PROTOCOL.certkeys - platform/external/openssh - Gitiles

 This document describes a simple public-key certificate authentication
 system for use by SSH.

 Background
 ----------

 The SSH protocol currently supports a simple public key authentication
 mechanism. Unlike other public key implementations, SSH eschews the use
 of X.509 certificates and uses raw keys. This approach has some benefits
 relating to simplicity of configuration and minimisation of attack
 surface, but it does not support the important use-cases of centrally
 managed, passwordless authentication and centrally certified host keys.

 These protocol extensions build on the simple public key authentication
 system already in SSH to allow certificate-based authentication. The
 certificates used are not traditional X.509 certificates, with numerous
 options and complex encoding rules, but something rather more minimal: a
 key, some identity information and usage options that have been signed
 with some other trusted key.

 A sshd server may be configured to allow authentication via certified
 keys, by extending the existing ~/.ssh/authorized_keys mechanism to
 allow specification of certification authority keys in addition to
 raw user keys. The ssh client will support automatic verification of
 acceptance of certified host keys, by adding a similar ability to
 specify CA keys in ~/.ssh/known_hosts.

 Certified keys are represented using new key types:

     ssh-rsa-cert-v01@openssh.com
     ssh-dss-cert-v01@openssh.com
     ecdsa-sha2-nistp256-cert-v01@openssh.com
     ecdsa-sha2-nistp384-cert-v01@openssh.com
     ecdsa-sha2-nistp521-cert-v01@openssh.com

 These include certification information along with the public key
 that is used to sign challenges. ssh-keygen performs the CA signing
 operation.

 Protocol extensions
 -------------------

 The SSH wire protocol includes several extensibility mechanisms.
 These modifications shall take advantage of namespaced public key
 algorithm names to add support for certificate authentication without
 breaking the protocol - implementations that do not support the
 extensions will simply ignore them.

 Authentication using the new key formats described below proceeds
 using the existing SSH "publickey" authentication method described
 in RFC4252 section 7.

 New public key formats
 ----------------------

 The certificate key types take a similar high-level format (note: data
 types and encoding are as per RFC4251 section 5). The serialised wire
 encoding of these certificates is also used for storing them on disk.

 #define SSH_CERT_TYPE_USER    1
 #define SSH_CERT_TYPE_HOST    2

 RSA certificate

     string    "ssh-rsa-cert-v01@openssh.com"
     string    nonce
     mpint     e
     mpint     n
     uint64    serial
     uint32    type
     string    key id
     string    valid principals
     uint64    valid after
     uint64    valid before
     string    critical options
     string    extensions
     string    reserved
     string    signature key
     string    signature

 DSA certificate

     string    "ssh-dss-cert-v01@openssh.com"
     string    nonce
     mpint     p
     mpint     q
     mpint     g
     mpint     y
     uint64    serial
     uint32    type
     string    key id
     string    valid principals
     uint64    valid after
     uint64    valid before
     string    critical options
     string    extensions
     string    reserved
     string    signature key
     string    signature

 ECDSA certificate

     string    "ecdsa-sha2-nistp256@openssh.com" |
               "ecdsa-sha2-nistp384@openssh.com" |
               "ecdsa-sha2-nistp521@openssh.com"
     string    nonce
     string    curve
     string    public_key
     uint64    serial
     uint32    type
     string    key id
     string    valid principals
     uint64    valid after
     uint64    valid before
     string    critical options
     string    extensions
     string    reserved
     string    signature key
     string    signature

 The nonce field is a CA-provided random bitstring of arbitrary length
 (but typically 16 or 32 bytes) included to make attacks that depend on
 inducing collisions in the signature hash infeasible.

 e and n are the RSA exponent and public modulus respectively.

 p, q, g, y are the DSA parameters as described in FIPS-186-2.

 curve and public key are respectively the ECDSA "[identifier]" and "Q"
 defined in section 3.1 of RFC5656.

 serial is an optional certificate serial number set by the CA to
 provide an abbreviated way to refer to certificates from that CA.
 If a CA does not wish to number its certificates it must set this
 field to zero.

 type specifies whether this certificate is for identification of a user
 or a host using a SSH_CERT_TYPE_... value.

 key id is a free-form text field that is filled in by the CA at the time
 of signing; the intention is that the contents of this field are used to
 identify the identity principal in log messages.

