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Published: Mar 30, 2021 License: MPL-2.0 Imports: 13 Imported by: 0


Content Signature

.. sectnum::
.. contents:: Table of Contents


As we rapidly increase the number of services that send configuration data to
Firefox agents, we also increase the probability of a service being
compromised to serve fraudulent data to our users. Content Signature implements
a signing protocol to protect the information sent from backend services to Firefox

Content signature adds a layer to TLS and certificate pinning.
As we grow our service infrastructure, the risk of a vulnerability on our public
endpoints increases, and an attacker could exploit a vulnerability to serve bad
data from trusted sites directly. TLS with certificate pinning prevents bad actors
from creating fraudulent Firefox services, but does not reduce the impact a break-in
would have on our users. Content signature provides this extra layer.

Finally, content signature helps us use Content Delivery Networks (CDN) without
worrying that a compromise would end-up serving bad data to our users.
Signing content at the source reduces pressure on the infrastructure
and allows us to rely on vendors without worrying about data integrity.

For more information, refer to Julien Vehent's presentation linked below:

.. image::


Content signatures are computed on data and served to Firefox either via a HTTP
response header or through a separate signature field in the data being transported.

Content signature have three main components: a signature mode (**mode**), an
ecdsa signature encoded with Base64 URL (**signature**) and the URL to a chain
of certificates that link to a trusted root (**x5u**). The example below shows
the JSON representation of a content signature:

.. code:: json

	  "mode": "p384ecdsa",
	  "signature": "gZimwQAsuCj_JcgxrIjw1wzON8WYN9YKp3I5I9NmOgnGLOJJwHDxjOA2QEnzN7bXBGWFgn8HJ7fGRYxBy1SHiDMiF8VX7V49KkanO9MO-RRN1AyC9xmghuEcF4ndhQaI",
	  "x5u": ""

* **mode** is a suite of algorithms used to issue the signature. Autograph uses three

  * **p384ecdsa** is the default used by firefox. It calculates signatures on the P-384
    NIST curve and uses SHA2-384 for hashes.

  * **p256ecdsa** uses the P-256 NIST curve and SHA256 for hashes

  * **p521ecdsa** uses the P-521 NIST curve and SHA512 for hashes

* **signature** contains the base64_url of the signature, computed using an elliptic
  curve and a hash algorithm that depends on the mode. The signature is issued by
  the private key of the end-entity cert referenced in the X5U. The decoded base64
  contains a binary string that is a DL/ECSSA representation of the R and S values
  (IEEE Std 1363-2000). This format concatenates R and S into a single value. To
  retrieve R and S, split the decoded base64 in the middle, and take R on the left
  and S on the right.

* **x5u** contains the location of the chain of trust that issued the signature.
  This file contains at least two certificates encoded in PEM format, where the
  first certificate is the end-entity that issued the signature, and the last
  certificate is the root of the PKI. Firefox is configured to only accept
  signatures from the internal PKI shared with AMO. This is controlled via the
  `security.content.signature.root_hash` preference, where the value is the
  hexadecimal of the sha256 of the DER of the root certificate.

When Firefox verifies a content signature, it first retrieves the X5U and checks
the signature validity using the end-entity certificate, the signature, and the
content being protected. Firefox then verifies the chain of trust of the
end-entity links to a root cert with a hash matching the one in Firefox.
Finally, to prevent application A from signing content for application B,
Firefox verifies the subject alternate name of the end-entity certificate
matches the one it expects. This is hardcoded for each component that uses
content signature. Onecrl, for example, uses the namespace
`` and only end-entity certificates that
have this subject alternate name can issue signatures for the OneCRL service.


The type of this signer is **contentsignature**.

Configuring an Autograph signer to issue content signature requires providing
the private ECDSA key and the X5U value to be used in signatures.

