SignatureValidator.java
// SPDX-FileCopyrightText: 2021 Paul Schaub <vanitasvitae@fsfe.org>
//
// SPDX-License-Identifier: Apache-2.0
package org.pgpainless.signature.consumer;
import java.security.NoSuchAlgorithmException;
import java.util.Arrays;
import java.util.Date;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import org.bouncycastle.bcpg.sig.KeyFlags;
import org.bouncycastle.bcpg.sig.NotationData;
import org.bouncycastle.bcpg.sig.SignatureCreationTime;
import org.bouncycastle.openpgp.PGPException;
import org.bouncycastle.openpgp.PGPPublicKey;
import org.bouncycastle.openpgp.PGPSignature;
import org.bouncycastle.openpgp.PGPSignatureList;
import org.bouncycastle.openpgp.PGPSignatureSubpacketVector;
import org.bouncycastle.openpgp.PGPUserAttributeSubpacketVector;
import org.pgpainless.algorithm.HashAlgorithm;
import org.pgpainless.algorithm.KeyFlag;
import org.pgpainless.algorithm.PublicKeyAlgorithm;
import org.pgpainless.algorithm.SignatureSubpacket;
import org.pgpainless.algorithm.SignatureType;
import org.pgpainless.exception.SignatureValidationException;
import org.pgpainless.implementation.ImplementationFactory;
import org.pgpainless.key.OpenPgpFingerprint;
import org.pgpainless.policy.Policy;
import org.pgpainless.signature.SignatureUtils;
import org.pgpainless.signature.subpackets.SignatureSubpacketsUtil;
import org.pgpainless.util.BCUtil;
import org.pgpainless.util.DateUtil;
import org.pgpainless.util.NotationRegistry;
/**
* A collection of validators that perform validation steps over signatures.
*/
public abstract class SignatureValidator {
public abstract void verify(PGPSignature signature) throws SignatureValidationException;
/**
* Check, whether there is the possibility that the given signature was created by the given key.
* {@link #verify(PGPSignature)} throws a {@link SignatureValidationException} if we can say with certainty that the signature
* was not created by the given key (e.g. if the sig carries another issuer, issuer fingerprint packet).
*
* If there is no information found in the signature about who created it (no issuer, no fingerprint),
* {@link #verify(PGPSignature)} will simply return since it is plausible that the given key created the sig.
*
* @param signingKey signing key
* @return validator that throws a {@link SignatureValidationException} if the signature was not possibly made by the given key.
*/
public static SignatureValidator wasPossiblyMadeByKey(PGPPublicKey signingKey) {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
OpenPgpFingerprint signingKeyFingerprint = OpenPgpFingerprint.of(signingKey);
Long issuer = SignatureSubpacketsUtil.getIssuerKeyIdAsLong(signature);
if (issuer != null) {
if (issuer != signingKey.getKeyID()) {
throw new SignatureValidationException("Signature was not created by " + signingKeyFingerprint + " (signature issuer: " + Long.toHexString(issuer) + ")");
}
}
OpenPgpFingerprint fingerprint = SignatureSubpacketsUtil.getIssuerFingerprintAsOpenPgpFingerprint(signature);
if (fingerprint != null) {
if (!fingerprint.equals(signingKeyFingerprint)) {
throw new SignatureValidationException("Signature was not created by " + signingKeyFingerprint + " (signature fingerprint: " + fingerprint + ")");
}
}
// No issuer information found, so we cannot rule out that we did not create the sig
}
};
}
/**
* Verify that a subkey binding signature - if the subkey is signing-capable - contains a valid primary key
* binding signature.
