Add A5 Cipher (#3292)

src/main/java/com/thealgorithms/ciphers/a5/A5Cipher.java

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+package com.thealgorithms.ciphers.a5;
+
+import java.util.BitSet;
+
+// https://en.wikipedia.org/wiki/A5/1
+public class A5Cipher {
+    private final A5KeyStreamGenerator keyStreamGenerator;
+    private static final int KEY_STREAM_LENGTH = 228; // 28.5 bytes so we need to pad bytes or something
+
+    public A5Cipher( BitSet sessionKey, BitSet frameCounter ) {
+        keyStreamGenerator = new A5KeyStreamGenerator();
+        keyStreamGenerator.initialize( sessionKey, frameCounter );
+    }
+
+    public BitSet encrypt( BitSet plainTextBits ) {
+        // create a copy
+        var result = new BitSet( KEY_STREAM_LENGTH );
+        result.xor( plainTextBits );
+
+        var key = keyStreamGenerator.getNextKeyStream();
+        result.xor( key );
+
+        return result;
+    }
+
+    public void resetCounter() {
+        keyStreamGenerator.reInitialize();
+    }
+
+}

src/main/java/com/thealgorithms/ciphers/a5/A5KeyStreamGenerator.java

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+package com.thealgorithms.ciphers.a5;
+
+import java.util.BitSet;
+
+// TODO: raise exceptions for improper use
+public class A5KeyStreamGenerator extends CompositeLFSR {
+    private BitSet initialFrameCounter;
+    private BitSet frameCounter;
+    private BitSet sessionKey;
+    private static final int INITIAL_CLOCKING_CYCLES = 100;
+    private static final int KEY_STREAM_LENGTH = 228; // 28.5 bytes so we need to pad bytes or something
+
+    @Override
+    public void initialize( BitSet sessionKey, BitSet frameCounter ) {
+        this.sessionKey = sessionKey;
+        this.frameCounter = (BitSet) frameCounter.clone();
+        this.initialFrameCounter = (BitSet) frameCounter.clone();
+        registers.clear();
+        LFSR lfsr1 = new LFSR( 19, 8, new int[]{ 13, 16, 17, 18 } );
+        LFSR lfsr2 = new LFSR( 22, 10, new int[]{ 20, 21 } );
+        LFSR lfsr3 = new LFSR( 23, 10, new int[]{ 7, 20, 21, 22 } );
+        registers.add( lfsr1 );
+        registers.add( lfsr2 );
+        registers.add( lfsr3 );
+        registers.forEach( lfsr -> lfsr.initialize( sessionKey, frameCounter ) );
+    }
+
+    public void reInitialize() {
+        this.initialize( sessionKey, initialFrameCounter );
+    }
+
+    public BitSet getNextKeyStream() {
+        for ( int cycle = 1; cycle <= INITIAL_CLOCKING_CYCLES; ++cycle )
+            this.clock();
+
+        BitSet result = new BitSet( KEY_STREAM_LENGTH );
+        for ( int cycle = 1; cycle <= KEY_STREAM_LENGTH; ++cycle ) {
+            boolean outputBit = this.clock();
+            result.set( cycle - 1, outputBit );
+        }
+
+        reInitializeRegisters();
+        return result;
+    }
+
+    private void reInitializeRegisters() {
+        incrementFrameCounter();
+        registers.forEach( lfsr -> lfsr.initialize( sessionKey, frameCounter ) );
+    }
+
+    private void incrementFrameCounter() {
+        Utils.increment( frameCounter, FRAME_COUNTER_LENGTH );
+    }
+}

src/main/java/com/thealgorithms/ciphers/a5/BaseLFSR.java

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+package com.thealgorithms.ciphers.a5;
+
+import java.util.BitSet;
+
+public interface BaseLFSR {
+    void initialize(BitSet sessionKey, BitSet frameCounter);
+    boolean clock();
+    int SESSION_KEY_LENGTH = 64;
+    int FRAME_COUNTER_LENGTH = 22;
+}

