"Alphanumeric" hash - A-Z, 0-9

I'm looking for a function that will generate an "alphanumeric hash". Given a source string, it produces a determinate result string that can contain any letter a-z or digit 0-9, and cannot be reverse-engineered to produce the source. This will be used to generate passwords for a system based on secret data, so strings between 8 and 12 characters are ideal and a secure hash would also be ideal. I'm thinking I can use a normal bitwise hash, XOR-fold it to 64 bits (if I use, for instance, SHA256) and then take the result 5 bits at a time (producing a number 0-31) and look up the character code to use from an indexed ordered collection. There are 26 letters and 10 digits meaning I'll have to leave a few out (probably removing characters that could be mistaken for others if handwritten). 64 bits, 5 bits at a time, will produce a 12-character string with 4 bits left over. However, I'm worried about two things: first, introducing bias by taking a non-power-of-2 number of bits; and second, what to do with the leftover bits. Do I use them as-is knowing there will only be 16 possibilities, do I leave them off (and lose data possibly introducing bias), or do I incorporate one more bit to make a 13-character string (and where should the last bit come from)? EDIT: Here's my current stab at it; it takes an enumerable of bytes (like the byte array produced by most hash algorithms) and returns a string: ``` /// <summary> /// Converts an IEnumerable of bytes to a string representation which can have any lowercase letter a-z except for l, o, q and z, and any digit 0-9. /// Uses 5 bits of the byte array at a time to generate numbers from 0 to 31, which are then translated to letters or numbers. /// </summary> /// <param name="toConvert">the byte array to convert.</param> /// <returns>A string containing the alphanumeric case-insensitive representation of the bytes in the array.</returns> public static string ToInsensitiveAlphaNumericString(this IEnumerable<byte> toConvert) { var chars = new[] { 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'm', 'n', 'p', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9' }; var enumerator = toConvert.GetEnumerator(); enumerator.MoveNext(); int buffer = enumerator.Current; short bufferLength = 8; const int valueLength = 5; var builder = new StringBuilder(); while (true) { var value = buffer >> (bufferLength - valueLength); builder.Append(chars[value]); buffer = buffer - (value << (bufferLength - valueLength)); bufferLength -= valueLength; if(bufferLength < valueLength ) { if (enumerator.MoveNext()) { buffer = (buffer << 8) + enumerator.Current; bufferLength += 8; } else { //here's the main question; to include, or not to include? if (bufferLength > 0) builder.Append(chars[buffer]); break; } } } return builder.ToString(); } ```