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descriptor: Formal ABNF for digest values #655

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descriptor: Formal ABNF for digest values #655

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11 changes: 6 additions & 5 deletions descriptor.md
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Expand Up @@ -64,12 +64,12 @@ If the identifier can be communicated in a secure manner, one can retrieve the c
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 lowercase hex-encoded result of the hash.

The digest string MUST match the following grammar:
The digest string MUST match the following [ABNF][rfc5234]:

```
digest := algorithm ":" hex
algorithm := /[a-z0-9_+.-]+/
hex := /[a-f0-9]+/
```ABNF
digest = algorithm ":" hex
algorithm = 1*(DIGIT / %x61-7A / "+" / "." / "_" / "-") ; [0-9a-z+._-]+
hex = 1*(DIGIT / %x61-66) ; [0-9a-f]+
```

Some example digest strings include the following:
Expand Down Expand Up @@ -148,6 +148,7 @@ In the following example, the descriptor indicates that the referenced manifest
```

[rfc3986]: https://tools.ietf.org/html/rfc3986
[rfc5234]: https://tools.ietf.org/html/rfc5234
[rfc6838]: https://tools.ietf.org/html/rfc6838
[rfc6838-s4.2]: https://tools.ietf.org/html/rfc6838#section-4.2
[rfc7230-s2.7]: https://tools.ietf.org/html/rfc7230#section-2.7