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Notation Extensibility for Signing and Verification

Keys and associated certificates used for signing artifacts using Notation could be available to users through varied solutions that provide secure key generation, storage and cryptographic operations. Some are well established with standards like PIV and PKCS #11 implemented by hardware tokens, smart cards. More recent options which use varied authentication and API protocols are remote key management services and signing services by third party vendors and cloud service providers. Notation will support a few built-in integrations with standard providers, and will provide plugin interfaces for users, and vendors to implement their own integrations with the solutions they use. This allows a plugin publisher to implement, test, release and patch their solutions independent of Notation’s development and release cycle. This document provides specification for the plugin model, and interfaces to implement. This specification aims to work both for existing and future signature formats adopted by Notary Project.

Terminology

  • Plugin Publisher - A user, organization, open source project or 3rd party vendor that creates a Notation plugin for internal or public distribution.
  • Plugin - A component external to Notation that can integrate as one of the steps in Notation’s workflow for signature generation or verification. A Notation plugin can be distributed as a single executable file, or an archive file (zip or tar.gz).
  • Default provider - Signing and verification mechanisms built into Notation itself to provide default experience without requiring to install/configure additional plugins. [We are yet to define what is included in the default experience].

Plugin mechanism

Requirements

  • A plugin publisher MUST be able to distribute and patch a Notation plugin independently of Notation’s release cycle.
  • The plugin mechanism MUST work across commonly used OS platforms like Linux, Windows and macOS.
  • The plugin interface contract MUST be versioned. This version is different from Notation's release version. It will be used to accommodate for additional capabilities and contract changes post initial release of Notation.
  • A plugin MAY implement a subset of capabilities (features) available in plugin contract. E.g A plugin may implement signing feature, but not verification.
  • Notation and plugins MAY be updated independently in an environment.
    • Notation MUST work with a plugin that implements a matching or lower minor version of the plugin contract. Notation SHALL NOT support using a plugin with higher version of plugin contract.
    • A plugin MUST support a single plugin contract version, per major version.

Notation will invoke plugins as executable, pass parameters using command line arguments, and use standard IO streams to pass request/response payloads. This mechanism is used as Go language (used to develop Notation Go library) does not have a in built support to load and execute plugins that works across OS platforms. Other mechanisms like gRPC require every plugin to be implemented as a service/daemon.

Package and release a plugin

Notation supports the installation of a plugin from an https URL or from a file in the filesystem. To support the plugin installation and management using notation plugin commands, the plugin publisher must adhere to the following conventions:

  • Plugin executable file MUST follow the naming convention notation-{plugin-name}. On windows OS, the file extension .exe is REQUIRED.
  • The plugin distribution format MUST be either .zip, .tar.gz, or a single plugin executable file. If the release format is single plugin executable file, it is highly recommended to compress it into an archive for installation efficiency consideration.
  • If the archive format is .zip or .tar.gz, there MUST be one and only one plugin executable file within each archive.
  • Plugin publisher MUST provide SHA256 checksum for each released archive or single plugin executable file if they want to enable users to install a plugin from an https URL.
  • The plugin archive MAY contain License files. Also, it's recommended to include licenses in the archive.
  • Currently, Notation facilitates the installation of plugin executable files and archives, with a size limit of less than 256 MiB. Consequently, the plugin executable file and archive size MUST be less than 256 MiB. Note: Although users have the option to install plugins larger than 256 MiB, they will be unable to utilize the notation plugin install command in such cases.

For example, an archive of a Notation plugin helloworld for Linux AMD64 machine notation-helloworld_1.0.1_linux_amd64.tar.gz includes these files:

notation-helloworld_1.0.1_linux_amd64.tar.gz
├── notation-helloworld (required)
├── LICENSE (optional)
└── Dependencies (required if present)

Plugin lifecycle management

Installation

Plugin publisher will provide instructions to download and install the plugin. Plugins intended for public distribution should also include instructions for users to verify the authenticity of the plugin.

Plugin installation methods

Notation offers plugin management commands that allow users to install plugins from various sources, including https URL and filesystem.