*
* @param primaryKey primary key
* @param subkey subkey
* @param policy policy
* @param validationDate reference date for signature verification
* @return validator
*/
public static SignatureValidator hasValidPrimaryKeyBindingSignatureIfRequired(PGPPublicKey primaryKey, PGPPublicKey subkey, Policy policy, Date validationDate) {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
if (!PublicKeyAlgorithm.fromId(signature.getKeyAlgorithm()).isSigningCapable()) {
// subkey is not signing capable -> No need to process embedded sigs
return;
}
KeyFlags keyFlags = SignatureSubpacketsUtil.getKeyFlags(signature);
if (keyFlags == null) {
return;
}
if (!KeyFlag.hasKeyFlag(keyFlags.getFlags(), KeyFlag.SIGN_DATA)
&& !KeyFlag.hasKeyFlag(keyFlags.getFlags(), KeyFlag.CERTIFY_OTHER)) {
return;
}
try {
PGPSignatureList embeddedSignatures = SignatureSubpacketsUtil.getEmbeddedSignature(signature);
boolean hasValidPrimaryKeyBinding = false;
Map<PGPSignature, Exception> rejectedEmbeddedSigs = new ConcurrentHashMap<>();
for (PGPSignature embedded : embeddedSignatures) {
if (SignatureType.valueOf(embedded.getSignatureType()) == SignatureType.PRIMARYKEY_BINDING) {
try {
signatureStructureIsAcceptable(subkey, policy).verify(embedded);
signatureIsEffective(validationDate).verify(embedded);
correctPrimaryKeyBindingSignature(primaryKey, subkey).verify(embedded);
hasValidPrimaryKeyBinding = true;
break;
} catch (SignatureValidationException e) {
rejectedEmbeddedSigs.put(embedded, e);
}
}
}
if (!hasValidPrimaryKeyBinding) {
throw new SignatureValidationException("Missing primary key binding signature on signing capable subkey " + Long.toHexString(subkey.getKeyID()), rejectedEmbeddedSigs);
}
} catch (PGPException e) {
throw new SignatureValidationException("Cannot process list of embedded signatures.", e);
}
}
};
}
/**
* Verify that a signature has an acceptable structure.
*
* @param signingKey signing key
* @param policy policy
* @return validator
*/
public static SignatureValidator signatureStructureIsAcceptable(PGPPublicKey signingKey, Policy policy) {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
signatureIsNotMalformed(signingKey).verify(signature);
signatureDoesNotHaveCriticalUnknownNotations(policy.getNotationRegistry()).verify(signature);
signatureDoesNotHaveCriticalUnknownSubpackets().verify(signature);
signatureUsesAcceptableHashAlgorithm(policy).verify(signature);
signatureUsesAcceptablePublicKeyAlgorithm(policy, signingKey).verify(signature);
}
};
}
/**
* Verify that a signature was made using an acceptable {@link PublicKeyAlgorithm}.
*
* @param policy policy
* @param signingKey signing key
* @return validator
*/
private static SignatureValidator signatureUsesAcceptablePublicKeyAlgorithm(Policy policy, PGPPublicKey signingKey) {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
PublicKeyAlgorithm algorithm = PublicKeyAlgorithm.fromId(signingKey.getAlgorithm());
try {
int bitStrength = BCUtil.getBitStrength(signingKey);
if (!policy.getPublicKeyAlgorithmPolicy().isAcceptable(algorithm, bitStrength)) {
throw new SignatureValidationException("Signature was made using unacceptable key. " +
algorithm + " (" + bitStrength + " bits) is not acceptable according to the public key algorithm policy.");
}
} catch (NoSuchAlgorithmException e) {
throw new SignatureValidationException("Cannot determine bit strength of signing key.", e);
}
}
};
}
/**
* Verify that a signature uses an acceptable {@link HashAlgorithm}.
*
* @param policy policy
* @return validator
*/
private static SignatureValidator signatureUsesAcceptableHashAlgorithm(Policy policy) {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
HashAlgorithm hashAlgorithm = HashAlgorithm.fromId(signature.getHashAlgorithm());
Policy.HashAlgorithmPolicy hashAlgorithmPolicy = getHashAlgorithmPolicyForSignature(signature, policy);
if (!hashAlgorithmPolicy.isAcceptable(signature.getHashAlgorithm())) {
throw new SignatureValidationException("Signature uses unacceptable hash algorithm " + hashAlgorithm);
}
}
};
}
/**
* Return the applicable {@link Policy.HashAlgorithmPolicy} for the given {@link PGPSignature}.
