ML-DSA with ruby/openssl

Ruby’s openssl gem is more or less the de facto source of cryptography in Ruby. It supports largely unopinionated bindings on top of OpenSSL’s EVP_PKEY. While ruby/openssl has many helpers for classical algorithms like RSA and ECDSA, its generic bindings on top of EVP_PKEY make most ML-DSA functionality available.

Since ruby/openssl provides Ruby bindings on top of OpenSSL, using ML-DSA requires building the gem against OpenSSL 3.5 or later. Version 3.2 of the ruby/openssl gem is also required for some of the key import and export APIs.

Some quick notes:

1. I’ve omitted the require 'openssl' in snippets.
2. I’m showing ML-DSA-44, but you can change it to ML-DSA-65 or ML-DSA-87 for other parameter sets.

Generating a Key

key = OpenSSL::PKey.generate_key("ML-DSA-44")

Importing a Seed

seed = "\x01" * 32 # Example seed
key = OpenSSL::PKey.generate_key("ML-DSA-44", hexseed: seed.unpack1("H*"))

⚠️ Note: Take care to ensure hexseed is not nil or "" (empty string). OpenSSL interprets this to mean generating a new key.

⚠️ Note: While generate_key has a seed option as well, you should import the seed as a hex string because a plain string will fail to import if it contains \x00 octets. \x00 is valid for seeds, so hex should be used to ensure \x00 bytes are preserved properly.

Exporting a Seed

key = get_ml_dsa_key()
key.get_param("seed")

⚠️ Note: get_param at the time of writing is new and may not be available. If this API is not available to you yet, unfortunately there is no easy way to retrieve the seed. The two options for getting the seed without get_param are to use to_text, which requires awkward parsing, or perform a PKCS#8 export and extract the seed from the PKCS#8 private key.

Importing a Private Key or Public Key

key = OpenSSL::PKey.new_raw_private_key("ML-DSA-44", private_key)
key = OpenSSL::PKey.new_raw_public_key("ML-DSA-44", public_key)

Exporting a Private Key or Public Key

key = get_ml_dsa_key()
private_key = key.raw_private_key
public_key = key.raw_public_key

Signing

key = get_ml_dsa_key()
data_to_sign = get_data_to_sign()
signature = key.sign(nil, data_to_sign)

Signing, Deterministic

key = get_ml_dsa_key()
data_to_sign = get_data_to_sign()
signature = key.sign(nil, data_to_sign, deterministic: 1)

⚠️ Note: Hedged, or non-deterministic, is the better option to use. There are some uses for deterministic signatures, but unless you know you should be using them, it’s better to use hedged.

Signing with Context

key = get_ml_dsa_key()
data_to_sign = get_data_to_sign()
signature = key.sign(nil, data_to_sign, "context-string": "example context") # Hedged
signature = key.sign(nil, data_to_sign, "context-string": "example context", deterministic: 1) # Deterministic

⚠️ Note: If your context is a binary string, it should be passed in hex form for the same reasons as seed:

signature = key.sign(nil, data_to_sign, "hexcontext-string": "example\x00context".unpack1("H*"))

Signing with External-Mu

key = get_ml_dsa_key()
mu_to_sign = calculate_mu()
signature = key.sign(nil, mu_to_sign, mu: 1) # Hedged
signature = key.sign(nil, mu_to_sign, mu: 1, deterministic: 1) # Deterministic

⚠️ Note: External-Mu mode lets callers supply the 64-byte message representative that ML-DSA normally computes internally. Constructing External-Mu requires careful construction; it’s not just a simple hash over the data you want to sign like RSA or ECDSA offer. Calculating mu is an exercise left for another post. Right now it is impossible to calculate with ruby/openssl because it requires squeezing output from SHAKE-256, which ruby/openssl doesn’t support currently, but it will in the future.

⚠️ Note: context-string is ignored when using mu. That’s because mu’s calculation already accounts for the context, so it won’t be used as part of signing or verifying.

