I have constructed a stream cipher from a secure hash algorithm and a HMAC. Here is a brief description of the algorithm:

Let: (Actually Objective-C styled pseudo code)

• [data SHA512Hash]: SHA-512 hash of data,
• [data SHA512HMAC:key]: SHA-512 HMAC of data with key,
• [data xor:another]: bitwise XOR of two data and another which have the same length,
• [data bytesTo:index]: first index bytes of data,
• [data removeFirstBytes:count]: remove first count bytes from data,
• [data length]: length of data.
• [data concat:other]: Concatenate other after data.
• [Data new]: allocate an empty buffer.

Pseudocode: (Or actual Objective-C code?)

Data hkey = [key SHA512]; // Hash to make lengths match.
Data segKey = [IV SHA512];

Data dest = [Data new]; // This will be the output
Data last = (Some constant)

do {
    // Cut off hash-sized chunks of source data
    Data segment = [data bytesTo:[hkey length]];
    [data removeFirstBytes:[hkey length]];

    // Derive a new segment key
    segKey = [segKey SHA512HMAC:hkey]; // or [segKey SHA512HMAC:[hkey concat:last]]? If so, how to decrypt?

    // XOR
    last = [segment xor:[segKey bytesTo:[segment length]]] // Truncates to make length match
    dest = [dest concat:last];
} while ([data length] > 0)

From some other questions I am suggested that this is SHA-512 HMAC running in OFB mode but what about the change in the code comment?

This code seem to decipher itself when given the same key.

EDIT Here is this cipher implemented in Objective-C: https://github.com/xcvista/CGIKit/blob/master/MSBooster/MSBooster/NSData%2BMSTrivialCryptography.m (Free software, 2-clause BSD license.)