposal.
Clients are of course free to offer stronger cipher suites, e.g., Clients are of course free to offer stronger cipher suites, e.g.,
using AES-256; when they do, the server SHOULD prefer the stronger using AES-256; when they do, the server SHOULD prefer the stronger
cipher suite unless there are compelling reasons (e.g., seriously cipher suite unless there are compelling reasons (e.g., seriously
degraded performance) to choose otherwise. degraded performance) to choose otherwise.
This document does not change the mandatory-to-implement TLS cipher This document does not change the mandatory-to-implement TLS cipher
suite(s) prescribed by TLS or application protocols using TLS. To suite(s) prescribed by TLS or application protocols using TLS. To
maximize interoperability, RFC 5246 mandates implementation of the maximize interoperability, RFC 5246 mandates implementation of the
TLS_RSA_WITH_AES_128_CBC_SHA cipher suite, which is significantly TLS_RSA_WITH_AES_128_CBC_SHA cipher suite, which is significantly
weaker than the cipher suites recommended here. Implementers should weaker than the cipher suites recommended here (the GCM mode does not
consider the interoperability gain against the loss in security when suffer from the same weakness, caused by the order of MAC-then-
deploying that cipher suite. Other application protocols specify Encrypt in TLS [Krawczyk2001], since it uses an Authenticated
other cipher suites as mandatory to implement (MTI). Encryption with Associated Data (AEAD) mode of operation).
Implementers should consider the interoperability gain against the
loss in security when deploying that cipher suite. Other application
protocols specify other cipher suites as mandatory to implement
(MTI).
Note that some profiles of TLS 1.2 use different cipher suites. For Note that some profiles of TLS 1.2 use different cipher suites. For
example, [RFC6460] defines a profile that uses the example, [RFC6460] defines a profile that uses the
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and
TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suites. TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suites.
[RFC4492] allows clients and servers to negotiate ECDH parameters [RFC4492] allows clients and servers to negotiate ECDH parameters
(curves). Both clients and servers SHOULD include the "Supported (curves). Both clients and servers SHOULD include the "Supported
Elliptic Curves" extension [RFC4492]. For interoperability, clients Elliptic Curves" extension [RFC4492]. For interoperability, clients
and servers SHOULD support the NIST P-256 (secp256r1) curve and servers SHOULD support the NIST P-256 (secp256r1) curve
skipping to change at page 11, line 4 skipping to change at page 11, line 19
Note that some profiles of TLS 1.2 use different cipher suites. For Note that some profiles of TLS 1.2 use different cipher suites. For
example, [RFC6460] defines a profile that uses the example, [RFC6460] defines a profile that uses the
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and
TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suites. TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suites.
[RFC4492] allows clients and servers to negotiate ECDH parameters [RFC4492] allows clients and servers to negotiate ECDH parameters
(curves). Both clients and servers SHOULD include the "Supported (curves). Both clients and servers SHOULD include the "Supported
Elliptic Curves" extension [RFC4492]. For interoperability, clients Elliptic Curves" extension [RFC4492]. For interoperability, clients
and servers SHOULD support the NIST P-256 (secp256r1) curve and servers SHOULD support the NIST P-256 (secp256r1) curve
[RFC4492]. In addition, clients SHOULD send an ec_point_formats [RFC4492]. In addition, clients SHOULD send an ec_point_formats
extension with a single element, "uncompressed". extension with a single element, "uncompressed".
4.3. Public Key Length 4.3. Public Key Length
When using the cipher suites recommended in this document, two public When using the cipher suites recommended in this document, two public
keys are normally used in the TLS handshake: one for the Diffie- keys are normally used in the TLS handshake: one for the Diffie-
Hellman key agreement and one for server authentication. Where a Hellman key agreement and one for server authentication. Where a
client certificate is used, a third public key is added. client certificate is used, a third public key is added.
With a key exchange based on modular Diffie-Hellman ("DHE" cipher With a key exchange based on modular Diffie-Hellman ("DHE" cipher
suites), DH key lengths of at least 2048 bits are RECOMMENDED. suites), DH key lengths of at least 2048 bits are RECOMMENDED.
