I am using iText for PDF signing.
My PDF form has 3 signature fields. The first two fields are for approval signatures and the last field is for certification signature. Certification signature is always signed after the approval signatures and Append mode is set to true for all signatures.
My question is after the certification signature is signed (certification level set to CERTIFIED_NO_CHANGES_ALLOWED) the validated approval signatures will become invalidated while the certification signature is validated.
How should I go about fixing this?
Certification signature is always signed after the approval signatures
This is not allowed.
The PDF specification ISO 32000-1 indicates that
The signature dictionary of a certification signature shall be the value of a signature field and shall contain a ByteRange entry. It may also be referenced from the DocMDP entry in the permissions dictionary (see 12.8.4, “Permissions”). The signature dictionary shall contain a signature reference dictionary (see Table 253) that has a DocMDP transform method.
(section 12.8.1 Digital Signatures / General)
The DocMDP transform method shall be used to detect modifications relative to a signature field that is signed by the author of a document (the person applying the first signature). A document can contain only one signature field that contains a DocMDP transform method; it shall be the first signed field in the document. It enables the author to specify what changes shall be permitted to be made the document and what changes invalidate the author’s signature.
(section 12.8.2.2.1 Digital Signatures / Transform Methods / DocMDP / General)
I.e. a certification signature has a DocMDP transform method and the signature having a DocMDP transform method must be the first one signed in the document.
What you try to achieve, though, might be possible otherwise if PDF2 features are ok with you, cf. Bruno's white paper Digital Signatures for PDF documents section 2.5.5 Locking fields and documents after signing:
Since PDF 1.5, it’s possible to lock specific fields when signing a document. In PDF 2.0, there will be a new feature that allows you to define the document-level permissions that are already available for certification signatures, but now also for approval signatures.
For details (which go beyond the size of an answer here) read that section...
Related
I have PDF document with one LTV signature (document contains DSS dictionary). Why iText check if the signature covers whole document returns FALSE, if the only information added after signing was DSS? No other changes were made in the document.
In iText 7.1.7 method SignatureUtil.signatureCoversWholeDocument does not take into account DSS byte range. For me DSS is part of the signature and validation check should count with this.
How to ignore/exclude DSS bytes at the end of the PDF document when calling signatureCoversWholeDocument method?
Why iText check if the signature covers whole document returns FALSE, if the only information added after signing was DSS? No other changes were made in the document.
But you yourself say that something has been added after signing. Thus, the signature does not cover the whole document. Thus, if iText reported something different here, it would be wrong.
In iText 7.1.7 method SignatureUtil.signatureCoversWholeDocument does not take into account DSS byte range. For me DSS is part of the signature and validation check should count with this.
DSS is not part of the signature. Please re-read the specifications.
SignatureUtil.signatureCoversWholeDocument checks whether the signed byte ranges of the given signature encompass the whole PDF except the gap left during signing for embedding the CMS signature container.
If there is any addition to the document thereafter, the method will return false even if the addition only contains DSS, actually even if the addition contains no new objects, and also even if only a trailing eol is added.
How to ignore/exclude DSS bytes at the end of the PDF document when calling signatureCoversWholeDocument method?
What you need is something else. What you need is code that checks whether additions applied to a PDF after a signature are allowed or disallowed. iText does not contain a method for that, so you have to implement that code yourself or look for a third-party implementation.
Please be aware that this task is not easy. Even the implementation thereof in Adobe Acrobat (which may serve as a kind of reference here) errs every so often, and in either direction.
I'm writing a parse tool to extract each field of P12 file in C language, OpenSSL is too huge for my project.
After reading PKCS# series documents and ASN.1 documents, I understand the basic parse step.
I use OpenSSL to generate a self-signed P12 file，there're some questions during parsing:
Why Certificate is not stored in a Safebag, in my case it's stored in EncryptedData field?
What does the localKeyId attribute mean? it has an OctetString, what is the OctectString used for?
Why the contentType of encryptedContentInfo is id-data? I think it should be id-encryptedData. In my case, Certificate is stored in encryptedContentInfo field.
Thanks,
CZ
The PKCS12 standard also available as rfc7292 formally allows a very wide range and combination of options, but in practice only a few of these options are used. There are basically 3 levels:
the file has type/structure PFX consisting mostly of a PKCS7/CMS ContentInfo which theoretically can be 'data' or 'signed-data' but in practice is always the former (with the nominally optional MacData appended) and contains
'AuthenticatedSafe' which is a sequence of one or more (almost always more) ContentInfo(s) each of which (separately) may be encrypted or not and contains (after decryption if applicable)
a sequence of one of more 'bag'(s) each of which contains actual data of a certain type such as an encrypted privatekey or a certificate along with optional attributes.
In practice there is usually:
one CI (at level 2) with PKCS7/CMS type 'encrypted-data' using a very weak algorithm (RC2-40) containing one or more CertBag(s) each containing a cert plus any attributes for it, and
one or more CI(s) (each) with type 'data' containing a PKCS8ShroudedKeyBag containing an encrypted privatekey (using PKCS8 as stated) usually using a strong algorithm commonly 3DES, plus attributes.