Plugin installation path

To enumerate all available plugins the PLUGIN_DIRECTORY is scanned based on per OS:

OS PLUGIN_DIRECTORY
Unix $XDG_CONFIG_HOME/notation/plugins
Windows %AppData%/notation/plugins
Darwin $HOME/Library/Application Support/notation/plugins

Each plugin executable must be located under $PLUGIN_DIRECTORY/{plugin-name} directory, with executable named as notation-{plugin-name}.

Any directory found inside $PLUGIN_DIRECTORY is considered potential plugin "candidates". Anything found which is not a directory is ignored and is not considered as a plugin candidate.

To be considered a valid plugin a candidate must pass each of these "plugin candidate tests":

  • The directory must contain an executable named notation-{plugin-name}.
  • The executable MUST be a regular file, symlinks are not supported. Implementation MUST validate that the executable is a regular file, before executing it, and fail it it does not meet this condition.
  • On Windows, executables must have a .exe suffix.
  • Must, where relevant, have appropriate OS "execute" permissions (e.g. Unix x bit set) for the current user.
  • Must actually be executed successfully and when executed with the subcommand get-plugin-metadata must produce a valid JSON metadata (and nothing else) on its standard output (schema to be discussed later).

Commands

Notation provides various commands, including install, list, and uninstall, to manage plugin lifecycle. For more information, please refer to the notation plugin CLI specification.

Using a plugin for signing

  • To use a plugin for signing, the user associates the plugin as part of registering a signing key. E.g.
    • notation key add --name "mysigningkey" --id "keyid" --plugin "com.example.nv2plugin"
    • In the example, the command registers a signing key in /notation/config.json, where mysigningkey is a friendly key name to refer during signing operation from the CLI, id is an key identifier known to plugin that is used for signing, and the value of plugin specifies it's using a plugin located at ~/notation/plugins/com.example.nv2plugin/notation-com.example.nv2plugin.
{
  "signingKeys": {  
    "default": "",  
    "keys": [
        {  
          "name": "mysigningkey",  
          "id" : "keyid",
          "plugin": "com.example.nv2plugin",
          // Optional configuration as required by the plugin
          "pluginConfig" : {   
            "key1" : "value1",
            "key 2" : "value 2"
          }
        }
    ]  
  }
}

Plugin configuration

Plugins may require additional configuration to work correctly, that could be set out of band, or provided by notation when it invokes a plugin. To support plugin configuration through notation, the key configuration provides an optional pluginConfig map of of key value pairs, that is passed as-is by notation to the plugin.

  • To use this feature, plugin authors MUST define and document the set of plugin configuration keys-values, which is set by users when they associate a plugin with signing key.
  • Plugin authors SHOULD NOT use plugin configuration to store sensitive configuration in plaintext, such as authentication keys used by the plugin etc.

For the previous example, plugin config can be set using command line arguments as part of registering a key with the notation key add command. E.g.

  • notation key add "mysigningkey" --id "keyid" --plugin "com.example.nv2plugin" --plugin-config key1=value1 --plugin-config "key 2"="value 2"

Plugin config can be also set/overriden during signing with the notation sign command. Following example overrides value for key 2 already set in config.json by previous command.

  • notation sign $IMAGE --key "mysigningkey" --plugin-config "key 2"=newValue2

Plugin contract

  • Notation will invoke the plugin executable for each command (e.g. sign, verify), pass inputs through stdin and get output through stdout and stderr.
  • The command will be passed as the first argument to the plugin e.g. notation-{plugin-name} <command>. A JSON request is passed using stdin. The plugin is expected to return a JSON response through stdout with a 0 exit code for successful response, and a non-zero exit code with a JSON error response in stderr for error response. Each command defines its request, response and error contract. To avoid any additional content like debug or info level logging from dependencies and inbuilt libraries, the plugin implementation should redirect any output to stdout on initialization, and only send the JSON response away from stdout when the command execution completes. E.g. For golang, set os.Stdout to point to a log file.
  • Every request JSON will contain a contractVersion top level attribute whose value will indicate the plugin contract version. Contract version is revised when there are changes to command request/response, new plugin commands are introduced, and supported through Notation.
  • To maintain forward compatibility plugin implementors MUST ignore unrecognized attributes in command request which are introduced in minor version updates of the plugin contract.
  • For an error response, every command returns a non-zero exit code 1, with an OPTIONAL JSON error response in stderr. It is recommended to return an error response to help user troubleshoot the error. There is no need to send different exit codes for different error conditions, as Notation (which will call the plugin and parse the response) will use error-code in the error response to interpret different error conditions if needed. Notation will attempt to parse the error response in stderr when exit code is 1, else treat it as a general error for any other non-zero exit codes.
{
  // Each command defines expected error codes
  "errorCode" : "<error code>",  
  // Plugin defined error message.
  "errorMessage" : "User friendly error message",

