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OCI Content Descriptors

An OCI image consists of several different components, arranged in a Merkle Directed Acyclic Graph (DAG). References between components in the graph are expressed through Content Descriptors. A Content Descriptor (or simply Descriptor) describes the disposition of the targeted content. A Content Descriptor includes the type of the content, a content identifier (digest), and the byte-size of the raw content.

Descriptors SHOULD be embedded in other formats to securely reference external content.

Other formats SHOULD use descriptors to securely reference external content.

Properties

A descriptor consists of a set of properties encapsulated in key-value fields.

The following fields contain the primary properties that constitute a Descriptor:

  • mediaType string

    This REQUIRED property contains the MIME type of the referenced content.

    The OCI image specification defines several of its own MIME types for resources defined in the specification.

  • digest string

    This REQUIRED property is the digest of the targeted content, conforming to the requirements outlined in Digests and Verification. Retrieved content SHOULD be verified against this digest when consumed via untrusted sources.

  • size int64

    This REQUIRED property specifies the size, in bytes, of the raw content. This property exists so that a client will have an expected size for the content before processing. If the length of the retrieved content does not match the specified length, the content SHOULD NOT be trusted.

  • urls array

    This OPTIONAL property specifies a list of URLs from which this object MAY be downloaded.

Reserved

The following field keys MUST NOT be used in descriptors specified in other OCI specifications:

  • data string

    This key is RESERVED for future versions of the specification.

All other fields may be included in other OCI specifications. Extended Descriptor field additions proposed in other OCI specifications SHOULD first be considered for addition into this specification.

Digests and Verification

The digest property of a Descriptor acts as a content identifier, enabling content addressability. It uniquely identifies content by taking a collision-resistant hash of the bytes. If the identifier can be communicated in a secure manner, one can retrieve the content from an insecure source, calculate the digest independently, and be certain that the correct content was obtained.

The value of the digest property, the digest string, is a serialized hash result, consisting of an algorithm portion and a hex portion. The algorithm identifies the methodology used to calculate the digest; the hex portion is the hex-encoded result of the hash.

The digest string matches the following grammar:

digest      := algorithm ":" hex
algorithm   := /[a-z0-9_+.-]+/
hex         := /[A-Fa-f0-9]+/

Some example digest strings include the following:

digest description
sha256:6c3c624b58dbbcd3c0dd82b4c53f04194d1247c6eebdaab7c610cf7d66709b3b Common sha256 based digest

Before consuming content targeted by a descriptor from untrusted sources, the byte content SHOULD be verified against the digest. Before calculating the digest, the size of the content SHOULD be verified to reduce hash collision space. Heavy processing before calculating a hash SHOULD be avoided. Implementations MAY employ some canonicalization of the underlying content to ensure stable content identifiers.

Algorithms

While the algorithm component of the digest does allow one to utilize a wide variety of algorithms, compliant implementations SHOULD use SHA-256.

Let's use a simple example in pseudo-code to demonstrate a digest calculation: A digest is calculated by the following pseudo-code, where H is the selected hash algorithm, identified by string <alg>:

let ID(C) = Descriptor.digest
let C = <bytes>
let D = '<alg>:' + EncodeHex(H(C))
let verified = ID(C) == D

Above, we define the content identifier as ID(C), extracted from the Descriptor.digest field. Content C is a string of bytes. Function H returns a the hashs of C in bytes and is passed to function EncodeHex to obtain the digest. The result verified is true if ID(C) is equal to D, confirming that C is the content identified by D. After verification, the following is true:

D == ID(C) == '<alg>:' + EncodeHex(H(C))

The digest is confirmed as the content identifier by independently calculating the digest.

SHA-256

SHA-256 is a collision-resistant hash function, chosen for ubiquity, reasonable size and secure characteristics. Implementations MUST implement SHA-256 digest verification for use in descriptors.

Examples

The following example describes a Manifest with a content identifier of "sha256:5b0bcabd1ed22e9fb1310cf6c2dec7cdef19f0ad69efa1f392e94a4333501270", of size 7682 bytes:

{
  "mediaType": "application/vnd.oci.image.manifest.v1+json",
  "size": 7682,
  "digest": "sha256:5b0bcabd1ed22e9fb1310cf6c2dec7cdef19f0ad69efa1f392e94a4333501270"
}