Is the seed block in Argon2 (H0) 64-bit or 64-byte?
In core.h of the Argon2 reference implementation, we can see that ARGON2_PREHASH_DIGEST_LENGTH = 64.  In core.c, we can see that ARGON2_PREHASH_DIGEST_LENGTH and ARGON2_PREHASH_SEED_LENGTH are used as numbers of bytes (to allocate a buffer in bytes and to specify the size in bytes of a BLAKE2b hash).

We can also reason that it makes no sense for the password, salt, optional secret key, and associated data to figure into the hash only via a 64-bit intermediate quantity—that would open it to preimage attacks on the intermediate quantity.

Please send email to the authors of the internet-draft identifying the incorrect ‘64-bit’ units!

Update: The November 22, 2018 revision of the Argon2 internet-draft, draft-irtf-cfrg-argon2-04, corrects the typo:


  Establish H_0 as the 64-byte value as shown below.
  
  —draft-irtf-cfrg-argon2-04, §3.2 ‘Argon2 Operation’