Rationale: For various reasons, in practice DH keys are typically Rationale: For various reasons, in practice DH keys are typically
generated in lengths that are powers of two (e.g., 2^10 = 1024 bits, generated in lengths that are powers of two (e.g., 2^10 = 1024 bits,
2^11 = 2048 bits, 2^12 = 4096 bits). Because a DH key of 1228 bits 2^11 = 2048 bits, 2^12 = 4096 bits). Because a DH key of 1228 bits
would be roughly equivalent to only an 80-bit symmetric key would be roughly equivalent to only an 80-bit symmetric key
[RFC3766], it is better to use keys longer than that for the "DHE" [RFC3766], it is better to use keys longer than that for the "DHE"
family of cipher suites. A DH key of 1926 bits would be roughly family of cipher suites. A DH key of 1926 bits would be roughly
equivalent to a 100-bit symmetric key [RFC3766] and a DH key of 2048 equivalent to a 100-bit symmetric key [RFC3766] and a DH key of 2048
bits might be sufficient for at least the next 10 years. See bits might be sufficient for at least the next 10 years
Section 4.4 for additional information on the use of modular Diffie- [NIST.SP.800-56A]. See Section 4.4 for additional information on the
Hellman in TLS. use of modular Diffie-Hellman in TLS.
As noted in [RFC3766], correcting for the emergence of a TWIRL As noted in [RFC3766], correcting for the emergence of a TWIRL
machine would imply that 1024-bit DH keys yield about 65 bits of machine would imply that 1024-bit DH keys yield about 65 bits of
equivalent strength and that a 2048-bit DH key would yield about 92 equivalent strength and that a 2048-bit DH key would yield about 92
bits of equivalent strength. bits of equivalent strength.
With regard to ECDH keys, the IANA named curve registry contains With regard to ECDH keys, the IANA named curve registry contains
160-bit elliptic curves which are considered to be roughly equivalent 160-bit elliptic curves which are considered to be roughly equivalent
to only an 80-bit symmetric key [ECRYPT-II]. The use of curves of to only an 80-bit symmetric key [ECRYPT-II]. Curves of less than
less than 192-bits is NOT RECOMMENDED. 192-bits SHOULD NOT be used.
When using RSA servers SHOULD authenticate using certificates with at When using RSA servers SHOULD authenticate using certificates with at
least a 2048-bit modulus for the public key. In addition, the use of least a 2048-bit modulus for the public key. In addition, the use of
the SHA-256 hash algorithm is RECOMMENDED (see [CAB-Baseline] for the SHA-256 hash algorithm is RECOMMENDED (see [CAB-Baseline] for
more details). Clients SHOULD indicate to servers that they request more details). Clients SHOULD indicate to servers that they request
SHA-256, by using the "Signature Algorithms" extension defined in SHA-256, by using the "Signature Algorithms" extension defined in
TLS 1.2. TLS 1.2.
4.4. Modular vs. Elliptic Curve DH Cipher Suites 4.4. Modular vs. Elliptic Curve DH Cipher Suites
Not all TLS implementations support both modular and elliptic curve Not all TLS implementations support both modular and elliptic curve
Diffie-Hellman groups, as required by Section 4.2. Some Diffie-Hellman groups, as required by Section 4.2. Some
implementations are severely limited in the length of DH values. implementations are severely limited in the length of DH values.
When such implementations need to be accommodated, we recommend using When such implementations need to be accommodated, the following are
(in priority order): RECOMMENDED (in priority order):
1. Elliptic Curve DHE with negotiated parameters [RFC5289] 1. Elliptic Curve DHE with negotiated parameters [RFC5289]
2. TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 [RFC5288], with 2048-bit 2. TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 [RFC5288], with 2048-bit
Diffie-Hellman parameters Diffie-Hellman parameters
3. TLS_DHE_RSA_WITH_AES_128_GCM_SHA256, with 1024-bit parameters. 3. TLS_DHE_RSA_WITH_AES_128_GCM_SHA256, with 1024-bit parameters.
