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From my perspective and Iroha1 experience hex format is the most convenient. It is compact enough, readable and can be easily serialized/deserialized on any language or from the console.
Proposal 2.
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Utilize multihashes to manage keys and signatures inside Iroha
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Multihash is a format for storing hashes/keys/signatures in a way that also self-describes the type of that hash/key/signature. We can distringuish multiple key types inside Iroha using this format.
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