zig-jwt/jwt.zig

const std = @import("std");
const testing = std.testing;
const ValueTree = std.json.ValueTree;
const base64url = std.base64.url_safe_no_pad;

const Algorithm = enum {
    HS256,
};

const JWTType = enum {
    JWS,
    JWE,
};

pub fn validate(allocator: *std.mem.Allocator, algorithm: Algorithm, key: []const u8, tokenText: []const u8) !ValueTree {
    // 1.   Verify that the JWT contains at least one period ('.')
    //      character.
    // 2.   Let the Encoded JOSE Header be the portion of the JWT before the
    //      first period ('.') character.
    var end_of_jose_base64 = std.mem.indexOfScalar(u8, tokenText, '.') orelse return error.InvalidFormat;
    const jose_base64 = tokenText[0..end_of_jose_base64];

    // 3.   Base64url decode the Encoded JOSE Header following the
    //      restriction that no line breaks, whitespace, or other additional
    //      characters have been used.
    var jose_json = try allocator.alloc(u8, try base64url.Decoder.calcSizeForSlice(jose_base64));
    defer allocator.free(jose_json);
    try base64url.Decoder.decode(jose_json, jose_base64);

    // 4.   Verify that the resulting octet sequence is a UTF-8-encoded
    //      representation of a completely valid JSON object conforming to
    //      RFC 7159 [RFC7159]; let the JOSE Header be this JSON object.

    // TODO: Make sure the JSON parser confirms everything above

    var parser = std.json.Parser.init(allocator, false);
    defer parser.deinit();

    var cty_opt = @as(?[]const u8, null);
    defer if (cty_opt) |cty| allocator.free(cty);

    var jwt_tree = try parser.parse(jose_json);
    defer jwt_tree.deinit();

    // 5.   Verify that the resulting JOSE Header includes only parameters
    //      and values whose syntax and semantics are both understood and
    //      supported or that are specified as being ignored when not
    //      understood.

    var jwt_root = jwt_tree.root;
    if (jwt_root != .Object) return error.InvalidFormat;

    {
        var alg_val = jwt_root.Object.get("alg") orelse return error.InvalidFormat;
        if (alg_val != .String) return error.InvalidFormat;
        const alg = std.meta.stringToEnum(Algorithm, alg_val.String) orelse return error.InvalidAlgorithm;

        // Make sure that the algorithm matches: https://auth0.com/blog/critical-vulnerabilities-in-json-web-token-libraries/
        if (alg != algorithm) return error.InvalidAlgorithm;

        // TODO: Determine if "jku"/"jwk" need to be parsed and validated

        if (jwt_root.Object.get("crit")) |crit_val| {
            if (crit_val != .Array) return error.InvalidFormat;
            const crit = crit_val.Array;
            if (crit.items.len == 0) return error.InvalidFormat;

            // TODO: Implement or allow extensions?
            return error.UnknownExtension;
        }
    }

    // 6.   Determine whether the JWT is a JWS or a JWE using any of the
    //      methods described in Section 9 of [JWE].

    const jwt_type = determine_jwt_type: {
        // From Section 9 of the JWE specification:
        // > o  If the object is using the JWS Compact Serialization or the JWE
        // >    Compact Serialization, the number of base64url-encoded segments
        // >    separated by period ('.') characters differs for JWSs and JWEs.
        // >    JWSs have three segments separated by two period ('.') characters.
        // >    JWEs have five segments separated by four period ('.') characters.
        switch (std.mem.count(u8, tokenText, ".")) {
            2 => break :determine_jwt_type JWTType.JWS,
            4 => break :determine_jwt_type JWTType.JWE,
            else => return error.InvalidFormat,
        }
    };