Verifying

key = get_ml_dsa_key()
signature = signature_to_verify()
data_to_verify = get_data_to_verify()
result = key.verify(nil, signature, data_to_verify)

Verifying with Context

key = get_ml_dsa_key()
signature = signature_to_verify()
data_to_verify = get_data_to_verify()
result = key.verify(nil, signature, data_to_verify, "context-string": "example context") # UTF-8 context string
result = key.verify(nil, signature, data_to_verify, "hexcontext-string": "example context".unpack1("H*")) # Same context, hex encoded

Verifying with External-Mu

key = get_ml_dsa_key()
signature = signature_to_verify()
mu_to_verify = calculate_mu_to_verify()
result = key.verify(nil, signature, mu_to_verify, mu: 1)

.NET's Cryptographic One-Shots

Over the past few releases in .NET, formerly .NET Core, there has been progress on making cryptographic primitives like AES, SHA, etc. better for developers to use.

“Better” is an interesting point of conversation with cryptographic API design. To a developer, better may mean more throughput, less allocations, or a simply less cumbersome API. To a framework or library author, it means thinking about how developers will use, or mis-use, an API.

Let’s look at AES encryption as it was in the .NET Framework days:

using System.Security.Cryptography;

byte[] data = default;

using (Aes aes = Aes.Create())
{
    byte[] key = default; // Get a key from somewhere
    byte[] iv = default; // Get a unique IV from somewhere

    using ICryptoTransform transform = aes.CreateEncryptor(key, iv);
    byte[] encrypted = transform.TransformFinalBlock(data, 0, data.Length);
}

This may only be a dozen or so lines of a code, but not all of it is straight forward. What is a “transform”? What is a “block”? What does “final” mean?

The APIs expose quite a bit of functionality, and are confusingly named. TransformFinalBlock for example, despite having “Block” in it’s name, is almost always going to be capable of encrypting more than one block. It also means the data doesn’t need to be block aligned, so it handles padding appropriately. Since none of that may be understood, developers often times work around perceived problems, like handling individual blocks. While this API design offers the most flexibility for developers, it also offers the most complexity.

This complexity exists for a small group of people. Most developers have some small amount of data they want to encrypt, and when they don’t, they have a Stream.

Complex APIs that are prone to misuse are problematic in a security context. A misused cryptographic API almost always harms intended goal of the cryptographic API, whether that be secrecy, integrity, etc.

For .NET, this became a concern when AES GCM and CCM were exposed in .NET Core 3.1. The AesGcm and AesCcm classes do not follow the design of AES prior. In addition to not inheriting from Aes or SymmetricAlgorithm, they also do not expose any block functionality. This was discussed at length, and instead these types offer simple Encrypt or Decrypt APIs that takes data and return data, or write to an existing buffer.

This, while less flexible, resolves many concerns about misusing the AES GCM cipher mode. Primarily among those concerns was releasing unauthenticated data. Streaming decryption is, put simply, difficult to do safely.

“Streaming” decryption doesn’t necessarily mean the use of System.IO.Stream. Rather, it means processing a block of plaintext or ciphertext a block at a time and doing something with it in the middle of encrypting or decrypting. This is often perceived as desirable when handling large amounts of data. After all, if I have a 12 gigabyte file, I can’t just put that in a byte array and encrypt it. Rather, processing it in chunks lets me handle it in memory.

In pseudo code, let’s say I wanted to decrypt a file and send it over the network:

# NOTE: This is an example of doing things improperly
encryptedFileStream = getFile()
stream = getStream()

loop {
    data = encryptedFileStream.read(128 / 8) # One AES block size

    if (data.length == 128 / 8) {
        stream.write(decrypt(data))
    }
    else {
        stream.write(decryptFinal(data))
        stream.close()
        break
    }
}

Recall though that AES GCM is authenticated. That is, AES GCM can tell if your ciphertext has been modified while in storage or in transit. An important detail of this though is that GCM cannot authenticate until it has processed the entire ciphertext (when decryptFinal is called.)

This is breaks down because as we are decrypting, we’re sending (releasing) the plaintext before AES GCM has been able to authenticate the entire cipher text. If the person, tool, whatever on the other end of the network is processing that decrypted data in real time, then they have processed unauthentic data and it’s too late to go back and tell them “Never mind, that data I send you a few seconds ago might have been tampered with.”

There are correct ways to do this, but are also still difficult to do correctly. You could break the file up in to small chunks and treat them as individual ciphertexts. However then you need to worry about many nonces, ensuring chunks are processed in the right order, a chunk isn’t missing, or replayed, etc.

Before long you’ve invented a cryptographic protocol. This is largely why many folks will recommend using something that is well understood and robust rather than trying to build it yourself. Though not a primitive cipher, this kind of problem falls in the “roll your own cryptography” bucket.

It’s rather easy to accidentally roll your own cryptography, especially so when working with “streaming” data.

