One day, Nobita found that his computer is extremely slow. After several hours' work, he finally found that it was a virus that made his poor computer slow and the virus was activated by a misoperation of opening an attachment of an email.
Nobita did use an outstanding anti-virus software, however, for some strange reason, this software did not check email attachments. Now Nobita decide to detect viruses in emails by himself.
To detect an virus, a virus sample (several binary bytes) is needed. If these binary bytes can be found in the email attachment (binary data), then the attachment contains the virus.
Note that attachments (binary data) in emails are usually encoded in base64. To encode a binary stream in base64, first write the binary stream into bits. Then take 6 bits from the stream in turn, encode these 6 bits into a base64 character according the following table:
That is, translate every 3 bytes into 4 base64 characters. If the original binary stream contains 3k + 1 bytes, where k is an integer, fill last bits using zero when encoding and append '==' as padding. If the original binary stream contains 3k + 2 bytes, fill last bits using zero when encoding and append '=' as padding. No padding is needed when the original binary stream contains 3k bytes.
| Value | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 |
| Encoding | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | a | b | c | d | e | f |
| Value | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | 46 | 47 | 48 | 49 | 50 | 51 | 52 | 53 | 54 | 55 | 56 | 57 | 58 | 59 | 60 | 61 | 62 | 63 |
| Encoding | g | h | i | j | k | l | m | n | o | p | q | r | s | t | u | v | w | x | y | z | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | + | / |
For example, to encode 'hello' into base64, first write 'hello' as binary bits, that is: 01101000 01100101 01101100 01101100 01101111
Then, take 6 bits in turn and fill last bits as zero as padding (zero padding bits are marked in bold): 011010 000110 010101 101100 011011 000110 111100
They are 26 6 21 44 27 6 60 in decimal. Look up the table above and use corresponding characters: aGVsbG8
Since original binary data contains 1 * 3 + 2 bytes, padding is needed, append '=' and 'hello' is finally encoded in base64: aGVsbG8=
Section 5.2 of RFC 1521 describes how to encode a binary stream in base64 much more detailedly:
Click here to see Section 5.2 of RFC 1521 if you have interest
The Base64 Content-Transfer-Encoding is designed to represent arbitrary sequences of octets in a form that need not be humanly readable. The encoding and decoding algorithms are simple, but the encoded data are consistently only about 33 percent larger than the unencoded data. This encoding is virtually identical to the one used in Privacy Enhanced Mail (PEM) applications, as defined in RFC 1421. The base64 encoding is adapted from RFC 1421, with one change: base64 eliminates the "*" mechanism for embedded clear text. A 65-character subset of US-ASCII is used, enabling 6 bits to be represented per printable character. (The extra 65th character, "=", is used to signify a special processing function.) NOTE: This subset has the important property that it is represented identically in all versions of ISO 646, including US ASCII, and all characters in the subset are also represented identically in all versions of EBCDIC. Other popular encodings, such as the encoding used by the uuencode utility and the base85 encoding specified as part of Level 2 PostScript, do not share these properties, and thus do not fulfill the portability requirements a binary transport encoding for mail must meet. The encoding process represents 24-bit groups of input bits as output strings of 4 encoded characters. Proceeding from left to right, a 24-bit input group is formed by concatenating 3 8-bit input groups. These 24 bits are then treated as 4 concatenated 6-bit groups, each of which is translated into a single digit in the base64 alphabet. When encoding a bit stream via the base64 encoding, the bit stream must be presumed to be ordered with the most-significant-bit first. That is, the first bit in the stream will be the high-order bit in the first byte, and the eighth bit will be the low-order bit in the first byte, and so on. Each 6-bit group is used as an index into an array of 64 printable characters. The character referenced by the index is placed in the output string. These characters, identified in Table 1, below, are selected so as to be universally representable, and the set excludes characters