draft-ietf-curdle-gss-keyex-sha2-09.txt   draft-ietf-curdle-gss-keyex-sha2-10.txt 
Internet Engineering Task Force S. Sorce Internet Engineering Task Force S. Sorce
Internet-Draft H. Kario Internet-Draft H. Kario
Updates: 4462 (if approved) Red Hat, Inc. Updates: 4462 (if approved) Red Hat, Inc.
Intended status: Standards Track Jun 11, 2019 Intended status: Standards Track Jul 22, 2019
Expires: December 13, 2019 Expires: January 23, 2020
GSS-API Key Exchange with SHA2 GSS-API Key Exchange with SHA2
draft-ietf-curdle-gss-keyex-sha2-09 draft-ietf-curdle-gss-keyex-sha2-10
Abstract Abstract
This document specifies additions and amendments to RFC4462. It This document specifies additions and amendments to RFC4462. It
defines a new key exchange method that uses SHA-2 for integrity and defines a new key exchange method that uses SHA-2 for integrity and
deprecates weak DH groups. The purpose of this specification is to deprecates weak DH groups. The purpose of this specification is to
modernize the cryptographic primitives used by GSS Key Exchanges. modernize the cryptographic primitives used by GSS Key Exchanges.
Status of This Memo Status of This Memo
skipping to change at page 1, line 34 skipping to change at page 1, line 34
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 https://datatracker.ietf.org/drafts/current/. Drafts is at https://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 December 13, 2019. This Internet-Draft will expire on January 23, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 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
(https://trustee.ietf.org/license-info) in effect on the date of (https://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
skipping to change at page 2, line 27 skipping to change at page 2, line 27
8.1. New Finite Field DH mechanisms . . . . . . . . . . . . . 10 8.1. New Finite Field DH mechanisms . . . . . . . . . . . . . 10
8.2. New Elliptic Curve DH mechanisms . . . . . . . . . . . . 10 8.2. New Elliptic Curve DH mechanisms . . . . . . . . . . . . 10
8.3. GSSAPI Delegation . . . . . . . . . . . . . . . . . . . . 10 8.3. GSSAPI Delegation . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 11 9.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 12 9.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction 1. Introduction
SSH GSS-API Methods [RFC4462] allows the use of GSSAPI for SSH GSS-API Methods [RFC4462] allows the use of GSSAPI [RFC2743] for
authentication and key exchange in SSH. It defines three exchange authentication and key exchange in SSH. It defines three exchange
methods all based on DH groups and SHA-1. This document updates methods all based on DH groups and SHA-1. This document updates
RFC4462 with new methods intended to support environments that desire RFC4462 with new methods intended to support environments that desire
to use the SHA-2 cryptographic hash functions. to use the SHA-2 cryptographic hash functions.
2. Rationale 2. Rationale
Due to security concerns with SHA-1 [RFC6194] and with MODP groups Due to security concerns with SHA-1 [RFC6194] and with MODP groups
with less than 2048 bits [NIST-SP-800-131Ar1] we propose the use of with less than 2048 bits [NIST-SP-800-131Ar1] we propose the use of
the SHA-2 [RFC6234] based hashes with DH group14, group15, group16, hashes based on SHA-2 [RFC6234] with DH group14, group15, group16,
group17 and group18 [RFC3526]. Additionally we add support for key group17 and group18 [RFC3526]. Additionally we add support for key
exchange based on Elliptic Curve Diffie Hellman with the NIST P-256, exchange based on Elliptic Curve Diffie Hellman with the NIST P-256,
P-384 and P-521 as well as the X25519 and X448 curves. Following the P-384 and P-521 [SEC2v2] as well as the X25519 and X448 [RFC7748]
practice of [RFC8268] only SHA-256 and SHA-512 hashes are used for DH curves. Following the practice of [RFC8268] only SHA-256 and SHA-512
groups. For NIST curves the same curve-to-hashing algorithm pairing hashes are used for DH groups. For NIST curves the same curve-to-
used in [RFC5656] is adopted for consistency. hashing algorithm pairing used in [RFC5656] is adopted for
consistency.
3. Document Conventions 3. Document Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 >RFC2119 [RFC2119] RFC8174 [RFC8174] when, and only when, they 14 RFC2119 [RFC2119] RFC8174 [RFC8174] when, and only when, they
appear in all capitals, as shown here. appear in all capitals, as shown here.