In .NET then, AES GCM and CCM do not support encrypting individual blocks. This still does not solve the issue of ensuring chunks are handled appropriately when handling large amounts of data. For that, higher level tools are still recommended. However it removes the temptation for streaming AES GCM, for which any attempt to use is almost always incorrect. Since it is very difficult to use correctly with no practical use cases, it isn’t offered.

Toward Better APIs

Simple APIs are important to making them misuse resistent, and .NET has gotten better at that over the past few releases.

Like AesGcm, the SymmetricAlgorithm and its derivatives like Aes, TripleDES, etc. all offer similar one-shot APIs starting in .NET 6 in the form of EncryptCbc, EncryptEcb or DecryptCbc and DecryptEcb.

using System.Security.Cryptography;

byte[] data = default;

using (Aes aes = Aes.Create())
{
    byte[] key = default; // Get a key from somewhere
    byte[] iv = default; // Get a unique IV from somewhere

    aes.Key = key;

    // Encrypt all the data at once
    byte[] encrypted = aes.EncryptCbc(data, iv);
}

There is no ICryptoTransform that needs to be reasoned about or disposed, and there is no need to worry about blocks, padding, etc. Where possible, one shots have been added in most places, and made static where possible.

For hashing, prior to .NET 5 it would look something like this:

// Prior to .NET 5
using System.Security.Cryptography;

byte[] data = default; // Some data

using (SHA256 hash = SHA256.Create())
{
    byte[] digest = hash.ComputeHash(data);
}

Rather than creating an instance of a hash algorithm, HashData now exists:

// Starting in .NET 5
using System.Security.Cryptography;

byte[] data = default; // Some data
byte[] digest = SHA256.HashData(data);

This is much easier to reason about. The method is static, there is no stateful hash object that needs to be instantiated, no need to remember to dispose of it, and no need to worry about thread safety. Not only are the one shots easier to use, they almost always offer better performance, either in throughput or reduced allocations. These one shots are not simple wrappers around HashAlgorithm.Create() and then hashing something. They internally do not allocate on the managed heap at all. Everyone benefits here: the APIs are simpler, and developers get better performance.

For .NET 6, the one shot hashing APIs were brought to the HMAC classes as well, offering the same improved APIs and better performance.

Also for .NET 6 PBKDF2 got the same treatment with Rfc2898DeriveBytes.Pbkdf2.

using System.Security.Cryptography;

byte[] salt = RandomNumberGenerator.GetBytes(32);
byte[] prk = Rfc2898DeriveBytes.Pbkdf2(
    userPassword,
    salt,
    iterations: 200_000,
    HashAlgorithmName.SHA256,
    outputLength: 32);

All of these APIs also offer modern amenities, like working ReadOnlySpan<byte> for input data and being able to write to a Span<byte> for output data.

I’m happy with .NETs move toward easier to use APIs for cryptographic primitives. I still largely believe many developers should use higher level concepts rather than these basic building blocks. However, for those that need the building blocks, they are getting better.

Dos and Don'ts of stackalloc

In .NET Core 2.1 a small but well-received feature was the ability to “safely” allocate a segment of data on the stack, using stackalloc, when used with Span<T>.

Before Span<T>, stackalloc required being in an unsafe context:

unsafe {
    byte* data = stackalloc byte[256];
}

The use of unsafe, along with the little number of APIs in .NET that could work with pointers, was enough to deter a lot of people from using it. As a result, it remained a relatively niche feature. The introduction of Span<T> now means this can be done without being in an unsafe context now:

Span<byte> data = stackalloc byte[256];

The .NET team has also been working diligently to add Span<T> APIs where it makes sense. There is now more appeal and possibility to use stackalloc.

stackalloc is desirable in some performance sensitive areas. It can be used in places where small arrays were used, with the advantage that it does not allocate on the heap - and thus does not apply pressure to the garbage collector. stackalloc is not a general purpose drop-in for arrays. They are limited in a number of ways that other posts explain well enough, and require the use of Span<T>.

Recently I vented a bit about stackalloc on Twitter, as one does on Twitter, specifically the community’s fast embrace of it, without discussing or well-documenting some of stackalloc’s sharp edges. I’m going to expand on that here, and make an argument for stackalloc still being unsafe and requiring some thought about being used.

