Added prefix for value contract storage

[pierluca]

There's a weakness in Dela (#258) whereby the (existing) smart contracts store key=values without any prefix for the keys in a shared key/value store. Effectively this would allow any malicious user to overwrite other smart contracts' data.

This fixes it for the value contract.

We probably need to do something similar for https://github.com/dedis/d-voting

@PascalinDe @lanterno for your information, the current key/value store is limited to keys of 32 bytes. Do reach out if adding a prefix could be a problem for D-Voting (we haven't checked).

[PascalinDe]

LGTM

[pierluca]

I haven't checked the CI, but I expect some integration tests will be broken.
More work is needed, but it should give a pointer to the general direction.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
2 Code Smells

11.8% Coverage
0.4% Duplication

[coveralls]

Pull Request Test Coverage Report for Build 5764771956

• 16 of 16 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 98.739%

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Totals
Change from base Build 5764444164:	0.03%
Covered Lines:	14645
Relevant Lines:	14832

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💛 - Coveralls

[PascalinDe]

I haven't checked the CI, but I expect some integration tests will be broken. More work is needed, but it should give a pointer to the general direction.

ah, I didn't check the tests since I'm used to a "the dev's responsability approach"

can you mark PR's that are not meant for a merging review as draft please? I don't mind reviewing WIP, but it gives me an idea of what to look out for 🙂