In order:
- unspecified, it may be any block cipher (with the desired output block and key size);
- unspecified, it may be any constant of the right block size;
- unspecified, it may be any padding type that suites the key size.
Davies-Meyer is a construction for a compression function, not an actual algorithm that can be used out of the box. Usually these parameters would be described for a specific algorithm that employs the Davies-Meyer construction.
In principle they could be left as configuration parameters as well (similar to HMAC where the hash is a configuration parameter, e.g. HMAC-SHA-256). However, for hash functions this is not common.
One of the uses of these kind of abstractions is that it is possible to create a proof for them, independent of the block cipher used - you'd have to assume that the block cipher isn't broken for the particular use case of course.
Here is the relevant part of the Wikipedia article on one way functions:
The hash function can only be considered secure if at least the
following conditions are met:
- The block cipher has no special properties that distinguish it from ideal ciphers, such as weak keys or keys that lead to identical or
related encryptions (fixed points or key-collisions).
- The resulting hash size is big enough. According to the birthday attack a security level of 280 (generally assumed to be infeasible to
compute today)[citation needed] is desirable thus the hash size should
be at least 160 bits.
- The last block is properly length padded prior to the hashing. (See Merkle–Damgård construction.) Length padding is normally implemented
and handled internally in specialised hash functions like SHA-1 etc.
The constructions presented below: Davies–Meyer,
Matyas–Meyer–Oseas, Miyaguchi–Preneel and Hirose have been shown to be
secure under [the] black-box analysis.
For a specific hash-function implementation that uses Davies-Meyer you can have a look at SHA-2 hash family. Note that SHA-2 doesn't use a named block cipher; it uses a specialized cipher specific to SHA-2. The padding mode is bit padding, but it has a length indication. The IV's are specified for each hash function separately, cannot miss them.
