caucase - Certificate Authority for Users, Certificate Authority for SErvices

Overview

The goal of caucase is to automate certificate issuance and renewal without constraining how the certificate will be used.

For example, there is no assumption that the certificate will be used to secure HTTP, nor to serve anything at all: you may need certificates to authenticate users, or sign your mails, or secure an SQL server socket.

As an unfortunate consequence, it is not possible for caucase to automatically validate a signing request content against a service (ex: as one could check the certificate for an HTTPS service was requested by someone with the ability to make it serve a special file).

This also means that, while caucase imposes RFC-recommended constraints on many certificate fields and extensions to be worthy of trust, it imposes no constraint at all on subject and alternate subject certificate fields.

To still allow certificates to be used, caucase uses itself to authenticate users (humans or otherwise) who implement the validation procedure: they tell caucase what certificates to emit. Once done, any certificate can be prolonged at a simple request of the key holder while the to-renew certificate is still valid (not expired, not revoked).

Bootstrapping the system (creating the first service certificate for caucased to operate on HTTPS, and creating the first user certificate to control further certificate issuance) works by caucase automatically signing a set number of certificates upon submission.

Vocabulary

Caucase manipulates the following asymmetric cryptography concepts.

Key pair: A private key and corresponding public key. The public key can be derived from the private key, but not the other way around. As a consequence, the private key is itself considered to be a key pair.
Certificate: A certificate is the assurante, by a certificate authority, that a given public key and set of attributes belong to an authorised entity. Abbreviated cert or crt. A certificate is by definition signed by a CA.
Certificate Authority: An entry, arbitrarily trusted (but worthy of trust by its actions and decision) which can issue certificates. Abbreviated CA.
Certificate signing request: A document produced by an entity desiring to get certified, which they send to a certificate authority. The certificate signing request contains the public key and desired set of attributes that the CA should pronounce itself on. The CA has all liberty to issue a different set of attributes, or to not issue a certificate.
Certificate revocation list: Lists the certificates which were issued by a CA but which should not be trusted anymore. This can happen for a variety of reasons: the private key was compromised, or its owning entity should not be trusted anymore (ex: entity's permission to access to protected service was revoked).
PEM: A serialisation mechanism commonly used for various cryptographic data pieces. It relies on base64 so it is 7-bits-safe (unlike DER), and is very commonly supported. Caucase exclusively uses PEM format.

Validity period

Cryptographic keys wear out as are used and as they age.

Of course, they do not bit-rot nor become thinner with use. But each time one uses a key and each minute an attacker had access to a public key, fractions of the private key bits are inevitably leaked, weakening it overall.

So keys must be renewed periodically to preserve intended security level. So there is a limited life span to each certificate, including the ones emitted by caucase.

The unit duration for caucase-emitted certificates is the "normal" certificate life span. It default to 93 days from the moment the certificate was signed, or about 3 months.

Then the CA certificate has a default life span of 4 "normal" certificate validity periods. As CA renewal happens in caucase without x509-level cross signing (by decision, to avoid relying on intermediate CA support on certificate presenter side and instead rely on more widespread multi-CA-certificate support on verifier side), there is a hard lower bound of 3 validity periods, under which the CA certificate cannot be reliably renewed without risking certificate validation issues for emitted "normal" certificates. CA certificate renewal is composed of 2 phases:

Passive distribution phase: current CA certificate has a remaining life span of less than 2 "normal" certificate life spans: a new CA certificate is generated and distributed on-demand (on "normal" certificate renewal and issuance, on CRL retrieval with caucase tools...), but not used to sign anything.
Active use phase: new CA certificate is valid for more than one "normal" certificate life span. This means that all existing certificates which are still in active use had to be renewed at least once since the new CA certificate exists. This means all the certificate holders had the opportunity to learn about the new CA certificate. So the new CA certificate starts being used to sign new certificates, and the old CA certificate falls out of use as its signed "normal" certificates expire.

By default, all caucase tools will generate a new private key unrelated to the previous one on each certificate renewal.

Lastly, there is another limited validity period, although not for the same reasons: the list of revoked certificates also has a maximum life span. In caucase, the CRL is re-generated whenever it is requested and:

there is no previous CRL
previous CRL expired
any revocation happened since previous CRL was created

Commands

Caucase provides several commands to work with certificates.

caucase

Reference caucase "one-shot" client.

This command is intended to be used for isolated actions:

listing and signing pending certificate signature requests
revoking certificates

It is also able to submit certificate signing requests, retrieve signed certificates, requesting certificate renewals and updating both CA certificates and revocation lists, but you may be interested in using caucase-updater for this instead.

caucase-updater

Reference caucase certificate renewal daemon.

Monitors a key pair, corresponding CA certificate and CRL, and renew them before expiration.

When the key-pair lacks a signed certificate, issues a pre-existing CSR to caucase server and waits for the certificate to be issued.

caucase-probe

Caucase server availability tester.

Performs minimal checks to verify a caucase server is available at given URL.

caucase-rerequest

Utility allowing to re-issue a CSR using a locally-generated private key.

