您可以直接使用实际的OpenSSH VisualHostKey代码，该代码位于OpenSSH源代码中的key.c文件中的key_fingerprint_randomart()函数中。该算法相当简单，可以将任何字节数组作为输入。在OpenSSH中，输入是密钥的加密哈希值；您也可以这样做。
(如在OpenSSH源代码中定义的那样，该函数还需要一个指向密钥结构本身的指针，但这仅用于在图片顶部打印密钥的类型和大小。)
事实上，由于该代码是自由许可的，因此让我在此处包含一份副本。这是从OpenSSH 6.1中提取的，$OpenBSD: key.c,v 1.99 2012/05/23 03:28:28 djm Exp $:
/*
* Copyright (c) 2000, 2001 Markus Friedl. All rights reserved.
* Copyright (c) 2008 Alexander von Gernler. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
* IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
* THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
/*
* Draw an ASCII-Art representing the fingerprint so human brain can
* profit from its built-in pattern recognition ability.
* This technique is called "random art" and can be found in some
* scientific publications like this original paper:
*
* "Hash Visualization: a New Technique to improve Real-World Security",
* Perrig A. and Song D., 1999, International Workshop on Cryptographic
* Techniques and E-Commerce (CrypTEC '99)
* sparrow.ece.cmu.edu/~adrian/projects/validation/validation.pdf
*
* The subject came up in a talk by Dan Kaminsky, too.
*
* If you see the picture is different, the key is different.
* If the picture looks the same, you still know nothing.
*
* The algorithm used here is a worm crawling over a discrete plane,
* leaving a trace (augmenting the field) everywhere it goes.
* Movement is taken from dgst_raw 2bit-wise. Bumping into walls
* makes the respective movement vector be ignored for this turn.
* Graphs are not unambiguous, because circles in graphs can be
* walked in either direction.
*/
/*
* Field sizes for the random art. Have to be odd, so the starting point
* can be in the exact middle of the picture, and FLDBASE should be >=8 .
* Else pictures would be too dense, and drawing the frame would
* fail, too, because the key type would not fit in anymore.
*/
#define FLDBASE 8
#define FLDSIZE_Y (FLDBASE + 1)
#define FLDSIZE_X (FLDBASE * 2 + 1)
static char *
key_fingerprint_randomart(u_char *dgst_raw, u_int dgst_raw_len, const Key *k)
{
/*
* Chars to be used after each other every time the worm
* intersects with itself. Matter of taste.
*/
char *augmentation_string = " .o+=*BOX@%&#/^SE";
char *retval, *p;
u_char field[FLDSIZE_X][FLDSIZE_Y];
u_int i, b;
int x, y;
size_t len = strlen(augmentation_string) - 1;
retval = xcalloc(1, (FLDSIZE_X + 3) * (FLDSIZE_Y + 2));
/* initialize field */
memset(field, 0, FLDSIZE_X * FLDSIZE_Y * sizeof(char));
x = FLDSIZE_X / 2;
y = FLDSIZE_Y / 2;
/* process raw key */
for (i = 0; i < dgst_raw_len; i++) {
int input;
/* each byte conveys four 2-bit move commands */
input = dgst_raw[i];
for (b = 0; b < 4; b++) {
/* evaluate 2 bit, rest is shifted later */
x += (input & 0x1) ? 1 : -1;
y += (input & 0x2) ? 1 : -1;
/* assure we are still in bounds */
x = MAX(x, 0);
y = MAX(y, 0);
x = MIN(x, FLDSIZE_X - 1);
y = MIN(y, FLDSIZE_Y - 1);
/* augment the field */
if (field[x][y] < len - 2)
field[x][y]++;
input = input >> 2;
}
}
/* mark starting point and end point*/
field[FLDSIZE_X / 2][FLDSIZE_Y / 2] = len - 1;
field[x][y] = len;
/* fill in retval */
snprintf(retval, FLDSIZE_X, "+--[%4s %4u]", key_type(k), key_size(k));
p = strchr(retval, '\0');
/* output upper border */
for (i = p - retval - 1; i < FLDSIZE_X; i++)
*p++ = '-';
*p++ = '+';
*p++ = '\n';
/* output content */
for (y = 0; y < FLDSIZE_Y; y++) {
*p++ = '|';
for (x = 0; x < FLDSIZE_X; x++)
*p++ = augmentation_string[MIN(field[x][y], len)];
*p++ = '|';
*p++ = '\n';
}
/* output lower border */
*p++ = '+';
for (i = 0; i < FLDSIZE_X; i++)
*p++ = '-';
*p++ = '+';
return retval;
}
它似乎与OpenSSH代码的其余部分没有显著的依赖关系，除了const Key *k参数外，该参数仅在一个行中作为key_type()和key_size()函数（或宏？）的参数使用。非标准类型u_char和u_int似乎只是unsigned char和unsigned int的别名，而xcalloc()函数似乎只是标准calloc()的替代或包装。