Annotation of src/usr.bin/cksum/cksum.1, Revision 1.12
1.12 ! aaron 1: .\" $OpenBSD: cksum.1,v 1.11 2000/03/05 20:09:20 aaron Exp $
1.1 deraadt 2: .\" $NetBSD: cksum.1,v 1.8 1995/09/02 05:45:15 jtc Exp $
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38: .\" @(#)cksum.1 8.2 (Berkeley) 4/28/95
39: .\"
40: .Dd April 28, 1995
41: .Dt CKSUM 1
1.8 aaron 42: .Os
1.1 deraadt 43: .Sh NAME
1.9 aaron 44: .Nm cksum ,
45: .Nm sum
1.1 deraadt 46: .Nd display file checksums and block counts
47: .Sh SYNOPSIS
48: .Nm cksum
49: .Op Fl o Ar \&1 No \&| Ar \&2
50: .Op Ar file ...
51: .Nm sum
52: .Op Ar file ...
53: .Sh DESCRIPTION
54: The
1.11 aaron 55: .Nm cksum
1.1 deraadt 56: utility writes to the standard output three whitespace separated
57: fields for each input file.
58: These fields are a checksum
59: .Tn CRC ,
60: the total number of octets in the file and the file name.
61: If no file name is specified, the standard input is used and no file name
62: is written.
63: .Pp
64: The
65: .Nm sum
66: utility is identical to the
1.11 aaron 67: .Nm cksum
1.1 deraadt 68: utility, except that it defaults to using historic algorithm 1, as
69: described below.
70: It is provided for compatibility only.
71: .Pp
72: The options are as follows:
1.12 ! aaron 73: .Bl -tag -width Ds
1.10 deraadt 74: .It Fl o Ar \&1 No \&| Ar \&2
1.1 deraadt 75: Use historic algorithms instead of the (superior) default one.
76: .Pp
77: Algorithm 1 is the algorithm used by historic
78: .Bx
1.8 aaron 79: systems as the
1.1 deraadt 80: .Xr sum 1
81: algorithm and by historic
82: .At V
83: systems as the
84: .Xr sum
85: algorithm when using the
86: .Fl r
87: option.
88: This is a 16-bit checksum, with a right rotation before each addition;
89: overflow is discarded.
90: .Pp
91: Algorithm 2 is the algorithm used by historic
92: .At V
93: systems as the
94: default
95: .Xr sum
96: algorithm.
97: This is a 32-bit checksum, and is defined as follows:
98: .Bd -unfilled -offset indent
99: s = sum of all bytes;
100: r = s % 2^16 + (s % 2^32) / 2^16;
101: cksum = (r % 2^16) + r / 2^16;
102: .Ed
103: .Pp
104: Both algorithm 1 and 2 write to the standard output the same fields as
105: the default algorithm except that the size of the file in bytes is
106: replaced with the size of the file in blocks.
107: For historic reasons, the block size is 1024 for algorithm 1 and 512
108: for algorithm 2.
109: Partial blocks are rounded up.
110: .El
111: .Pp
112: The default
113: .Tn CRC
114: used is based on the polynomial used for
115: .Tn CRC
116: error checking
117: in the networking standard
118: .St -iso8802-3
119: The
120: .Tn CRC
121: checksum encoding is defined by the generating polynomial:
122: .Pp
123: .Bd -unfilled -offset indent
124: G(x) = x^32 + x^26 + x^23 + x^22 + x^16 + x^12 +
125: x^11 + x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1
126: .Ed
127: .Pp
128: Mathematically, the
129: .Tn CRC
130: value corresponding to a given file is defined by
131: the following procedure:
132: .Bd -filled -offset indent
133: The
134: .Ar n
135: bits to be evaluated are considered to be the coefficients of a mod 2
136: polynomial M(x) of degree
137: .Ar n Ns \-1 .
138: These
139: .Ar n
140: bits are the bits from the file, with the most significant bit being the most
141: significant bit of the first octet of the file and the last bit being the least
142: significant bit of the last octet, padded with zero bits (if necessary) to
143: achieve an integral number of octets, followed by one or more octets
144: representing the length of the file as a binary value, least significant octet
145: first.
146: The smallest number of octets capable of representing this integer are used.
147: .Pp
148: M(x) is multiplied by x^32 (i.e., shifted left 32 bits) and divided by
149: G(x) using mod 2 division, producing a remainder R(x) of degree <= 31.
150: .Pp
151: The coefficients of R(x) are considered to be a 32-bit sequence.
152: .Pp
153: The bit sequence is complemented and the result is the CRC.
154: .Ed
155: .Pp
156: The
1.11 aaron 157: .Nm cksum
1.1 deraadt 158: and
159: .Nm sum
1.7 aaron 160: utilities exit 0 on success or >0 if an error occurred.
1.3 bitblt 161: .Sh WARNING
162: Do not use
1.11 aaron 163: .Nm cksum
164: or
1.8 aaron 165: .Nm sum
1.11 aaron 166: to detect hostile binary modifications.
167: An attacker can trivially produce backdoored daemons which have the same
168: CRC as the standard versions.
169: Use a cryptographic checksum (such as MD5) instead.
1.1 deraadt 170: .Sh SEE ALSO
1.6 deraadt 171: .Xr md5 1 ,
172: .Xr rmd160 1 ,
173: .Xr sha1 1
1.5 deraadt 174: .Pp
1.1 deraadt 175: The default calculation is identical to that given in pseudo-code
176: in the following
177: .Tn ACM
178: article.
179: .Rs
180: .%T "Computation of Cyclic Redundancy Checks Via Table Lookup"
181: .%A Dilip V. Sarwate
182: .%J "Communications of the \\*(tNACM\\*(sP"
183: .%D "August 1988"
184: .Re
185: .Sh STANDARDS
186: The
1.11 aaron 187: .Nm cksum
1.4 denny 188: utility is compliant with the
189: .St -p1003.2-92
190: specification.
1.1 deraadt 191: .Sh HISTORY
192: The
1.11 aaron 193: .Nm cksum
1.1 deraadt 194: utility appeared in
195: .Bx 4.4 .
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