 "valid principals" is a string containing zero or more principals as
 strings packed inside it. These principals list the names for which this
 certificate is valid; hostnames for SSH_CERT_TYPE_HOST certificates and
 usernames for SSH_CERT_TYPE_USER certificates. As a special case, a
 zero-length "valid principals" field means the certificate is valid for
 any principal of the specified type. XXX DNS wildcards?

 "valid after" and "valid before" specify a validity period for the
 certificate. Each represents a time in seconds since 1970-01-01
 00:00:00. A certificate is considered valid if:

     valid after <= current time < valid before

 criticial options is a set of zero or more key options encoded as
 below. All such options are "critical" in the sense that an implementation
 must refuse to authorise a key that has an unrecognised option.

 extensions is a set of zero or more optional extensions. These extensions
 are not critical, and an implementation that encounters one that it does
 not recognise may safely ignore it.

 Generally, critical options are used to control features that restrict
 access where extensions are used to enable features that grant access.
 This ensures that certificates containing unknown restrictions do not
 inadvertently grant access while allowing new protocol features to be
 enabled via extensions without breaking certificates' backwards
 compatibility.

 The reserved field is currently unused and is ignored in this version of
 the protocol.

 signature key contains the CA key used to sign the certificate.
 The valid key types for CA keys are ssh-rsa, ssh-dss and the ECDSA types
 ecdsa-sha2-nistp256, ecdsa-sha2-nistp384, ecdsa-sha2-nistp521. "Chained"
 certificates, where the signature key type is a certificate type itself
 are NOT supported. Note that it is possible for a RSA certificate key to
 be signed by a DSS or ECDSA CA key and vice-versa.

 signature is computed over all preceding fields from the initial string
 up to, and including the signature key. Signatures are computed and
 encoded according to the rules defined for the CA's public key algorithm
 (RFC4253 section 6.6 for ssh-rsa and ssh-dss, RFC5656 for the ECDSA
 types).

 Critical options
 ----------------

 The critical options section of the certificate specifies zero or more
 options on the certificates validity. The format of this field
 is a sequence of zero or more tuples:

     string       name
     string       data

 Options must be lexically ordered by "name" if they appear in the
 sequence. Each named option may only appear once in a certificate.

 The name field identifies the option and the data field encodes
 option-specific information (see below). All options are
 "critical", if an implementation does not recognise a option
 then the validating party should refuse to accept the certificate.

 The supported options and the contents and structure of their
 data fields are:

 Name                    Format        Description
 -----------------------------------------------------------------------------
 force-command           string        Specifies a command that is executed
                                       (replacing any the user specified on the
                                       ssh command-line) whenever this key is
                                       used for authentication.

 source-address          string        Comma-separated list of source addresses
                                       from which this certificate is accepted
                                       for authentication. Addresses are
                                       specified in CIDR format (nn.nn.nn.nn/nn
                                       or hhhh::hhhh/nn).
                                       If this option is not present then
                                       certificates may be presented from any
                                       source address.

 Extensions
 ----------

 The extensions section of the certificate specifies zero or more
 non-critical certificate extensions. The encoding and ordering of
 extensions in this field is identical to that of the critical options,
 as is the requirement that each name appear only once.

 If an implementation does not recognise an extension, then it should
 ignore it.

 The supported extensions and the contents and structure of their data
 fields are:

 Name                    Format        Description
 -----------------------------------------------------------------------------
 permit-X11-forwarding   empty         Flag indicating that X11 forwarding
                                       should be permitted. X11 forwarding will
                                       be refused if this option is absent.

 permit-agent-forwarding empty         Flag indicating that agent forwarding
                                       should be allowed. Agent forwarding
                                       must not be permitted unless this
                                       option is present.

 permit-port-forwarding  empty         Flag indicating that port-forwarding
                                       should be allowed. If this option is
                                       not present then no port forwarding will
                                       be allowed.

 permit-pty              empty         Flag indicating that PTY allocation
                                       should be permitted. In the absence of
                                       this option PTY allocation will be
                                       disabled.

 permit-user-rc          empty         Flag indicating that execution of
                                       ~/.ssh/rc should be permitted. Execution
                                       of this script will not be permitted if
                                       this option is not present.

