schema: further decompose the digest grammar

The digest grammar is now further decomposed into compontents to remove
reliance on regex grouping syntax.

We also extend the character set to include `=` sign which enables
variable length base64 encoding.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

descriptor.md

@@ -69,9 +69,11 @@ The _algorithm_ specifies the cryptographic hash function and encoding used for
 A digest string MUST match the following grammar:
 
 ```
-digest      := algorithm ":" encoded
-algorithm   := /[a-z0-9]+(?:[+._-][a-z0-9]+)*/
-encoded     := /[a-zA-Z0-9_-]+/
+digest                := algorithm ":" encoded
+algorithm             := algorithm-component [algorithm-separator algorithm-component]*
+algorithm-component   := /[a-z0-9]+/
+algorithm-separator   := /[+._-]/
+encoded               := /[a-zA-Z0-9=_-]+/
 ```
 
 Some example digest strings include the following:
@@ -81,7 +83,7 @@ digest                                                                  | algori
 sha256:6c3c624b58dbbcd3c0dd82b4c53f04194d1247c6eebdaab7c610cf7d66709b3b | [SHA-256](#sha-256) | Yes       |
 sha512:401b09eab3c013d4ca54922bb802bec8fd5318192b0a75f201d8b3727429080fb337591abd3e44453b954555b7a0812e1081c39b740293f765eae731f5a65ed1 | [SHA-256](#sha-512) | Yes      |
 multihash+base58:QmRZxt2b1FVZPNqd8hsiykDL3TdBDeTSPX9Kv46HmX4Gx8`        | Multihash           | No        |
-sha256+b64:LCa0a2j_xo_5m0U8HTBBNBNCLXBkg7-g-YpeiGJm564                  | SHA-256 with base64 | No        |
+sha256+b64u:LCa0a2j_xo_5m0U8HTBBNBNCLXBkg7-g-YpeiGJm564                  | SHA-256 with urlsafe base64 | No        |
 
 Please see [Registered Algorithms](#registered-identifiers) for a list of supported algorithms.
 

schema/defs-descriptor.json

@@ -9,7 +9,7 @@
     "digest": {
       "description": "the cryptographic checksum digest of the object, in the pattern '<algorithm>:<encoded>'",
       "type": "string",
-      "pattern": "^[a-z0-9]+(?:[+._-][a-z0-9]+)*:[a-zA-Z0-9_-]+$"
+      "pattern": "^[a-z0-9]+(?:[+._-][a-z0-9]+)*:[a-zA-Z0-9=_-]+$"
     },
     "urls": {
       "description": "a list of urls from which this object may be downloaded",

schema/descriptor_test.go

@@ -278,7 +278,15 @@ func TestDescriptor(t *testing.T) {
 		},
 		{
 			descriptor: `{
-				"digest": "sha256+b64:LCa0a2j_xo_5m0U8HTBBNBNCLXBkg7-g-YpeiGJm564",
+				"digest": "sha256+b64u:LCa0a2j_xo_5m0U8HTBBNBNCLXBkg7-g-YpeiGJm564",
+				"size": 1000000,
+				"mediaType": "application/vnd.oci.image.config.v1+json"
+			}`,
+		},
+		{
+			// test for those who cannot use modulo arithmetic to recover padding.
+			descriptor: `{
+				"digest": "sha256+b64u.unknownlength:LCa0a2j_xo_5m0U8HTBBNBNCLXBkg7-g-YpeiGJm564=",
 				"size": 1000000,
 				"mediaType": "application/vnd.oci.image.config.v1+json"
 			}`,

schema/fs.go

@@ -237,17 +237,17 @@ ERcrb5b9zhBc4s2zO7r2jN/2xKhin3+/McttXS9NB/Cle+p+BgAA///HjexwPQQAAA==
 
 	"/defs-descriptor.json": {
 		local:   "defs-descriptor.json",
-		size:    921,
-		modtime: 1493324159,
+		size:    922,
+		modtime: 1493750367,
 		compressed: `
-H4sIAAAAAAAA/6STX2/TMBTF3/spLl7FgDZN4QFp0TQ0sXcextOmrrqzb+I74j+yXU1l6ndHTrK0KwKB
-9pDIPro+5/5unKcJgFAUZWCf2FlRgbiimi3nXQSPIbHctBggOfjmyX51NiFbCnA1HHMBrj1Jrlli5zHv
-TUcXUUHOARCGFOP3radRAhCscqpOyceqLJ0nK58z4sKFpoxSk8GSDTZUqjG13LvNn71S7y1iCmybve4x
-JQod3t3tZXGDxc9lcbbaL9+cTN8Wd+vFbPW0nH/89HlX/lvZVHQRuz5JKG4opkO6o+EmTSDD1ifXBPSa
-JUhN8kfcGOjPgqshF7n7B5JpDmy77QAAp+fYNi5w0uaiOs+jUqQuTv9zAgPX7N2X6na2WBerUXn/ocrr
-y+JmWZyti9XsCHAT2vgXPISWe4ZcCHVwBh41Sw1JcxygwOAW7gmUe7StQ0Xq9/YxBNzuZU5kDnP/DAog
-ahcM5o8gNoHFoO9eYKC1LuHL2/maq3joNzY9DVRnP0V1XDxEZ0/Kg7+iNOivu9779zDnSX52k18BAAD/
-/7fEAguZAwAA
+H4sIAAAAAAAA/6STX2/TMBTF3/spLl7FgDZN4QFp0Ria2DsP42lTV93ZN/Ed8R/ZrqYy9bsjJ1naFYFA
+PCSyj67Pub8b52kCIBRFGdgndlZUIK6oZst5F8FjSCw3LQZIDr56sl+cTciWAlwNx1yAa0+Sa5bYecx7
+09FFVJBzAIQhxfht62mUAASrnKpT8rEqS+fJyueMuHChKaPUZLBkgw2Vakwt927zZ6/Ue4uYAttmr3tM
+iUKHd3d7Wdxg8WNZnK32y1cn09fF3XoxWz0t5+8/fNyVf1c2FV3Erk8SihuK6ZDuaLhJE8iw9ck1Ab1m
+CVKT/B43Bvqz4GrIRe7+gWSaA9tuOwDA6Tm2jQuctLmozvOoFKmL03+cwMA1e/O5up0t1sVqVN6+q/L6
+srhZFmef1sVqdkS4CW38Ax9Cyz1ELoQ6OAOPmqWGpDkOVGBwC/cEyj3a1qEi9Wv/GAJu9zInMoe5vycF
+ELULBvNXEJvAYtB3LzDQWpfw5fX8n7t46Dc2PQ1UZz9FdVw8RGdPyoPfojTor7ve+/cw50l+dpOfAQAA
+//8aH/C2mgMAAA==
 `,
 	},