My answer here shows the first levels of parsing to find the encryption details; further parsing requires decryption as shown (for a specific case) in my answer here.
As mentioned all bags can have attributes; in practice depending on the implementation some bags may have the 'friendlyName' attribute with a value intended for people to use, and if a matching privatekey and cert are present they both have a 'localKeyId' attribute with the same value to tie them together, as explained in my answer to a different but related Q. 'localKeyId' is not intended for people to use and you should not normally present it to people.
I don't understand why JWS unprotected headers exist.
For some context: a JWS unprotected header contains parameters that are not integrity protected and can only be used per-signature with JSON Serialization.
If they could be used as a top-level header, I could see why someone could want to include a mutable parameter (that wouldn't change the signature). However, this is not the case.
Can anyone think of a use-case or know why they are included in the spec?
Thanks!
JWS Spec
The answer by Florent leaves me unsatisfied.
Regarding the example of using a JWT to sign a hash of a document... the assertion is that the algorithm and keyID would be "sensitive data" that needs to be "protected". By which I suppose he means "signed". But there's no need to sign the algorithm and keyID.
Example
Suppose Bob creates a signed JWT, that contains an unprotected header asserting alg=HS256 and keyid=XXXX1 . This JWT is intended for transmission to Alice.
Case 1
Suppose Mallory intercepts the signed JWT sent by Bob. Mallory then creates a new unprotected header, asserting alg=None.
The receiver (Alice) is now responsible for verifying the signature on the payload. Alice must not be satisfied with "no signature"; in fact Alice must not rely on a client (sender) assertion to determine which signing algorithm is acceptable for her. Therefore Alice rejects the JWT with the contrived "no signature" header.
Case 2
Suppose Mallory contrives a header with alg=RS256 and keyId=XXX1. Now Alice tries to validate the signature and finds either:
the algorithm is not compliant
the key specified for that algorithm does not exist
Therefore Alice rejects the JWT.
Case 3
Suppose Mallory contrives a header with alg=HS256 and keyId=ZZ3. Now Alice tries to validate the signature and finds the key is unknown, and rejects the JWT.
In no case does the algorithm need to be part of the signed material. There is no scenario under which an unprotected header leads to a vulnerability or violation of integrity.
Getting Back to the Original Question
The original question was: What is the purpose of an unprotected JWT header?
Succinctly, the purpose of an unprotected JWS header is to allow transport of some metadata that can be used as hints to the receiver. Like alg (Algorithm) and kid (Key ID). Florent suggests that stuffing data into an unprotected header could lead to efficiency. This isn't a good reason. Here is the key point: The claims in the unprotected header are hints, not to be relied upon or trusted.
A more interesting question is: What is the purpose of a protected JWS header? Why have a provision that signs both the "header" and the "payload"? In the case of a JWS Protected Header, the header and payload are concatenated and the result is signed. Assuming the header is JSON and the payload is JSON, at this point there is no semantic distinction between the header and payload. So why have the provision to sign the header at all?
One could just rely on JWS with unprotected headers. If there is a need for integrity-protected claims, put them in the payload. If there is a need for hints, put them in the unprotected header. Sign the payload and not the header. Simple.
This works, and is valid. But it presumes that the payload is JSON. This is true with JWT, but not true with all JWS. RFC 7515, which defines JWS, does not require the signed payload to be JSON. Imagine the payload is a digital image of a medical scan. It's not JSON. One cannot simply "attach claims" to that. Therefore JWS allows a protected header, such that the (non JSON) payload AND arbitrary claims can be signed and integrity checked.
In the case where the payload is non-JSON and the header is protected, there is no facility to include "extra non signed headers" into the JWS. If there is a need for sending some data that needs to be integrity checked and some that are simply "hints", there really is only one container: the protected header. And the hints get signed along with the real claims.
One could avoid the need for this protected-header trick, by just wrapping a JSON hash around the data-to-be-signed. For example:
{
"image" : "qw93u9839839...base64-encoded image data..."
}
And after doing so, one could add claims to this JSON wrapper.
{
"image" : "qw93u9839839...base64-encoded image data..."
"author" : "Whatever"
}
And those claims would then be signed and integrity-proected.
But in the case of binary data, encoding it to a string to allow encapsulation into a JSON may inflate the data significantly. A JWS with a non-JSON payload avoids this.
