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25 
26 package com.sun.crypto.provider;
27 
28 import javax.crypto.ShortBufferException;
29 import java.util.Arrays;
30 
31 /**
32  * This class implements padding as specified in the PKCS#5 standard.
33  *
34  * @author Gigi Ankeny
35  *
36  *
37  * @see Padding
38  */
39 final class PKCS5Padding implements Padding {
40 
41     private int blockSize;
42 
PKCS5Padding(int blockSize)43     PKCS5Padding(int blockSize) {
44         this.blockSize = blockSize;
45     }
46 
47     /**
48      * Adds the given number of padding bytes to the data input.
49      * The value of the padding bytes is determined
50      * by the specific padding mechanism that implements this
51      * interface.
52      *
53      * @param in the input buffer with the data to pad
54      * @param off the offset in <code>in</code> where the padding bytes
55      * are appended
56      * @param len the number of padding bytes to add
57      *
58      * @exception ShortBufferException if <code>in</code> is too small to hold
59      * the padding bytes
60      */
padWithLen(byte[] in, int off, int len)61     public void padWithLen(byte[] in, int off, int len)
62         throws ShortBufferException
63     {
64         if (in == null)
65             return;
66 
67         int idx = Math.addExact(off, len);
68         if (idx > in.length) {
69             throw new ShortBufferException("Buffer too small to hold padding");
70         }
71 
72         byte paddingOctet = (byte) (len & 0xff);
73         Arrays.fill(in, off, idx, paddingOctet);
74         return;
75     }
76 
77     /**
78      * Returns the index where the padding starts.
79      *
80      * <p>Given a buffer with padded data, this method returns the
81      * index where the padding starts.
82      *
83      * @param in the buffer with the padded data
84      * @param off the offset in <code>in</code> where the padded data starts
85      * @param len the length of the padded data
86      *
87      * @return the index where the padding starts, or -1 if the input is
88      * not properly padded
89      */
unpad(byte[] in, int off, int len)90     public int unpad(byte[] in, int off, int len) {
91         if ((in == null) ||
92             (len == 0)) { // this can happen if input is really a padded buffer
93             return 0;
94         }
95         int idx = Math.addExact(off, len);
96         byte lastByte = in[idx - 1];
97         int padValue = (int)lastByte & 0x0ff;
98         if ((padValue < 0x01)
99             || (padValue > blockSize)) {
100             return -1;
101         }
102 
103         int start = idx - padValue;
104         if (start < off) {
105             return -1;
106         }
107 
108         for (int i = start; i < idx; i++) {
109             if (in[i] != lastByte) {
110                 return -1;
111             }
112         }
113         return start;
114     }
115 
116     /**
117      * Determines how long the padding will be for a given input length.
118      *
119      * @param len the length of the data to pad
120      *
121      * @return the length of the padding
122      */
padLength(int len)123     public int padLength(int len) {
124         int paddingOctet = blockSize - (len % blockSize);
125         return paddingOctet;
126     }
127 }
128