DON’T: Use variable allocation lengths

A large risk with using stackalloc is running out of stack space. If you’ve ever written a method that is recursive and went too deep, you’ll eventually receive a StackOverflowException. The StackOverflowException is a bit special in that it is one of the exceptions that cannot be caught. When a stack overflow occurs, the process immediately exits. Allocating too much with stackalloc has the same effect - it causes a stack overflow and the process immediately terminates.

This is particularly worrisome when the allocation’s length is determined by user input:

Span<char> buffer = stackalloc char[userInput.Length]; //DON'T

This allows users to take down your process, an effective denial-of-service.

Using a constant also reduces the risk of arithmetic or overflow mistakes. Currently, .NET Core’s CLR interprets that amount to be allocated as an unsigned integer. This means that an arithmetic over or under flow may result in a stack overflow.

Span<byte> b = stackalloc byte[(userInput % 64) - 1]; //DON'T

DO: Use a constant for allocation size

Instead, it’s better to use a constant value for stackalloc, always. It immediately resolves any ambiguities about how much is allocated on the stack.

Span<char> buffer = stackalloc char[256]; //better

Once you have an allocated buffer, you can use Span’s Slice funtionality to adjust it to the correct size:

Span<char> buffer = stackalloc char[256];
Span<char> input = buffer.Slice(0, userInput.Length);

DON’T: Use stackalloc in non-constant loops

Even if you allocate a fixed length amount of data on the stack, doing so in a loop can be dangerous as well, especially if the number of the iterations the loop makes is driven by user input:

for (int i = 0; i < userInput; i++) { // DON'T
    Span<char> buffer = stackalloc char[256];
}

This also can cause a denial of service, since this allows someone to control the number of stack allocations, though not the length of the allocation.

DO: Allocate outside of loops

Span<char> buffer = stackalloc char[256]; //better
for (int i = 0; i < userInput; i++) {
    //Do something with buffer
}

Allocating outside of the loop is the best solution. This is not only safer, but also better for performance.

DON’T: Allocate a lot on the stack

It’s tempting to allocate as much as nearly possible on the stack:

Span<byte> data = stackalloc byte[8000 * 1024]; // DON'T

You may find that this runs fine on Linux, but fails on Windows with a stack overflow. Different operating systems, architectures, and environments, have different stacks limits. Linux typically allows for a larger stack than Windows by default, and other hosting scenarios such as in an IIS worker process come with even lower limits. An embedded environment may have a stack of only a few kilobytes.

DO: Conservatively use the stack

The stack should be used for small allocations only. How much depends on the size of each element being allocated. It’s also desirable to not allocate many large structs, either.

I won’t prescribe anything specific, but anything larger than a kilobyte is a point of concern. You can allocate on the heap depending on how much you need. A typical pattern might be:

const int MaxStackSize = 256;
Span<byte> buffer =
    userInput > MaxStackSize
      ? new byte[userInput]
      : stackalloc byte[MaxStackSize];

Span<byte> data = buffer.Slice(0, userInput);

This will allocate on the stack for small amounts, still in a constant amount, or if too large, will use a heap-allocated array. This pattern may also make it easier to use ArrayPool, if you choose, which also does not guarantee that the returned array is exactly the requested size:

const int MaxStackSize = 256;
byte[]? rentedFromPool = null;
Span<byte> buffer =
    userInput > MaxStackSize
    ? (rentedFromPool = ArrayPool<byte>.Shared.Rent(userInput))
    : stackalloc byte[MaxStackSize];

// Use data
Span<byte> data = buffer.Slice(0, userInput);

// Return from pool, if we rented
if (rentedFromPool is object) {
    // DO: if using ArrayPool, think carefully about clearing
    // or not clearing the array.
    ArrayPool<byte>.Shared.Return(rentedFromPool, clearArray: true);
}

DON’T: Assume stack allocations are zero initialized

Most normal uses of stackalloc result in zero-initialized data. This behavior is however not guaranteed, and can change depending if the application is built for Debug or Release, and other contents of the method. Therefore, don’t assume that any of the elements in a stackalloced Span<T> are initialized to something by default. For example:

Span<byte> buffer = stackalloc byte[sizeof(int)];
byte lo = 1;
byte hi = 1;
buffer[0] = lo;
buffer[1] = hi;
// DONT: depend on elements at 2 and 3 being zero-initialized
int result = BinaryPrimitives.ReadInt32LittleEndian(buffer);

In this case, we might expect the result to be 257, every time. However if the stackalloc does not zero initialize the buffer, then the contents of the upper-half of the integer will not be as expected.

