[SOLVED] Use HKDF in bouncy-castle FIPS

Use HKDF in bouncy-castle FIPS

I need to be able to use HKDF algorithm on my input key in bouncy-castle FIPS library. In usual bouncy-castle library there is this straightforward way of doing this

HKDFBytesGenerator hkdfBytesGenerator = new HKDFBytesGenerator(new SHA256Digest());
HKDFParameters hkdfParameters = new HKDFParameters(inputKey, salt, info);
hkdfBytesGenerator.init(hkdfParameters);
byte[] hkdf = new byte[32];
hkdfBytesGenerator.generateBytes(hkdf, 0, 32);

In bouncy-castle FIPS however I couldn't yet figure out how to perform the same operation. I need to be able to provide both salt and info parameters. HKDFKeyBuilder#build only performs "extraction" part without "expanding" and therefore doesn't have info as a parameter. Similarly named HKDFBytesGenerator however doesn't perform "extraction" and therefore doesn't contain salt.

Solution

The answer was next to me the whole time. As I mentioned HKDFKeyBuilder only performs "extraction" and HKDFBytesGenerator only performs "expanding", so you need to use both. You need to first extract key with HKDFKeyBuilder and then expand it with HKDFBytesGenerator. The code should look similar to this depending on your other parameters

HKDFKeyBuilder hkdfKeyBuilder = FipsKDF.HKDF_KEY_BUILDER.withSalt(salt).withPrf(AgreementKDFPRF.SHA256_HMAC);
byte[] extractedKey = hkdfKeyBuilder.build(inputKey).getKey();
AgreementOperatorFactory factory = new AgreementOperatorFactory();
KDFCalculator<AgreementKDFParameters> kdfCalculator = factory.createKDFCalculator(FipsKDF.HKDF.withPRF(AgreementKDFPRF.SHA256_HMAC).using(extractedKey).withIV(info));
byte[] hkdf = new byte[32];
kdfCalculator.generateBytes(hkdf);
return hkdf;