How to choose constants in a cryptographic function? A number of cryptographic functions have constants built in. For example, the constants used in RFC 2104 for HMAC, or the constants used in s-boxes (e.g., DES and AES), or MD5.  In general, how are constants such as these generated so as not to arouse suspicion of tampering or weakening?
There are some approaches.


In many algorithms it for the security doesn't really matter what constant is used, as long as it is not too simple, like initialization vectors for hash functions. (And of course, we need to use always the same number.) Then mathematical constants like binary expansions of irrational numbers like $\sqrt{2}$ (or roots of other numbers), $e$, $\pi$ can be used, to show that one didn't select the numbers to create a back door. This is known as a nothing up my sleeve number, the linked Wikipedia article contains some more examples.
The padding constants for HMAC, 0x36 for ipad and 0x5C for opad, are repeated (for the hash function's block size) and XORed with the key to generate the prefixes for the two hashing steps. 0x36 = 0b00110110 and 0x5C = 0b01011100 - these two values are about "as different as possible", to avoid any attacks which rely on a similar hash state after the key block for inner and outer hash. This is mainly a heuristic, as far as I understand.
In the DES S-boxes, the constants were randomly generated and then tested for resistance against differential cryptanalysis. As differential cryptanalysis was not yet known officially, and NSA didn't explain what they did (one only could see that they gave it back with other S-boxes), there was quite some suspicion that they put in a backdoor, while they actually made the algorithm harder to break.
In AES, the S-boxes are an implementation of a simple mathematical function (inversion in $\mathbb F_{2^8}$, followed by a linear transformation). One wouldn't need an S-box here, it just helps to efficiently implement it.
In the hash function Skein (one of the SHA-3 candidates), the initialization vector (initial state of the iteration) is not a fixed arbitrary number, but is gained by hashing a "configuration vector" (where we use 0 as init state). If one uses Skein only in common uses (e.g. a simple hash), i.e. always with the same configuration, one can hardcode the resulting hash state into the implementation instead of re-running it each time.