Rationale: Although Elliptic Curve Cryptography is widely deployed Rationale: Although Elliptic Curve Cryptography is widely deployed
there are some communities where its uptake has been limited for there are some communities where its uptake has been limited for
skipping to change at page 12, line 33 skipping to change at page 12, line 48
o There are no standardized, widely implemented protocol mechanisms o There are no standardized, widely implemented protocol mechanisms
to negotiate the DH groups or parameter lengths supported by to negotiate the DH groups or parameter lengths supported by
client and server. client and server.
o Many servers choose DH parameters of 1024 bits or fewer. o Many servers choose DH parameters of 1024 bits or fewer.
o There are widely deployed client implementations that reject o There are widely deployed client implementations that reject
received DH parameters if they are longer than 1024 bits. In received DH parameters if they are longer than 1024 bits. In
addition, several implementations do not perform appropriate addition, several implementations do not perform appropriate
validation of group parameters and are vulnerable to attacks validation of group parameters and are vulnerable to attacks
referenced in Section 2.9 of [I-D.ietf-uta-tls-attacks] referenced in Section 2.9 of [RFC7457]
We note that with DHE and ECDHE cipher suites, the TLS master key Note that with DHE and ECDHE cipher suites, the TLS master key only
only depends on the Diffie-Hellman parameters and not on the strength depends on the Diffie-Hellman parameters and not on the strength of
of the RSA certificate; moreover, 1024 bit modular DH parameters are the RSA certificate; moreover, 1024 bit modular DH parameters are
generally considered insufficient at this time. generally considered insufficient at this time.
With modular ephemeral DH, deployers SHOULD carefully evaluate With modular ephemeral DH, deployers ought to carefully evaluate
interoperability vs. security considerations when configuring their interoperability vs. security considerations when configuring their
TLS endpoints. TLS endpoints.
4.5. Truncated HMAC 4.5. Truncated HMAC
Implementations MUST NOT use the Truncated HMAC extension, defined in Implementations MUST NOT use the Truncated HMAC extension, defined in
Section 7 of [RFC6066]. Section 7 of [RFC6066].
Rationale: the extension does not apply to the AEAD cipher suites Rationale: the extension does not apply to the AEAD cipher suites
recommended above. However it does apply to most other TLS cipher recommended above. However it does apply to most other TLS cipher
suites. Its use has been shown to be insecure in [PatersonRS11]. suites. Its use has been shown to be insecure in [PatersonRS11].
5. Applicability Statement 5. Applicability Statement
The deployment recommendations of this document address the operators The deployment recommendations of this document address the operators
of application layer services that are most commonly used on the of application layer services that are most commonly used on the
Internet, including, but not limited to: Internet, including, but not limited to:
o Operators of web servers that wish to protect HTTP with TLS. o Operators of web services that wish to protect HTTP with TLS.
o Operators of email servers who wish to protect the application- o Operators of email services who wish to protect the application-
layer protocols with TLS (e.g., IMAP, POP3 or SMTP). layer protocols with TLS (e.g., IMAP, POP3 or SMTP).
o Operators of instant-messaging services who wish to protect their o Operators of instant-messaging services who wish to protect their
application-layer protocols with TLS (e.g., XMPP or IRC). application-layer protocols with TLS (e.g., XMPP or IRC).
5.1. Security Services 5.1. Security Services
This document provides recommendations for an audience that wishes to This document provides recommendations for an audience that wishes to
secure their communication with TLS to achieve the following: secure their communication with TLS to achieve the following:
skipping to change at page 13, line 41 skipping to change at page 14, line 8
o Authentication: an end-point of the TLS communication is o Authentication: an end-point of the TLS communication is
authenticated as the intended entity to communicate with. authenticated as the intended entity to communicate with.
With regard to authentication, TLS enables authentication of one or With regard to authentication, TLS enables authentication of one or
both end-points in the communication. Although some TLS usage both end-points in the communication. Although some TLS usage
scenarios do not require authentication, those scenarios are not in scenarios do not require authentication, those scenarios are not in
scope for this document (a rationale for this decision is provided scope for this document (a rationale for this decision is provided
under Section 5.2). under Section 5.2).
