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If you knew your keys were uniformly distributed you would never use a varint encoding to store them because you'd stand a very high chance of encoding them into a field longer than a primitive integer. A varint can only hold 28 bits of number in the first four bytes, so your odds of getting 5-byte output is 15/16 i.e. very likely. If you really had to encode them I would use either two fixed64 fields, the experimental fixed128 type, or 'bytes' with the exactly-36-byte-long string representation of a UUID. In no case could I imagine packing a huge vector of random numbers into a protobuf int32 field.