    // 7.   Depending upon whether the JWT is a JWS or JWE, there are two
    //      cases:
    const message_base64 = get_message: {
        switch (jwt_type) {
            // If the JWT is a JWS, follow the steps specified in [JWS] for
            // validating a JWS.  Let the Message be the result of base64url
            // decoding the JWS Payload.
            .JWS => {
                var section_iter = std.mem.split(tokenText, ".");
                std.debug.assert(section_iter.next() != null);
                const payload_base64 = section_iter.next().?;
                const signature_base64 = section_iter.rest();

                var signature = try allocator.alloc(u8, try base64url.Decoder.calcSizeForSlice(signature_base64));
                defer allocator.free(signature);
                try base64url.Decoder.decode(signature, signature_base64);

                switch (algorithm) {
                    .HS256 => {
                        const gen_sig = generate_signature_hmac_sha256(key, jose_base64, payload_base64);
                        if (!std.mem.eql(u8, signature, &gen_sig)) {
                            return error.InvalidSignature;
                        }
                    },
                }

                break :get_message try allocator.dupe(u8, payload_base64);
            },
            .JWE => {
                // Else, if the JWT is a JWE, follow the steps specified in
                // [JWE] for validating a JWE.  Let the Message be the resulting
                // plaintext.
                return error.Unimplemented;
            },
        }
    };
    defer allocator.free(message_base64);

    // 8.   If the JOSE Header contains a "cty" (content type) value of
    //      "JWT", then the Message is a JWT that was the subject of nested
    //      signing or encryption operations.  In this case, return to Step
    //      1, using the Message as the JWT.
    if (jwt_root.Object.get("cty")) |cty_val| {
        if (cty_val != .String) return error.InvalidFormat;
        return error.Unimplemented;
    }

    // 9.   Otherwise, base64url decode the Message following the
    //      restriction that no line breaks, whitespace, or other additional
    //      characters have been used.
    var message = try allocator.alloc(u8, try base64url.Decoder.calcSizeForSlice(message_base64));
    defer allocator.free(message);
    try base64url.Decoder.decode(message, message_base64);

    // 10.  Verify that the resulting octet sequence is a UTF-8-encoded
    //      representation of a completely valid JSON object conforming to
    //      RFC 7159 [RFC7159]; let the JWT Claims Set be this JSON object.
    var message_parser = std.json.Parser.init(allocator, true);
    defer message_parser.deinit();

    var message_tree = try message_parser.parse(message);
    return message_tree;
}

const HmacSha256 = std.crypto.auth.hmac.sha2.HmacSha256;
pub fn generate_signature_hmac_sha256(key: []const u8, protectedHeaderBase64: []const u8, payloadBase64: []const u8) [HmacSha256.mac_length]u8 {
    var h = HmacSha256.init(key);
    h.update(protectedHeaderBase64);
    h.update(".");
    h.update(payloadBase64);

    var out: [HmacSha256.mac_length]u8 = undefined;
    h.final(&out);

    return out;
}

test "validate jws hmac sha-256" {
    const token = "eyJ0eXAiOiJKV1QiLA0KICJhbGciOiJIUzI1NiJ9.eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6Ly9leGFtcGxlLmNvbS9pc19yb290Ijp0cnVlfQ.dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk";
    const key_base64 = "AyM1SysPpbyDfgZld3umj1qzKObwVMkoqQ-EstJQLr_T-1qS0gZH75aKtMN3Yj0iPS4hcgUuTwjAzZr1Z9CAow";

    var key = try std.testing.allocator.alloc(u8, try base64url.Decoder.calcSizeForSlice(key_base64));
    defer std.testing.allocator.free(key);
    try base64url.Decoder.decode(key, key_base64);

    var claims_tree = try validate(std.testing.allocator, .HS256, key, token);
    defer claims_tree.deinit();

    var claims = claims_tree.root;

    try std.testing.expectEqualSlices(u8, "joe", claims.Object.get("iss").?.String);
    try std.testing.expectEqual(@as(i64, 1300819380), claims.Object.get("exp").?.Integer);
    try std.testing.expectEqual(true, claims.Object.get("http://example.com/is_root").?.Bool);
}