  // Optional plugin defined additional key value pairs related to the error.
  "errorMetadata" : {
    "key1" : "value1",
    "key2" : "value2"
  }
}

Plugin metadata

  • Every plugin MUST implement a metadata discovery command called get-plugin-metadata.
  • Notation will invoke this command as part of signing and verification workflows to discover the capabilities of the plugin and subsequently invoke commands corresponding to the capabilities. Notation will invoke the get-plugin-metadata command every time a signing or verification workflow is invoked to discover metadata. Notation will not cache the response of get-plugin-metadata command as it involves invalidating cache when a plugin is updated, detecting a plugin update can be non trivial. The get-plugin-metadata command is expected to return only static data, and plugin implementors should avoid making remote calls in this command.

get-plugin-metadata

Request

{
  // Optional plugin configuration, map of string-string
  "pluginConfig" : { }
}

Response All response attributes are required.

{
  // Plugin name that matches its install dir
  // e.g. "com.example.nv2plugin".
  "name" : "<plugin name>",  
  // Plugin description.
  "description" : "description>",
  // Plugin publisher controlled version.
  "version" : "<version>",
  // Plugin webpage for support or documentation.
  "url" : "<URL>",
  
  // List of contract versions supported by the plugin, one per major version
  "supportedContractVersions" : [ ],
  
  // List of one or more capabilities supported by plugin.
  // Valid values are 
  //    SIGNATURE_GENERATOR.RAW
  //    SIGNATURE_GENERATOR.ENVELOPE
  //    SIGNATURE_VERIFIER.TRUSTED_IDENTITY
  //    SIGNATURE_VERIFIER.REVOCATION_CHECK
  //
  // A signing plugin implements either SIGNATURE_GENERATOR.RAW
  // or SIGNATURE_GENERATOR.ENVELOPE capability.
  //
  // A verification plugin implements 
  // one or more of SIGNATURE_VERIFIER capabilities.
  "capabilities" : [
    
  ]
}

plugin-name - Plugin name uses reverse domain name notation to avoid plugin name collisions.

supported-contract-versions - The list of contract versions supported by the plugin. Currently this list must include only one version, per major version. Post initial release, Notation may add new features through plugins, in the form of new commands (e.g. tsa-sign for timestamping), or additional request and response parameters. Notation will publish updates to plugin interface along with appropriate contract version update. Backwards compatible changes (changes for which older version of plugin continue to work with versions of Notation using newer contract version) like new optional parameters on existing contracts, and new commands will be supported through minor version contract updates, breaking changes through major version updates. To maintain forward compatibility plugin implementors MUST ignore unrecognized attributes in command request which are introduced in minor version updates of the plugin contract. Plugin get-plugin-metadata command returns the contract version a plugin supports. Notation will evaluate the minimum plugin version required to satisfy a user's request, and reject the request if the plugin does not support the required version.

capabilities - Non empty list of features supported by a plugin. Each capability such as SIGNATURE_GENERATOR.RAW requires one of more commands to be implemented by the plugin. When new features are available for plugins to implement, an implementation may choose to not implement it, and therefore will not include the feature in capabilities. Notation will evaluate the capability required to satisfy a user’s request, and reject the request if the plugin does not support the required capability.

Signing interfaces

Requirements

  • The interface MUST NOT be tied to a specific signature envelope format.

Notation will support plugins to be developed against the following interfaces - Signature Generator, and Signature Envelope Generator. These interfaces target abstraction levels that satisfy most plugin integration scenarios.

