1container-signature(5)(Container) container-signature(5)(Container)
2Miloslav Trmač March 2017
6
9 This document describes the format of container signatures, as imple‐
10 mented by the github.com/containers/image/signature package.
11 Most users should be able to consume these signatures by using the
14 github.com/containers/image/signature package (preferably through the
15 higher-level signature.PolicyContext interface) without having to care
16 about the details of the format described below. This documentation
17 exists primarily for maintainers of the package and to allow indepen‐
18 dent reimplementations.
19
22 The signature provides an end-to-end authenticated claim that a con‐
23 tainer image has been approved by a specific party (e.g. the creator of
24 the image as their work, an automated build system as a result of an
25 automated build, a company IT department approving the image for pro‐
26 duction) under a specified identity (e.g. an OS base image / specific
27 application, with a specific version).
28 A container signature consists of a cryptographic signature which iden‐
31 tifies and authenticates who signed the image, and carries as a signed
32 payload a JSON document. The JSON document identifies the image being
33 signed, claims a specific identity of the image and if applicable, con‐
34 tains other information about the image.
35 The signatures do not modify the container image (the layers, configu‐
38 ration, manifest, …); e.g. their presence does not change the manifest
39 digest used to identify the image in docker/distribution servers;
40 rather, the signatures are associated with an immutable image. An im‐
41 age can have any number of signatures so signature distribution systems
42 SHOULD support associating more than one signature with an image.
43
46 As distributed, the container signature is a blob which contains a
47 cryptographic signature in an industry-standard format, carrying a
48 signed JSON payload (i.e. the blob contains both the JSON document and
49 a signature of the JSON document; it is not a “detached signature” with
50 independent blobs containing the JSON document and a cryptographic sig‐
51 nature).
52 Currently the only defined cryptographic signature format is an OpenPGP
55 signature (RFC 4880), but others may be added in the future. (The blob
56 does not contain metadata identifying the cryptographic signature for‐
57 mat. It is expected that most formats are sufficiently self-describing
58 that this is not necessary and the configured expected public key pro‐
59 vides another indication of the expected cryptographic signature for‐
60 mat. Such metadata may be added in the future for newly added crypto‐
61 graphic signature formats, if necessary.)
62 Consumers of container signatures SHOULD verify the cryptographic sig‐
65 nature against one or more trusted public keys (e.g. defined in a pol‐
66 icy.json signature verification policy file ⟨containers-pol‐
67 icy.json.5.md⟩) before parsing or processing the JSON payload in any
68 way, in particular they SHOULD stop processing the container signature
69 if the cryptographic signature verification fails, without even start‐
70 ing to process the JSON payload.
71 (Consumers MAY extract identification of the signing key and other
74 metadata from the cryptographic signature, and the JSON payload, with‐
75 out verifying the signature, if the purpose is to allow managing the
76 signature blobs, e.g. to list the authors and image identities of sig‐
77 natures associated with a single container image; if so, they SHOULD
78 design the output of such processing to minimize the risk of users con‐
79 sidering the output trusted or in any way usable for making policy de‐
80 cisions about the image.)
81 OpenPGP signature verification
84 When verifying a cryptographic signature in the OpenPGP format, the
85 consumer MUST verify at least the following aspects of the signature
86 (like the github.com/containers/image/signature package does):
87 • The blob MUST be a “Signed Message” as defined RFC 4880 sec‐
90 tion 11.3. (e.g. it MUST NOT be an unsigned “Literal Mes‐
91 sage”, or any other non-signature format).
92 • The signature MUST have been made by an expected key trusted
94 for the purpose (and the specific container image).
95 • The signature MUST be correctly formed and pass the crypto‐
97 graphic validation.
98 • The signature MUST correctly authenticate the included JSON
100 payload (in particular, the parsing of the JSON payload MUST
101 NOT start before the complete payload has been cryptographi‐
102 cally authenticated).
103 • The signature MUST NOT be expired.
105 The consumer SHOULD have tests for its verification code which verify
109 that signatures failing any of the above are rejected.
110
113 The payload of the cryptographic signature is a JSON document (RFC
114 7159). Consumers SHOULD parse it very strictly, refusing any signature
115 which violates the expected format (e.g. missing members, incorrect
116 member types) or can be interpreted ambiguously (e.g. a duplicated mem‐
117 ber in a JSON object).
