PdfReader pdfReader = new PdfReader(IN_PDF_FILE);
AcroFields fields = pdfReader.getAcroFields();
fields.getFields();
ArrayList<String> signatureNames = fields.getSignatureNames();
for (String signatureName : signatureNames) {
   log.info("signatureName: " + signatureName);
   PdfPKCS7 pkcs7 = fields.verifySignature(signatureName, JCP.PROVIDER_NAME);
   log.info("pkcs7.verify(): " + pkcs7.verify());
}

package com.itextpdf.text.pdf.security;

import java.security.GeneralSecurityException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import org.bouncycastle.jcajce.provider.digest.MD2.Digest;

public class BouncyCastleDigest implements ExternalDigest {
    public BouncyCastleDigest() {
    }

    public MessageDigest getMessageDigest(String hashAlgorithm) throws GeneralSecurityException {
        String oid = DigestAlgorithms.getAllowedDigests(hashAlgorithm);
        if (oid == null) {
            throw new NoSuchAlgorithmException(hashAlgorithm);
        } else if (oid.equals("1.2.840.113549.2.2")) {
            return new Digest();
        } else if (oid.equals("1.2.840.113549.2.5")) {
            return new org.bouncycastle.jcajce.provider.digest.MD5.Digest();
        } else if (oid.equals("1.3.14.3.2.26")) {
            return new org.bouncycastle.jcajce.provider.digest.SHA1.Digest();
        } else if (oid.equals("2.16.840.1.101.3.4.2.4")) {
            return new org.bouncycastle.jcajce.provider.digest.SHA224.Digest();
        } else if (oid.equals("2.16.840.1.101.3.4.2.1")) {
            return new org.bouncycastle.jcajce.provider.digest.SHA256.Digest();
        } else if (oid.equals("2.16.840.1.101.3.4.2.2")) {
            return new org.bouncycastle.jcajce.provider.digest.SHA384.Digest();
        } else if (oid.equals("2.16.840.1.101.3.4.2.3")) {
            return new org.bouncycastle.jcajce.provider.digest.SHA512.Digest();
        } else if (oid.equals("1.3.36.3.2.2")) {
            return new org.bouncycastle.jcajce.provider.digest.RIPEMD128.Digest();
        } else if (oid.equals("1.3.36.3.2.1")) {
            return new org.bouncycastle.jcajce.provider.digest.RIPEMD160.Digest();
        } else if (oid.equals("1.3.36.3.2.3")) {
            return new org.bouncycastle.jcajce.provider.digest.RIPEMD256.Digest();
        } else if (oid.equals("1.2.643.2.2.9")) {
            return new org.bouncycastle.jcajce.provider.digest.GOST3411.Digest();
        } else {
            throw new NoSuchAlgorithmException(hashAlgorithm);
        }
    }
}