If deployers deviate from the recommendations given in this document, If deployers deviate from the recommendations given in this document,
they MUST verify that they do not need one of the foregoing security they need to be aware that they might lose access to one of the
services. foregoing security services.
This document applies only to environments where confidentiality is This document applies only to environments where confidentiality is
required. It recommends algorithms and configuration options that required. It recommends algorithms and configuration options that
enforce secrecy of the data-in-transit. enforce secrecy of the data-in-transit.
This document also assumes that data integrity protection is always This document also assumes that data integrity protection is always
one of the goals of a deployment. In cases where integrity is not one of the goals of a deployment. In cases where integrity is not
required, it does not make sense to employ TLS in the first place. required, it does not make sense to employ TLS in the first place.
There are attacks against confidentiality-only protection that There are attacks against confidentiality-only protection that
utilize the lack of integrity to also break confidentiality (see for utilize the lack of integrity to also break confidentiality (see for
instance [DegabrieleP07] in the context of IPsec). instance [DegabrieleP07] in the context of IPsec).
The intended audience covers those services that are most commonly This document addresses itself to application protocols that are most
used on the Internet. Typically, all communication between TLS commonly used on the Internet with TLS and DTLS. Typically, all
clients and TLS servers requires all three of the above security communication between TLS clients and TLS servers requires all three
services. This is particularly true where TLS clients are user of the above security services. This is particularly true where TLS
agents like Web browsers or email software. clients are user agents like Web browsers or email software.
This document does not address the rarer deployment scenarios where This document does not address the rarer deployment scenarios where
one of the above three properties is not desired, such as the use one of the above three properties is not desired, such as the use
case described under Section 5.2 below. Another example of an case described under Section 5.2 below. As another scenario where
audience not needing confidentiality is the following: a monitored confidentiality is not needed, consider a monitored network where the
network where the authorities in charge of the respective traffic authorities in charge of the respective traffic domain require full
domain require full access to unencrypted (plaintext) traffic, and access to unencrypted (plaintext) traffic, and where users
where users collaborate and send their traffic in the clear. collaborate and send their traffic in the clear.
5.2. Unauthenticated TLS and Opportunistic Security 5.2. Unauthenticated TLS and Opportunistic Security
Several important applications use TLS to protect data between a TLS Several important applications use TLS to protect data between a TLS
client and a TLS server, but do so without the TLS client necessarily client and a TLS server, but do so without the TLS client necessarily
verifying the server's certificate. This practice is often called verifying the server's certificate. This practice is often called
"unauthenticated TLS". The reader is referred to "unauthenticated TLS". The reader is referred to
[I-D.ietf-dane-smtp-with-dane] for an example and an explanation of [I-D.ietf-dane-smtp-with-dane] for an example and an explanation of
why this less secure practice will likely remain common in the why this less secure practice will likely remain common in the
context of SMTP (especially for MTA-to-MTA communications). The context of SMTP (especially for MTA-to-MTA communications). The
practice is also encountered in similar contexts such as server-to- practice is also encountered in similar contexts such as server-to-
server traffic on the XMPP network (where multi-tenant hosting server traffic on the XMPP network (where multi-tenant hosting
environments make it difficult for operators to obtain proper environments make it difficult for operators to obtain proper
certificates for all of the domains they service). certificates for all of the domains they service).
Furthermore, in some scenarios the use of TLS itself is optional, Furthermore, in some scenarios the use of TLS itself is optional,
i.e. the client decides dynamically ("opportunistically") whether to i.e. the client decides dynamically ("opportunistically") whether to
use TLS with a particular server or to connect in the clear. This use TLS with a particular server or to connect in the clear. This
practice, often called "opportunistic security", and is described at practice, often called "opportunistic security", is described at
length in Section 2 of [I-D.farrelll-mpls-opportunistic-encrypt]. length in [RFC7435].