* Revocation signatures are being policed using a different policy than non-revocation signatures.
*
* @param signature signature
* @param policy revocation policy for revocation sigs, normal policy for non-rev sigs
* @return policy
*/
private static Policy.HashAlgorithmPolicy getHashAlgorithmPolicyForSignature(PGPSignature signature, Policy policy) {
SignatureType type = SignatureType.valueOf(signature.getSignatureType());
Policy.HashAlgorithmPolicy hashAlgorithmPolicy;
if (type == SignatureType.CERTIFICATION_REVOCATION || type == SignatureType.KEY_REVOCATION || type == SignatureType.SUBKEY_REVOCATION) {
hashAlgorithmPolicy = policy.getRevocationSignatureHashAlgorithmPolicy();
} else {
hashAlgorithmPolicy = policy.getSignatureHashAlgorithmPolicy();
}
return hashAlgorithmPolicy;
}
/**
* Verify that a signature does not carry critical unknown notations.
*
* @param registry notation registry of known notations
* @return validator
*/
public static SignatureValidator signatureDoesNotHaveCriticalUnknownNotations(NotationRegistry registry) {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
List<NotationData> hashedNotations = SignatureSubpacketsUtil.getHashedNotationData(signature);
for (NotationData notation : hashedNotations) {
if (!notation.isCritical()) {
continue;
}
if (!registry.isKnownNotation(notation.getNotationName())) {
throw new SignatureValidationException("Signature contains unknown critical notation '" + notation.getNotationName() + "' in its hashed area.");
}
}
}
};
}
/**
* Verify that a signature does not contain critical unknown subpackets.
*
* @return validator
*/
public static SignatureValidator signatureDoesNotHaveCriticalUnknownSubpackets() {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
PGPSignatureSubpacketVector hashedSubpackets = signature.getHashedSubPackets();
for (int criticalTag : hashedSubpackets.getCriticalTags()) {
try {
SignatureSubpacket.fromCode(criticalTag);
} catch (IllegalArgumentException e) {
throw new SignatureValidationException("Signature contains unknown critical subpacket of type " + Long.toHexString(criticalTag));
}
}
}
};
}
/**
* Verify that a signature is effective right now.
*
* @return validator
*/
public static SignatureValidator signatureIsEffective() {
return signatureIsEffective(new Date());
}
/**
* Verify that a signature is effective at the given reference date.
*
* @param validationDate reference date for signature verification
* @return validator
*/
public static SignatureValidator signatureIsEffective(Date validationDate) {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
signatureIsAlreadyEffective(validationDate).verify(signature);
signatureIsNotYetExpired(validationDate).verify(signature);
}
};
}
/**
* Verify that a signature was created prior to the given reference date.
*
* @param validationDate reference date for signature verification
* @return validator
*/
public static SignatureValidator signatureIsAlreadyEffective(Date validationDate) {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
Date signatureCreationTime = SignatureSubpacketsUtil.getSignatureCreationTime(signature).getTime();
// Hard revocations are always effective
if (SignatureUtils.isHardRevocation(signature)) {
return;
}
if (signatureCreationTime.after(validationDate)) {
throw new SignatureValidationException("Signature was created at " + signatureCreationTime + " and is therefore not yet valid at " + validationDate);
}
}
};
}
/**
* Verify that a signature is not yet expired.
*
* @param validationDate reference date for signature verification
* @return validator
*/
public static SignatureValidator signatureIsNotYetExpired(Date validationDate) {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
// Hard revocations do not expire
if (SignatureUtils.isHardRevocation(signature)) {
return;
}
Date signatureExpirationTime = SignatureSubpacketsUtil.getSignatureExpirationTimeAsDate(signature);
if (signatureExpirationTime != null && signatureExpirationTime.before(validationDate)) {
throw new SignatureValidationException("Signature is already expired (expiration: " + signatureExpirationTime + ", validation: " + validationDate + ")");
}
}
};
}
/**
* Verify that a signature is not malformed.