4. New Diffie-Hellman Key Exchange methods 4. New Diffie-Hellman Key Exchange methods
This document adopts the same naming convention defined in [RFC4462] This document adopts the same naming convention defined in [RFC4462]
to define families of methods that cover any GSS-API mechanism used to define families of methods that cover any GSS-API mechanism used
with a specific Diffie-Hellman group and SHA-2 Hash combination. with a specific Diffie-Hellman group and SHA-2 Hash combination.
The following new key exchange algorithms are defined:
+--------------------------+--------------------------------+ +--------------------------+--------------------------------+
| Key Exchange Method Name | Implementation Recommendations | | Key Exchange Method Name | Implementation Recommendations |
+--------------------------+--------------------------------+ +--------------------------+--------------------------------+
| gss-group14-sha256-* | SHOULD/RECOMMENDED | | gss-group14-sha256-* | SHOULD/RECOMMENDED |
| gss-group15-sha512-* | MAY/OPTIONAL | | gss-group15-sha512-* | MAY/OPTIONAL |
| gss-group16-sha512-* | SHOULD/RECOMMENDED | | gss-group16-sha512-* | SHOULD/RECOMMENDED |
| gss-group17-sha512-* | MAY/OPTIONAL | | gss-group17-sha512-* | MAY/OPTIONAL |
| gss-group18-sha512-* | MAY/OPTIONAL | | gss-group18-sha512-* | MAY/OPTIONAL |
+--------------------------+--------------------------------+ +--------------------------+--------------------------------+
Each key exchange method is implicitly registered by this document. Table 1: New key exchange algorithms
The IESG is considered to be the owner of all these key exchange
methods; this does NOT imply that the IESG is considered to be the
owner of the underlying GSS-API mechanism.
Each method in any family of methods specifies GSS-API-authenticated Each key exchange method prefix is registered by this document. The
Diffie-Hellman key exchanges as described in Section 2.1 of IESG is the change controller of all these key exchange methods; this
[RFC4462]. The method name for each method is the concatenation of does NOT imply that the IESG is considered to be in control of the
the family name prefix with the Base64 encoding of the MD5 hash corresponding GSS-API mechanism.
[RFC1321] of the ASN.1 DER encoding [ISO-IEC-8825-1] of the
underlying GSS-API mechanism's OID. Base64 encoding is described in
Section 6.8 of [RFC2045].
Family method refences Each method in any family of methods (Table 2) specifies GSS-API-
authenticated Diffie-Hellman key exchanges as described in
Section 2.1 of [RFC4462]. The method name for each method (Table 1)
is the concatenation of the family name prefix with the Base64
encoding of the MD5 hash [RFC1321] of the ASN.1 DER encoding
[ISO-IEC-8825-1] of the corresponding GSS-API mechanism's OID.
Base64 encoding is described in Section 4 of [RFC4648].
+---------------------+-------------+-------------+-----------------+ +---------------------+-------------+-------------+-----------------+
| Family Name prefix | Hash | Group | Reference | | Family Name prefix | Hash | Group | Reference |
| | Function | | | | | Function | | |
+---------------------+-------------+-------------+-----------------+ +---------------------+-------------+-------------+-----------------+
| gss-group14-sha256- | SHA-256 | 2048-bit | Section 3 of | | gss-group14-sha256- | SHA-256 | 2048-bit | Section 3 of |
| | | MODP | [RFC3526] | | | | MODP | [RFC3526] |
| gss-group15-sha512- | SHA-512 | 3072-bit | Section 4 of | | gss-group15-sha512- | SHA-512 | 3072-bit | Section 4 of |
| | | MODP | [RFC3526] | | | | MODP | [RFC3526] |
| gss-group16-sha512- | SHA-512 | 4096-bit | Section 5 of | | gss-group16-sha512- | SHA-512 | 4096-bit | Section 5 of |
| | | MODP | [RFC3526] | | | | MODP | [RFC3526] |
| gss-group17-sha512- | SHA-512 | 6144-bit | Section 6 of | | gss-group17-sha512- | SHA-512 | 6144-bit | Section 6 of |
| | | MODP | [RFC3526] | | | | MODP | [RFC3526] |
| gss-group18-sha512- | SHA-512 | 8192-bit | Section 7 of | | gss-group18-sha512- | SHA-512 | 8192-bit | Section 7 of |
| | | MODP | [RFC3526] | | | | MODP | [RFC3526] |
+---------------------+-------------+-------------+-----------------+ +---------------------+-------------+-------------+-----------------+
Table 2: Family method references
5. New Elliptic Curve Diffie-Hellman Key Exchange methods 5. New Elliptic Curve Diffie-Hellman Key Exchange methods
In [RFC5656] new SSH key exchange algorithms based on Elliptic Curve In [RFC5656] new SSH key exchange algorithms based on Elliptic Curve
Cryptography are introduced. We reuse much of section 4 of [RFC5656] Cryptography are introduced. We reuse much of section 4 of [RFC5656]
to define GSS-API-authenticated ECDH Key Exchanges. to define GSS-API-authenticated ECDH Key Exchanges.