Signature Generator

This interface targets plugins that integrate with providers of basic cryptographic operations. E.g. Local PIV/PKCS#11 hardware tokens, remote KMS, or key vault services. Plugins that target this interface will only generate a raw signature given a payload to sign. Notation will package this signature into a signature envelope, and generate the signature manifest. Notation will also generate the TSA signature if indicated by the user. The plugin does not need to be signature envelope format aware, and will continue to work if Notary Project adopts additional signature formats.

Signing workflow using plugin

  1. Given a user request to sign oci-artifact, with signing key keyName (the friendly key name)
  2. Pull the image manifest using oci-artifact url, and construct a descriptor
  3. Append any user provided metadata and metadata defined in Notary Project signature specification as descriptor annotations.
  4. Determine if the registered key uses a plugin
  5. Execute the plugin with get-plugin-metadata command
    1. If plugin supports capability SIGNATURE_GENERATOR.RAW
      1. Execute the plugin with describe-key command, set request.keyId and the optional request.pluginConfig to corresponding values associated with signing key keyName in config.json.
      2. Generate the payload to be signed for JWS envelope format.
        1. Create the JWS protected headers collection and set alg to value corresponding to describe-key.response.keySpec as per signature algorithm selection.
        2. Create the Notary Project signature Payload (JWS Payload) as defined here.
        3. The payload to sign is then created as - ASCII(BASE64URL(UTF8(ProtectedHeaders)) ‘.’ BASE64URL(JWSPayload))
      3. Execute the plugin with generate-signature command.
        1. Set request.keyId and the optional request.pluginConfig to corresponding values associated with signing key keyName in config.json.
        2. Set request.payload as base64 encoded payload to sign (the JWS payload to sign is double encoded, this is a shortcoming of using plugin contract with JSON encoding).
        3. Set keySpec to value returned by describe-key command in response.keySpec, and hashAlgorithm to hash algorithm corresponding to the key spec, as per signature algorithm selection. The algorithm specified in hashAlgorithm MUST be used by the plugin to hash the payload (request.payload) as part of signature generation.
      4. Validate the generated signature, return an error if any of the checks fails.
        1. Check if response.signingAlgorithm is one of supported signing algorithms.
        2. Check that the plugin did not modify request.payload before generating the signature, and the signature is valid for the given payload. Verify the hash of the request.payload against response.signature, using the public key of signing certificate (leaf certificate) in response.certificateChain along with the response.signingAlgorithm. This step does not include certificate chain validation (certificate chain leads to a trusted root configured in Notation's Trust Store), or revocation check.
        3. Check that the response.certificateChain conforms to Certificate Requirements.
      5. Assemble the signature envelope using response.signature, response.signingAlgorithm and response.certificateChain. If the signing scheme is notary.x509, implementations SHOULD also request and include TSA timestamp countersignature in this step. The certReq field in the timestamping request MUST be set to true.
      6. Generate a signature manifest for the given signature envelope.
    2. Else if, plugin supports capability SIGNATURE_GENERATOR.ENVELOPE (covered in next section)
    3. Return an error

describe-key

This command is used to get metadata for a given key.

Request

{
  "contractVersion" : "<major-version.minor-version>",

  // Key id associated with signing key (keyName)
  // in config.json /signingKeys/keys
  "keyId": "<key id>",

  // Optional plugin configuration, map of string-string
  "pluginConfig" : { }
}

keyId : Required field that has the key identifier (keyId) associated with signing key keyName in config.json.

pluginConfig : Optional field for plugin configuration. For details, see Plugin Configuration section.

Response

{
   // The same key id as passed in the request.
  "keyId" : "<key id>",
  "keySpec" : "<key type and size>"
}

keySpec : One of following supported key types - RSA-2048, RSA-3072, RSA-4096, EC-256, EC-384, EC-521.

NOTE: This command can also be used as part of notation key describe {key-name} which will include the following output

  • Summary of key definition from config.json
  • If the key has an associated plugin
    • Output of plugin discover command
    • Output of describe-key command for the specific key

generate-signature

This command is used to generate the raw signature for a given payload.