It can be argued that the recommendations provided in this document It can be argued that the recommendations provided in this document
ought to apply equally to unauthenticated TLS as well as ought to apply equally to unauthenticated TLS as well as
authenticated TLS. That would keep TLS implementations and authenticated TLS. That would keep TLS implementations and
deployments in sync, which is a desirable property given that servers deployments in sync, which is a desirable property given that servers
can be used simultaneously for unauthenticated TLS and for can be used simultaneously for unauthenticated TLS and for
authenticated TLS (indeed, a server cannot know whether a client authenticated TLS (indeed, a server cannot know whether a client
might attempt authenticated or unauthenticated TLS). On the other might attempt authenticated or unauthenticated TLS). On the other
hand, it has been argued that some of the recommendations in this hand, it has been argued that some of the recommendations in this
document might be too strict for unauthenticated scenarios and that document might be too strict for unauthenticated scenarios and that
skipping to change at page 17, line 5 skipping to change at page 17, line 22
Forward secrecy ensures in such cases that the session keys cannot be Forward secrecy ensures in such cases that the session keys cannot be
determined even by an attacker who obtains the long-term keys some determined even by an attacker who obtains the long-term keys some
time after the conversation. It also protects against an attacker time after the conversation. It also protects against an attacker
who is in possession of the long-term keys, but remains passive who is in possession of the long-term keys, but remains passive
during the conversation. during the conversation.
Forward secrecy is generally achieved by using the Diffie-Hellman Forward secrecy is generally achieved by using the Diffie-Hellman
scheme to derive session keys. The Diffie-Hellman scheme has both scheme to derive session keys. The Diffie-Hellman scheme has both
parties maintain private secrets and send parameters over the network parties maintain private secrets and send parameters over the network
as modular powers over certain cyclic groups. The properties of the as modular powers over certain cyclic groups. The properties of the
so-called Discrete Logarithm Problem (DLP) allow to derive the so-called Discrete Logarithm Problem (DLP) allow the parties to
session keys without an eavesdropper being able to do so. There is derive the session keys without an eavesdropper being able to do so.
currently no known attack against DLP if sufficiently large There is currently no known attack against DLP if sufficiently large
parameters are chosen. A variant of the Diffie-Hellman scheme uses parameters are chosen. A variant of the Diffie-Hellman scheme uses
Elliptic Curves instead of the originally proposed modular Elliptic Curves instead of the originally proposed modular
arithmetics. arithmetics.
Unfortunately, many TLS/DTLS cipher suites were defined that do not Unfortunately, many TLS/DTLS cipher suites were defined that do not
feature forward secrecy, e.g., TLS_RSA_WITH_AES_256_CBC_SHA256. We feature forward secrecy, e.g., TLS_RSA_WITH_AES_256_CBC_SHA256. This
thus advocate strict use of forward-secrecy-only ciphers. document therefore advocates strict use of forward-secrecy-only
ciphers.
7.4. Diffie-Hellman Exponent Reuse 7.4. Diffie-Hellman Exponent Reuse
For performance reasons, many TLS implementations reuse Diffie- For performance reasons, many TLS implementations reuse Diffie-
Hellman and Elliptic Curve Diffie-Hellman exponents across multiple Hellman and Elliptic Curve Diffie-Hellman exponents across multiple
connections. Such reuse can result in major security issues: connections. Such reuse can result in major security issues:
o If exponents are reused for a long time (e.g., more than a few o If exponents are reused for a long time (e.g., more than a few
hours), an attacker who gains access to the host can decrypt hours), an attacker who gains access to the host can decrypt
previous connections. In other words, exponent reuse negates the previous connections. In other words, exponent reuse negates the
effects of forward secrecy. effects of forward secrecy.
o TLS implementations that reuse exponents should test the DH public o TLS implementations that reuse exponents should test the DH public
key they receive for group membership, in order to avoid some key they receive for group membership, in order to avoid some
known attacks. These tests are not standardized in TLS at the known attacks. These tests are not standardized in TLS at the
time of writing. See [RFC6989] for recipient tests required of time of writing. See [RFC6989] for recipient tests required of
IKEv2 implementations that reuse DH exponents. IKEv2 implementations that reuse DH exponents.
