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Use managed RSASSA-PSS with SHA-1

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OpenSSL (the underlying RSA implementation on Linux) has deprecated SHA-1 signing. Used a managed implementation so that it does not error.
This commit is contained in:
Mbucari 2025-08-15 14:44:56 -06:00 committed by MBucari
parent 716b1923a4
commit 5ceda408da
1 changed files with 85 additions and 5 deletions
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90
Source/AudibleUtilities/Widevine/Device.cs
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@ -1,5 +1,6 @@
using System;
using System.IO;
using System.Numerics;
using System.Security.Cryptography;
#nullable enable
@ -56,18 +57,97 @@ internal class Device
public byte[] SignMessage(byte[] message)
{
using var sha1 = SHA1.Create();
var digestion = sha1.ComputeHash(message);
return CdmKey.SignHash(digestion, HashAlgorithmName.SHA1, RSASignaturePadding.Pss);
var digestion = SHA1.HashData(message);
return PssSha1Signer.SignHash(CdmKey, digestion);
}
public bool VerifyMessage(byte[] message, byte[] signature)
{
using var sha1 = SHA1.Create();
var digestion = sha1.ComputeHash(message);
var digestion = SHA1.HashData(message);
return CdmKey.VerifyHash(digestion, signature, HashAlgorithmName.SHA1, RSASignaturePadding.Pss);
}
public byte[] DecryptSessionKey(byte[] sessionKey)
=> CdmKey.Decrypt(sessionKey, RSAEncryptionPadding.OaepSHA1);
/// <summary>
/// Completely managed implementation of RSASSA-PSS using SHA-1.
/// https://github.com/bcgit/bc-csharp/blob/master/crypto/src/crypto/signers/PssSigner.cs
/// </summary>
private static class PssSha1Signer
{
private const int Sha1DigestSize = 20;
private const int Trailer = 0xBC;
public static byte[] SignHash(RSA rsa, ReadOnlySpan<byte> hash)
{
ArgumentOutOfRangeException.ThrowIfNotEqual(hash.Length, Sha1DigestSize);
var parameters = rsa.ExportParameters(true);
var Modulus = new BigInteger(parameters.Modulus, isUnsigned: true, isBigEndian: true);
var Exponent = new BigInteger(parameters.D, isUnsigned: true, isBigEndian: true);
var emBits = rsa.KeySize - 1;
var block = new byte[(emBits + 7) / 8];
var firstByteMask = (byte)(0xFFU >> ((block.Length * 8) - emBits));
Span<byte> mDash = new byte[8 + 2 * Sha1DigestSize];
hash.CopyTo(mDash.Slice(8));
var h = SHA1.HashData(mDash);
block[^(2 * (Sha1DigestSize + 1))] = 1;
byte[] dbMask = MaskGeneratorFunction1(h, 0, h.Length, block.Length - Sha1DigestSize - 1);
for (int i = 0; i != dbMask.Length; i++)
block[i] ^= dbMask[i];
h.CopyTo(block, block.Length - Sha1DigestSize - 1);
block[0] &= firstByteMask;
block[^1] = Trailer;
var input = new BigInteger(block, isUnsigned: true, isBigEndian: true);
var result = BigInteger.ModPow(input, Exponent, Modulus);
return result.ToByteArray(isUnsigned: true, isBigEndian: true);
}
private static byte[] MaskGeneratorFunction1(byte[] Z, int zOff, int zLen, int length)
{
byte[] mask = new byte[length];
byte[] hashBuf = new byte[Sha1DigestSize];
byte[] C = new byte[4];
int counter = 0;
using var sha = SHA1.Create();
for (; counter < (length / Sha1DigestSize); counter++)
{
ItoOSP(counter, C);
sha.TransformBlock(Z, zOff, zLen, null, 0);
sha.TransformFinalBlock(C, 0, C.Length);
sha.Hash!.CopyTo(mask, counter * Sha1DigestSize);
}
if ((counter * Sha1DigestSize) < length)
{
ItoOSP(counter, C);
sha.TransformBlock(Z, zOff, zLen, null, 0);
sha.TransformFinalBlock(C, 0, C.Length);
Array.Copy(sha.Hash!, 0, mask, counter * Sha1DigestSize, mask.Length - (counter * Sha1DigestSize));
}
return mask;
}
private static void ItoOSP(int i, byte[] sp)
{
sp[0] = (byte)((uint)i >> 24);
sp[1] = (byte)((uint)i >> 16);
sp[2] = (byte)((uint)i >> 8);
sp[3] = (byte)((uint)i >> 0);
}
}
}