Intended to be used in conjunction with caucase-updater when user cannot generate the CSR on the system where the certificate is desired (ex: automated HTTPS server deployment), where user is not the intended audience for caucase-produced certificate:

User generates a CSR on their own system, and signs it with any key (it will not be needed later
User sends the CSR to the system where the certificate is desired
User gets caucase-rerequest to run on this CSR, producing a new private key and a CSR similar to issued one, but signed with this new private key
From then on, caucase-updater can take over

This way, no private key left their original system, and user could still freely customise certificate extensions.

caucase-key-id

Utility displaying the identifier of given key, or the identifier of keys involved in given backup file.

Allows identifying users which hold a private key candidate for restoring a caucased backup (see Restoration procedure).

caucased

Reference caucase server daemon.

This daemon provides access to both CAU and CAS services over both HTTP and HTTPS.

It handles its own certificate issuance and renewal, so there is no need to use caucase-updater for this service.

CORS

caucased implements CORS protection: when receiving a cross-origin request, it will respond with 401 Unauthorized, with the WWW-Authenticate header set to a custom scheme ("cors") with an "url" parameter containing an URI template with one variable field: "return" (more on it later).

Upon receiving this response, the application is expected to render the URL template and redirect the user to resulting URL. There, the user will be informed of the cross-origin access attempt, and offered the choice to grant or deny access to given origin.

Once their decision is made, their browser will receive a cookie remembering this decision, and they will be redirected to the URL received in the "return" field received upon above-described redirection.

Then, the application should retry the original request, which will be accompanied by that cookie.

Backups

Loosing the CA private key prevents issuing any new certificate trusted by services which trusted the CA. Also, it prevents issuing any new CRL. Recovering from such total loss requires starting a new CA and rolling it out to all services which used the previous one. This is very time-costly.

So backups are required.

On the other hand, if someone gets their hand on the CA private key, they can issue certificates for themselves, allowing them to authenticate with services trusting the CA managed by caucase - including caucased itself if they issue a user certificate: they can then revoke existing certificates and cause a lot of damage.

So backups cannot happen in clear text, they must be encrypted.

But the danger of encrypted backups is that by definition they become worthless if they cannot be decrypted. So as many (trusted) entities as possible should be granted the ability to decrypt the backups.

The solution proposed by caucased is to encrypt produced backups in a way which allows any of the caucase users to decrypt the archive.

As these users are already entrusted with issuing certificates, this puts only a little more power in their hands than they already have. The little extra power they get is that by having unrestricted access to the CA private key they can issue certificates bypassing all caucase restrictions. The proposed parade is to only make the backups available to a limited subset of caucase users when there is an actual disaster, and otherwise keep it out of their reach. This mechanism is not handled by caucase.

As there are few trusted users, caucase can keep their still-valid certificates in its database for the duration of their validity with minimal size cost.

Backup procedure

Backups happen periodically as long as caucased is running. See --backup-period and --backup-directory.

As discussed above, produced files should be kept out of reach of caucase users until a disaster happens.

Restoration procedure

See caucased-manage --restore-backup.

To restore, one of the trusted users must voluntarily compromise their own private key, providing it to the administrator in charge of the restoration procedure. Restoration procedure will hence immediately revoke their certificate. They must also provide a CSR generated with a different private key, so that caucase can provide them with a new certificate, so they keep their access only via different credentials.

admin identifies the list of keys which can decipher a backup, and broadcasts that list to key holders
key holders manifest themselves
admin picks a key holder, requests them to provide their existing private key and to generate a new key and accompanying CSR
key holder provide requested items
admin initiates restoration with --restore-backup and provides key holder with replacement certificate
admin starts caucased, service is back online.

Backup file format

64bits: 'caucase0' magic string
32bits LE: header length
header: json-encoded header (see below)
encrypted byte stream (aka payload)

Header schema (inspired from s/mime, but s/mime tools available do not support at least iterative production or iterative generation):

{
  "description": "Caucase backup header",
  "required": ["algorithm", "key_list"],
  "properties": {
    "cipher": {
      "description": "Symetric ciher used for payload",
      "required": ["name"],
      "properties": {
        "name":
          "enum": ["aes256_cbc_pkcs7_hmac_10M_sha256"],
          "type": "string"
        },
        "parameter": {
          "description": "Name-dependend clear cipher parameter (ex: IV)",
          "type": "string"
        }
      }
      "type": "object"
    },
    "key_list": {
      "description": "Content key, encrypted with public keys",
      "minItems": 1,
      "items": {
        "required": ["id", "cipher", "key"],
        "properties": {
          "id": {
            "description": "Hex-encoded sha1 hash of the public key",
            "type": "string"
          },
          "cipher": {
            "description": "Asymetric cipher used for symetric key",
            "required": ["name"],
            "properties": {
              "name": {
                "enum": ["rsa_oaep_sha1_mgf1_sha1"],
                "type": "string"
              }
            },
            "type": "object"
          }
          "key": {
            "description": "Hex-encoded encrypted concatenation of signing and symetric encryption keys",
            "type": "string"
          }
        },
        "type": "object"
      },
      "type": "array"
    }
  },
  "type": "object"
}