7.5. Certificate Revocation 7.5. Certificate Revocation
Unfortunately, no mechanism exists at this time that we can recommend The following considerations and recommendations represent the
as a complete and efficient solution for the problem of checking the current state of the art regarding certificate revocation, even
revocation status of common public key certificates (a.k.a. PKIX though no complete and efficient solution exists for the problem of
certificates, [RFC5280]). The current state of the art is as checking the revocation status of common public key certificates
follows: (a.k.a. PKIX certificates, [RFC5280]):
o Although Certificate Revocation Lists (CRLs) are the most widely o Although Certificate Revocation Lists (CRLs) are the most widely
supported mechanism for distributing revocation information, they supported mechanism for distributing revocation information, they
have known scaling challenges that limit their usefulness (despite have known scaling challenges that limit their usefulness (despite
workarounds such as partitioned CRLS and delta CRLs). workarounds such as partitioned CRLS and delta CRLs).
o Proprietary mechanisms that embed revocation lists in the Web o Proprietary mechanisms that embed revocation lists in the Web
browser's configuration database cannot scale beyond a small browser's configuration database cannot scale beyond a small
number of the most heavily used Web servers. number of the most heavily used Web servers.
skipping to change at page 18, line 24 skipping to change at page 18, line 44
o OCSP stapling as defined in [RFC6066] does not extend to o OCSP stapling as defined in [RFC6066] does not extend to
intermediate certificates used in a certificate chain. Although intermediate certificates used in a certificate chain. Although
[RFC6961] addresses this shortcoming, it is a recent addition [RFC6961] addresses this shortcoming, it is a recent addition
without much deployment. without much deployment.
o Both CRLs and OSCP depend on relatively reliable connectivity to o Both CRLs and OSCP depend on relatively reliable connectivity to
the Internet, which might not be available to certain kinds of the Internet, which might not be available to certain kinds of
nodes (such as newly provisioned devices that need to establish a nodes (such as newly provisioned devices that need to establish a
secure connection in order to boot up for the first time). secure connection in order to boot up for the first time).
With regard to PKIX certificates, servers SHOULD support both OCSP With regard to PKIX certificates, servers SHOULD support the
[RFC6960] and OCSP stapling. To enable interoperability with the following as a best practice given the current state of the art and
widest range of clients, servers SHOULD support both the as a foundation for a possible future solution:
status_request extension defined in [RFC6066] and the
status_request_v2 extension defined in [RFC6961]. Servers also 1. OCSP [RFC6960]
SHOULD support the OCSP stapling extension defined in [RFC6961] as a
best practice given the current state of the art and as a foundation 2. Both the status_request extension defined in [RFC6066] and the
for a possible future solution. status_request_v2 extension defined in [RFC6961] (this might
enable interoperability with the widest range of clients)
3. The OCSP stapling extension defined in [RFC6961]
The foregoing considerations do not apply to scenarios where the The foregoing considerations do not apply to scenarios where the
DANE-TLSA resource record [RFC6698] is used to signal to a client DANE-TLSA resource record [RFC6698] is used to signal to a client
which certificate a server considers valid and good to use for TLS which certificate a server considers valid and good to use for TLS
connections. connections.
8. Acknowledgments 8. Acknowledgments
We would like to thank Uri Blumenthal, Viktor Dukhovni, Stephen Thanks to RJ Atkinson, Uri Blumenthal, Viktor Dukhovni, Stephen
Farrell, Daniel Kahn Gillmor, Paul Hoffman, Simon Josefsson, Watson Farrell, Daniel Kahn Gillmor, Paul Hoffman, Simon Josefsson, Watson
Ladd, Orit Levin, Ilari Liusvaara, Johannes Merkle, Bodo Moeller, Ladd, Orit Levin, Ilari Liusvaara, Johannes Merkle, Bodo Moeller,
Yoav Nir, Massimiliano Pala, Kenny Paterson, Patrick Pelletier, Tom Yoav Nir, Massimiliano Pala, Kenny Paterson, Patrick Pelletier, Tom
Ritter, Joe St. Sauver, Joe Salowey, Rich Salz, Brian Smith, Sean Ritter, Joe St. Sauver, Joe Salowey, Rich Salz, Brian Smith, Sean
Turner, and Aaron Zauner for their feedback and suggested Turner, and Aaron Zauner for their feedback and suggested
improvements. Thanks to Brian Smith, who has provided a great improvements. Thanks also to Brian Smith, who has provided a great
resource in his "Proposal to Change the Default TLS Ciphersuites resource in his "Proposal to Change the Default TLS Ciphersuites
Offered by Browsers" [Smith2013]. Finally, thanks to all others who Offered by Browsers" [Smith2013]. Finally, thanks to all others who
commented on the TLS, UTA, and other discussion lists but who are not commented on the TLS, UTA, and other discussion lists but who are not
mentioned here by name. mentioned here by name.
