How do I pass bytes32 to a solidity function call? about web3j

[parin13]

lets consider i have function whose input parameter is

• bytes32

• bytes32[ ] // array of type bytes 32

• uint8

which looks like:

 function abc(bytes32 id, bytes32[ ] name,uint8 version) returns(bool)
    {  
        //
     }

so now to provoke the function (using remix) you have to pass parameters which looks like:

"0x12",["0x1262","0x12","0x12"],8

[EbenezerGH]

After struggling with this for a while I decided to just pass my value as a string and convert it to a bytes32 once it enters the contract...

This works just fine.

    function stringToBytes32(string memory source) private pure returns (bytes32 result) {
        bytes memory tempEmptyStringTest = bytes(source);
        if (tempEmptyStringTest.length == 0) {
            return 0x0;
        }
        assembly {
        result := mload(add(source, 32))
        }
    }

[narayanprusty]

@conor10 @thedoctor Can you please answer this?

[thedoctor]

@qnimate hash.getBytes() is going to give you a java byte[] – you can convert that to a Bytes32 if and only if it has a length == 32. Check the length of hash.getBytes(). Depending on what kind of object hash is, getBytes() might not be what you want to call.

[narayanprusty]

@thedoctor hash is a Java string. My hash length is 64 because it's in hexadecimal format. bytes32 is supposed to be 32 length. How do I convert hash to 32 byte length? It's a sha256 hash.

[thedoctor]

Okay, yeah that's not what you want. If hash is a string, then getBytes will give you the byte encoding of the string in whatever your default charset is (probably utf8).

I'm guessing your hash is hex encoded? If so, you want to decode the hex to a byte array –– lots of ways to do that, here's one from ethereumJ that handles the 0x prefix:

new Bytes32(org.ethereum.jsonrpc.TypeConverter.StringHexToByteArray(hash))

[narayanprusty]

@thedoctor anyway to do it without using ethereumJ?

[thedoctor]

Yes, many:
https://stackoverflow.com/questions/8890174/in-java-how-do-i-convert-a-hex-string-to-a-byte
https://stackoverflow.com/questions/140131/convert-a-string-representation-of-a-hex-dump-to-a-byte-array-using-java
https://commons.apache.org/proper/commons-codec/
https://google.github.io/guava/releases/16.0/api/docs/com/google/common/io/BaseEncoding.html

Note: if your hash has a 0x prefix, you'll have to strip it first if you use any of those methods.

[narayanprusty]

@thedoctor I am using this to convert byte[] b = new BigInteger(s,16).toByteArray();

If the string length is 64 then the byte[] length shows as 33 but when the string length is 63 then the byte[] length shows as 32. Not sure what's wrong. Any idea?

The sha256 hash in should be a 64 length string right?

[narayanprusty]

@thedoctor seems like this is the issue http://stackoverflow.com/questions/33536253/java-the-length-of-the-byte-array-seems-to-be-wrong-from-converting-a-string-of

[narayanprusty]

Found solution

Arrays.copyOfRange(new BigInteger(hash, 16).toByteArray(), 1, 33)

@thedoctor Thanks a lot for the help.

[thedoctor]

Ahhhh, actually @qnimate that will eventually give you problems. You shouldn't use the BigInteger conversion method (as it says in the most-upvoted answer to the question you got that from) because it doesn't work with leading zeros.

If you want a quick one-liner with no dependencies, do this:

    javax.xml.bind.DatatypeConverter.parseHexBinary(hash);

General advice: When you're handling cryptographic primitives, even if they're not sensitive, it's worth your time to investigate thoroughly the proper way to handle them. Copying from SO without understanding what's happening is not advised.

[conor10]

@qnimate - web3j already has a number of helper functions for conversions betwen hex strings, BigIntegers and byte arrays. See https://github.com/web3j/web3j/blob/master/src/main/java/org/web3j/utils/Numeric.java#L157 for converting a hex string to a byte array.
@thedoctor - thanks for chipping in here.

[thedoctor]

Huh, look at that. I actually looked in that package for something that would avoid external dependencies last night – but I only checked Bytes Convert and Strings

[oneamitj]

This worked for me, only tested for ASCII, may/may not work for UTF... Is there any better method?

ASCII String to Bytes32:

public static Bytes32 stringToBytes32(String string) {
        byte[] byteValue = string.getBytes();
        byte[] byteValueLen32 = new byte[32];
        System.arraycopy(byteValue, 0, byteValueLen32, 0, byteValue.length);
        return new Bytes32(byteValueLen32);
    }

Bytes32 to ASCII String:

StringUtils.newStringUsAscii(varTypeBytes32.getValue())