This behavior will not always be observed. In Debug builds, it’s likely that you will see that stackalloc zero-initializes its contents every time, whereas in Release builds, you may find that the contents of a stackalloc are uninitialized.

Starting in .NET 5, developers can opt to explicitly skip zero-initializing stackalloc contents with the SkipLocalsInit attribute. Without it, whether or not stackalloc is default initialized is up to Roslyn.

DO: Initialize if required

Any item read from a stackalloced buffer should be explicitly assigned, or use Clear to explicitly clear the entire Span<T> and initialize it to defaults.

Span<byte> buffer = stackalloc byte[sizeof(int)];
buffer.Clear(); //explicit zero initialize
byte lo = 1;
byte hi = 1;
buffer[0] = lo;
buffer[1] = hi;
int result = BinaryPrimitives.ReadInt32LittleEndian(buffer);

Though not explicitly covered in this post, the same advice applies to arrays rented from the ArrayPool.

Summary

In summary, stackalloc needs to be used with care. Failing to do so can result in process termination, which is a denial-of-service: your program or web server aren’t running any more.

Import and Export RSA Key Formats in .NET Core 3

.NET Core 3.0 introduced over a dozen new APIs for importing and exporting RSA keys in different formats. Many of them are a variant of another with a slightly different API, but they are extremely useful for working with private and public keys from other systems that work with encoding keys.

RSA keys can be encoded in a variety of different ways, depending on if the key is public or private or protected with a password. Different programs will import or export RSA keys in a different format, etc.

Often times RSA keys can be described as “PEM” encoded, but that is already ambiguous as to how the key is actually encoded. PEM takes the form of:

-----BEGIN LABEL-----
content
-----END LABEL-----

The content between the labels is base64 encoded. The one that is probably the most often seen is BEGIN RSA PRIVATE KEY, which is frequently used in web servers like nginx, apache, etc:

-----BEGIN RSA PRIVATE KEY-----
MII...
-----END RSA PRIVATE KEY-----

The base64-encoded text is an RSAPrivateKey from the PKCS#1 spec, which is just an ASN.1 SEQUENCE of integers that make up the RSA key. The corresponding .NET Core 3 API for this is ImportRSAPrivateKey, or one of its overloads. If your key is “PEM” encoded, you need to find the base64 text between the label BEGIN and END headers, base64 decode it, and pass to ImportRSAPrivateKey. There is currently an API proposal to make reading PEM files easier. If your private key is DER encoded, then that just means you can read the content directly as bytes in to ImportRSAPrivateKey.

Here is an example:

var privateKey = "MII..."; //Get just the base64 content.
var privateKeyBytes = Convert.FromBase64String(privateKey);
using var rsa = RSA.Create();
rsa.ImportRSAPrivateKey(privateKeyBytes, out _);

When using openssl, the openssl rsa commands typically output RSAPrivateKey PKCS#1 private keys, for example openssl genrsa.

A different format for a private key is PKCS#8. Unlike the RSAPrivateKey from PKCS#1, a PKCS#8 encoded key can represent other kinds of keys than RSA. As such, the PEM label for a PKCS#8 key is “BEGIN PRIVATE KEY” (note the lack of “RSA” there). The key itself contains an AlgorithmIdentifer of what kind of key it is.

PKCS#8 keys can also be encrypted protected, too. In that case, the PEM label will be “BEGIN ENCRYPTED PRIVATE KEY”.

.NET Core 3 has APIs for both of these. Unencrypted PKCS#8 keys can be imported with ImportPkcs8PrivateKey, and encrypted PKCS#8 keys can be imported with ImportEncryptedPkcs8PrivateKey. Their usage is similar to ImportRSAPrivateKey.

Public keys have similar behavior. A PEM encoded key that has the label “BEGIN RSA PUBLIC KEY” should use ImportRSAPublicKey. Also like private keys, the public key has a format that self-describes the algorithm of the key called a Subject Public Key Info (SPKI) which is used heavily in X509 and many other standards. The PEM header for this is “BEGIN PUBLIC KEY”, and ImportSubjectPublicKeyInfo is the correct way to import these.

All of these APIs have export versions of themselves as well, so if you are trying to export a key from .NET Core 3 to a particular format, you’ll need to use the correct export API.