Additionally we utilize also the curves defined in Additionally, we also utilize the curves defined in
[I-D.ietf-curdle-ssh-curves] to complement the 3 classic NIST defined [I-D.ietf-curdle-ssh-curves] to complement the three classic NIST-
curves required by [RFC5656]. defined curves required by [RFC5656].
5.1. Generic GSS-API Key Exchange with ECDH 5.1. Generic GSS-API Key Exchange with ECDH
This section reuses much of the scheme defined in Section 2.1 of This section reuses much of the scheme defined in Section 2.1 of
[RFC4462] and combines it with the scheme defined in Section 4 of [RFC4462] and combines it with the scheme defined in Section 4 of
[RFC5656]; in particular, all checks and verification steps [RFC5656]; in particular, all checks and verification steps
prescribed in Section 4 of [RFC5656] apply here as well. prescribed in Section 4 of [RFC5656] apply here as well.
Key-agreement schemes ECDHE-Curve25519 and ECDHE-Curve448 perform the Key-agreement schemes ECDHE-Curve25519 and ECDHE-Curve448 perform the
Diffie-Helman protocol using the functions X25519 and X448, Diffie-Helman protocol using the functions X25519 and X448,
respectively. Implementations SHOULD compute these functions using respectively. Implementations MUST compute these functions using the
the algorithms described in [RFC7748]. When they do so, algorithms described in [RFC7748]. When they do so, implementations
implementations MUST check whether the computed Diffie-Hellman shared MUST check whether the computed Diffie-Hellman shared secret is the
secret is the all-zero value and abort if so, as described in all-zero value and abort if so, as described in Section 6 of
Section 6 of [RFC7748]. Alternative implementations of these [RFC7748]. Alternative implementations of these functions SHOULD
functions SHOULD abort when either input forces the shared secret to abort when either input forces the shared secret to one of a small
one of a small set of values, as discussed in Section 7 of [RFC7748]. set of values, as discussed in Section 7 of [RFC7748].
This section defers to [RFC7546] as the source of information on GSS- This section defers to [RFC7546] as the source of information on GSS-
API context establishment operations, Section 3 being the most API context establishment operations, Section 3 being the most
relevant. All Security Considerations described in [RFC7546] apply relevant. All Security Considerations described in [RFC7546] apply
here too. here too.
The parties generate each an ephemeral key pair, according to The parties each generate an ephemeral key pair, according to
Section 3.2.1 of [SEC1v2]. Keys are verified upon receipt by the Section 3.2.1 of [SEC1v2]. Keys are verified upon receipt by the
parties according to Section 3.2.3.1 of [SEC1v2]. parties according to Section 3.2.3.1 of [SEC1v2].
For NIST Curves the keys use the uncompressed point representation For NIST Curves the keys use the uncompressed point representation
and must be converted using the algorithm in Section 2.3.4 of and MUST be converted using the algorithm in Section 2.3.4 of
[SEC1v2]. If the conversion fails or the point is trasmitted using [SEC1v2]. If the conversion fails or the point is transmitted using
the compressed representation, the key exchange MUST fail. the compressed representation, the key exchange MUST fail.
