./vigenere_keylen.pl crypted.txt

Checking key length of  2: min  13, max  36 ( 36.11 % ) -->  5.56 -  2.01 =  3.55 %
Checking key length of  3: min   5, max  35 ( 14.29 % ) -->  8.11 -  1.16 =  6.95 %
Checking key length of  4: min   4, max  20 ( 20.00 % ) -->  6.18 -  1.24 =  4.94 %
Checking key length of  5: min   2, max  21 (  9.52 % ) -->  8.11 -  0.77 =  7.34 %
Checking key length of  6: min   4, max  15 ( 26.67 % ) -->  6.95 -  1.85 =  5.10 %
Checking key length of  7: min   2, max  13 ( 15.38 % ) -->  7.03 -  1.08 =  5.95 %
Checking key length of  8: min   2, max  10 ( 20.00 % ) -->  6.18 -  1.24 =  4.94 %
Checking key length of  9: min   1, max  13 (  7.69 % ) -->  9.03 -  0.69 =  8.34 %
Checking key length of 10: min   1, max  12 (  8.33 % ) -->  9.27 -  0.77 =  8.49 %
Checking key length of 11: min   1, max   9 ( 11.11 % ) -->  7.64 -  0.85 =  6.80 %
Checking key length of 12: min   1, max  10 ( 10.00 % ) -->  9.27 -  0.93 =  8.34 %
Checking key length of 13: min   1, max  18 (  5.56 % ) --> 18.07 -  1.00 = 17.07 %
Checking key length of 14: min   1, max   8 ( 12.50 % ) -->  8.65 -  1.08 =  7.57 %
Checking key length of 15: min   1, max  13 (  7.69 % ) --> 15.06 -  1.16 = 13.90 %
Checking key length of 16: min   1, max   7 ( 14.29 % ) -->  8.65 -  1.24 =  7.41 %
Checking key length of 17: min   1, max   7 ( 14.29 % ) -->  9.19 -  1.31 =  7.88 %
Checking key length of 18: min   1, max   6 ( 16.67 % ) -->  8.34 -  1.39 =  6.95 %
Checking key length of 19: min   1, max   7 ( 14.29 % ) --> 10.27 -  1.47 =  8.80 %
Checking key length of 20: min   1, max   7 ( 14.29 % ) --> 10.81 -  1.54 =  9.27 %

The key length seems to be: 13.