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# Documentation: BER/DER parsing recipes
## Builtin types
Most builtin types can be parsed by calling the `from_der` or `from_der` functions (see `FromBer` and `FromDer` traits for documentation).
For ex:
```rust
# use asn1_rs::*;
# let parser = |input| -> Result<(), Error> {
let (rem, result) = <u32>::from_der(input)?;
# Ok(()) };
```
Note: this crates makes extensive use of types annotation and turbofish operator, for example `<Type>::from_der()` or `TaggedExplicit::<u32, Error, 0>::from_der()`.
See table B-3 in <https://doc.rust-lang.org/book/appendix-02-operators.html> for reference on syntax.
## `SEQUENCE` and `SET`
The `SEQUENCE` and `SET` types are handled very similarly, so recipes will be given for `SEQUENCE`, but can be adapted to `SET` by replacing words.
### Parsing `SEQUENCE`
Usually, the sequence envelope does not need to be stored, so it just needs to be parsed to get the sequence content and parse it.
The methods [`from_ber_and_then`](crate::Sequence::from_ber_and_then()) and [`from_der_and_then`](crate::Sequence::from_der_and_then()) provide helpers for that:
```rust
# use asn1_rs::*;
# let parser = |input| -> Result<(), Error> {
let (rem, result) = Sequence::from_ber_and_then(input, |i| {
// first item is INTEGER
let (rem, a) = u32::from_der(input)?;
// second item is OCTET STRING
let (rem, b) = <&[u8]>::from_der(input)?;
Ok((rem, (a, b)))
})?;
// result has type (u32, &[u8])
assert_eq!(result.0, 0);
assert_eq!(result.1, b"\x00\x01");
# Ok(()) };
```
### Automatically deriving sequence parsers
The [`BerSequence`](crate::BerSequence) and [`DerSequence`](crate::DerSequence)
custom derive provide attributes to automatically derive a parser for a sequence.
For ex:
```rust
# use asn1_rs::*;
#[derive(DerSequence)]
pub struct S {
a: u32,
b: u16,
c: u16,
}
# let parser = |input| -> Result<(), Error> {
let (rem, result) = S::from_der(input)?;
# Ok(()) };
```
This will work for any field type that implements [`FromBer`](crate::FromBer) or [`FromDer`](crate::FromDer), respectively.
See [`derive`](mod@derive) documentation for more examples and documentation.
### Parsing `SEQUENCE OF`
`SEQUENCE OF T` can be parsed using either type `SequenceOf<T>` or `Vec<T>`:
```rust
# use asn1_rs::*;
# let parser = |input| -> Result<(), Error> {
let (rem, result) = SequenceOf::<u32>::from_der(input)?;
# Ok(()) };
```
or
```rust
# use asn1_rs::*;
# let parser = |input| -> Result<(), Error> {
let (rem, result) = <Vec<u32>>::from_der(input)?;
# Ok(()) };
```
`SET OF T` can be parsed using either `SetOf<T>`, `BTreeSet<T>` or `HashSet<T>`.
## `EXPLICIT` tagged values
### Parsing `EXPLICIT`, expecting a known tag
If you expect only a specific tag, use `TaggedExplicit`.
For ex, to parse a `[3] EXPLICIT INTEGER`:
```rust
# use asn1_rs::*;
# let parser = |input| -> Result<(), Error> {
let (rem, result) = TaggedExplicit::<u32, Error, 0>::from_der(input)?;
// result has type TaggedValue. Use `.as_ref()` or `.into_inner()`
// to access content
let tag = result.tag();
let class = result.class();
assert_eq!(result.as_ref(), &0);
# Ok(()) };
```
### Specifying the class
`TaggedExplicit` does not check the class, and accepts any class. It expects you to check the class after reading the value.
To specify the class in the parser, use `TaggedValue`:
```rust
# use asn1_rs::*;
# let parser = |input| -> Result<(), Error> {
// Note: the strange notation (using braces) is required by the compiler to use
// a constant instead of the numeric value.
let (rem, result) = TaggedValue::<u32, Error, Explicit, {Class::CONTEXT_SPECIFIC}, 0>::from_der(input)?;
# Ok(()) };
```
Note that `TaggedExplicit` is a type alias to `TaggedValue`, so the objects are the same.
### Accepting any `EXPLICIT` tag
To parse a value, accepting any class or tag, use `TaggedParser`.
```rust
# use asn1_rs::*;
# let parser = |input| -> Result<(), Error> {
let (rem, result) = TaggedParser::<Explicit, u32>::from_der(input)?;
// result has type TaggedParser. Use `.as_ref()` or `.into_inner()`
// to access content
let tag = result.tag();
let class = result.class();
assert_eq!(result.as_ref(), &0);
# Ok(()) };
```
### Optional tagged values
To parse optional tagged values, `Option<TaggedExplicit<...>>` can be used:
```rust
# use asn1_rs::*;
# let parser = |input| -> Result<(), Error> {
let (rem, result) = Option::<TaggedExplicit::<u32, Error, 0>>::from_der(input)?;
# Ok(()) };
```
The type `OptTaggedExplicit` is also provided as an alias:
```rust
# use asn1_rs::*;
# let parser = |input| -> Result<(), Error> {
let (rem, result) = OptTaggedExplicit::<u32, Error, 0>::from_der(input)?;
# Ok(()) };
```
## `IMPLICIT` tagged values
### Parsing `IMPLICIT`, expecting a known tag
If you expect only a specific tag, use `TaggedImplicit`.
For ex, to parse a `[3] EXPLICIT INTEGER`:
```rust
# use asn1_rs::*;
# let parser = |input| -> Result<(), Error> {
let (rem, result) = TaggedExplicit::<u32, Error, 0>::from_der(input)?;
// result has type TaggedValue. Use `.as_ref()` or `.into_inner()`
// to access content
let tag = result.tag();
let class = result.class();
assert_eq!(result.as_ref(), &0);
# Ok(()) };
```
### Specifying the class
`TaggedImplicit` does not check the class, and accepts any class. It expects you to check the class after reading the value.
To specify the class in the parser, use `TaggedValue`:
```rust
# use asn1_rs::*;
# let parser = |input| -> Result<(), Error> {
// Note: the strange notation (using braces) is required by the compiler to use
// a constant instead of the numeric value.
let (rem, result) = TaggedValue::<u32, Error, Implicit, { Class::CONTEXT_SPECIFIC }, 1>::from_der(input)?;
# Ok(()) };
```
Note that `TaggedImplicit` is a type alias to `TaggedValue`, so the objects are the same.
### Accepting any `IMPLICIT` tag
To parse a value, accepting any class or tag, use `TaggedParser`.
```rust
# use asn1_rs::*;
# let parser = |input| -> Result<(), Error> {
let (rem, result) = TaggedParser::<Implicit, u32>::from_der(input)?;
// result has type TaggedParser. Use `.as_ref()` or `.into_inner()`
// to access content
let tag = result.tag();
let class = result.class();
assert_eq!(result.as_ref(), &0);
# Ok(()) };
```
### Optional tagged values
To parse optional tagged values, `Option<TaggedImplicit<...>>` can be used:
```rust
# use asn1_rs::*;
# let parser = |input| -> Result<(), Error> {
let (rem, result) = Option::<TaggedImplicit::<u32, Error, 0>>::from_der(input)?;
# Ok(()) };
```
The type `OptTaggedImplicit` is also provided as an alias:
```rust
# use asn1_rs::*;
# let parser = |input| -> Result<(), Error> {
let (rem, result) = OptTaggedImplicit::<u32, Error, 0>::from_der(input)?;
# Ok(()) };
```
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