債券の無作為変数と暗号化のメッセージ翻訳

最近国債などの金融商品の価格計算について読んでいます。
Popular models for determining interest rates of bonds include the Ho-Lee, Hull-White, and Black-Derman-Toy models. All of these models represent dr (change in interest rate) in terms of the product of the volatility (sigma) and a stochastic term (dZ). I am confused about what exactly is meant by dZ. The literature I have read indicates that in formulas like:
dr = theta(t)dt + sigma*dZ, dZ represents a normally distributed random variable and therefore dr is also normally distributed. If dZ is normally distributed, though, what does "Z" represent? Is dZ a unit interval of Z (i.e. Z = INTEGRAL(dZ)) and is Z also normally distributed? I have yet to find confirmation of this, although perhaps I need to further study stochastic calculus.

暗号化の勉強を続けながらアルゴリズムをプログラムで実施しようと思っています。文字で作成されたメッセージをひとつな整数に変換する事が最初の手段です。
In particular, the RSA public key encryption algorithm uses the formula:
C = m^e mod n where C is the ciphertext, m is the (numerical representation of) the message), n is the product of two primes, and e another integer known to both parties. How the m is obtained is what I would like to investigate. As the number must fit within a fixed number of bits, it seems like simply using the bits of the concatenated ascii (assuming English message) characters would be impractical. Thus, there must be some sort of two way hash (as the other party must convert the integer back to ascii).