 $OpenBSD: PROTOCOL.certkeys,v 1.9 2012/03/28 07:23:22 djm Exp $
	This document describes a simple public-key certificate authentication
	system for use by SSH.

	Background
	----------

	The SSH protocol currently supports a simple public key authentication
	mechanism. Unlike other public key implementations, SSH eschews the use
	of X.509 certificates and uses raw keys. This approach has some benefits
	relating to simplicity of configuration and minimisation of attack
	surface, but it does not support the important use-cases of centrally
	managed, passwordless authentication and centrally certified host keys.

	These protocol extensions build on the simple public key authentication
	system already in SSH to allow certificate-based authentication. The
	certificates used are not traditional X.509 certificates, with numerous
	options and complex encoding rules, but something rather more minimal: a
	key, some identity information and usage options that have been signed
	with some other trusted key.

	A sshd server may be configured to allow authentication via certified
	keys, by extending the existing ~/.ssh/authorized_keys mechanism to
	allow specification of certification authority keys in addition to
	raw user keys. The ssh client will support automatic verification of
	acceptance of certified host keys, by adding a similar ability to
	specify CA keys in ~/.ssh/known_hosts.

	Certified keys are represented using new key types:

	ssh-rsa-cert-v01@openssh.com
	ssh-dss-cert-v01@openssh.com
	ecdsa-sha2-nistp256-cert-v01@openssh.com
	ecdsa-sha2-nistp384-cert-v01@openssh.com
	ecdsa-sha2-nistp521-cert-v01@openssh.com

	These include certification information along with the public key
	that is used to sign challenges. ssh-keygen performs the CA signing
	operation.

	Protocol extensions
	-------------------

	The SSH wire protocol includes several extensibility mechanisms.
	These modifications shall take advantage of namespaced public key
	algorithm names to add support for certificate authentication without
	breaking the protocol - implementations that do not support the
	extensions will simply ignore them.

	Authentication using the new key formats described below proceeds
	using the existing SSH "publickey" authentication method described
	in RFC4252 section 7.

	New public key formats
	----------------------

	The certificate key types take a similar high-level format (note: data
	types and encoding are as per RFC4251 section 5). The serialised wire
	encoding of these certificates is also used for storing them on disk.

	#define SSH_CERT_TYPE_USER 1
	#define SSH_CERT_TYPE_HOST 2

	RSA certificate

	string "ssh-rsa-cert-v01@openssh.com"
	string nonce
	mpint e
	mpint n
	uint64 serial
	uint32 type
	string key id
	string valid principals
	uint64 valid after
	uint64 valid before
	string critical options
	string extensions
	string reserved
	string signature key
	string signature

	DSA certificate

	string "ssh-dss-cert-v01@openssh.com"
	string nonce
	mpint p
	mpint q
	mpint g
	mpint y
	uint64 serial
	uint32 type
	string key id
	string valid principals
	uint64 valid after
	uint64 valid before
	string critical options
	string extensions
	string reserved
	string signature key
	string signature

	ECDSA certificate

	string "ecdsa-sha2-nistp256@openssh.com" \|
	"ecdsa-sha2-nistp384@openssh.com" \|
	"ecdsa-sha2-nistp521@openssh.com"
	string nonce
	string curve
	string public_key
	uint64 serial
	uint32 type
	string key id
	string valid principals
	uint64 valid after
	uint64 valid before
	string critical options
	string extensions
	string reserved
	string signature key
	string signature

	The nonce field is a CA-provided random bitstring of arbitrary length
	(but typically 16 or 32 bytes) included to make attacks that depend on
	inducing collisions in the signature hash infeasible.

	e and n are the RSA exponent and public modulus respectively.

	p, q, g, y are the DSA parameters as described in FIPS-186-2.

	curve and public key are respectively the ECDSA "[identifier]" and "Q"
	defined in section 3.1 of RFC5656.

	serial is an optional certificate serial number set by the CA to
	provide an abbreviated way to refer to certificates from that CA.
	If a CA does not wish to number its certificates it must set this
	field to zero.

	type specifies whether this certificate is for identification of a user
	or a host using a SSH_CERT_TYPE_... value.

	key id is a free-form text field that is filled in by the CA at the time
	of signing; the intention is that the contents of this field are used to
	identify the identity principal in log messages.

