Select four bytes and reduce to digits

Use a byte offset, clear the top bit, then take the decimal remainder.

• HS is exactly 20 bytes for base HOTP. Its byte 19 supplies the low-order four bits as an offset in 0…15. Copy the four consecutive bytes starting at that byte offset; mask their most significant bit to obtain an unsigned31-bit integer (§5.3).
• The official §5.4 example digest is1f8698690e02ca16618550ef7f19da8e945b555a. Its last byte5a has low nibblea, decimal 10. Select bytes=10…13:50 ef 7f 19. Clearing the high bit leaves50ef7f19, decimal1357872921.
• With Digit=6, reduce1357872921 modulo1000000 to get 872921. Display exactly six digits, preserving leading zeroes; illustrative remainder42 is000042 (Appendix C). This §5.4 example is separate from the Appendix D key/counter vectors.
• Implementations MUST extract at least six digits; considering seven or more is a SHOULD (§5.3). This guide illustrates six, and the bounded helper supports six through eight as explicitly discussed there. It does not impose the proposed nine-digit MUST NOT from Held for Document Update erratum 5792 as an accepted new requirement.
• Reported errata 6756 and7651 discuss alternate digest lengths and bit/byte notation. They are not verified corrections. This guide uses the unambiguous20-byte SHA-1 example in §5.4 and byte indexes. Reported erratum 5129 proposes fixed-offset Appendix D results; retain the published dynamic-offset values.