To summarize each PEM label and API pairing:

1. “BEGIN RSA PRIVATE KEY” => RSA.ImportRSAPrivateKey
2. “BEGIN PRIVATE KEY” => RSA.ImportPkcs8PrivateKey
3. “BEGIN ENCRYPTED PRIVATE KEY” => RSA.ImportEncryptedPkcs8PrivateKey
4. “BEGIN RSA PUBLIC KEY” => RSA.ImportRSAPublicKey
5. “BEGIN PUBLIC KEY” => RSA.ImportSubjectPublicKeyInfo

One gotcha with openssl is to pay attention to the output of the key format. A common enough task from openssl is “Given this PEM-encoded RSA private key, give me a PEM encoded public-key” and is often enough done like this:

openssl rsa -in key.pem -pubout

Even if key.pem is a PKCS#1 RSAPrivateKey (“BEGIN RSA PRIVATE KEY”), the -pubout option will output a SPKI (“BEGIN PUBLIC KEY”), not an RSAPublicKey (“BEGIN RSA PUBLIC KEY”). For that, you would need to use -RSAPublicKey_out instead of -pubout. The openssl pkey commands will also typically give you PKCS#8 or SPKI formatted keys.

Sometimes valid RSA signatures in .NET

One of the nice things about .NET Core being open source is following along with some of the issues that people report. I tend to keep an eye on System.Security tagged issues, since those tend to be at the intersection of things that interest me and things I can maybe help with.

A user filed an issue where .NET Framework considered a CMS valid, and .NET Core did not. This didn’t entirely surprise me. In the .NET Framework, the SignedCms class is heavily backed by Windows’ handling of CMS/PKCS#7. In .NET Core, the implementation is managed (sans the cryptography). The managed implementation adheres somewhat strictly to the CMS specification. As other issues have noticed, Windows’, thus .NET Framework’s, implementation was a little more relaxed in some ways.

This turned out not to be one of those cases. The CMS part was actually working just fine. What was failing was RSA itself. The core of the issue was that different implementations of RSA disagreed on the RSA signature’s validity.

That seems pretty strange!

When I talk about different implementations on Windows, I am usually referring to CAPI vs CNG, or RSACryptoServiceProvider and RSACng, respectively. For now, I’m keeping this post to the .NET Framework. We’ll bring .NET Core in to the discussion later.

There are two implementations because, well, Windows has two of them. CNG, or “Cryptography API: Next Generation” is the newer of the two and is intended to be future of cryptographic primitives on Windows. It shipped in Windows Vista, and offers functionality that CAPI cannot do. An example of that is PSS RSA signatures.

.NET Framework exposes these implementations as RSACryptoServiceProvider and RSACng. They should be interchangable, and CNG implementations should be used going forward. However, there is one corner case where the old, CAPI implementation considers a signature valid while the CNG one does not.

The issue can be demonstrated like so:

byte[] n = new byte[] { ... };
byte[] e = new byte[] { ... };
byte[] signature = new byte[] { ... };
var digest = new byte[] {
    0x68, 0xB4, 0xF9, 0x26, 0x34, 0x31, 0x25, 0xDD,
    0x26, 0x50, 0x13, 0x68, 0xC1, 0x99, 0x26, 0x71,
    0x19, 0xA2, 0xDE, 0x81, 
};
using (var rsa = new RSACng())
{
    rsa.ImportParameters(new RSAParameters {
        Modulus = n,
        Exponent = e
    });
    var valid = rsa.VerifyHash(digest, signature, HashAlgorithmName.SHA1,
                               RSASignaturePadding.Pkcs1);
    Console.WriteLine(valid);
}
using (var rsa = new RSACryptoServiceProvider())
{
    rsa.ImportParameters(new RSAParameters {
        Modulus = n,
        Exponent = e
    });
    var valid = rsa.VerifyHash(digest, signature, HashAlgorithmName.SHA1,
                               RSASignaturePadding.Pkcs1);
    Console.WriteLine(valid);
}

When used with one of the curious signatures that exhibits this behavior, such as the one in the GitHub link, the first result will be false, and the second will be true.

Nothing jumped out at me as being problematic. The signature padding is PKCS, the public exponent is the very typical 67,537, and the RSA key is sensible in size.

To make it stranger, this signature came off the timestamp of Firefox’s own signed installer. So why are the results different?

