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use random key for double hmac comparisons

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This commit is contained in:
Kyle Spearrin 2018-01-18 09:31:22 -05:00
parent 01878ef00c
commit 1390df48b6
2 changed files with 5 additions and 4 deletions
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2
src/App/Services/CryptoService.cs
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@ -386,7 +386,7 @@ namespace Bit.App.Services
if(EncKey?.MacKey != null && !string.IsNullOrWhiteSpace(encyptedValue.Mac)) if(EncKey?.MacKey != null && !string.IsNullOrWhiteSpace(encyptedValue.Mac))
{ {
var computedMacBytes = Crypto.ComputeMac(encyptedValue.CipherTextBytes, EncKey.MacKey); var computedMacBytes = Crypto.ComputeMac(encyptedValue.CipherTextBytes, EncKey.MacKey);
if(!Crypto.MacsEqual(EncKey.MacKey, computedMacBytes, encyptedValue.MacBytes)) if(!Crypto.MacsEqual(computedMacBytes, encyptedValue.MacBytes))
{ {
throw new InvalidOperationException("MAC failed."); throw new InvalidOperationException("MAC failed.");
} }

7
src/App/Utilities/Crypto.cs
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@ -95,7 +95,7 @@ namespace Bit.App.Utilities
if(key.MacKey != null && mac != null) if(key.MacKey != null && mac != null)
{ {
var computedMacBytes = ComputeMac(ct, iv, key.MacKey); var computedMacBytes = ComputeMac(ct, iv, key.MacKey);
if(!MacsEqual(key.MacKey, computedMacBytes, mac)) if(!MacsEqual(computedMacBytes, mac))
{ {
throw new InvalidOperationException("MAC failed."); throw new InvalidOperationException("MAC failed.");
} }
@ -148,10 +148,11 @@ namespace Bit.App.Utilities
// Safely compare two MACs in a way that protects against timing attacks (Double HMAC Verification). // 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/ // ref: https://www.nccgroup.trust/us/about-us/newsroom-and-events/blog/2011/february/double-hmac-verification/
public static bool MacsEqual(byte[] macKey, byte[] mac1, byte[] mac2) // ref: https://paragonie.com/blog/2015/11/preventing-timing-attacks-on-string-comparison-with-double-hmac-strategy
public static bool MacsEqual(byte[] mac1, byte[] mac2)
{ {
var algorithm = WinRTCrypto.MacAlgorithmProvider.OpenAlgorithm(MacAlgorithm.HmacSha256); var algorithm = WinRTCrypto.MacAlgorithmProvider.OpenAlgorithm(MacAlgorithm.HmacSha256);
var hasher = algorithm.CreateHash(macKey); var hasher = algorithm.CreateHash(RandomBytes(32));
hasher.Append(mac1); hasher.Append(mac1);
mac1 = hasher.GetValueAndReset(); mac1 = hasher.GetValueAndReset();