Robert Sparks and Dave Waltermire provided helpful reviews on behalf
of the General Area Review Team and the Security Directorate,
respectively.
The authors gratefully acknowledge the assistance of Leif Johansson
and Orit Levin as the working group chairs and Pete Resnick as the
sponsoring Area Director.
9. References 9. References
9.1. Normative References 9.1. Normative References
[I-D.ietf-tls-prohibiting-rc4]
Popov, A., "Prohibiting RC4 Cipher Suites", draft-ietf-
tls-prohibiting-rc4-01 (work in progress), October 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys", BCP 86, Public Keys Used For Exchanging Symmetric Keys", BCP 86,
RFC 3766, April 2004. RFC 3766, April 2004.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B. [RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
skipping to change at page 20, line 29 skipping to change at page 21, line 11
Smart, N., "ECRYPT II Yearly Report on Algorithms and Smart, N., "ECRYPT II Yearly Report on Algorithms and
Keysizes (2011-2012)", 2012, Keysizes (2011-2012)", 2012,
<http://www.ecrypt.eu.org/documents/D.SPA.20.pdf>. <http://www.ecrypt.eu.org/documents/D.SPA.20.pdf>.
[Heninger2012] [Heninger2012]
Heninger, N., Durumeric, Z., Wustrow, E., and J. Heninger, N., Durumeric, Z., Wustrow, E., and J.
Halderman, "Mining Your Ps and Qs: Detection of Widespread Halderman, "Mining Your Ps and Qs: Detection of Widespread
Weak Keys in Network Devices", Usenix Security Symposium Weak Keys in Network Devices", Usenix Security Symposium
2012, 2012. 2012, 2012.
[I-D.farrelll-mpls-opportunistic-encrypt]
Farrel, A. and S. Farrell, "Opportunistic Encryption in
MPLS Networks", draft-farrelll-mpls-opportunistic-
encrypt-02 (work in progress), February 2014.
[I-D.ietf-dane-smtp-with-dane] [I-D.ietf-dane-smtp-with-dane]
Dukhovni, V. and W. Hardaker, "SMTP security via Dukhovni, V. and W. Hardaker, "SMTP security via
opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-10 opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-10
(work in progress), May 2014. (work in progress), May 2014.
[I-D.ietf-dane-srv] [I-D.ietf-dane-srv]
Finch, T., Miller, M., and P. Saint-Andre, "Using DNS- Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
Based Authentication of Named Entities (DANE) TLSA Records Based Authentication of Named Entities (DANE) TLSA Records
with SRV Records", draft-ietf-dane-srv-06 (work in with SRV Records", draft-ietf-dane-srv-06 (work in
progress), June 2014. progress), June 2014.
[I-D.ietf-tls-downgrade-scsv] [I-D.ietf-tls-downgrade-scsv]
Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
Suite Value (SCSV) for Preventing Protocol Downgrade Suite Value (SCSV) for Preventing Protocol Downgrade
Attacks", draft-ietf-tls-downgrade-scsv-02 (work in Attacks", draft-ietf-tls-downgrade-scsv-02 (work in
progress), November 2014. progress), November 2014.
[I-D.ietf-tls-prohibiting-rc4]
Popov, A., "Prohibiting RC4 Cipher Suites", draft-ietf-
tls-prohibiting-rc4-01 (work in progress), October 2014.
[I-D.ietf-uta-tls-attacks]
Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
Current Attacks on TLS and DTLS", draft-ietf-uta-tls-
attacks-04 (work in progress), September 2014.