 Each signer is composed of an identifier and an ECDSA private key on the P-384
 NIST curve. To generate a key pair with openssl, use:

.. code:: bash

	$ openssl ecparam -name secp384r1 -genkey

The output from OpenSSL must be copied under the `privatekey` section of the
signer, as follows:

.. code:: yaml

    - id: appkey1
      type: contentsignature
      privatekey: |
          -----BEGIN EC PARAMETERS-----
          -----END EC PARAMETERS-----
          -----BEGIN EC PRIVATE KEY-----
          -----END EC PRIVATE KEY-----

Based on the `privatekey`, autograph will return the corresponding `publickey`
in the JSON responses. If you're using a PKI and want to verify signatures with
a X.509 certificate, you can generate this certificate based on the private key,
store it someplace, and tell autograph to return its location in the `x5u`

.. code:: bash

	# first make a CSR based on the private key
	$ openssl req -new -key /tmp/autograph-dev.key -out /tmp/autograph-dev.csr

	# then self sign the CSR
	$ openssl x509 -req -days 365 -in /tmp/autograph-dev.csr -signkey /tmp/autograph-dev.key -out /tmp/autograph-dev.crt

Store the CRT on `` and set the x5u value in `autograph.yaml`.

.. code:: yaml

	- id: appkey2
	  x5u: ""
      type: contentsignature
      privatekey: |
          -----BEGIN EC PARAMETERS-----

Signature requests

This signer support both the `/sign/data` and `/sign/hash` endpoints. When
signing data, the base64 of the data being signed must be passed in the `input`
field of the JSON signing request. When signing hashes, the `input` field must
contain the base64 of the hash being signed.

.. code:: json

			"input": "Y2FyaWJvdW1hdXJpY2UK",
			"keyid": "some_content_signer"

This signer doesn't support any option.




View Source
const (
	// Type of this signer is 'contentsignature'
	Type = "contentsignature"

	// P256ECDSA defines an ecdsa content signature on the P-256 curve
	P256ECDSA = "p256ecdsa"

	// P256ECDSABYTESIZE defines the bytes length of a P256ECDSA signature

	// P384ECDSA defines an ecdsa content signature on the P-384 curve
	P384ECDSA = "p384ecdsa"

	// P384ECDSABYTESIZE defines the bytes length of a P384ECDSA signature

	// P521ECDSA defines an ecdsa content signature on the P-521 curve
	P521ECDSA = "p521ecdsa"

	// P521ECDSABYTESIZE defines the bytes length of a P521ECDSA signature

	// SignaturePrefix is a string preprended to data prior to signing
	SignaturePrefix = "Content-Signature:\x00"


This section is empty.


This section is empty.


type ContentSignature

type ContentSignature struct {
	R, S     *big.Int // fields must be exported for ASN.1 marshalling
	HashName string
	Mode     string
	X5U      string
	ID       string
	Len      int
	Finished bool

ContentSignature contains the parsed representation of a signature

func Unmarshal

func Unmarshal(signature string) (sig *ContentSignature, err error)

Unmarshal parses a base64 url encoded content signature and returns it into a ContentSignature structure that can be verified.

Note this function does not set the X5U value of a signature.

func (*ContentSignature) Marshal

func (sig *ContentSignature) Marshal() (str string, err error)

Marshal returns the R||S signature is encoded in base64 URL safe, following DL/ECSSA format spec from IEEE Std 1363-2000.

func (*ContentSignature) String

func (sig *ContentSignature) String() string

func (*ContentSignature) VerifyData

func (sig *ContentSignature) VerifyData(input []byte, pubKey *ecdsa.PublicKey) bool

VerifyData verifies a signatures on its raw, untemplated, input using a public key

func (*ContentSignature) VerifyHash

func (sig *ContentSignature) VerifyHash(hash []byte, pubKey *ecdsa.PublicKey) bool

VerifyHash verifies a signature on its templated hash using a public key

type ContentSigner

type ContentSigner struct {
	// contains filtered or unexported fields

ContentSigner implements an issuer of content signatures

func New

func New(conf signer.Configuration) (s *ContentSigner, err error)

New initializes a ContentSigner using a signer configuration

func (*ContentSigner) Config

func (s *ContentSigner) Config() signer.Configuration

Config returns the configuration of the current signer

func (*ContentSigner) GetDefaultOptions

func (s *ContentSigner) GetDefaultOptions() interface{}

GetDefaultOptions returns nil because this signer has no option

func (*ContentSigner) SignData

func (s *ContentSigner) SignData(input []byte, options interface{}) (signer.Signature, error)

SignData takes input data, templates it, hashes it and signs it. The returned signature is of type ContentSignature and ready to be Marshalled.

func (*ContentSigner) SignHash

func (s *ContentSigner) SignHash(input []byte, options interface{}) (signer.Signature, error)

SignHash takes an input hash and returns a signature. It assumes the input data has already been hashed with something like sha384

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