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zbor - Zig CBOR

The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation (RFC8949). It is used in different protocols like the Client to Authenticator Protocol CTAP2 which is a essential part of FIDO2 authenticators/ Passkeys.

I have utilized this library in several projects throughout the previous year, primarily in conjunction with my FIDO2 library. I'd consider it stable. With the introduction of Zig version 0.11.0, this library will remain aligned with the most recent stable release. If you have any problems or want to share some ideas feel free to open an issue or write me a mail, but please be kind.

Getting started

First add this library as a dependency to your build.zig.zon file:

.{
    .name = "your-project",
    .version = 0.0.1,

    .dependencies = .{
        .zbor = .{
            // For a specific release use:
            // .url = "https://github.com/r4gus/zbor/archive/refs/tags/0.13.1.tar.gz",
            // For the current master use:
            .url = "https://github.com/r4gus/zbor/archive/master.tar.gz",
            .hash = <hash>,
        }
    },
    .paths = .{
        // Your paths...
    },
}

then within you build.zig add the following code:

// First fetch the dependency...
const zbor_dep = b.dependency("zbor", .{
    .target = target,
    .optimize = optimize,
});
const zbor_module = zbor_dep.module("zbor");

// If you have a module that has zbor as a dependency...
const your_module = b.addModule("your-module", .{
    .root_source_file = .{ .path = "src/main.zig" },
    .imports = &.{
        .{ .name = "zbor", .module = zbor_module },
    },
});

// Or as a dependency for a executable...
exe.root_module.addImport("zbor", zbor_module);

The easiest way to get the required hash is to use a wrong one and then copy the correct one from the error message.

Usage

This library lets you inspect and parse CBOR data without having to allocate additional memory.

Inspect CBOR data

To inspect CBOR data you must first create a new DataItem.

const cbor = @import("zbor");

const di = DataItem.new("\x1b\xff\xff\xff\xff\xff\xff\xff\xff") catch {
    // handle the case that the given data is malformed
};

DataItem.new() will check if the given data is well-formed before returning a DataItem. The data is well formed if it's syntactically correct.

To check the type of the given DataItem use the getType() function.

std.debug.assert(di.getType() == .Int);

Possible types include Int (major type 0 and 1) ByteString (major type 2), TextString (major type 3), Array (major type 4), Map (major type 5), Tagged (major type 6) and Float (major type 7).

Based on the given type you can the access the underlying value.

std.debug.assert(di.int().? == 18446744073709551615);

All getter functions return either a value or null. You can use a pattern like if (di.int()) |v| v else return error.Oops; to access the value in a safe way. If you've used DataItem.new() and know the type of the data item, you should be safe to just do di.int().?.

The following getter functions are supported:

• int - returns ?i65
• string - returns ?[]const u8
• array - returns ?ArrayIterator
• map - returns ?MapIterator
• simple - returns ?u8
• float - returns ?f64
• tagged - returns ?Tag
• boolean - returns ?bool

Iterators

The functions array and map will return an iterator. Every time you call next() you will either get a DataItem/ Pair or null.

const di = DataItem.new("\x98\x19\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x18\x18\x19");

var iter = di.array().?;
while (iter.next()) |value| {
  _ = value;
  // doe something
}

Encoding and decoding

Serialization

You can serialize Zig objects into CBOR using the stringify() function.

const allocator = std.testing.allocator;
var str = std.ArrayList(u8).init(allocator);
defer str.deinit();

const Info = struct {
    versions: []const []const u8,
};

const i = Info{
    .versions = &.{"FIDO_2_0"},
};

try stringify(i, .{}, str.writer());

Note: Compile time floats are always encoded as single precision floats (f32). Please use @floatCast before passing a float to stringify().

The stringify() function is convenient but also adds extra overhead. If you want full control over the serialization process you can use the following functions defined in zbor.build: writeInt, writeByteString, writeTextString, writeTag, writeSimple, writeArray, writeMap. For more details check out the manual serialization example and the corresponding source code.

Stringify Options

You can pass options to the stringify function to influence its behavior. Without passing any options, stringify will behave as follows:

• Enums will be serialized to their textual representation
• u8 slices will be serialized to byte strings
• For structs and unions:
  • null fields are skipped by default
  • fields of type std.mem.Allocator are always skipped.
  • the names of fields are serialized to text strings

You can modify that behavior by changing the default options, e.g.:

const EcdsaP256Key = struct {
    /// kty:
    kty: u8 = 2,
    /// alg:
    alg: i8 = -7,
    /// crv:
    crv: u8 = 1,
    /// x-coordinate
    x: [32]u8,
    /// y-coordinate
    y: [32]u8,

    pub fn new(k: EcdsaP256.PublicKey) @This() {                                                                                                                                         
        const xy = k.toUncompressedSec1();
        return .{
            .x = xy[1..33].*,
            .y = xy[33..65].*,
        };
    }
};

//...

try stringify(k, .{ .field_settings = &.{
    .{ .name = "kty", .field_options = .{ .alias = "1", .serialization_type = .Integer } },
    .{ .name = "alg", .field_options = .{ .alias = "3", .serialization_type = .Integer } },
    .{ .name = "crv", .field_options = .{ .alias = "-1", .serialization_type = .Integer } },
    .{ .name = "x", .field_options = .{ .alias = "-2", .serialization_type = .Integer } },
    .{ .name = "y", .field_options = .{ .alias = "-3", .serialization_type = .Integer } },
} }, str.writer());

Here we define a alias for every field of the struct and tell serialize that it should treat those aliases as integers instead of text strings.

