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Double HMAC comparison to prevent timing attacks

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This commit is contained in:
Kyle Spearrin 2017-04-27 12:14:45 -04:00
parent cf58c1b4b5
commit bcb7d88ed7
1 changed files with 40 additions and 5 deletions
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45
src/App/Services/CryptoService.cs
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@ -258,7 +258,7 @@ namespace Bit.App.Services
var cryptoKey = provider.CreateSymmetricKey(key.EncKey);
var iv = WinRTCrypto.CryptographicBuffer.GenerateRandom(provider.BlockLength);
var encryptedBytes = WinRTCrypto.CryptographicEngine.Encrypt(cryptoKey, plaintextBytes, iv);
var mac = key.MacKey != null ? ComputeMac(encryptedBytes, iv, key.MacKey) : null;
var mac = key.MacKey != null ? ComputeMacBase64(encryptedBytes, iv, key.MacKey) : null;
return new CipherString(key.EncryptionType, Convert.ToBase64String(iv),
Convert.ToBase64String(encryptedBytes), mac);
@ -314,9 +314,9 @@ namespace Bit.App.Services
if(key.MacKey != null && !string.IsNullOrWhiteSpace(encyptedValue.Mac))
{
var computedMac = ComputeMac(encyptedValue.CipherTextBytes,
var computedMacBytes = ComputeMac(encyptedValue.CipherTextBytes,
encyptedValue.InitializationVectorBytes, key.MacKey);
if(computedMac != encyptedValue.Mac)
if(!MacsEqual(key.MacKey, computedMacBytes, encyptedValue.MacBytes))
{
throw new InvalidOperationException("MAC failed.");
}
@ -359,7 +359,13 @@ namespace Bit.App.Services
return decryptedBytes;
}
private string ComputeMac(byte[] ctBytes, byte[] ivBytes, byte[] macKey)
private string ComputeMacBase64(byte[] ctBytes, byte[] ivBytes, byte[] macKey)
{
var mac = ComputeMac(ctBytes, ivBytes, macKey);
return Convert.ToBase64String(mac);
}
private byte[] ComputeMac(byte[] ctBytes, byte[] ivBytes, byte[] macKey)
{
if(macKey == null)
{
@ -380,7 +386,36 @@ namespace Bit.App.Services
var hasher = algorithm.CreateHash(macKey);
hasher.Append(ivBytes.Concat(ctBytes).ToArray());
var mac = hasher.GetValueAndReset();
return Convert.ToBase64String(mac);
return mac;
}
// Safely compare two MACs in a way that protects against timing attacks (Double HMAC Verification).
// ref: https://www.nccgroup.trust/us/about-us/newsroom-and-events/blog/2011/february/double-hmac-verification/
private bool MacsEqual(byte[] macKey, byte[] mac1, byte[] mac2)
{
var algorithm = WinRTCrypto.MacAlgorithmProvider.OpenAlgorithm(MacAlgorithm.HmacSha256);
var hasher = algorithm.CreateHash(macKey);
hasher.Append(mac1);
mac1 = hasher.GetValueAndReset();
hasher.Append(mac2);
mac2 = hasher.GetValueAndReset();
if(mac1.Length != mac2.Length)
{
return false;
}
for(int i = 0; i < mac2.Length; i++)
{
if(mac1[i] != mac2[i])
{
return false;
}
}
return true;
}
public SymmetricCryptoKey MakeKeyFromPassword(string password, string salt)