HTH
The RFC gives us examples of unprotected headers as follows:
A.6.2. JWS Per-Signature Unprotected Headers
Key ID values are supplied for both keys using per-signature Header Parameters. The two JWS Unprotected Header values used to represent these key IDs are:
{"kid":"2010-12-29"}
and
{"kid":"e9bc097a-ce51-4036-9562-d2ade882db0d"}
https://datatracker.ietf.org/doc/html/rfc7515#appendix-A.6.2
The use of kid in the example is likely not coincidence. Because JWS allows multiple signatures per payload, a cleartext hint system could be useful. For example, if a key is not available to the verifier (server), then you can skip decoding the protected header. The term "hint" is actually used in the kid definition:
4.1.4. "kid" (Key ID) Header Parameter
The "kid" (key ID) Header Parameter is a hint indicating which key was used to secure the JWS. This parameter allows originators to explicitly signal a change of key to recipients. The structure of the "kid" value is unspecified. Its value MUST be a case-sensitive string. Use of this Header Parameter is OPTIONAL.
https://datatracker.ietf.org/doc/html/rfc7515#section-4.1.4
If we look at Key Identification it mentions where you a kid does not have to be integrity protected (ie: part of unprotected headers): (emphasis mine)
6. Key Identification
It is necessary for the recipient of a JWS to be able to determine the key that was employed for the digital signature or MAC operation. The key employed can be identified using the Header Parameter methods described in Section 4.1 or can be identified using methods that are outside the scope of this specification. Specifically, the Header Parameters "jku", "jwk", "kid", "x5u", "x5c", "x5t", and "x5t#S256" can be used to identify the key used. These Header Parameters MUST be integrity protected if the information that they convey is to be utilized in a trust decision; however, if the only information used in the trust decision is a key, these parameters need not be integrity protected, since changing them in a way that causes a different key to be used will cause the validation to fail.
The producer SHOULD include sufficient information in the Header Parameters to identify the key used, unless the application uses another means or convention to determine the key used. Validation of the signature or MAC fails when the algorithm used requires a key (which is true of all algorithms except for "none") and the key used cannot be determined.
The means of exchanging any shared symmetric keys used is outside the scope of this specification.
https://datatracker.ietf.org/doc/html/rfc7515#section-6
Simplified, if you have a message that by somebody modifying the kid will refer to another key, then the signature itself will not match. Therefore you don't have to include the kid in the protected header. A good example of the first part, where the information they convey is to be utilized in a trust decision, is the ACME (aka the Let's Encrypt protocol). When creating an account, and storing the key data, you want to trust the kid. We want to store kid, so we need to make sure it's valid. After the server has stored the kid and can use it to get a key, we can push messages and reference the kid in unprotected header (not done by ACME but possible). Since we're only going to verify the signature, then the kid is used a hint or reference to which kid was used for the account. If that field is tampered with, then it'll point to a nonexistent of completely different key and fail the signature the check. That means the kid itself is "the only information used in the trust decision".
There's also more theoretical scenarios that, knowing how it works you can come up with.
For example: the idea of having multiple signatures that you can pass on (exchange). A signing authority can include one signature that can be for an intermediary (server) and another for the another recipient (end-user client). This is differentiated by the kid and the server doesn't need to verify or even decode the protected header or signature. Or perhaps, the intermediary doesn't have the client's secret in order to verify a signature.
For example, a multi-recipient message (eg: chat room) could be processed by a relay/proxy and using kid in the unprotected header, pass along a reconstructed compact JWS (${protected}.${payload}.${signature}) for each recipient based on kid (or any other custom unprotected header field, like userId or endpoint).
Another example, would be a server with access to many different keys and a cleartext kid would be faster than iterating and decoded each protected field to find which one.
From one perspective, all you're doing is skipping base64url decoding the protected header for performance, but if you're going to proxy/relay the data, then you're not polluting the protected header which is meant for another recipient.
Is there any way to verify the OpenSSL signature using only {signature,hashed message} pair, skipping the original file to be presented for verification?
I need to verify the signature with only {signature,hashed message} pair remotely so using the original file is cumbersome specially when its very large.
Is there any way to verify the OpenSSL signature using only hash value and without needing the original file?
Yes, but there are strings attached.
The scheme which requires the original message to be presented to the verifying function is a Signature Scheme with Appendix (SSA). A scheme like the old PKCS #1.0 signing is an example of it.
The scheme which does not require the original message is a Signature Scheme with Recovery (PSSR). In a PSSR, the encoded message is part of the signature and masked. A scheme like the new PKCS #2.0 PSSR signing is an example of it.
There are no schemes that take just a hash, as far as I know. You have to have the {message,signature} pair. Allowing the message to be disgorged from the signing or verification can be a security violation.
OpenSSL provides both of them, as does most other security libraries, like Botan, Crypto++, NSS, etc.
Also see RSA signature on TLS on Information Security Stack Exchange.
I have been trying to verify the signature with hash value remotely so using the original file is cumbersome specially when its very large.
That's the insecure thing signature schemes want to avoid....
This is the signature i got when i am signing a pdf
From which, is there any way to get Signatory(in my Signature Suniiii). Now I am able to extract location, date and Reason except Signatory.
First of all, the image you see includes textual information which can be freely chosen by the signing application. Usually for such visualizations the text is chosen to match the information invisibly stored in the signature field but some mischievous signer may insert differences. Thus, you first have to decide whether you want to get the text from the signature appearance or the values stored invisibly in the field.
If you want to read the exact text from the visualization, simply use the code from the answers to the question "Get Layer2 Text (Signature Description) from signature image using itextsharp".
If you want to read the exact content from the invisible field information, use the values of the signature field value dictionary as you already do for location, date, and reason, and also read the CMS signature container from the Contents, extract the signer certificate, and read its owner DN.