A GSS Context is established according to Section 4 of [RFC5656]; The A GSS Context is established according to Section 4 of [RFC5656]; The
client initiates the establishment using GSS_Init_sec_context() and client initiates the establishment using GSS_Init_sec_context() and
the server responds to it using GSS_Accept_sec_context(). For the the server responds to it using GSS_Accept_sec_context(). For the
negotiation, the client MUST set mutual_req_flag and integ_req_flag negotiation, the client MUST set mutual_req_flag and integ_req_flag
to "true". In addition, deleg_req_flag MAY be set to "true" to to "true". In addition, deleg_req_flag MAY be set to "true" to
request access delegation, if requested by the user. Since the key request access delegation, if requested by the user. Since the key
exchange process authenticates only the host, the setting of exchange process authenticates only the host, the setting of
anon_req_flag is immaterial to this process. If the client does not anon_req_flag is immaterial to this process. If the client does not
skipping to change at page 5, line 18 skipping to change at page 5, line 18
then anon_req_flag SHOULD be set to "true". Otherwise, this flag MAY then anon_req_flag SHOULD be set to "true". Otherwise, this flag MAY
be set to true if the client wishes to hide its identity. This key be set to true if the client wishes to hide its identity. This key
exchange process will exchange only a single message token once the exchange process will exchange only a single message token once the
context has been established, therefore the replay_det_req_flag and context has been established, therefore the replay_det_req_flag and
sequence_req_flag SHOULD be set to "false". sequence_req_flag SHOULD be set to "false".
The client MUST include its public key with the first message it The client MUST include its public key with the first message it
sends to the server during this process; if the server receives more sends to the server during this process; if the server receives more
than one key or none at all, the key exchange MUST fail. than one key or none at all, the key exchange MUST fail.
During GSS Context estalishment multiple tokens may be exchanged by During GSS Context establishment multiple tokens may be exchanged by
the client and the server. When the GSS Context is established the client and the server. When the GSS Context is established
(major_status is GSS_S_COMPLETE) the parties check that mutual_state (major_status is GSS_S_COMPLETE) the parties check that mutual_state
and integ_avail are both "true". If not the key exchange MUST fail. and integ_avail are both "true". If not the key exchange MUST fail.
Once a party receives the peer's public key it proceeds to compute a Once a party receives the peer's public key it proceeds to compute a
shared secret K. For NIST Curves the computation is done according shared secret K. For NIST Curves the computation is done according
to Section 3.3.1 of [SEC1v2] and the resulting value z is converted to Section 3.3.1 of [SEC1v2] and the resulting value z is converted
to the octet string K using the conversion defined in Section 2.3.5 to the octet string K using the conversion defined in Section 2.3.5
of [SEC1v2]. For curve25519 and curve448 the algorithms in Section 6 of [SEC1v2]. For curve25519 and curve448 the algorithms in Section 6
of [RFC7748] are used instead. of [RFC7748] are used instead.
skipping to change at page 6, line 40 skipping to change at page 6, line 40
Calls GSS_VerifyMIC( MIC, H ) Calls GSS_VerifyMIC( MIC, H )
This is implemented with the following messages: This is implemented with the following messages:
The client sends: The client sends:
byte SSH_MSG_KEXGSS_INIT byte SSH_MSG_KEXGSS_INIT
string output_token (from GSS_Init_sec_context()) string output_token (from GSS_Init_sec_context())
string Q_C, client's ephemeral public key octet string string Q_C, client's ephemeral public key octet string
The server may responds with: The server may respond with:
byte SSH_MSG_KEXGSS_HOSTKEY byte SSH_MSG_KEXGSS_HOSTKEY
string server public host key and certificates (K_S) string server public host key and certificates (K_S)
The server sends: The server sends:
byte SSH_MSG_KEXGSS_CONTINUE byte SSH_MSG_KEXGSS_CONTINUE
string output_token (from GSS_Accept_sec_context()) string output_token (from GSS_Accept_sec_context())
Each time the client receives the message described above, it makes Each time the client receives the message described above, it makes
skipping to change at page 8, line 12 skipping to change at page 8, line 12
GSS_S_COMPLETE, a protocol error has occurred and the key exchange GSS_S_COMPLETE, a protocol error has occurred and the key exchange
MUST fail. MUST fail.
If the client receives a SSH_MSG_KEXGSS_COMPLETE message and a call If the client receives a SSH_MSG_KEXGSS_COMPLETE message and a call
to GSS_Init_sec_context() does not result in a major_status code of to GSS_Init_sec_context() does not result in a major_status code of
GSS_S_COMPLETE, a protocol error has occurred and the key exchange GSS_S_COMPLETE, a protocol error has occurred and the key exchange
MUST fail. MUST fail.
