Appendix F. ID mapping in ITCH to FIX conversion

Values of OrderID, TradeID, MDEntryID fields are encoded from UInt64 to 12-symbol string.

First char in an encoded ID is prefix. Possible prefix values are:

'O' - Order,

'T' - On Book Trade,

'N' - Off Book Trade.

The rest 11 chars represent original ID in Base64 encoding (similar to Base64, but without '=' suffix and based on 62 characters set (10 chars ‘0’-‘9’ + 26 chars ‘A’-‘Z’ + 26 chars ‘a’-‘z’).