* A signature is malformed if it has no hashed creation time subpacket,
* it predates the creation time of the signing key, or it predates the creation date
* of the signing key binding signature.
*
* @param creator signing key
* @return validator
*/
public static SignatureValidator signatureIsNotMalformed(PGPPublicKey creator) {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
signatureHasHashedCreationTime().verify(signature);
signatureDoesNotPredateSigningKey(creator).verify(signature);
signatureDoesNotPredateSigningKeyBindingDate(creator).verify(signature);
}
};
}
/**
* Verify that a signature has a hashed creation time subpacket.
*
* @return validator
*/
public static SignatureValidator signatureHasHashedCreationTime() {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
SignatureCreationTime creationTime = SignatureSubpacketsUtil.getSignatureCreationTime(signature);
if (creationTime == null) {
throw new SignatureValidationException("Malformed signature. Signature has no signature creation time subpacket in its hashed area.");
}
}
};
}
/**
* Verify that a signature does not predate the creation time of the signing key.
*
* @param key signing key
* @return validator
*/
public static SignatureValidator signatureDoesNotPredateSigningKey(PGPPublicKey key) {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
Date keyCreationTime = key.getCreationTime();
Date signatureCreationTime = signature.getCreationTime();
if (keyCreationTime.after(signatureCreationTime)) {
throw new SignatureValidationException("Signature predates its signing key (key creation: " + keyCreationTime + ", signature creation: " + signatureCreationTime + ")");
}
}
};
}
/**
* Verify that a signature does not predate the binding date of the signing key.
*
* @param signingKey signing key
* @return validator
*/
public static SignatureValidator signatureDoesNotPredateSigningKeyBindingDate(PGPPublicKey signingKey) {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
if (signingKey.isMasterKey()) {
return;
}
boolean predatesBindingSig = true;
Iterator<PGPSignature> bindingSignatures = signingKey.getSignaturesOfType(SignatureType.SUBKEY_BINDING.getCode());
if (!bindingSignatures.hasNext()) {
throw new SignatureValidationException("Signing subkey does not have a subkey binding signature.");
}
while (bindingSignatures.hasNext()) {
PGPSignature bindingSig = bindingSignatures.next();
if (!bindingSig.getCreationTime().after(signature.getCreationTime())) {
predatesBindingSig = false;
}
}
if (predatesBindingSig) {
throw new SignatureValidationException("Signature was created before the signing key was bound to the key ring.");
}
}
};
}
/**
* Verify that a subkey binding signature is correct.
*
* @param primaryKey primary key
* @param subkey subkey
* @return validator
*/
public static SignatureValidator correctSubkeyBindingSignature(PGPPublicKey primaryKey, PGPPublicKey subkey) {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
if (primaryKey.getKeyID() == subkey.getKeyID()) {
throw new SignatureValidationException("Primary key cannot be its own subkey.");
}
try {
signature.init(ImplementationFactory.getInstance().getPGPContentVerifierBuilderProvider(), primaryKey);
boolean valid = signature.verifyCertification(primaryKey, subkey);
if (!valid) {
throw new SignatureValidationException("Signature is not correct.");
}
} catch (PGPException e) {
throw new SignatureValidationException("Cannot verify subkey binding signature correctness", e);
}
}
};
}
/**
* Verify that a primary key binding signature is correct.
*
* @param primaryKey primary key
* @param subkey subkey
* @return validator
*/
public static SignatureValidator correctPrimaryKeyBindingSignature(PGPPublicKey primaryKey, PGPPublicKey subkey) {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
try {
signature.init(ImplementationFactory.getInstance().getPGPContentVerifierBuilderProvider(), subkey);
boolean valid = signature.verifyCertification(primaryKey, subkey);
if (!valid) {
throw new SignatureValidationException("Primary Key Binding Signature is not correct.");
}
} catch (PGPException e) {
throw new SignatureValidationException("Cannot verify primary key binding signature correctness", e);
}
}
};
}
/**
* Verify that a direct-key signature is correct.