with particular significance to SMTP (e.g., ".", CR, LF) and to the encapsulation boundaries defined in this document (e.g., "-"). Table 1: The Base64 Alphabet Value Encoding Value Encoding Value Encoding Value Encoding 0 A 17 R 34 i 51 z 1 B 18 S 35 j 52 0 2 C 19 T 36 k 53 1 3 D 20 U 37 l 54 2 4 E 21 V 38 m 55 3 5 F 22 W 39 n 56 4 6 G 23 X 40 o 57 5 7 H 24 Y 41 p 58 6 8 I 25 Z 42 q 59 7 9 J 26 a 43 r 60 8 10 K 27 b 44 s 61 9 11 L 28 c 45 t 62 + 12 M 29 d 46 u 63 / 13 N 30 e 47 v 14 O 31 f 48 w (pad) = 15 P 32 g 49 x 16 Q 33 h 50 y The output stream (encoded bytes) must be represented in lines of no more than 76 characters each. All line breaks or other characters not found in Table 1 must be ignored by decoding software. In base64 data, characters other than those in Table 1, line breaks, and other white space probably indicate a transmission error, about which a warning message or even a message rejection might be appropriate under some circumstances. Special processing is performed if fewer than 24 bits are available at the end of the data being encoded. A full encoding quantum is always completed at the end of a body. When fewer than 24 input bits are available in an input group, zero bits are added (on the right) to form an integral number of 6-bit groups. Padding at the end of the data is performed using the '=' character. Since all base64 input is an integral number of octets, only the following cases can arise: (1) the final quantum of encoding input is an integral multiple of 24 bits; here, the final unit of encoded output will be an integral multiple of 4 characters with no "=" padding, (2) the final quantum of encoding input is exactly 8 bits; here, the final unit of encoded output will be two characters followed by two "=" padding characters, or (3) the final quantum of encoding input is exactly 16 bits; here, the final unit of encoded output will be three characters followed by one "=" padding character. Because it is used only for padding at the end of the data, the occurrence of any '=' characters may be taken as evidence that the end of the data has been reached (without truncation in transit). No such assurance is possible, however, when the number of octets transmitted was a multiple of three. Any characters outside of the base64 alphabet are to be ignored in base64-encoded data. The same applies to any illegal sequence of characters in the base64 encoding, such as "=====" Care must be taken to use the proper octets for line breaks if base64 encoding is applied directly to text material that has not been converted to canonical form. In particular, text line breaks must be converted into CRLF sequences prior to base64 encoding. The important thing to note is that this may be done directly by the encoder rather than in a prior canonicalization step in some implementations. NOTE: There is no need to worry about quoting apparent encapsulation boundaries within base64-encoded parts of multipart entities because no hyphen characters are used in the base64 encoding.
Here is a piece of ANSI C code that can encode binary data in base64. It contains a function, encode (infile, outfile), to encode binary file infile in base64 and output result to outfile.
Click here to see the reference C code if you have interest
static const char cb64[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
void encodeblock(unsigned char in[3], unsigned char out[4], int len) {
out[0] = cb64[ in[0] >> 2 ];
out[1] = cb64[ ((in[0] & 0x03) << 4) | ((in[1] & 0xf0) >> 4) ];
out[2] = (unsigned char) (len > 1 ? cb64[ ((in[1] & 0x0f) << 2) | ((in[2] & 0xc0) >> 6) ] : '=');
out[3] = (unsigned char) (len > 2 ? cb64[ in[2] & 0x3f ] : '=');
}
void encode(FILE *infile, FILE *outfile) {
unsigned char in[3], out[4];
int i, len;
while (!feof(infile)) {
len = 0;
for (i = 0; i < 3; i++) {
in[i] = (unsigned char) getc(infile);
if (!feof(infile)) {
len++;
} else {
in[i] = 0;
}
}
if (len) {
encodeblock(in, out, len);
for (i = 0; i < 4; i++) {
putc(out[i], outfile);
}
}
}
}
Input
Input contains multiple cases (about 15, of which most are small ones). The first line of each case contains an integer N (0 <= N <= 512). In the next N distinct lines, each line contains a sample of a kind of virus, which is not empty, has not more than 64 bytes in binary and is encoded in base64. Then, the next line contains an integer M (1 <= M <= 128). In the following M lines, each line contains the content of a file to be detected, which is not empty, has no more than 2048 bytes in binary and is encoded in base64.