118 Any violations of the JSON format or of other requirements in this doc‐
121 ument MAY be accepted if the JSON document can be recognized to have
122 been created by a known-incorrect implementation (see optional.creator
123 ⟨#optionalcreator⟩ below) and if the semantics of the invalid document,
124 as created by such an implementation, is clear.
125 The top-level value of the JSON document MUST be a JSON object with ex‐
128 actly two members, critical and optional, each a JSON object.
129 The critical object MUST contain a type member identifying the document
132 as a container signature (as defined below ⟨#criticaltype⟩) and signa‐
133 ture consumers MUST reject signatures which do not have this member or
134 in which this member does not have the expected value.
135 To ensure forward compatibility (allowing older signature consumers to
138 correctly accept or reject signatures created at a later date, with
139 possible extensions to this format), consumers MUST reject the signa‐
140 ture if the critical object, or any of its subobjects, contain any mem‐
141 ber or data value which is unrecognized, unsupported, invalid, or in
142 any other way unexpected. At a minimum, this includes unrecognized
143 members in a JSON object, or incorrect types of expected members.
144 For the same reason, consumers SHOULD accept any members with unrecog‐
147 nized names in the optional object, and MAY accept signatures where the
148 object member is recognized but unsupported, or the value of the member
149 is unsupported. Consumers still SHOULD reject signatures where a mem‐
150 ber of an optional object is supported but the value is recognized as
151 invalid.
152
155 An example of the full format follows, with detailed description below.
156 To reiterate, consumers of the signature SHOULD perform successful
157 cryptographic verification, and MUST reject unexpected data in the
158 critical object, or in the top-level object, as described above.
159 {
162 "critical": {
163 "type": "atomic container signature",
164 "image": {
165 "docker-manifest-digest": "sha256:817a12c32a39bbe394944ba49de563e085f1d3c5266eb8e9723256bc4448680e"
166 },
167 "identity": {
168 "docker-reference": "docker.io/library/busybox:latest"
169 }
170 },
171 "optional": {
172 "creator": "some software package v1.0.1-35",
173 "timestamp": 1483228800,
174 }
175 }
176 critical
180 This MUST be a JSON object which contains data critical to correctly
181 evaluating the validity of a signature.
182 Consumers MUST reject any signature where the critical object contains
185 any unrecognized, unsupported, invalid or in any other way unexpected
186 member or data.
187 critical.type
190 This MUST be a string with a string value exactly equal to atomic con‐
191 tainer signature (three words, including the spaces).
192 Signature consumers MUST reject signatures which do not have this mem‐
195 ber or this member does not have exactly the expected value.
196 (The consumers MAY support signatures with a different value of the
199 type member, if any is defined in the future; if so, the rest of the
200 JSON document is interpreted according to rules defining that value of
201 critical.type, not by this document.)
202 critical.image
205 This MUST be a JSON object which identifies the container image this
206 signature applies to.
207 Consumers MUST reject any signature where the critical.image object
210 contains any unrecognized, unsupported, invalid or in any other way un‐
211 expected member or data.
212 (Currently only the docker-manifest-digest way of identifying a con‐
215 tainer image is defined; alternatives to this may be defined in the fu‐
216 ture, but existing consumers are required to reject signatures which
217 use formats they do not support.)
218 critical.image.docker-manifest-digest
221 This MUST be a JSON string, in the github.com/opencontainers/go-di‐
222 gest.Digest string format.
223 The value of this member MUST match the manifest of the signed con‐
226 tainer image, as implemented in the docker/distribution manifest ad‐
227 dressing system.
228 The consumer of the signature SHOULD verify the manifest digest against
231 a fully verified signature before processing the contents of the image
232 manifest in any other way (e.g. parsing the manifest further or down‐
233 loading layers of the image).
234 Implementation notes: * A single container image manifest may have sev‐
237 eral valid manifest digest values, using different algorithms. * For
238 “signed” docker/distribution schema 1 ⟨https://github.com/docker/dis‐
239 tribution/blob/master/docs/spec/manifest-v2-1.md⟩ manifests, the mani‐
240 fest digest applies to the payload of the JSON web signature, not to
241 the raw manifest blob.
242 critical.identity
245 This MUST be a JSON object which identifies the claimed identity of the
246 image (usually the purpose of the image, or the application, along with
247 a version information), as asserted by the author of the signature.
248 Consumers MUST reject any signature where the critical.identity object
251 contains any unrecognized, unsupported, invalid or in any other way un‐
252 expected member or data.
