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The Bouncy Castle Java APIs for the OpenPGP Protocol. The APIs are designed primarily to be used in conjunction with the BC LTS provider but may also be used with other providers providing cryptographic services.

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package org.bouncycastle.bcpg;

public class HashUtils
{

    /**
     * Return the length of the salt per hash algorithm, used in OpenPGP v6 signatures.
     *
     * @see 
     *     OpenPGP - Salt Size declarations
     * @param hashAlgorithm hash algorithm tag
     * @return size of the salt for the given hash algorithm in bytes
     */
    public static int getV6SignatureSaltSizeInBytes(int hashAlgorithm)
    {
        switch (hashAlgorithm)
        {
            case HashAlgorithmTags.SHA256:
            case HashAlgorithmTags.SHA224:
            case HashAlgorithmTags.SHA3_256:
            case HashAlgorithmTags.SHA3_256_OLD:
                return 16;
            case HashAlgorithmTags.SHA384:
                return 24;
            case HashAlgorithmTags.SHA512:
            case HashAlgorithmTags.SHA3_512:
            case HashAlgorithmTags.SHA3_512_OLD:
                return 32;
            default:
                throw new IllegalArgumentException("Salt size not specified for Hash Algorithm with ID " + hashAlgorithm);
        }
    }

    /**
     * Return true, if the encountered saltLength matches the value the specification gives for the hashAlgorithm.
     *
     * @param hashAlgorithm hash algorithm tag
     * @param saltSize encountered salt size
     * @return true if the encountered size matches the spec
     * @implNote LibrePGP allows for zero-length signature salt values, so this method only works for IETF OpenPGP v6.
     */
    public boolean saltSizeMatchesSpec(int hashAlgorithm, int saltSize)
    {
        try
        {
            return saltSize == getV6SignatureSaltSizeInBytes(hashAlgorithm);
        }
        catch (IllegalArgumentException e) // Unknown algorithm or salt size is not specified for the hash algo
        {
            return false;
        }
    }
}