Request

{
  "contractVersion" : "<major-version.minor-version>",

  // Key id associated with signing key (keyName)
  // in config.json /signingKeys/keys
  "keyId": "<key id>",

  // Optional plugin configuration, map of string-string
  "pluginConfig" : { },

  // The key spec for the given key id
  "keySpec" : "<key type and size>",

  // Hash algorithm associated with the key spec, plugin must 
  // hash the payload using this hash algorithm
  "hashAlgorithm" : "SHA-256" | "SHA-384" | "SHA-512",

  // Payload to sign, this is base64 encoded
  "payload" : "<base64 encoded payload to be signed>",

}

keyId : Required field that has the key identifier (keyId) associated with signing key keyName in config.json.

pluginConfig : Optional field for plugin configuration. For details, see Plugin Configuration section.

keySpec : Required field that has one of following supported key types - RSA-2048, RSA-3072, RSA-4096, EC-256, EC-384, EC-521. Specifies the key type and size for the key.

hashAlgorithm : Required field that specifies Hash algorithm corresponding to the signature algorithm determined by keySpec for the key.

payload : Required field that contains base64 encoded payload to be signed. For JWS, the payload to sign is base64 encoded (a second time) before sending to the plugin.

Response

All response attributes are required.

{    
  // The same key id as passed in the request.
  "keyId" : "<key id>",
  "signature" : "<Base64 encoded signature>",
  "signingAlgorithm" : "<signing algorithm>",
  "certificateChain": ["Base64(DER(leafCert))","Base64(DER(intermediateCACert))","Base64(DER(rootCert))"]
}

signingAlgorithm : One of following supported signing algorithms, Notation uses this validate the signature, and to set the appropriate attribute in signature envelope (e.g. JWS alg). Supported values are

  • RSASSA-PSS-SHA-256: RSASSA-PSS with SHA-256
  • RSASSA-PSS-SHA-384: RSASSA-PSS with SHA-384
  • RSASSA-PSS-SHA-512: RSASSA-PSS with SHA-512
  • ECDSA-SHA-256: ECDSA on secp256r1 with SHA-256
  • ECDSA-SHA-384: ECDSA on secp384r1 with SHA-384
  • ECDSA-SHA-512: ECDSA on secp521r1 with SHA-512

certificateChain : Ordered list of certificates starting with leaf certificate and ending with root certificate.

Error codes for describe-key and generate-signature

  • VALIDATION_ERROR - Any of the required request fields was empty, or a value was malformed/invalid. Includes condition where the key referenced by keyId was not found.
  • UNSUPPORTED_CONTRACT_VERSION - The contract version used in the request is unsupported.
  • ACCESS_DENIED - Authentication/authorization error to use given key.
  • TIMEOUT - The operation to generate signature timed out and can be retried by Notation.
  • THROTTLED - The operation to generate signature was throttles and can be retried by Notation.
  • ERROR - Any general error that does not fall into previous error categories.

Signature Envelope Generator

This interface targets plugins that in addition to signature generation want to generate the complete signature envelope. This interface allows plugins to have full control over the generated signature envelope, and can append additional signed and unsigned metadata, including timestamp countersignatures. The plugin must be signature envelope format aware, and implement new formats when Notary Project adopts new formats.

