Add HashUtils helper for looking up salt sizes for digest algorithms

pg/src/main/java/org/bouncycastle/bcpg/HashUtils.java

@@ -0,0 +1,53 @@
+package org.bouncycastle.bcpg;
+
+public class HashUtils
+{
+
+    /**
+     * Return the length of the salt per hash algorithm, used in OpenPGP v6 signatures.
+     *
+     * @see <a href="https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-13.html#hash-algorithms-registry">
+     *     Salt Size declarations</a>
+     * @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;
+        }
+    }
+}