HOTP, the HMAC-based One-Time Password algorithm from RFC 4226, uses a "dynamic truncation" function to turn the 20 byte HMAC-SHA-1 value into a 31 bit string. The dynamic truncation (from Section 5.3) works like this (and is probably useless):
DT(String) // String = String...String Let OffsetBits be the low-order 4 bits of String Offset = StToNum(OffsetBits) // 0 <= OffSet <= 15 Let P = String[OffSet]...String[OffSet+3] Return the Last 31 bits of P
TOTP (RFC 6238) allows using SHA-256 and SHA-512 as the HMAC hash in HOTP, but doesn't seem to define a new dynamic truncation function for use with them:
TOTP implementations MAY use HMAC-SHA-256 or HMAC-SHA-512 functions, based on SHA-256 or SHA-512 [SHA2] hash functions, instead of the HMAC-SHA-1 function that has been specified for the HOTP computation in [RFC4226].
Should I use low 4 bits of
String as offset, low 4 bits of
String[length-1], or perhaps some other number of bits or a completely different truncation algorithm?