src/main/java/com/thealgorithms/ciphers/a5/CompositeLFSR.java

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+package com.thealgorithms.ciphers.a5;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+import java.util.TreeMap;
+
+public abstract class CompositeLFSR implements BaseLFSR {
+    protected final List<LFSR> registers = new ArrayList<>();
+
+    /**
+     * Implements irregular clocking using the clock bit for each register
+     * @return the registers discarded bit xored value
+     */
+    @Override
+    public boolean clock() {
+        boolean majorityBit = getMajorityBit();
+        boolean result = false;
+        for ( var register : registers ) {
+            result ^= register.getLastBit();
+            if ( register.getClockBit() == majorityBit )
+                register.clock();
+        }
+        return result;
+    }
+
+    private boolean getMajorityBit() {
+        Map<Boolean, Integer> bitCount = new TreeMap<>();
+        bitCount.put( false, 0 );
+        bitCount.put( true, 0 );
+
+        registers.forEach( lfsr -> bitCount.put( lfsr.getClockBit(), bitCount.get( lfsr.getClockBit() ) + 1 ) );
+        return bitCount.get( false ) <= bitCount.get( true );
+    }
+}

src/main/java/com/thealgorithms/ciphers/a5/LFSR.java

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+package com.thealgorithms.ciphers.a5;
+
+import java.util.BitSet;
+
+public class LFSR implements BaseLFSR {
+    private final BitSet register;
+    private final int length;
+    private final int clockBitIndex;
+    private final int[] tappingBitsIndices;
+
+    public LFSR( int length, int clockBitIndex, int[] tappingBitsIndices ) {
+        this.length = length;
+        this.clockBitIndex = clockBitIndex;
+        this.tappingBitsIndices = tappingBitsIndices;
+        register = new BitSet( length );
+    }
+
+    @Override
+    public void initialize( BitSet sessionKey, BitSet frameCounter ) {
+        register.clear();
+        clock( sessionKey, SESSION_KEY_LENGTH );
+        clock( frameCounter, FRAME_COUNTER_LENGTH );
+    }
+
+    private void clock( BitSet key, int keyLength ) {
+        // We start from reverse because LFSR 0 index is the left most bit
+        // while key 0 index is right most bit, so we reverse it
+        for ( int i = keyLength - 1; i >= 0; --i ) {
+            var newBit = key.get( i ) ^ xorTappingBits();
+            pushBit( newBit );
+        }
+    }
+
+    @Override
+    public boolean clock() {
+        return pushBit( xorTappingBits() );
+    }
+
+    public boolean getClockBit() {
+        return register.get( clockBitIndex );
+    }
+
+    public boolean get( int bitIndex ) {
+        return register.get( bitIndex );
+    }
+
+    public boolean getLastBit() {
+        return register.get( length - 1 );
+    }
+
+    private boolean xorTappingBits() {
+        boolean result = false;
+        for ( int i : tappingBitsIndices ) {
+            result ^= register.get( i );
+        }
+        return result;
+    }
+
+    private boolean pushBit( boolean bit ) {
+        boolean discardedBit = rightShift();
+        register.set( 0, bit );
+        return discardedBit;
+    }
+
+    private boolean rightShift() {
+        boolean discardedBit = get( length - 1 );
+        for ( int i = length - 1; i > 0; --i ) {
+            register.set( i, get( i - 1 ) );
+        }
+        register.set( 0, false );
+        return discardedBit;
+    }
+
+    @Override
+    public String toString() {
+        return register.toString();
+    }
+}

src/main/java/com/thealgorithms/ciphers/a5/Utils.java

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+package com.thealgorithms.ciphers.a5;
+
+// Source http://www.java2s.com/example/java-utility-method/bitset/increment-bitset-bits-int-size-9fd84.html
+//package com.java2s;
+//License from project: Open Source License
+
+import java.util.BitSet;
+
+public class Utils {
+    public static boolean increment( BitSet bits, int size ) {
+        int i = size - 1;
+        while ( i >= 0 && bits.get( i ) ) {
+            bits.set( i--, false );/*from w w w  . j a  v a  2s  .c o  m*/
+        }
+        if ( i < 0 ) {
+            return false;
+        }
+        bits.set( i, true );
+        return true;
+    }
+}