Sample calls to MI _CIPHER builtin from ILE RPG:
Listing Of Available Encryption Algorithms by Scott Klement
Encrypt/Decrypt Using RC4 Algorithm by Scott Klement

Off-site references:

iSeries Machine Interface Technical Reference from IBM

Midrange.com's MIDRANGE-L archives MD5 via RPG? by Bruce Vining

iSeries NEWS article Tales from the AS/400 CRYPT by Gene Gaunt (requires membership)

iSeries NEWS article Data Integrity With One-Way Hash Functions by Gene Gaunt (requires membership)

iSeries NEWS article Twofish Encryption on the AS/400 by Gene Gaunt (requires membership)