[Kleinjung2010] [Kleinjung2010]
Kleinjung, T., "Factorization of a 768-Bit RSA Modulus", Kleinjung, T., "Factorization of a 768-Bit RSA Modulus",
CRYPTO 10, 2010, <https://eprint.iacr.org/2010/006.pdf>. CRYPTO 10, 2010, <https://eprint.iacr.org/2010/006.pdf>.
[Krawczyk2001]
Krawczyk, H., "The order of encryption and authentication
for protecting communications (Or: how secure is SSL?)",
CRYPTO 01, 2001, <https://eprint.iacr.org/2001/045.pdf>.
[Multiple-Encryption]
Merkle, R. and M. Hellman, "On the security of multiple
encryption", Communications of the ACM 24, 1981,
<http://dl.acm.org/citation.cfm?id=358718>.
[NIST.SP.800-56A]
Barker, E., Chen, L., Roginsky, A., and M. Smid,
"Recommendation for Pair-Wise Key Establishment Schemes
Using Discrete Logarithm Cryptography", NIST Special
Publication 800-56A, 2013, <http://nvlpubs.nist.gov/
nistpubs/SpecialPublications/NIST.SP.800-56Ar2.pdf>.
[POODLE] Moeller, B., Duong, T., and K. Kotowicz, "This POODLE [POODLE] Moeller, B., Duong, T., and K. Kotowicz, "This POODLE
Bites: Exploiting the SSL 3.0 Fallback", 2014, <https:// Bites: Exploiting the SSL 3.0 Fallback", 2014, <https://
www.openssl.org/~bodo/ssl-poodle.pdf>. www.openssl.org/~bodo/ssl-poodle.pdf>.
[PatersonRS11] [PatersonRS11]
Paterson, K., Ristenpart, T., and T. Shrimpton, "Tag size Paterson, K., Ristenpart, T., and T. Shrimpton, "Tag size
does matter: attacks and proofs for the TLS record does matter: attacks and proofs for the TLS record
protocol", 2011, protocol", 2011,
<http://dx.doi.org/10.1007/978-3-642-25385-0_20>. <http://dx.doi.org/10.1007/978-3-642-25385-0_20>.
skipping to change at page 22, line 43 skipping to change at page 23, line 25
RFC 6960, June 2013. RFC 6960, June 2013.
[RFC6961] Pettersen, Y., "The Transport Layer Security (TLS) [RFC6961] Pettersen, Y., "The Transport Layer Security (TLS)
Multiple Certificate Status Request Extension", RFC 6961, Multiple Certificate Status Request Extension", RFC 6961,
June 2013. June 2013.
[RFC6989] Sheffer, Y. and S. Fluhrer, "Additional Diffie-Hellman [RFC6989] Sheffer, Y. and S. Fluhrer, "Additional Diffie-Hellman
Tests for the Internet Key Exchange Protocol Version 2 Tests for the Internet Key Exchange Protocol Version 2
(IKEv2)", RFC 6989, July 2013. (IKEv2)", RFC 6989, July 2013.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, December 2014.
[RFC7457] Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
Known Attacks on Transport Layer Security (TLS) and
Datagram TLS (DTLS)", RFC 7457, February 2015.
[Smith2013] [Smith2013]
Smith, B., "Proposal to Change the Default TLS Smith, B., "Proposal to Change the Default TLS
Ciphersuites Offered by Browsers.", 2013, <https:// Ciphersuites Offered by Browsers.", 2013, <https://
briansmith.org/browser-ciphersuites-01.html>. briansmith.org/browser-ciphersuites-01.html>.
[Soghoian2011] [Soghoian2011]
Soghoian, C. and S. Stamm, "Certified lies: Detecting and Soghoian, C. and S. Stamm, "Certified lies: Detecting and
defeating government interception attacks against SSL.", defeating government interception attacks against SSL.",
Proc. 15th Int. Conf. Financial Cryptography and Data Proc. 15th Int. Conf. Financial Cryptography and Data
Security , 2011. Security , 2011.
 End of changes. 51 change blocks. 
135 lines changed or deleted 173 lines changed or added

This html diff was produced by rfcdiff 1.48. The latest version is available from http://tools.ietf.org/tools/rfcdiff/