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Diffstat (limited to 'src/main/model/asn1/BitString.java')
-rw-r--r-- | src/main/model/asn1/BitString.java | 95 |
1 files changed, 95 insertions, 0 deletions
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; + } +} |