Correct data corruption tests (#613)

Replace "256 - byte" with "~byte" as an example of data corruption.
I don't know what the person writing that thought, but if the byte
happens to be equal exactly 128 then 256 - 128 = 128, nothing gets
corrupted, and the tests fail. They do occasionally fail, when the
stars are just right, this was attributed to some issue in the token
parsers of Secure Cell which ignored some of the bytes. Now all that
code has been updated to be more strict so *any* changes in the data
should be reliably detected and result in a failure.

src/wrappers/themis/wasm/test/test.js

@@ -94,7 +94,7 @@ describe('wasm-themis', function() {
             })
             it('detect data corruption', function() {
                 let key = new themis.KeyPair().privateKey
-                key[20] = 256 - key[20]
+                key[20] = ~key[20]
                 assert.throws(() => new themis.PrivateKey(key))
             })
             it('handles type mismatches', function() {
@@ -197,7 +197,7 @@ describe('wasm-themis', function() {
             it('detects corrupted data', function() {
                 let cell = new themis.SecureCellSeal(masterKey1)
                 let encrypted = cell.encrypt(testInput)
-                encrypted[20] = 256 - encrypted[20]
+                encrypted[20] = ~encrypted[20]
                 assert.throws(() => cell.decrypt(encrypted),
                     expectError(ThemisErrorCode.FAIL)
                 )
@@ -299,15 +299,15 @@ describe('wasm-themis', function() {
             it('detects corrupted data', function() {
                 let cell = new themis.SecureCellTokenProtect(masterKey1)
                 let result = cell.encrypt(testInput)
-                result.data[5] = 256 - result.data[5]
+                result.data[5] = ~result.data[5]
                 assert.throws(() => cell.decrypt(result.data, result.token),
                     expectError(ThemisErrorCode.FAIL)
                 )
             })
             it('detects corrupted token', function() {
                 let cell = new themis.SecureCellTokenProtect(masterKey1)
                 let result = cell.encrypt(testInput)
-                result.token[8] = 256 - result.token[8]
+                result.token[8] = ~result.token[8]
                 assert.throws(() => cell.decrypt(result.data, result.token),
                     expectError(ThemisErrorCode.FAIL)
                 )
@@ -380,7 +380,7 @@ describe('wasm-themis', function() {
             it('does not detect corrupted data', function() {
                 let cell = new themis.SecureCellContextImprint(masterKey1)
                 let encrypted = cell.encrypt(testInput, testContext)
-                encrypted[5] = 256 - encrypted[5]
+                encrypted[5] = ~encrypted[5]
                 let decrypted = cell.decrypt(encrypted, testContext)
                 assert.notDeepStrictEqual(testInput, decrypted)
             })
@@ -446,7 +446,7 @@ describe('wasm-themis', function() {
             it('detects corrupted data', function() {
                 let secureMessage = new themis.SecureMessage(new themis.KeyPair())
                 let encrypted = secureMessage.encrypt(testInput)
-                encrypted[10] = 256 - encrypted[10]
+                encrypted[10] = ~encrypted[10]
                 assert.throws(() => secureMessage.decrypt(encrypted),
                     expectError(ThemisErrorCode.FAIL)
                 )
@@ -532,7 +532,7 @@ describe('wasm-themis', function() {
                 let signer = new themis.SecureMessageSign(keyPair.privateKey)
                 let verifier = new themis.SecureMessageVerify(keyPair.publicKey)
                 let signed = signer.sign(testInput)
-                signed[12] = 256 - signed[12]
+                signed[12] = ~signed[12]
                 assert.throws(() => verifier.verify(signed),
                     expectError(ThemisErrorCode.FAIL)
                 )