5.2. ECDH Key Exchange Methods 5.2. ECDH Key Exchange Methods
The following new key exchange methods are defined:
+--------------------------+--------------------------------+ +--------------------------+--------------------------------+
| Key Exchange Method Name | Implementation Recommendations | | Key Exchange Method Name | Implementation Recommendations |
+--------------------------+--------------------------------+ +--------------------------+--------------------------------+
| gss-nistp256-sha256-* | SHOULD/RECOMMENDED | | gss-nistp256-sha256-* | SHOULD/RECOMMENDED |
| gss-nistp384-sha384-* | MAY/OPTIONAL | | gss-nistp384-sha384-* | MAY/OPTIONAL |
| gss-nistp521-sha512-* | MAY/OPTIONAL | | gss-nistp521-sha512-* | MAY/OPTIONAL |
| gss-curve25519-sha256-* | SHOULD/RECOMMENDED | | gss-curve25519-sha256-* | SHOULD/RECOMMENDED |
| gss-curve448-sha512-* | MAY/OPTIONAL | | gss-curve448-sha512-* | MAY/OPTIONAL |
+--------------------------+--------------------------------+ +--------------------------+--------------------------------+
Each key exchange method is implicitly registered by this document. Table 3: New key exchange methods
The IESG is considered to be the owner of all these key exchange
methods; this does NOT imply that the IESG is considered to be the
owner of the underlying GSS-API mechanism.
Each method in any family of methods specifies GSS-API-authenticated Each key exchange method prefix is registered by this document. The
Elliptic Curve Diffie-Hellman key exchanges as described in IESG is the change controller of all these key exchange methods; this
Section 5.1. The method name for each method is the concatenation of does NOT imply that the IESG is considered to be in control of the
the family method name with the Base64 encoding of the MD5 hash corresponding GSS-API mechanism.
[RFC1321] of the ASN.1 DER encoding [ISO-IEC-8825-1] of the
underlying GSS-API mechanism's OID. Base64 encoding is described in
Section 6.8 of [RFC2045].
Family method refences Each method in any family of methods (Table 4) specifies GSS-API-
authenticated Elliptic Curve Diffie-Hellman key exchanges as
described in Section 5.1. The method name for each method (Table 3)
is the concatenation of the family method name with the Base64
encoding of the MD5 hash [RFC1321] of the ASN.1 DER encoding
[ISO-IEC-8825-1] of the corresponding GSS-API mechanism's OID.
Base64 encoding is described in Section 4 of [RFC4648].
+------------------------+----------+---------------+---------------+ +------------------------+----------+---------------+---------------+
| Family Name prefix | Hash | Parameters / | Definition | | Family Name prefix | Hash | Parameters / | Definition |
| | Function | Function Name | | | | Function | Function Name | |
+------------------------+----------+---------------+---------------+ +------------------------+----------+---------------+---------------+
| gss-nistp256-sha256- | SHA-256 | secp256r1 | Section 2.4.2 | | gss-nistp256-sha256- | SHA-256 | secp256r1 | Section 2.4.2 |
| | | | of [SEC2v2] | | | | | of [SEC2v2] |
| gss-nistp384-sha384- | SHA-384 | secp384r1 | Section 2.5.1 | | gss-nistp384-sha384- | SHA-384 | secp384r1 | Section 2.5.1 |
| | | | of [SEC2v2] | | | | | of [SEC2v2] |
| gss-nistp521-sha512- | SHA-512 | secp521r1 | Section 2.6.1 | | gss-nistp521-sha512- | SHA-512 | secp521r1 | Section 2.6.1 |
| | | | of [SEC2v2] | | | | | of [SEC2v2] |
| gss-curve25519-sha256- | SHA-256 | X22519 | Section 5 of | | gss-curve25519-sha256- | SHA-256 | X22519 | Section 5 of |
| | | | [RFC7748] | | | | | [RFC7748] |
| gss-curve448-sha512- | SHA-512 | X448 | Section 5 of | | gss-curve448-sha512- | SHA-512 | X448 | Section 5 of |
| | | | [RFC7748] | | | | | [RFC7748] |
+------------------------+----------+---------------+---------------+ +------------------------+----------+---------------+---------------+
Table 4: Family method refences
6. Deprecated Algorithms 6. Deprecated Algorithms
Because they have small key lengths and are no longer strong in the Because they have small key lengths and are no longer strong in the
face of brute-force attacks, the algorithms in the following table face of brute-force attacks, the algorithms in the following table
are considered deprecated and SHOULD NOT be used. are considered deprecated and SHOULD NOT be used.
