src/usr.bin/ssh/PROTOCOL.certkeys - diff

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Diff for /src/usr.bin/ssh/PROTOCOL.certkeys between version 1.7 and 1.11

version 1.7, 2010/08/04 05:40:39

version 1.11, 2017/05/16 16:54:05

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The SSH protocol currently supports a simple public key authentication

mechanism. Unlike other public key implementations, SSH eschews the

mechanism. Unlike other public key implementations, SSH eschews the use

use of X.509 certificates and uses raw keys. This approach has some

of X.509 certificates and uses raw keys. This approach has some benefits

benefits relating to simplicity of configuration and minimisation

relating to simplicity of configuration and minimisation of attack

of attack surface, but it does not support the important use-cases

surface, but it does not support the important use-cases of centrally

of centrally managed, passwordless authentication and centrally

managed, passwordless authentication and centrally certified host keys.

certified host keys.

These protocol extensions build on the simple public key authentication

system already in SSH to allow certificate-based authentication.

system already in SSH to allow certificate-based authentication. The

The certificates used are not traditional X.509 certificates, with

certificates used are not traditional X.509 certificates, with numerous

numerous options and complex encoding rules, but something rather

options and complex encoding rules, but something rather more minimal: a

more minimal: a key, some identity information and usage options

key, some identity information and usage options that have been signed

that have been signed with some other trusted key.

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

keys, by extending the existing ~/.ssh/authorized_keys mechanism to

to allow specification of certification authority keys in addition

allow specification of certification authority keys in addition to

to raw user keys. The ssh client will support automatic verification

raw user keys. The ssh client will support automatic verification of

of acceptance of certified host keys, by adding a similar ability

acceptance of certified host keys, by adding a similar ability to

to specify CA keys in ~/.ssh/known_hosts.

specify CA keys in ~/.ssh/known_hosts.

Certified keys are represented using two new key types:

Certified keys are represented using new key types:

ssh-rsa-cert-v01@openssh.com and ssh-dss-cert-v01@openssh.com that

include certification information along with the public key that is used

to sign challenges. ssh-keygen performs the CA signing operation.

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

-------------------

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New public key formats

----------------------

The ssh-rsa-cert-v01@openssh.com and ssh-dss-cert-v01@openssh.com key

The certificate key types take a similar high-level format (note: data

types take a similar high-level format (note: data types and

types and encoding are as per RFC4251 section 5). The serialised wire

encoding are as per RFC4251 section 5). The serialised wire encoding of

encoding of these certificates is also used for storing them on disk.

these certificates is also used for storing them on disk.

#define SSH_CERT_TYPE_USER 1

#define SSH_CERT_TYPE_HOST 2

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string signature key

string signature

ECDSA certificate

string "ecdsa-sha2-nistp256-v01@openssh.com" |

"ecdsa-sha2-nistp384-v01@openssh.com" |

"ecdsa-sha2-nistp521-v01@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

ED25519 certificate

string "ssh-ed25519-cert-v01@openssh.com"

string nonce

string pk

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.

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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.

pk is the encoded Ed25519 public key as defined by

draft-josefsson-eddsa-ed25519-03.

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

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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?

any principal of the specified type.

"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.

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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 signature key field contains the CA key used to sign the

The valid key types for CA keys are ssh-rsa and ssh-dss. "Chained"

certificate. The valid key types for CA keys are ssh-rsa,

certificates, where the signature key type is a certificate type itself

ssh-dss, ssh-ed25519 and the ECDSA types ecdsa-sha2-nistp256,

are NOT supported. Note that it is possible for a RSA certificate key to

ecdsa-sha2-nistp384, ecdsa-sha2-nistp521. "Chained" certificates, where

be signed by a DSS CA key and vice-versa.

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

Ed25519 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).

(RFC4253 section 6.6 for ssh-rsa and ssh-dss, RFC5656 for the ECDSA

types), and draft-josefsson-eddsa-ed25519-03 for Ed25519.

Critical options

----------------

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string data

Options must be lexically ordered by "name" if they appear in the

sequence.

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

No critical options are defined for host certificates at present. The

data fields are:

supported user certificate options and the contents and structure of

their data fields are:

Name Format Description

-----------------------------------------------------------------------------

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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.

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

No extensions are defined for host certificates at present. The

fields are:

supported user certificate extensions and the contents and structure of

their data fields are:

Name Format Description

-----------------------------------------------------------------------------