*
* @param signer signing key
* @param signee signed key
* @return validator
*/
public static SignatureValidator correctSignatureOverKey(PGPPublicKey signer, PGPPublicKey signee) {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
try {
signature.init(ImplementationFactory.getInstance().getPGPContentVerifierBuilderProvider(), signer);
boolean valid;
if (signer.getKeyID() != signee.getKeyID()) {
valid = signature.verifyCertification(signer, signee);
} else {
valid = signature.verifyCertification(signee);
}
if (!valid) {
throw new SignatureValidationException("Signature is not correct.");
}
} catch (PGPException e) {
throw new SignatureValidationException("Cannot verify direct-key signature correctness", e);
}
}
};
}
/**
* Verify that a signature is a certification signature.
*
* @return validator
*/
public static SignatureValidator signatureIsCertification() {
return signatureIsOfType(
SignatureType.POSITIVE_CERTIFICATION,
SignatureType.CASUAL_CERTIFICATION,
SignatureType.GENERIC_CERTIFICATION,
SignatureType.NO_CERTIFICATION);
}
/**
* Verify that a signature type equals one of the given {@link SignatureType SignatureTypes}.
*
* @param signatureTypes one or more signature types
* @return validator
*/
public static SignatureValidator signatureIsOfType(SignatureType... signatureTypes) {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
SignatureType type = SignatureType.valueOf(signature.getSignatureType());
boolean valid = false;
for (SignatureType allowed : signatureTypes) {
if (type == allowed) {
valid = true;
break;
}
}
if (!valid) {
throw new SignatureValidationException("Signature is of type " + type + " while only " + Arrays.toString(signatureTypes) + " are allowed here.");
}
}
};
}
/**
* Verify that a signature over a user-id is correct.
*
* @param userId user-id
* @param certifiedKey key carrying the user-id
* @param certifyingKey key that created the signature.
* @return validator
*/
public static SignatureValidator correctSignatureOverUserId(String userId, PGPPublicKey certifiedKey, PGPPublicKey certifyingKey) {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
try {
signature.init(ImplementationFactory.getInstance().getPGPContentVerifierBuilderProvider(), certifyingKey);
boolean valid = signature.verifyCertification(userId, certifiedKey);
if (!valid) {
throw new SignatureValidationException("Signature over user-id '" + userId + "' is not correct.");
}
} catch (PGPException e) {
throw new SignatureValidationException("Cannot verify signature over user-id '" + userId + "'.", e);
}
}
};
}
/**
* Verify that a signature over a user-attribute packet is correct.
*
* @param userAttributes user attributes
* @param certifiedKey key carrying the user-attributes
* @param certifyingKey key that created the certification signature
* @return validator
*/
public static SignatureValidator correctSignatureOverUserAttributes(PGPUserAttributeSubpacketVector userAttributes, PGPPublicKey certifiedKey, PGPPublicKey certifyingKey) {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
try {
signature.init(ImplementationFactory.getInstance().getPGPContentVerifierBuilderProvider(), certifyingKey);
boolean valid = signature.verifyCertification(userAttributes, certifiedKey);
if (!valid) {
throw new SignatureValidationException("Signature over user-attribute vector is not correct.");
}
} catch (PGPException e) {
throw new SignatureValidationException("Cannot verify signature over user-attribute vector.", e);
}
}
};
}
public static SignatureValidator signatureWasCreatedInBounds(Date notBefore, Date notAfter) {
return new SignatureValidator() {
@Override
public void verify(PGPSignature signature) throws SignatureValidationException {
Date timestamp = signature.getCreationTime();
if (notBefore != null && timestamp.before(notBefore)) {
throw new SignatureValidationException("Signature was made before the earliest allowed signature creation time. Created: " +
DateUtil.formatUTCDate(timestamp) + " Earliest allowed: " + DateUtil.formatUTCDate(notBefore));
}
if (notAfter != null && timestamp.after(notAfter)) {
throw new SignatureValidationException("Signature was made after the latest allowed signature creation time. Created: " +
DateUtil.formatUTCDate(timestamp) + " Latest allowed: " + DateUtil.formatUTCDate(notAfter));
}
}
};
}
}