Jeremy Barton from Microsoft on .NET Core made the observation that the padding in the RSA signature itself is incorrect, but in a way that CAPI tollerates and CNG does not, at least by default. Let’s look at the raw signature. To do that, we need the public key and signature on disk, and we can poke at them with OpenSSL.

Using the command:

openssl rsautl -verify -in sig.bin -inkey key.der \
    -pubin -hexdump -raw -keyform der

We get the following output:

0000 - 00 01 ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
0010 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
0020 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
0030 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
0040 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
0050 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
0060 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
0070 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
0080 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
0090 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
00a0 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
00b0 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
00c0 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
00d0 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
00e0 - ff ff ff ff ff ff ff ff-ff ff ff 00 68 b4 f9 26
00f0 - 34 31 25 dd 26 50 13 68-c1 99 26 71 19 a2 de 81

This is a PKCS#1 v1.5 padded signature, as indicated by by starting with 00 01. The digest at the end can be seen, 68 b4 f9 26 ... 19 a2 de 81 which matches the digest above, so we know that the signature is for the right digest.

What is not correct in this signature is how the digest is encoded. The signature contains the bare digest. It should be encoded as an ASN.1 sequence along with the AlgorithmIdentifer of the digest:

DigestInfo ::= SEQUENCE {
	digestAlgorithm AlgorithmIdentifier,
	digest OCTET STRING
}

This goes back all the way to a document (warning: link is to an ftp:// site) written in 1993 by RSA labratories explaining how PKCS#1 v1.5 works,and was standardized in to an RFC in 1998.

The RSA signature we have only contains the raw digest. It is not part of a DigestInfo. If the digest were properly encoded, it would look something like this:

0000 - 00 01 ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
0010 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
0020 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
0030 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
0040 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
0050 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
0060 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
0070 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
0080 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
0090 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
00a0 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
00b0 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
00c0 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff
00d0 - ff ff ff ff ff ff ff ff-ff ff ff ff 00 30 21 30
00e0 - 09 06 05 2b 0e 03 02 1a-05 00 04 14 68 b4 f9 26
00f0 - 34 31 25 dd 26 50 13 68-c1 99 26 71 19 a2 de 81

The signature now includes DigestInfo along with the OID 1.3.14.3.2.26 to indicate that the digest is SHA1.

At this point we know what the difference is, and the original specification in part 10.1.2 makes it fairly clear that the “data” should be a digest and should be encoded as DigestInfo, not a bare digest.

The source of this signature is from Verisign's timestamp authority at http://timestamp.verisign.com/​scripts/​timstamp.dll. After checking with someone at DigiCert (now running this TSA), it was launched in May 1995.

I suspect that the TSA is old enough that the implementation was made before the specification was complete or simply got the specification wrong and no one noticed. Bringing this back to CNG and CAPI, CNG can validate this signatures, but you must explicitly tell CNG that the signature does not have an object identifier. BCRYPT_PKCS1_PADDING_INFO’s documentation has the detail there, but gist of it is

If there is no OID in the signature, then verification fails unless this member is NULL.

This would be used with {B,N}CryptVerifySignature. To bring this back around to the .NET Framework, how do we use RSACng and give null in for the padding algorithm? The short answer is: you cannot. If you try, you will get an explicit ArgumentException saying that the hash algorithm name cannot be null.

For .NET Framework, this solution “keep using RSACryptoServiceProvider”. If you need to validate these signatures, chances are you do not need to use CNG’s newer capabilities like PSS since these malformed signatures appear to be coming from old systems. Higher level things like SignedCms and SignedXml use RSACryptoServiceProvider by default, so they will continue to work.

To bring in .NET Core, the situation is a little more difficult. If you are using SignedCms like so:

var signedCms = new SignedCms();
signedCms.Decode(File.ReadAllBytes("cms-with-sig.bin"));
signedCms.CheckSignature(true);

This will start throwing when you migrate to .NET Core. .NET Core will use CNG when run on Windows to validate RSA signatures for SignedCms and SignedXml. This is currently not configurable, either. When used with SignedCms, it ultimately calls the X509Certificate2.GetRSAPublicKey() extension method, and that will always return an implementation based on CNG.

If you are using SignedCms on .NET Core and need to validate a CMS that is signed with these problematic signatures, you are currently out of luck using in-the-box components. As far as other platforms go, both macOS and Linux environments for .NET Core will agree with CNG - that the signature is invalid.

The good news is, these signatures are not easy to come by. So far, only the old Verisign timestamp authority is known to have produced signatures like this.