Signing workflow using plugin

  1. Given a user request to sign image, with keyName (friendly key name)
  2. Pull the image manifest using image url, and construct a descriptor
  3. Append any user provided metadata and metadata defined in Notary Project signature specification as descriptor annotations.
  4. Determine if the registered key uses a plugin
  5. Execute the plugin with get-plugin-metadata command
    1. If plugin supports capability SIGNATURE_GENERATOR.ENVELOPE
      1. Execute the plugin with generate-envelope command. Set request.keyId and the optional request.pluginConfig to corresponding values associated with signing key keyName in config.json. Set request.payload to base64 encoded the Notary Project signature Payload, request.payloadType to application/vnd.cncf.notary.payload.v1+json and request.signatureEnvelopeType to a pre-defined type (application/jose+json for JWS).
      2. If plugin supports timestamping under signing scheme notary.x509, the plugin SHOULD request and include TSA timestamp countersignature in the signature envelope at this step. The timestamp countersignature MUST be RFC 3161 compliant. The certReq field in the timestamping request MUST be set to true.
      3. response.signatureEnvelope contains the base64 encoded signature envelope, value of response.signatureEnvelopeType MUST match request.signatureEnvelopeType.
      4. Validate the generated signature envelope, return an error if any of the checks fails.
        1. Check if response.signatureEnvelopeType is a supported envelope type and response.signatureEnvelope's format matches response.signatureEnvelopeType.
        2. Check if the signing algorithm in the signature envelope is one of supported signing algorithms.
        3. Check that the targetArtifact descriptor in JWSPayload in response.signatureEnvelope matches request.payload. Plugins MAY append additional annotations but MUST NOT replace/override existing descriptor attributes and annotations.
        4. Check that response.signatureEnvelope can be verified using the public key and signing algorithm specified in the signing certificate, which is embedded as part of certificate chain in response.signatureEnvelope. This step does not include certificate chain validation (certificate chain leads to a trusted root configured in Notation), or revocation check.
        5. Check that the certificate chain in response.signatureEnvelope and timestamp countersignature confirm to certificate requirements.
      5. Generate a signature manifest for the given signature envelope, and append response.annotations to manifest annotations.
    2. Else if plugin supports capability SIGNATURE_GENERATOR.RAW (covered in previous section)
    3. Return an error

generate-envelope

This command is used to generate the complete signature envelope for a given payload.

Request All request attributes are required.

{
  "contractVersion" : "<major-version.minor-version>",
  
  // Key id associated with signing key (keyName)
  // in config.json /signingKeys/keys
  "keyId": "<key id>",

  // Optional plugin configuration, map of string-string
  "pluginConfig" : { },

  // Optional signature expiry duration in seconds, number
  "expiryDurationInSeconds" : 1234567,

  "payload" : "<Base64 encoded payload to be signed>",
  
  // The type of payload - currently a descriptor
  "payloadType" : "application/vnd.cncf.notary.payload.v1+json",
  
  // The expected response signature envelope
  "signatureEnvelopeType" : "application/jose+json"
}

keyId : Required field that has the key identifier (keyId) associated with signing key keyName in config.json.

pluginConfig : Optional field for plugin configuration. For details, see Plugin Configuration section.

expiryDurationInSeconds : Optional field which contains signature expiry duration(in seconds) as integer number.

signatureEnvelopeType - defines the type of signature envelope expected from the plugin. As Notation clients need to be updated in order to parse and verify new signature formats, the default signature format can only be changed with new major version releases of Notation. Users however can opt into using an updated signature envelope format supported by Notation, by passing an optional parameter. e.g. notation sign $IMAGE --key {key-name} --signature-format {some-new-type}

Response All response attributes are required.

{
   "signatureEnvelope": "<Base64 encoded signature envelope>",
   "signatureEnvelopeType" : "application/jose+json",
   
  // Annotations to be appended to Signature Manifest annotations
  "annotations" : {
    "key1" : "value1",
    "key2" : "value2"
  }
}

Error codes for generate-envelope

  • VALIDATION_ERROR - Any of the required request fields was empty, or a value was malformed/invalid. Includes condition where the key referenced by keyId was not found, payload type or signature envelope type in the request is unsupported by the plugin.
  • UNSUPPORTED_CONTRACT_VERSION - The contract version used in the request is unsupported.
  • ACCESS_DENIED - Authentication/authorization error to use given key.
  • TIMEOUT - The operation to generate signature timed out and can be retried by Notation.
  • THROTTLED - The operation to generate signature was throttles and can be retried by Notation.
  • ERROR - Any general error that does not fall into previous error categories.

Verification extensibility

Requirements

  • The interface MUST NOT be tied to a specific signature envelope format.
  • The interface MUST NOT allow a plugin to customize the complete signature verification workflow. Certain steps like identifying the applicable trust policy for the artifact, signature envelope schema validation, integrity checks, certificate chain validation against trust store will be performed by Notation, and cannot be customized by a plugin.
  • The interface MUST allow plugins to customize verification logic for specific supported steps
    • Trusted Identity validation
    • Revocation check validation
  • The interface MAY be extended in future to support additional steps which can be customized through a plugin.
  • The interface MUST be agnostic of sequencing of steps in signature verification workflow as implemented in Notation or other implementations.
  • The interface MUST allow processing of extended attributes that are not part of the Notary Project signature standard attributes.