	"valid principals" is a string containing zero or more principals as
	strings packed inside it. These principals list the names for which this
	certificate is valid; hostnames for SSH_CERT_TYPE_HOST certificates and
	usernames for SSH_CERT_TYPE_USER certificates. As a special case, a
	zero-length "valid principals" field means the certificate is valid for
	any principal of the specified type. XXX DNS wildcards?

	"valid after" and "valid before" specify a validity period for the
	certificate. Each represents a time in seconds since 1970-01-01
	00:00:00. A certificate is considered valid if:

	valid after <= current time < valid before

	criticial options is a set of zero or more key options encoded as
	below. All such options are "critical" in the sense that an implementation
	must refuse to authorise a key that has an unrecognised option.

	extensions is a set of zero or more optional extensions. These extensions
	are not critical, and an implementation that encounters one that it does
	not recognise may safely ignore it.

	Generally, critical options are used to control features that restrict
	access where extensions are used to enable features that grant access.
	This ensures that certificates containing unknown restrictions do not
	inadvertently grant access while allowing new protocol features to be
	enabled via extensions without breaking certificates' backwards
	compatibility.

	The reserved field is currently unused and is ignored in this version of
	the protocol.

	signature key contains the CA key used to sign the certificate.
	The valid key types for CA keys are ssh-rsa, ssh-dss and the ECDSA types
	ecdsa-sha2-nistp256, ecdsa-sha2-nistp384, ecdsa-sha2-nistp521. "Chained"
	certificates, where the signature key type is a certificate type itself
	are NOT supported. Note that it is possible for a RSA certificate key to
	be signed by a DSS or ECDSA CA key and vice-versa.

	signature is computed over all preceding fields from the initial string
	up to, and including the signature key. Signatures are computed and
	encoded according to the rules defined for the CA's public key algorithm
	(RFC4253 section 6.6 for ssh-rsa and ssh-dss, RFC5656 for the ECDSA
	types).

	Critical options
	----------------

	The critical options section of the certificate specifies zero or more
	options on the certificates validity. The format of this field
	is a sequence of zero or more tuples:

	string name
	string data

	Options must be lexically ordered by "name" if they appear in the
	sequence. Each named option may only appear once in a certificate.

	The name field identifies the option and the data field encodes
	option-specific information (see below). All options are
	"critical", if an implementation does not recognise a option
	then the validating party should refuse to accept the certificate.

	The supported options and the contents and structure of their
	data fields are:

	Name Format Description
	-----------------------------------------------------------------------------
	force-command string Specifies a command that is executed
	(replacing any the user specified on the
	ssh command-line) whenever this key is
	used for authentication.

	source-address string Comma-separated list of source addresses
	from which this certificate is accepted
	for authentication. Addresses are
	specified in CIDR format (nn.nn.nn.nn/nn
	or hhhh::hhhh/nn).
	If this option is not present then
	certificates may be presented from any
	source address.

	Extensions
	----------

	The extensions section of the certificate specifies zero or more
	non-critical certificate extensions. The encoding and ordering of
	extensions in this field is identical to that of the critical options,
	as is the requirement that each name appear only once.

	If an implementation does not recognise an extension, then it should
	ignore it.

	The supported extensions and the contents and structure of their data
	fields are:

	Name Format Description
	-----------------------------------------------------------------------------
	permit-X11-forwarding empty Flag indicating that X11 forwarding
	should be permitted. X11 forwarding will
	be refused if this option is absent.

	permit-agent-forwarding empty Flag indicating that agent forwarding
	should be allowed. Agent forwarding
	must not be permitted unless this
	option is present.

	permit-port-forwarding empty Flag indicating that port-forwarding
	should be allowed. If this option is
	not present then no port forwarding will
	be allowed.

	permit-pty empty Flag indicating that PTY allocation
	should be permitted. In the absence of
	this option PTY allocation will be
	disabled.

	permit-user-rc empty Flag indicating that execution of
	~/.ssh/rc should be permitted. Execution
	of this script will not be permitted if
	this option is not present.

	$OpenBSD: PROTOCOL.certkeys,v 1.9 2012/03/28 07:23:22 djm Exp $