Deprecated Algorithms Deprecated Algorithms
+--------------------------+--------------------------------+ +--------------------------+--------------------------------+
| Key Exchange Method Name | Implementation Recommendations | | Key Exchange Method Name | Implementation Recommendations |
skipping to change at page 10, line 47 skipping to change at page 10, line 47
Although a new cryptographic primitive is used with these methods the Although a new cryptographic primitive is used with these methods the
actual key exchange closely follows the key exchange defined in actual key exchange closely follows the key exchange defined in
[RFC5656]; therefore all the original Security Considerations as well [RFC5656]; therefore all the original Security Considerations as well
as those expressed in [RFC5656] apply. as those expressed in [RFC5656] apply.
8.3. GSSAPI Delegation 8.3. GSSAPI Delegation
Some GSSAPI mechanisms can act on a request to delegate credentials Some GSSAPI mechanisms can act on a request to delegate credentials
to the target host when the deleg_req_flag is set. In this case, to the target host when the deleg_req_flag is set. In this case,
extra care must be taken to ensure that the acceptor being extra care must be taken to ensure that the acceptor being
authenticated matches the target the user intended. Some mechanisms authenticated matches the target the user intended. Some mechanism
implementations (like commonly used krb5 libraries) may use insecure implementations (such as commonly used krb5 libraries) may use
DNS resolution to canonicalize the target name; in these cases insecure DNS resolution to canonicalize the target name; in these
spoofing a DNS response that points to an attacker-controlled machine cases spoofing a DNS response that points to an attacker-controlled
may results in the user silently delegating credentials to the machine may result in the user silently delegating credentials to the
attacker, who can then impersonate the user at will. attacker, who can then impersonate the user at will.
9. References 9. References
9.1. Normative References 9.1. Normative References
[I-D.ietf-curdle-ssh-curves] [I-D.ietf-curdle-ssh-curves]
Adamantiadis, A., Josefsson, S., and M. Baushke, "Secure Adamantiadis, A., Josefsson, S., and M. Baushke, "Secure
Shell (SSH) Key Exchange Method using Curve25519 and Shell (SSH) Key Exchange Method using Curve25519 and
Curve448", draft-ietf-curdle-ssh-curves-08 (work in Curve448", draft-ietf-curdle-ssh-curves-08 (work in
progress), June 2018. progress), June 2018.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992, DOI 10.17487/RFC1321, April 1992,
<https://www.rfc-editor.org/info/rfc1321>. <https://www.rfc-editor.org/info/rfc1321>.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<https://www.rfc-editor.org/info/rfc2045>.
[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, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743,
DOI 10.17487/RFC2743, January 2000,
<https://www.rfc-editor.org/info/rfc2743>.
[RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP) [RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)", Diffie-Hellman groups for Internet Key Exchange (IKE)",
RFC 3526, DOI 10.17487/RFC3526, May 2003, RFC 3526, DOI 10.17487/RFC3526, May 2003,
<https://www.rfc-editor.org/info/rfc3526>. <https://www.rfc-editor.org/info/rfc3526>.
[RFC4462] Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch, [RFC4462] Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch,
"Generic Security Service Application Program Interface "Generic Security Service Application Program Interface
(GSS-API) Authentication and Key Exchange for the Secure (GSS-API) Authentication and Key Exchange for the Secure
Shell (SSH) Protocol", RFC 4462, DOI 10.17487/RFC4462, May Shell (SSH) Protocol", RFC 4462, DOI 10.17487/RFC4462, May
2006, <https://www.rfc-editor.org/info/rfc4462>. 2006, <https://www.rfc-editor.org/info/rfc4462>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm [RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm
Integration in the Secure Shell Transport Layer", Integration in the Secure Shell Transport Layer",
RFC 5656, DOI 10.17487/RFC5656, December 2009, RFC 5656, DOI 10.17487/RFC5656, December 2009,
<https://www.rfc-editor.org/info/rfc5656>. <https://www.rfc-editor.org/info/rfc5656>.
[RFC7546] Kaduk, B., "Structure of the Generic Security Service [RFC7546] Kaduk, B., "Structure of the Generic Security Service
(GSS) Negotiation Loop", RFC 7546, DOI 10.17487/RFC7546, (GSS) Negotiation Loop", RFC 7546, DOI 10.17487/RFC7546,
May 2015, <https://www.rfc-editor.org/info/rfc7546>. May 2015, <https://www.rfc-editor.org/info/rfc7546>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
 End of changes. 27 change blocks. 
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