See Options and FieldSettings in src/parse.zig for all available options!

Deserialization

You can deserialize CBOR data into Zig objects using the parse() function.

const e = [5]u8{ 1, 2, 3, 4, 5 };
const di = DataItem.new("\x85\x01\x02\x03\x04\x05");

const x = try parse([5]u8, di, .{});

try std.testing.expectEqualSlices(u8, e[0..], x[0..]);

Parse Options

You can pass options to the parse function to influence its behaviour.

This includes:

• allocator - The allocator to be used. This is required if your data type has any pointers, slices, etc.
• duplicate_field_behavior - How to handle duplicate fields (.UseFirst, .Error)
• ignore_unknown_fields - Ignore unknown fields (default is true)
• field_settings - Lets you specify aliases for struct fields
• from_cborParse - Flag to break infinity loops (see Overriding parse)

Builder

You can also dynamically create CBOR data using the Builder.

const allocator = std.testing.allocator;

var b = try Builder.withType(allocator, .Map);
try b.pushTextString("a");
try b.pushInt(1);
try b.pushTextString("b");
try b.enter(.Array);
try b.pushInt(2);
try b.pushInt(3);
//try b.leave();            <-- you can leave out the return at the end
const x = try b.finish();
defer allocator.free(x);

// { "a": 1, "b": [2, 3] }
try std.testing.expectEqualSlices(u8, "\xa2\x61\x61\x01\x61\x62\x82\x02\x03", x);

Commands

• The push* functions append a data item
• The enter function takes a container type and pushes it on the builder stack
• The leave function leaves the current container. The container is appended to the wrapping container
• The finish function returns the CBOR data as owned slice

Overriding stringify

You can override the stringify function for structs and tagged unions by implementing cborStringify.

const Foo = struct {
    x: u32 = 1234,
    y: struct {
        a: []const u8 = "public-key",
        b: u64 = 0x1122334455667788,
    },

    pub fn cborStringify(self: *const @This(), options: Options, out: anytype) !void {

        // First stringify the 'y' struct
        const allocator = std.testing.allocator;
        var o = std.ArrayList(u8).init(allocator);
        defer o.deinit();
        try stringify(self.y, options, o.writer());

        // Then use the Builder to alter the CBOR output
        var b = try build.Builder.withType(allocator, .Map);
        try b.pushTextString("x");
        try b.pushInt(self.x);
        try b.pushTextString("y");
        try b.pushByteString(o.items);
        const x = try b.finish();
        defer allocator.free(x);

        try out.writeAll(x);
    }
};

The StringifyOptions can be used to indirectly pass an Allocator to the function.

Please make sure to set from_cborStringify to true when calling recursively into stringify(self) to prevent infinite loops.

Overriding parse

You can override the parse function for structs and tagged unions by implementing cborParse. This is helpful if you have aliases for your struct members.

const EcdsaP256Key = struct {
    /// kty:
    kty: u8 = 2,
    /// alg:
    alg: i8 = -7,
    /// crv:
    crv: u8 = 1,
    /// x-coordinate
    x: [32]u8,
    /// y-coordinate
    y: [32]u8,

    pub fn cborParse(item: DataItem, options: Options) !@This() {
        _ = options;
        return try parse(@This(), item, .{
            .from_callback = true, // prevent infinite loops
            .field_settings = &.{
                .{ .name = "kty", .field_options = .{ .alias = "1" } },
                .{ .name = "alg", .field_options = .{ .alias = "3" } },
                .{ .name = "crv", .field_options = .{ .alias = "-1" } },
                .{ .name = "x", .field_options = .{ .alias = "-2" } },
                .{ .name = "y", .field_options = .{ .alias = "-3" } },
            },
        });
    }
};

The Options can be used to indirectly pass an Allocator to the function.

Please make sure to set from_callback to true when calling recursively into parse(self) to prevent infinite loops.

Structs with fields of type std.mem.Allocator

If you have a struct with a field of type std.mem.Allocator you have to override the stringify funcation for that struct, e.g.:

pub fn cborStringify(self: *const @This(), options: cbor.StringifyOptions, out: anytype) !void {
    _ = options;

    try cbor.stringify(self, .{
        .from_cborStringify = true,
        .field_settings = &.{
            .{ .name = "allocator", .options = .{ .skip = true } },
        },
    }, out);
}

When using parse make sure you pass a allocator to the function. The passed allocator will be assigned to the field of type std.mem.Allocator.

ArrayBackedSlice

This library offers a convenient function named ArrayBackedSlice, which enables you to create a wrapper for an array of any size and type. This wrapper implements the cborStringify and cborParse methods, allowing it to seamlessly replace slices (e.g., []const u8) with an array.

test "ArrayBackedSlice test" {
    const allocator = std.testing.allocator;

    const S64B = ArrayBackedSlice(64, u8, .Byte);
    var x = S64B{};
    try x.set("\x01\x02\x03\x04");

    var str = std.ArrayList(u8).init(allocator);
    defer str.deinit();

    try stringify(x, .{}, str.writer());
    try std.testing.expectEqualSlices(u8, "\x44\x01\x02\x03\x04", str.items);

    const di = try DataItem.new(str.items);
    const y = try parse(S64B, di, .{});

    try std.testing.expectEqualSlices(u8, "\x01\x02\x03\x04", y.get());
}