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/BitString.java | 95 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 95 insertions(+)
 create mode 100644 src/main/model/asn1/BitString.java

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

diff --git a/src/main/model/asn1/BitString.java b/src/main/model/asn1/BitString.java
new file mode 100644
index 0000000..0561f24
--- /dev/null
+++ b/src/main/model/asn1/BitString.java
@@ -0,0 +1,95 @@
+package model.asn1;
+
+import model.asn1.exceptions.ParseException;
+import model.asn1.parsing.BytesReader;
+import ui.Utils;
+
+import java.math.BigInteger;
+
+/**
+ * Represents the ASN.1 BIT STRING (0x3) type. Bit strings represent bytes by padding to the LSB (like a bitstream).
+ * The bits are encoded as multiple bytes, but the unused bits are stored on the <b>lowest</b> part.
+ * For example, consider:
+ * <pre>
+ *     0b 00000001 10111001 01110111 (pad to the highest)
+ *     Will be encoded into
+ *     0b 01101110 01011101 11000000 (pad to the lowest)
+ *                            ^^^^^^
+ * </pre>
+ * Before the encoded value, there will be another byte denoting how many padding bits are added to the right.
+ * That is, the final encoding is:
+ * <pre>
+ *     0b 00000110 01101110 01011101 11000000
+ *        ^  6   ^ ^ Original Number  ^^Pad^
+ * </pre>
+ *
+ * BIT STRING has nothing to do with encoding bytes as printable strings (base10 or base16 or ASCII).
+ */
+public class BitString extends ASN1Object {
+    /**
+     * The X.680 universal class tag assignment.
+     */
+    public static final Tag TAG = new Tag(TagClass.UNIVERSAL, false, 0x3);
+
+    private final int unused;
+    private final Byte[] val;
+
+    /**
+     * EFFECT: Init with tags, unused, and val. For tags, see {@link ASN1Object}.
+     * REQUIRES: 0 <= unused < 8, the last byte in val must have the lowest $unused bits zero.
+     */
+    public BitString(Tag tag, Tag parentTag,
+                     final int unused,
+                     final Byte[] val) {
+        super(tag, parentTag);
+        this.unused = unused;
+        this.val = val;
+    }
+
+    /**
+     * EFFECT: Parse the input DER.
+     *   Throws {@link ParseException} if the input is invalid:
+     *    - Unused is not in 0 <= unused < 8
+     *    - The last byte does not have its lowest $unused bits zero
+     *    - Other issues found according to {@link ASN1Object}
+     */
+    public BitString(BytesReader encoded, boolean hasParentTag) throws ParseException {
+        super(encoded, hasParentTag);
+        this.unused = encoded.require(1, true)[0];
+        if (unused < 0 || unused > 7) {
+            throw new ParseException("Illegal unused byte: " + unused);
+        }
+        this.val = encoded.require(getLength() - 1, true);
+        if ((byte) (val[val.length - 1] << (8 - unused)) != 0) {
+            throw new ParseException(String.format("The last byte: 0x%02X does not have %d zero bits.",
+                    val[val.length - 1],
+                    unused));
+        }
+    }
+
+    /**
+     * EFFECTS: Get the converted form that has padding on MSB. The leftmost zero byte, if any, is removed.
+     */
+    public Byte[] getConvertedVal() {
+        return Utils.byteToByte(new BigInteger(Utils.byteToByte(val)).shiftRight(unused).toByteArray());
+    }
+
+    /**
+     * EFFECTS: Encode into DER.
+     */
+    @Override
+    public Byte[] encodeValueDER() {
+        Byte[] arr = new Byte[val.length + 1];
+        arr[0] = (byte) unused;
+        System.arraycopy(val, 0, arr, 1, val.length);
+        return arr;
+    }
+
+    public int getUnused() {
+        return unused;
+    }
+
+    public Byte[] getVal() {
+        return val;
+    }
+}
-- 
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