ILP and signatures

Cryptographic signatures are a simple way for ledgers to securely validate
the outcome of the external conditions upon which a transfer is escrowed.
Any one-way function can be used [18]. Using asymmetric cryptography, the
ledger escrows funds pending the presentation of a valid signature for a
pre-defined public key and message or hash. The ledger can then easily
validate the signature when it is presented and determine if the condition
has been met.

http://interledger.org/interledger.pdf

Question: are signatures a necessary component in the ledgers?