target = "https://tools.ietf.org/rfc/rfc8446#4.2.9"

# 4.2.9.  Pre-Shared Key Exchange Modes
#
# In order to use PSKs, clients MUST also send a
# "psk_key_exchange_modes" extension.  The semantics of this extension
# are that the client only supports the use of PSKs with these modes,
# which restricts both the use of PSKs offered in this ClientHello and
# those which the server might supply via NewSessionTicket.
# 
# A client MUST provide a "psk_key_exchange_modes" extension if it
# offers a "pre_shared_key" extension.  If clients offer
# "pre_shared_key" without a "psk_key_exchange_modes" extension,
# servers MUST abort the handshake.  Servers MUST NOT select a key
# exchange mode that is not listed by the client.  This extension also
# restricts the modes for use with PSK resumption.  Servers SHOULD NOT
# send NewSessionTicket with tickets that are not compatible with the
# advertised modes; however, if a server does so, the impact will just
# be that the client's attempts at resumption fail.
# 
# The server MUST NOT send a "psk_key_exchange_modes" extension.
# 
#    enum { psk_ke(0), psk_dhe_ke(1), (255) } PskKeyExchangeMode;
# 
#    struct {
#        PskKeyExchangeMode ke_modes<1..255>;
#    } PskKeyExchangeModes;
# 
# psk_ke:  PSK-only key establishment.  In this mode, the server
#    MUST NOT supply a "key_share" value.
# 
# psk_dhe_ke:  PSK with (EC)DHE key establishment.  In this mode, the
#    client and server MUST supply "key_share" values as described in
#    Section 4.2.8.
# 
# Any future values that are allocated must ensure that the transmitted
# protocol messages unambiguously identify which mode was selected by
# the server; at present, this is indicated by the presence of the
# "key_share" in the ServerHello.

[[spec]]
level = "MUST"
quote = '''
In order to use PSKs, clients MUST also send a
"psk_key_exchange_modes" extension.
'''

[[spec]]
level = "MUST"
quote = '''
A client MUST provide a "psk_key_exchange_modes" extension if it
offers a "pre_shared_key" extension.
'''

[[spec]]
level = "MUST"
quote = '''
If clients offer
"pre_shared_key" without a "psk_key_exchange_modes" extension,
servers MUST abort the handshake.
'''

[[spec]]
level = "MUST"
quote = '''
Servers MUST NOT select a key
exchange mode that is not listed by the client.
'''

[[spec]]
level = "SHOULD"
quote = '''
Servers SHOULD NOT
send NewSessionTicket with tickets that are not compatible with the
advertised modes; however, if a server does so, the impact will just
be that the client's attempts at resumption fail.
'''

[[spec]]
level = "MUST"
quote = '''
The server MUST NOT send a "psk_key_exchange_modes" extension.
'''

[[spec]]
level = "MUST"
quote = '''
In this mode, the server
MUST NOT supply a "key_share" value.
'''

[[spec]]
level = "MUST"
quote = '''
In this mode, the
client and server MUST supply "key_share" values as described in
Section 4.2.8.
'''