How unique are Trello board ids as I am able to see some common prefix (6 character match out of 24 character id) while creating board from Trello website within span of small interval.
Are those ids are cryptographically random or how difficult for normal user to guess other id based on some ids.
Note Need this information to assess security implication of storing this in my client side application
Trello also uses a shortId, which is 8 characters. So I created a card and go the key “Qe5yM5cI” which can contain upper and lower case letters and numbers.
Since there are 26 letters in the English alphabet (both upper and lower case) and 10 digits (0-9), there are a total of 26 + 26 + 10 = 62 possible characters that can be used for each position in the key.
To calculate the number of possible permutations of an 8-character key using a 62-character alphabet, we can use the formula for permutations:
n! / (n - r)!
where n is the number of characters in the alphabet (62), and r is the length of the key (8).
Plugging in the values, we get:
62! / (62 - 8)!
= 62 x 61 x 60 x 59 x 58 x 57 x 56 x 55
= 208,304,424,296,960,000
Therefore, there are approximately 208 trillion possible permutations for an 8-character key using a 62-character alphabet. So, 24-character length ID aside, the 8 digit one seems to have a 1 in 208 trillion chance of collision. It would be like winning the lottery 700,000 times in a row – if you could even live that long. 
Simple answer: very unique.