From d342a45d98c4795b3a3fe1aaef5236ad4a782b55 Mon Sep 17 00:00:00 2001
From: Yuuta Liang <yuutaw@students.cs.ubc.ca>
Date: Thu, 12 Oct 2023 12:10:33 +0800
Subject: Implement data structures from X.680, X.501, X.509, and PKCS#10, with
 X.690 encoding / decoding support

The implementation took four days, and it is still a little bit rough. Updated version should arrive soon.

Signed-off-by: Yuuta Liang <yuutaw@students.cs.ubc.ca>
---
 src/main/model/asn1/Tag.java | 109 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 109 insertions(+)
 create mode 100644 src/main/model/asn1/Tag.java

(limited to 'src/main/model/asn1/Tag.java')

diff --git a/src/main/model/asn1/Tag.java b/src/main/model/asn1/Tag.java
new file mode 100644
index 0000000..15c144f
--- /dev/null
+++ b/src/main/model/asn1/Tag.java
@@ -0,0 +1,109 @@
+package model.asn1;
+
+import model.asn1.exceptions.ParseException;
+import model.asn1.parsing.BytesReader;
+
+/**
+ * Represents the metadata (tag) of an ASN.1 type.
+ */
+public class Tag implements Encodable {
+    private final TagClass cls;
+    private final boolean constructive;
+    private final int number;
+
+    /**
+     * EFFECTS: Construct the ASN.1 tag with the given class, constructive / primitive, and number.
+     * REQUIRES: number > 0 (X.680$8.2) and number <= 31 (no high tag number support currently);
+     * the constructive or primitive value must follow the X.680 spec, and it must be primitive if it
+     * can be both constructive or primitive (according to X.690 DER).
+     */
+    public Tag(TagClass cls, boolean constructive, int number) {
+        this.cls = cls;
+        this.constructive = constructive;
+        this.number = number;
+    }
+
+    /**
+     * EFFECTS: Initialize the tag by parsing class / constructive / number from the encoded DER bytes.
+     *  {@link ParseException} is thrown if the input is invalid:
+     *    - The encoded array must have at least one byte.
+     *    - The tag number is zero if the class is UNIVERSAL.
+     * REQUIRES: The highest two bits must contain the class, and then the constructive bit, and finally the low 5 bits
+     * must contain the tag number <= 31.
+     * MODIFIES: encoded (one byte read)
+     */
+    public Tag(BytesReader encoded) throws ParseException {
+        final Byte val = encoded.require(1, true)[0];
+        // Take the highest two bits
+        // -64 = 2's complement of 0b11000000
+        final int highestTwo = val & -64;
+        if (highestTwo == TagClass.UNIVERSAL.getVal()) {
+            this.cls = TagClass.UNIVERSAL;
+        } else if (highestTwo == TagClass.APPLICATION.getVal()) {
+            this.cls = TagClass.APPLICATION;
+        } else if (highestTwo == TagClass.PRIVATE.getVal()) {
+            this.cls = TagClass.PRIVATE;
+        } else {
+            this.cls = TagClass.CONTEXT_SPECIFIC;
+        }
+
+        // Detect the constructive bit by only keeping the corresponding bit and shifting it to the lowestbit.
+        this.constructive = (val & 0x20) >> 5 == 1;
+
+        // Parse the number by clearing the high 3 bits.
+        this.number = val & 0x1F;
+
+        if (this.cls == TagClass.UNIVERSAL && this.number == 0) {
+            throw new ParseException(String.format("The tag number must not be zero for UNIVERSAL tags"
+                            + "(byte 0x%02X @ %d)", val, encoded.getIndex()));
+        }
+    }
+
+    /**
+     * EFFECTS: Encode that tag as DER bytes, as follows:
+     * HI 7  6 | 5   | 4  3  2  1  0 LO
+     *   Class | C/P | Tag Number
+     * Notes, In the domain of this application (PKI), a single byte is always returned
+     * (as nothing requires high tag number). However, the return type is held as byte[]
+     * to 1) compliant with the spec, 2) reserve for future scalability.
+     */
+    @Override
+    public Byte[] encodeDER() {
+        // Fill the low 5bits with tag number
+        byte value = (byte) number;
+        // Fill the bit in-between with constructive bit
+        if (constructive) {
+            value |= 0x20; // 0b00100000: Enable the 5th bit
+        } else {
+            value &= 0xDF; // 0b11011111: Disable the 5th bit
+        }
+        // Fill the high two bits with tag class
+        value |= cls.getVal();
+        return new Byte[] { value };
+    }
+
+    /**
+     * EFFECTS: Throw {@link ParseException} if this tag is not exactly the given tag. Useful for parsing.
+     */
+    public void enforce(Tag tag) throws ParseException {
+        if (this.number != tag.number
+                || this.constructive != tag.constructive
+                || this.cls != tag.cls) {
+            throw new ParseException(String.format("Illegal tag: Expected %s[%d] but got %s[%d].",
+                    tag.cls, tag.number,
+                    cls, number));
+        }
+    }
+
+    public TagClass getCls() {
+        return cls;
+    }
+
+    public boolean isConstructive() {
+        return constructive;
+    }
+
+    public int getNumber() {
+        return number;
+    }
+}
-- 
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