Guidelines for Verification plugin publishers

  • Usage of extended signed attributes which are marked critical in signature will have implications on portability of the signature. The environment where verification occurs will require dependencies on either a compatible verification plugin in addition to Notation, or a compliant verification tool that understands the extended signed attributes. Therefore, signatures intended for public distribution which require broad signature portability SHOULD avoid extended signed attributes which are marked critical.

  • Signatures which require a plugin for verification may be distributed privately (e.g. within an organization) or publicly (e.g. via a public registry). If the plugin publisher wants their plugin used publicly they SHOULD publish specifications for the verification logic the plugin performs and test vectors. This allows implementations of the Notary Project signature specification to perform the same logic themselves, if they choose to.

Signature Verifier

Verification workflow using plugin

  1. If signature envelope contains Verification Plugin attribute, check if the plugin with the given name exists, else fail signature verification.
    1. For the resolved plugin, call the get-plugin-metadata plugin command to get plugin version and capabilities.
    2. If signature envelope contains Verification plugin minimum version attribute.
      1. Validate that Verification plugin minimum version and plugin version are in SemVer format
      2. Validate that plugin version is greater than or equal to Verification plugin minimum version
      3. Fail signature verification if these validations fail
    3. Validate if plugin capabilities contains any SIGNATURE_VERIFIER capabilities
      1. Fail signature verification if a matching plugin with SIGNATURE_VERIFIER capability cannot be found. Include Verification Plugin Name attribute as a hint in error response.
  2. Complete steps Identify applicable trust policy and Proceed based on signature verification level from signature verification workflow.
  3. Complete steps Validate Integrity, Validate Expiry and Validate Trust Store from signature verification workflow.
  4. Based on the signature verification level, each validation may be enforced, logged or skipped.
  5. Populate verify-signature request. NOTE: The processing order of remaining known attributes does not matter as long as they are processed before the end of signature verification workflow.
    1. Set request.signature.criticalAttributes to the set of standard Notary Project signature attributes that are marked critical, from the signature envelope.
    2. Set request.signature.criticalAttributes.extendedAttributes to extended attributes that Notation does not recognize. Notation only supports JSON primitive types for critical extended attributes. If a signature producer required complex types, it MUST set the value to a primitive type by encoding it (e.g. Base64), and MUST be decoded by the plugin which processed the attribute.
    3. Set request.signature.unprocessedAttributes to set of critical attribute names that are marked critical, but unknown to Notation, and therefore cannot be processed by Notation.
    4. Set request.signature.certificateChain to ordered array of certificates from signing envelope.
    5. Set request.trustPolicy.trustedIdentities to corresponding values from user configured trust policy that is applicable for given artifact and was identified in step 2.
    6. Set request.trustPolicy.signatureVerification to the set of verification checks that are supported by the plugin, and are required to be performed as per step 2. Notation may not populate some checks in this array, if it determined them as “skipped” as per user’s trust policy.
  6. Process verify-signature.response
    1. Validate response.verificationResults map keys match the set of verifications requested in request.trustPolicy.signatureVerification . Fail signature verification if they don't match.
    2. For each element in response.verificationResults map, process each result as follows
      1. If verificationResult.success is true, proceed to next element.
      2. Else if verificationResult.success is false, check if the failure should be enforced or logged based on value of signatureVerification level in Trust Policy. Proceed to next element if failure is logged, else fail signature verification with verificationResult.reason.
    3. Validate values in response.processedAttributes match the set of values in request.signature.unprocessedAttributes. If they don't, fail signature verification, as the plugin did not process a critical attribute that is unknown to the caller.
  7. Perform any remaining steps in signature verification workflow which are not covered by plugin's verification capabilities. These steps MUST process any remaining critical attributes. Fail signature verification if any critical attributes are unprocessed at the end of this step.