There is a blank line after each case.
<h4< dd="">Output
For each case, output M lines. The ith line contains the number of kinds of virus detected in the ith file.
Output a blank line after each case.
<h4< dd="">Sample Input
3 YmFzZTY0 dmlydXM= dDog 1 dGVzdDogdmlydXMu 1 QA== 2 QA== ICAgICAgICA=
<h4< dd="">Sample Output
2 1 0
<h4< dd="">Hint
In the first sample case, there are three virus samples: base64, virus and t: , the data to be checked is test: virus., which contains the second and the third, two virus samples.
解码过程很恶心，不能用char存
题意：先给n串加密后的字符，然后m串加密后的字符，解码之后求n对应每个m的匹配数，很显然的ac自动机
加密过程是先用对应ascii表的标号来代替字符，然后把这些数字转换成8位的二进制，全部连起来，然后每6位算一个数，用二进制算成整数，最后用这些整数来映射给定的表
题解：反解密就好了，就是特别容易错
代码：
#include <iostream> #include <stdio.h> #include <string.h> #include <algorithm> #include <queue> using namespace std; struct Trie { int next[520*64][256],fail[520*64],end[520*64]; int root,L; int newnode() { for(int i = 0;i < 256;i++) next[L][i] = -1; end[L++] = -1; return L-1; } void init() { L = 0; root = newnode(); } void insert(unsigned char buf[],int len,int id) { int now = root; for(int i = 0;i < len;i++) { if(next[now][buf[i]] == -1) next[now][buf[i]] = newnode(); now = next[now][buf[i]]; } end[now] = id; } void build() { queue<int>Q; fail[root] = root; for(int i = 0;i < 256;i++) if(next[root][i] == -1) next[root][i] = root; else { fail[next[root][i]]=root; Q.push(next[root][i]); } while(!Q.empty()) { int now = Q.front(); Q.pop(); for(int i = 0;i < 256;i++) if(next[now][i] == -1) next[now][i] = next[fail[now]][i]; else { fail[next[now][i]] = next[fail[now]][i]; Q.push(next[now][i]); } } } bool used[520]; int query(unsigned char buf[],int len,int n) { memset(used,false,sizeof(used)); int now = root; for(int i = 0;i < len;i++) { now = next[now][buf[i]]; int temp = now; while( temp!=root ) { if(end[temp] != -1) used[end[temp]]=true; temp = fail[temp]; } } int res = 0; for(int i = 0;i < n;i++) if(used[i]) res++; return res; } }; unsigned char buf[2050]; int tot; char str[4000]; unsigned char s[4000]; unsigned char Get(char ch) { if( ch>='A'&&ch<='Z' )return ch-'A'; if( ch>='a'&&ch<='z' )return ch-'a'+26; if( ch>='0'&&ch<='9' )return ch-'0'+52; if( ch=='+' )return 62; else return 63; } void change(unsigned char str[],int len) { int t=0; for(int i=0;i<len;i+=4) { buf[t++]=((str[i]<<2)|(str[i+1]>>4)); if(i+2 < len) buf[t++]=( (str[i+1]<<4)|(str[i+2]>>2) ); if(i+3 < len) buf[t++]= ( (str[i+2]<<6)|str[i+3] ); } tot=t; } Trie ac; int main() { // freopen("in.txt","r",stdin); // freopen("out.txt","w",stdout); int n,m; while(scanf("%d",&n) == 1) { ac.init(); for(int i = 0;i < n;i++) { scanf("%s",str); int len = strlen(str); while(str[len-1]=='=')len--; for(int j = 0;j < len;j++) { s[j] = Get(str[j]); } change(s,len); ac.insert(buf,tot,i); } ac.build(); scanf("%d",&m); while(m--) { scanf("%s",str); int len=strlen(str); while(str[len-1]=='=')len--; for(int j = 0;j < len;j++) s[j] = Get(str[j]); change(s,len); printf("%d ",ac.query(buf,tot,n)); } printf(" "); } return 0; }