253 (Currently only the docker-reference way of claiming an image iden‐
256 tity/purpose is defined; alternatives to this may be defined in the fu‐
257 ture, but existing consumers are required to reject signatures which
258 use formats they do not support.)
259 critical.identity.docker-reference
262 This MUST be a JSON string, in the github.com/docker/distribution/ref‐
263 erence string format, and using the same normalization semantics (where
264 e.g. busybox:latest is equivalent to docker.io/library/busybox:latest).
265 If the normalization semantics allows multiple string representations
266 of the claimed identity with equivalent meaning, the critical.iden‐
267 tity.docker-reference member SHOULD use the fully explicit form (in‐
268 cluding the full host name and namespaces).
269 The value of this member MUST match the image identity/purpose expected
272 by the consumer of the image signature and the image (again, accounting
273 for the docker/distribution/reference normalization semantics).
274 In the most common case, this means that the critical.identity.docker-
277 reference value must be equal to the docker/distribution reference used
278 to refer to or download the image. However, depending on the specific
279 application, users or system administrators may accept less specific
280 matches (e.g. ignoring the tag value in the signature when pulling the
281 :latest tag or when referencing an image by digest), or they may re‐
282 quire critical.identity.docker-reference values with a completely dif‐
283 ferent namespace to the reference used to refer to/download the image
284 (e.g. requiring a critical.identity.docker-reference value which iden‐
285 tifies the image as coming from a supplier when fetching it from a com‐
286 pany-internal mirror of approved images). The software performing this
287 verification SHOULD allow the users to define such a policy using the
288 policy.json signature verification policy file format ⟨containers-pol‐
289 icy.json.5.md⟩.
290 The critical.identity.docker-reference value SHOULD contain either a
293 tag or digest; in most cases, it SHOULD use a tag rather than a digest.
294 (See also the default matchRepoDigestOrExact matching semantics in pol‐
295 icy.json ⟨containers-policy.json.5.md#signedby⟩.)
296 optional
299 This MUST be a JSON object.
300 Consumers SHOULD accept any members with unrecognized names in the op‐
303 tional object, and MAY accept a signature where the object member is
304 recognized but unsupported, or the value of the member is valid but un‐
305 supported. Consumers still SHOULD reject any signature where a member
306 of an optional object is supported but the value is recognized as in‐
307 valid.
308 optional.creator
311 If present, this MUST be a JSON string, identifying the name and ver‐
312 sion of the software which has created the signature.
313 The contents of this string is not defined in detail; however each im‐
316 plementation creating container signatures:
317 • SHOULD define the contents to unambiguously define the soft‐
320 ware in practice (e.g. it SHOULD contain the name of the soft‐
321 ware, not only the version number)
322 • SHOULD use a build and versioning process which ensures that
324 the contents of this string (e.g. an included version number)
325 changes whenever the format or semantics of the generated sig‐
326 nature changes in any way; it SHOULD not be possible for two
327 implementations which use a different format or semantics to
328 have the same optional.creator value
329 • SHOULD use a format which is reasonably easy to parse in soft‐
331 ware (perhaps using a regexp), and which makes it easy enough
332 to recognize a range of versions of a specific implementation
333 (e.g. the version of the implementation SHOULD NOT be only a
334 git hash, because they don’t have an easily defined ordering;
335 the string should contain a version number, or at least a date
336 of the commit).
337 Consumers of container signatures MAY recognize specific values or sets
341 of values of optional.creator (perhaps augmented with optional.time‐
342 stamp), and MAY change their processing of the signature based on these
343 values (usually to accommodate violations of this specification in past
344 versions of the signing software which cannot be fixed retroactively),
345 as long as the semantics of the invalid document, as created by such an
346 implementation, is clear.
347 If consumers of signatures do change their behavior based on the op‐
350 tional.creator value, they SHOULD take care that the way they process
351 the signatures is not inconsistent with strictly validating signature
352 consumers. (I.e. it is acceptable for a consumer to accept a signature
353 based on a specific optional.creator value if other implementations
354 would completely reject the signature, but it would be very undesirable
355 for the two kinds of implementations to accept the signature in differ‐
356 ent and inconsistent situations.)
357 optional.timestamp
360 If present, this MUST be a JSON number, which is representable as a
361 64-bit integer, and identifies the time when the signature was created
362 as the number of seconds since the UNIX epoch (Jan 1 1970 00:00 UTC).
363format signature container-signature(5)(Container)