verify-signature command

Request

{
  "contractVersion" : "<major-version.minor-version>",
  
   "signature" : {
    // Array of all the Notary Project defined critical attributes and their values
    // in the signature envelope. Agnostic of header names and value serialization
    // in specific envelope formats like JWS or COSE.
    "criticalAttributes" : 
    { 
       "contentType" : "application/vnd.cncf.notary.payload.v1+json",
       // One of notary.x509 or notary.x509.signingAuthority
       "signingScheme" : "notary.x509" | "notary.x509.signingAuthority",
       // Value is always RFC 3339 formatted date time string
       "expiry": "2022-10-06T07:01:20Z", 
       // if signingScheme is notary.x509.signingAuthority
       "authenticSigningTime": "2022-04-06T07:01:20Z",
       // Name of the verification plugin
       "verificationPlugin": "com.example.nv2plugin",
       // Optional plugin minimum version, if present.
       "verificationPluginMinVersion": "1.0.0",
       // Map of extended attributes
       // Extended attributes values support JSON primitive values string, number and null
       "extendedAttributes" : {
           // Map of extended attributes
          // Extended attributes values support JSON primitive values string, number and null.
           "name" : "<primitive-value>" 
       }
    },
   
    // Array of names of critical attributes that plugin 
    // caller does not understand and does not intend to process.
    "unprocessedAttributes" : [ ],

    // Certificate chain from signature envelope.
    "certificateChain" : [ ]
  },

  "trustPolicy" : {
    // Array of trusted identities as specified in 
    // trust policy.
    "trustedIdentities": [],
    // Array of verification checks to be performed by the plugin
    "signatureVerification" : [ 
        "SIGNATURE_VERIFIER.TRUSTED_IDENTITY",
        "SIGNATURE_VERIFIER.REVOCATION_CHECK"
        ]
  },
  
  // Optional plugin configuration, map of string-string
  "pluginConfig" : { }
}

The request can be extended in future by including other elements of signature envelope and trust policy as required to customize additional steps in verification.

Response All response attributes are required.

{
  "verificationResults" : {
        // Map of results. Key must match the set of verification capabilities 
        // in verify-signature request's trustPolicy.signatureVerification attribute.
        "SIGNATURE_VERIFIER.TRUSTED_IDENTITY" :
        {
            // The result of a verification check
            "success" : true | false,
            // Reason for check being successful or not,
            // required if value of success attribute is false.
            "reason" : ""
        },
        "SIGNATURE_VERIFIER.REVOCATION_CHECK" :
        {
            // The result of a verification check
            "success" : true | false,
            // Reason for check being successful or not,
            // required if value of success attribute is false.
            "reason" : ""
        }
  },
  // Array of strings containing critical attributes processed by the plugin.
  "processedAttributes" : [ 
  ]
}

verificationResults : Verifications performed by the plugin. This is a map where the keys MUST match set of verify capabilities in verify-signature request's trustPolicy.signatureVerification attribute, and values are objects with following attributes

  • success (required): The boolean verification result.
  • reason (optional): Reason associated with verification being successful or not, REQUIRED if value of success field is false.

processedAttributes (required): Array of strings containing critical attributes processed by the plugin. Values must be one or more of attribute names in verify-signature request's signature.unprocessedAttributes.

Error codes for verify-signature

  • VALIDATION_ERROR - Any of the required request fields was empty, or a value was malformed/invalid.
  • UNSUPPORTED_CONTRACT_VERSION - The contract version used in the request is unsupported.
  • ACCESS_DENIED - Authentication/authorization during signature verification, this may be due to external calls made by the plugin.
  • TIMEOUT - The verification process timed out and can be retried by Notation.
  • THROTTLED - The verification process was throttled and can be retried by Notation.
  • ERROR - Any general error that does not fall into previous error categories.

FAQ

Q: Will Notation generate timestamp countersignature for Signature Envelope Generator plugin or its responsibility of plugin publisher?

A : The Signature Envelope Generator plugin has full control over the generated signature envelope including any timestamp countersignature. Notation will NOT add/update/delete any timestamp countersignature in an envelope that's generated by a Signature Envelope Generator plugin.